ALBERT

Notes: Summary In long-term organized cultures of newborn mouse cerebellum, granule cell neurons were studied with silver impregnation and electron microscopy. In silver impregnated cultures, small neurons are defined as granule cell neurons from their size, morphology and location. There are also occasional large nerve endings with the morphology typical of mossy fiber endings. In correlative electron micrographs, granule cell neurons revealed a faithful reproduction of characteristic structures seen in vivo. The fine structural details of cerebellar glomeruli and myelinated granule cell bodies developed in vitro were also described.

Notes: Summary The relations between adrenergic and cholinergic terminals were studied in rat iris and rat heart with the electron microscope. Adrenergic terminals were identified by treating the animals with 5-hydroxydopamine, which produces dense-cored synaptic vesicles in adrenergic terminals in tissues fixed in glutaraldehyde and osmium. The specificity of this observation was verified. It was found that adrenergic and cholinergic nerve terminals often come in close contact with one another, the distance between the adjoining membranes being about 250 Å. At times, faint membrane thickenings could be observed in these places. The available pharmacological, physiological, and morphological evidence leaves little room for doubt that cholinergic terminal fibres can influence the adrenergic fibres. From mainly morphological evidence, it is also postulated that adrenergic terminals influence cholinergic ones.

Notes: Summary Ultrastructural studies have been carried out on the innervation of the interstitial gland of the mouse ovary. In addition Falck's fluorescence method was applied. 1. Fluorescence microscopy: In the ovarian stroma green fluorescent nerve fibers are frequently to be found in the surroundings of large and small vessels. Seldom small fibers invade blocks of interstitial cells; however, their final ramification is not discernible. 2. Electron microscopy: Terminal fibers of the autonomic nervous system reach the cells of the interstitial gland from all sides. They may be independent from the course of the vessels. Many axons penetrate the basal membrane and come into close contact with interstitial cells, partly by forming large swellings (boutons), which may be deeply embedded into the cytoplasm of the innervated cell. The synaptic cleft is about 200 Å wide. Specialized pre- and postsynaptic membranes have not been found. The innervated cells show no peculiarities. The possible function of the synapses is discussed.

Notes: Summary The fine structure of the nerves of the ovarian stroma of the domestic fowl is described for the first time. In the fowl, the nerves are concentrated upon blood vessels, smoth muscles and mainly, the thecal gland with the steroid-producing cells. Myelinated as well as unmyelinated nerve fibers were observed. Numerous axon terminals representing adrenergic and also presumptive cholinergic nerve fibers are regularly seen in membranous contact with steroid-producing cells. In these axon terminals microvesicles are oriented towards the steroid-producing cells indicating a specialization of the surface from axon-to-cell contact. Evidence has been presented that there is a membranous neuro-humoral contact between the peripheral autonomie nervous system and the steroid-producing cells in the ovary. The present investigation has demonstrated that there is morphologic evidence for a nervous control of steroid-producing cells. The physiological importance of this neuro-humoral contact is discussed.

Notes: Summary Pituicytes of Rana pipiens could be classified into two types, pale and dense, according to their relative densities of cytoplasm and the populations of free ribosomes and cell organelles. An intermediate type of pituicyte was also recognized. Lipid droplet such as are typical in the cytoplasm of mammalian pituicytes, are not in the cytoplasm of either types of frog pituicyte. Both types have long cytoplasmic processes which run among the nerve fibers, and some of them end at the pericapillary space. Nerve endings making synapse-like contacts with the cell bodies or the processes of the pituicyte are frequent. According to the structures and sizes of granules and vesicles in the nerve endings, these endings are classified into one of three types: 1) A, which appears to be a peptidergic neuronal ending containing dense granules 1,200–2,000 Å in diameter and small clear vesicles 300–400 Å in diameter; 2) B, which appear to be monoaminergic endings containing cored vesicles 600–1,000 Å in diameter and small clear vesicles 300–500 Å in diameter; 3) C, which appear to be cholinergic endings containing only small clear vesicles. Type C endings are relatively rare. In the synaptic area the axonal membranes appose those of the pituicytes across a gap of about 200 Å and numerous “presynaptic” vesicles are clustered or accumulated near the presynaptic membranes.

Notes: Summary Ependyma and subependymal regions of the lateral ventricles of cats (area of Nucleus caudatus and Corpus callosum) were fixed by perfusion and investigated electronmicroscopically. Intraventricular axons showing a beaded shape with smaller and thicker parts were found. The varicosities contain empty vesicles, dense core granules and mitochondria. Beside those certainly neuronal elements there exist intraventricular cell processes originating from ependymal cells and free cells. The neuronal processes make desmosome-like contacts with the ependyma. These regions of contact are surrounded by basket-like arranged microvilli. It is discussed, whether the contacts function like synapses or as receptors for a substance, which is still unknown. Contacts do not depend on the shape of ependymal cells. They exist with the ependyma of Nucleus caudatus (grey substance) as well as with the ependyma of Corpus callosum (white substance). Because of the different contents of cell-organelles one can differentiate three different sorts of processes situated inter- or intracellularly in the ependyma. Neuronal processes within the lateral ventricles are more numerous than those found within the ependymal layer.

Notes: Summary Synaptosomes (nerve-ending particles) prepared from the cerebral cortex of rats, aged 2–21 days, were examined after fixation in glutaraldehyde and osmium tetroxide, in order to study the development of the contact region between their pre- and postsynaptic components. Synaptosomes were present at all ages studied, although they increased in number and underwent morphological changes, e.g. a decrease in ribosomes and increase in synaptic vesicles, during development. The seventh postnatal day appeared to be a critical period for development, as many adult features were first observed at this time. The synaptic contact region was characterized by desmosome-like symmetrical thickenings until the fourth postnatal day, after which it became increasingly asymmetrical. The postsynaptic thickening, which had been undifferentiated until the seventh day, underwent a focalization of its material at this stage to form postsynaptic densities typical of the adult synapse. Cleft material was present at all ages, although for the first few days it was sparse and comparatively unorganized. An attempt has been made to distinguish between synaptosomes derived from axosomatic and axodendritic endings. The possible significance of the early desmosome-like thickenings is discussed, and the role of the cleft material and post-synaptic densities in the development of the synapse is analysed.

Notes: Summary The pars tuberalis of the hypophysis of Rana temporaria presents the general structural and the cytological characteristics of an endocrine gland. It is composed of elongated cells with long, branching processes ending on the external basement membrane of the pericapillary space. The pars tuberalis cells produce secretory granules which are accumulated in the pericapillary endings of the processes. Corresponding to its separate localization, the pars tuberalis of Rana temporaria has a separate vascularization of which the efferent capillaries anastomose with the capillary plexus of the median eminence. The general direction of the blood flow of the pars tuberalis is towards the capillaries of the median eminence. Also, the secretory products of the pars tuberalis pass into the blood stream of the hypophysial portal system. Several characteristics of the pars tuberalis show that its function must be different from that of the pars distalis of the hypophysis. Moreover, in contrast with the pars distalis, the activity of the pars tuberalis is not regulated by neurohumoral factors. The results show that a role of the pars tuberalis in the regulation of the activity of the pars distalis of the hypophysis is not excluded.

Notes: Summary Electron microscopic studies and three-dimensional graphic reconstructions from serial sections have shown that the large axon terminals of synaptic glomeruli in the ventrobasal nucleus of the rat are invaginated by spine-like protrusions from the astrocyte processes surrounding the glomeruli (“micro-trophospongium”). The astrocytic protrusions are similar in dimensions and internal morphology to the synapse-bearing dendritic excrescences that also invaginate the large axon terminals. Consequently astrocytic protrusions may be overlooked, or confused with dendritic excrescences sectioned at a non-synaptic level. The intimate neuronal—neuroglial relationship at such large axon terminals may reflect ion-exchange or metabolic interactions between the astrocytes and the axon terminal.

Notes: Summary The innervation of the adrenal cortex of the rat and the pig is investigated with the electron microscope. Nerve fibers containing synaptic and two types of dense-cored vesicles come into contact with endocrine cells. There are no specialized pre- and postsynaptic membranes. The synaptic cleft is about 200 Å wide. Generally the basement membrane between nerve and cell is absent. These observations are discussed on the base of more recent experimental findings. Small fibers having an average diameter of about 0.2 to 0.5 μ and containing only tubules and filaments are considered to represent parts of an afferent nervous system.

Notes: Summary The innervation of the mesenteric microvasculature was studied in fetal and neonatal rabbits with the aid of methods demonstrating fluorescence of catecholamines and cholinesterase activity as well as a silver impregnation procedure. The results showed that: (1) adrenergic nerve fibers were present, coursing independently in the mesentery by day twenty-one of gestation, and were found routinely in the adventitia of arterioles and venules by day 25 of gestation; (2) cholinesterase positive cells and fibers of the myenteric plexus were present by day 18 of gestation but cholinergic fibers were not present in the mesentery until day 26; the latter not being associated with blood vessels; and (3) nerve fibers in the mesentery thought to be sensory stained positively with the Holmes silver method on day 18 of gestation.

Notes: Summary The sciatic nerves were analysed using light and electron microscopes in the mouse, rat, guinea pig, cat, and dog (see Methods of G. Schnepp et al., 1971). 1. In light microscopic material, the ratio “g” between mean axon diameter and mean fibre diameter increased from 0.4 for small fibres to 0.6 for the thick fibres. For a given fibre diameter the myelin sheaths in rats and dogs were the thickest, while those in mice were the thinnest. 2. The ratio “g” in electron micrographs on the other hand was constant at 0.6 over a wide range of fibre diameters. The ultrastructural values agreed with those postulated by Rushton (1951) for optimal nerve conduction. In the electron microscopic material, the myelin sheaths were the thickest in the rat. 3. A linear proportionality was confirmed to exist between internodal length and increasing fibre size. Independently of fibre size, in all sciatic nerves, a few fibres were found with internodal length of about 300 μ. 4. In electron micrographs, no species differences were observed in the structure or distribution of unmyelinated nerve fibres.

Notes: Summary Quantitative electron microscopic studies have been carried out on the human thymus. According to the equation L v =(2n)/F (Hennig, 1963) we have calculated that there is less than 0.204 mm nerve per 1 mm3 thymus tissue inside the blood-thymus-barrier (level of significance of 0.95). This result is compared to the degree of innervation in brown adipose tissue, which contains more than 160 mm nerve per 1 mm3 tissue. The biological significance of the paucity of neuronal elements in the thymus is undetermined.

Notes: Summary 1. The Harderian gland in rabbits, representing the type of a tubulo-alveolar gland, is located on the medial and posterior aspect of the eyeball and consists of two different parts, a small white lobe and a larger red one. The secretory cells in the tubular endpieces of both lobes are lipids containing cells. The lipid droplets can be stained with Sudan IV and Sudan black B. The luminal surface of both cell types is characterized by an alcianophilia at pH 2,5. 2. The tubules of both lobes have a single layer of columnar epithelium. The lipid vacuoles in the cells of the red lobe are large, these of the white lobe small. The multilocular cytoplasm of all cells contains many free ribosomes and high amounts of mitochondria lying very closely together. All cells exhibit numerous and large Golgi-zones but only few ergastoplasm membranes. 3. The lateral surfaces of the secretory cells are connected by elaborate junctional complexes (Zonulae occludentes, zonulae adhaerentes, desmosomes). These lateral surfaces are increased by intercellular canaliculi. 4. Before being released into the glandular lumen, the limiting membranes of adjacent lipid droplets fuse, thus forming a large lipid vacuole. Extrusion generally is characterized by the coalescence of the limiting membrane with the plasmalemma, the formation of an opening at the cell surface and the discharge of the secretory lipid material. In the course of another mechanism of extrusion, the fat vacuoles are transported to the apical part of the cell where consequently the plasmamembrane bulges into the lumen. Eventually the fat vacuole is pinched off surrounded by a thin cytoplasmic envelope. 5. Terminal fibers of the autonomic nervous system penetrate the basal membrane and can be found closely attached to the secretory or myoepithelial cells, partly by forming large swellings, which may be deeply embedded into the cytoplasm of the innervated cell. These terminal parts of the axons contain groups of synaptic and dense-cored vesicles, mitochondria and neurotubuli. Specific pre- and postsynaptic membranes have not been observed. The possible function of the harderian gland is discussed.

Notes: Summary Electron microscopic studies have been carried out on the innervation of the mammalian anterior pituitary and parathyroids. The total area of grid squares (2.25·10−2mm2) examined was 2000 per gland and species. In the pituitary pars distalis and in the parenchyma of the parathyroid gland we did not observe a single axon profile. According to the equation $$L_V = \frac{{2n}}{F}$$ proposed by Hennig (1963) we have calculated that there might be—if any—0.133 mm of nerves per 1 mm3 tissue in those two endocrine glands (level of significance 0.95). Comparing these results to the degree of innervation in brown adipose tissue containing more than 160 mm nerve per 1 mm3 tissue we can not imagine that such a small degree of innervation is of any biological importance. In the pituitary pars tuberalis two types of axon terminals have been found both inside and outside the basement membrane surrounding the epithelial complexes. One type contains “synaptic” and two populations of smaller dense-cored vesicles, the other one contains a population of larger granules which have some properties of the classical elementary granules. Further investigations have to clarify the functional significance of those nerve endings.

Notes: Summary The organization of the α-lobes of the corpora pedunculata in the brain of the cricket Acheta domesticus L. has been investigated in the light and electron microscopes. The cylindrical fibre complex is composed of branches of “mushroom-body” fibres (intrinsic fibres) and extrinsic fibres, which penetrate the α-lobe. Intrinsic fibres (IF) run through the α-lobe in the same direction, but not strictly parallel to each other or to the axis of the α-lobe. Extrinsic fibres (EF) and their fine branches are often arranged perpendicular to the axis of the α-lobe. There is some evidence that different IF zones occur in the α-lobe when passing from its periphery to its centre. The distribution of EF may reflect a structural order when passing from the base of the lobe to its top. Numerous polarized synapses connect the IF with the EF. The IF show clusters of vesicles and presynaptic figures especially in their “blebs”, which can be seen in Golgi preparations for light microscopy. Two types of EF are distinguished on the basis of their synaptic junctions: (1) postsynaptic EF (abundant) and (2) EF with pre- and postsynaptic sites (perhaps restricted to some regions of the α-lobe). Presynaptic IF converge on EF, which may transfer excitation from the α-lobe to different parts of the brain and nervous system.

Notes: Summary The abdominal ganglia of the crayfish Astacus pallipes contain numerous vertebrate-like synapses which are characterized by presynaptic vesicles, darkened pre- and post-synaptic membranes, cleft material, and post-synaptic “fuzz”. Such synapses occur throughout the ganglia but are most easily found dorsally, where the neuropile is relatively coarse. The neuropile is far from homogeneous. Regional variations in fiber size, in degree of profile tortuosity, and in kind, magnitude, and distribution of vesicular content result in conspicuous textural variations. The structural polarity of synapses between the lateral giant fibers and other neurons is consistent with known physiological polarity and, hence, validates our criteria for recognition of synapses within the ganglion.

Notes: Summary The penis retractor muscle of Helix pomatia is passed by thick nerve trunks, which probably are nerve nets. In the contracted muscle, they are folded meanderlike, in the extended they are pulled smooth. Four types of vesicles different in size and contents can be distinguished in the nerves. Varicose fibres accompany the muscle cells. Frequently the terminals are running in a groove of the muscle fibre. In certain axon types occur presynaptic accumulations of vesicles and thickenings of the terminal membrane. Terminal and muscle cell are separated by a cleft of 300 AE width. The muscle fibre membrane has no subsynaptic infoldings. Beside these axons thin, naked neurites are running through the connective tissue. They are characterized by a high content of neurotubuli. One part of the axons presumably possesses a monoaminergic transmitter. After the glutaraldehyde-dichromate-reaction they contain dense grana, whose diameters are mainly below 1000 AE. The nerve trunks fluoresce after exposure to paraformaldehyde vapour, excited with UV-light, green to green-yellow. The maximum of the excitation was determined at 413 nm and the maximum of emission at 496 respectively at 510 nm. It is concluded, that both, a catecholamine and 5-HT are responsible for the fluorescence. Extraction and paperchromatographic separation lead to the opinion, that the catecholamine is dopamine.

Notes: Summary First appearance, structure and topological relations of Bowmans' glands in the regio olfactoria of white mice are described. The importance of these glands for the formation of the terminal mucous cover of the olfactory epithelium is discussed. In the last quarter of intrauterine life the glands of Bowman reach the lamina propria. In the terminal portion of the glands dark cells with many secretory droplets and pale cells with only a few of them can be seen. Secretory active cells are localized in the basal part of the olfactory epithelium as well. When entering the lamina propria the irregular wide basement membrane of the glands joins that one of the epithelium. It is possible to follow up this joined basement membrane for a short distance between the glands and the cells of the olfactory epithelium. Peripheral to the very basal part of the olfactory epithelium there is no basement membrane around the glands' tissue. Receptors and sustentacular cells are separated from the gland only by a normal intercellular space. The epithelium of the ducts consists of dark and light cells as well. They are connected with the sustentacular cells by desmosomelike contacts. In its prolongation the lumen of Bowman's duct is lined by the apical portions of the sustentacular cells and their microvilli, and by dendrites, olfactory vesicles, and sensory cilia of the receptor cells. In the region of cilia and microvilli one can see masses of secretion which have confluenced. In the intervillous space its special affinity to the receptor's membranes is evident. During the intrauterine phase of life no terminal mucous cover could be demonstrated. The drying effect of the air as a possible reason for the origin of the terminal mucous cover is discussed.

Notes: Summary Light microscopy and transverse electron microscopy has been employed to study the olfactory organs in 82 specimens of freshwater adapted young and homing adult Baltic sea trout Salmo trutta trutta L. In both sensory and indifferent epithelium the olfactory mucosa has scattered cells of a type that has not been described in any olfactory organ before. They are called labyrinth cells and are characterized by an extensive, turtuous, interconnected tubular system of smooth endoplasmatic reticulum intimately connected with numerous mitochondria. This cell type is similar to chloride and other cells which probably are involved in electrolyte transport in fish gills and pseudobranch, the rectal gland in elasmobranchs and the nasal gland in reptiles and birds. It is suggested that the olfactory organ in fish is serially homologous with the pseudobranch.

Notes: Summary Recent investigations have suggested specific differences in back and belly skin in anurans which appear to influence the quality of reflex responses obtained from various areas of the animals body. The present investigation represents a comparative morphological study of back and belly skin in control and skin-graftedRana pipiens, with special regard to the neuroepithelial relationships. A distinct difference in pigmentation of back and belly skin was observed. Intra-epithelial Merkel cells were present in all skin samples studied. The origins of the numerous unclassifiable cells in the Merkel cell region are discussed in relation to a presumed coordinating function of the Merkel cell during epithelial differentiation. Epitheliomesenchymal interactions were observed in the richly innvervated dermal regions. Two types of morphologically different intra-epithelial nerve endings were observed. These observations are discussed in relation to earlier observations on vertebrate skin and in relation to misdirected reflex responses obtained in skin-grafted anurans.

Notes: Summary Structural changes in drosopterinosomes (red pigment granules) of Rana japonica in the process of erythrophore differentiation were studied by light and electron microscopy. On the basis of the degree of pterinosome differentiation, three types can be recognized: Typ-I drosopterinosomes appear first during metamorphosis and have clear limiting membranes and amorphous materials within. Those of type-II are found in abundance shortly after metamorphosis and have inner structures, consisting of fibrillae and/or small lamellae in dense concentric arrangement. Type-III is found abundantly in adults and acquires an almost homogeneously electron-dense mature morphology, probably from the deposition of electron-dense materials. On the basis of counts of pterinosomes, a successive transformation from type I to III is suggested. The differences among red drosopterinosomes, yellow sepiapterinosomes in xanthophore and melanosomes are not always distinguishable electron microscopically. Discrimination is possible by careful examination of lamellar patterns characteristic of the respective granules and by a simultaneous application of light and electron microscopy. From this viewpoint, a re-evaluation of the identification of granules previously reported was effected.

Notes: Summary The surface of 4 granule-containing cells, in a cluster within the rat superior cervical ganglion, was studied by a serial sampling technique for electron microscopy. The result shows that all the 4 cells receive one, or three afferent synaptic boutons from the preganglionic fibers impinging upon their somata, and a somatic efferent synapse exists at two locations on each soma of the 2 of these cells. The postsynaptic element of the efferent synapse is observed to be represented by non-vesiculated and vesiculated segments of dendrites, soma and a possible axon collateral of the adrenergic principal neuron of the ganglion. There is a remarkably constant development of the attachment plaque between the granule-containing cells themselves, representing 1.7–2.3% of surface area for each cell. The surface area exposed to the extracellular space (covered only by a basal lamina) varies from 0.1 to 2.3% of the total perikaryal surface of the 4 cells. A tendency is noted that those cells without efferent synapses possess a more extensive area exposed to extracellular space than those forming somatic efferent synapse to the postganglionic elements.

Notes: Summary The lobules of the Harderian glands of young male and female hamsters are widely separated by connective tissue and are made up of alveolar cells containing small vacuoles. There is considerable increase in the size of the lobules by 12 days. Based on size distribution of vacuoles two types of alveolar cells can be recognized in the male hamsters 4 weeks and older. The Harderian glands of 2-week animals of both sexes contain a black-brown pigment. The appearance of pigment is accompanied by a proliferation of membranes. In the female the amount of pigment increases with increasing age until 4 weeks of age and then remains at a relatively stable level. In the male no pigment was found after 4 weeks of age. Membrane-bounded juxtanuclear structures are present in the alveolar cells of both sexes until 3 weeks of age. These structures appear to play different roles in the male and female hamsters. The differentiation of the alveolar cells into light and dark types must take place during the 3rd week of postnatal development. A possible relationship between the levels of gonadal hormones and the occurrence of pigment at different ages in both sexes is also discussed.

Notes: Summary The olfactory and accessory lobes of the crayfish, Cherax destructor contain glomeruli. Light microscope and electron microscope studies show that these glomeruli are the only regions of synaptic activity in the lobes and that at least four separate sets of axons meet within the glomeruli. The olfactory glomeruli are column shaped, complex structures with no large single pre- or postsynaptic elements. The accessory lobe glomeruli follow a more conventional pattern and each has one large axon ending in a terminal arborization where it makes synaptic contact with large numbers of smaller fibres. The large fibre is presynaptic.

Notes: Summary Ultrastructural evidence is given of the occurrence of nervous elements in the mesoglea of Ctenophores based on the presence of the typical synapses of this phylum. In Beroids, nervous fibers from the ectodermal nerve-net cross the epithelial basal membrane and run through the mesoglea; they are devoid of any ensheathing cell. These neurites build highly differentiated synapses upon the muscles and upon peculiar cells, tentatively named mesenchymal cells. In Cydippids, nerve fibers and nerve cell-bodies have been observed in the mesoglea of the tentacles. The mesogleal core of each tentacle contains mesenchymal cells and a thick strand of neurons and neurites, forming a kind of elongated ganglion. Neurites of either the axial neurones or the epithelial nerve-net neurones form numerous radial nerve strands across the tentacular muscles. Interneural, neuro-muscular and neuro-mesenchymal junctions are very frequent in the tentacle. As far as the organization of the mesoglea is concerned, the Ctenophora thus appear closer to Turbellaria than to Cnidaria.

Notes: Summary The antimitotic drugs colchicine and vinblastine were applied to the melanophores of isolated scales of Pterophyllum scalare. The time course of inhibition of pigment migrations was studied quantitatively with the photoelectric method. Within one hour of treatment with 5 mM colchicine, a partial and reversible inhibition of pigment movements is obtained. During this time, the number of microtubules per cell process does not differ significantly from controls when pigment is dispersed. However, their number increases concomitantly with the decrease in rate of aggregation induced by KCl up to about the same value as in dispersed pigment cells. This indicates a close connection between pigment movements and the microtubule turnover known to occur in angelfish melanophores. When applied in a concentration of 0,2 mM, vinblastine has a total and irreversible effect on pigment movements, with an especially rapid and profound inhibition of pigment dispersion. The fine structure of vinblastine-treated melanophores is characterized by an almost complete loss of microtubules and the appearance of crystalloid bodies, which differ in fine structure from vinblastine-induced crystals of all other cell types so far examined. Possibly their peculiar structure is related to special properties of melanophore microtubules, such as rapid turnover and relatively high resistance to colchicine.

Notes: Summary The locus coeruleus of cat is populated by two types of neurons: medium sized ones, with plump cell bodies and relatively short dendrites; and small ones, with triangular bodies and relatively long dendrites. The former type is regarded here as typical of the centre, whereas the second type could simply represent displaced neurons from the adjacent griseum centrale. Electron microscopy failed to reveal any outstanding richness in pigment granules in kittens up to five weeks old. Very characteristic somatic appendages were found, mostly in the medium sized neurons. These somatic “spines” communicate with the perikaryon by means of a narrow neck region. A complex, multilayered, glial sheath surrounds the cells. This glial sheath is pierced by the somatic appendages, which are not surrounded by glia and make contact with axonal knobs. Typical dendritic spines appear to be absent. Axodendritic synapses are made on medium sized dendritic trunks. By and large, most of the synaptic vesicles present in the centre are of the small, clear-centered type. However, dense core vesicles extremely variegated in size and appearance were found, both in presynaptic and postsynaptic profiles. The possibility that dense core vesicles should be regarded as atypical lysosomes rich in by-products of the metabolism of catecholamines (melanine) has been considered.

Notes: Summary The anatomy of the blood supply to the gills of the dogfish, Scyliorhinus canicula, is described. The anatomical basis for a counter-current exchange system at the respiratory surfaces is reported. Within the interbranchial septum there is a capillary network joining all the afferent branchial arterioles of the gill. The structure of the walls of the corpus cavernosum is found to be of smooth muscle cells supported by a basal lamina and connective tissue and lined by endothelial cells containing phagocytic vesicles. Both the capillary network and corpus cavernosum are suggested to function in smoothing the pressure pulses of the blood flow. Pre- and post-lamellar vessels and pre- and post-lamellar sphincters are described. The sphincters are thought to control the number of secondary lamellae physiologically in the respiratory circuit, and by retaining blood within nonperfused lamellae to act in conjunction with pillar cells (contracting in antagonism to the hydrostatic skeleton of the blood) to maintain the rigidity of secondary lamellae in the water current. Whorls of cells of unknown function are found within the interbranchial septum. In the epithelium lining the water channel large cells having a complexly branching plasma membrane and a very large central vacuole occurs. The cytoplasm lining the lumen contains numerous vacuoles each surrounded by a double membrane.

Notes: Summary There is a giant dopamine-containing cell (GDC) in the left pedal ganglion of Planorbis corneus. Some presynaptic endings of the GDC are located within the visceral and left parietal ganglia, other endings are located peripherally. Dense-cored vesicles of 50–250 nm diameter were observed in the perikaryon and primary axon of the GDC. Electron microscope histochemistry suggests that these vesicles contain dopamine. Vesicles with a similar appearance are present in some axonal processes located in areas of the nervous system known to contain presynaptic endings of the GDC. This neurone offers unique advantages for studying the role of neuronal dopamine.

Notes: Summary The present study was undertaken to re-examine the central nucleus of the cat in Nissl stained sections, to describe its fine structural characteristics, and to compare the lateral subdivision of this nucleus with the putamen. In Nissl preparations, it was observed that the lateral subdivision of the central nucleus contains a fairly homogeneous population of small cells while the putamen contains both small and large cells. It is the presence of a few large cells at the lateral periphery of the lateral part of the central nucleus that makes the boundary between the two nuclei indistinct. Examination of the fine structure of the medial and lateral subdivisions of the central nucleus revealed the presence of many boutons containing flattened vesicles and fewer with spheroid vesicles. In contrast, most of the boutons in the putamen have spheroid vesicles. It is concluded that the lateral subdivision of the central nucleus may be distinguished from the putamen, except at its most lateral border, by its homogeneous population of small cells and its many boutons containing flattened vesicles.

Notes: Summary Lateral line organs in young salamanders of the species Ambystoma mexicanum were investigated with scanning and transmission electron microscopy. They were found to differ from the lateral line organs in adult animals (1) by being lower, having short hair cells and supporting cells, (2) by the hair cells having areas of lateral contact, (3) by the occasional presence, at the edge of the organ, of hair cells at an early developmental stage. Two types of nerve endings are seen: (1) afferent, and (2) less commonly, efferent vesiculated ones. Synaptic bodies have been seen in the cytoplasm without association to afferent synapses.

Notes: Summary Qualitative and quantitative studies were made to determine the amount of nerve fiber supplying corpora lutea (CL) of rats during the oestrous cycle and pregnancy and sow CL during days 4–6 after ovulation. Fluorescence microscopy of freeze-dried, paraformaldehyde treated (Falck-Hillarp method) rat ovaries reveals adrenergic nerve fibers which run along with vessels and form a network among interstitial gland cells. Nerve fibers do not enter the granulosa cell layer in follicles or CL. In the CL circumference both vascular and non-vascular nerves occur the latter being related to the fibromuscular layer and probably innervating smooth muscle cells. No striking differences exist between the innervation of the ovary in non-pregnant and pregnant rats. Bodian and methylene blue staining did not contribute to a more detailed knowledge of rat ovary nerve supply. Electron microscopic quantitative analysis of rat and pig CL (rat: day 18 of pregnancy; pig: day 4–6 after ovulation) revealed no axon profiles in 2.000 grid squares (one square measuring 2.25×10-2 mm2) of randomly taken CL sections. Thus it was possible to calculate an upper limit of 133 μm of nerve fibers per 1 mm3 CL tissue, in case there were any at all.

Notes: Summary The number of Golgi cells per unit volume was determined in different regions of the cerebellar cortex of man and of ten other mammals. Despite the general belief in the uniform architecture of the cerebellar cortex, regional differences in the distribution of Golgi cells were found. In the inferior parts of the vermis, the number of Golgi cells per unit volume is twice that in the corresponding hemispheres. In addition, there are differences between the anterior and inferior parts of the vermis. These differences are a feature of the cytoarchitecture of the cerebellum in man and all the investigated mammals. The ratio of Purkinje cells to Golgi cells was also determined and found to differ in different species. In man, this ratio is 1∶1.5, while in the monkey and cat it is almost 1∶1.9 and in the rat 1∶3.3. These differences in the ratio of Purkinje cells to Golgi cells are discussed from the point of view of cerebellar evolution.

Notes: Summary The structure of the aesthete organs of the chiton Lepidochitona cinereus (L.) is described by light and electron microscopy. The observations do not discount a sensory function for these organs, but possible sensory structures such as microvilli and cilia are poorly organised. Most of the aesthete is taken up with large cells actively forming secretory products and apparently passing them proximally in the aesthete canal towards the animal. Comparisons are drawn with the brachiopod caecae.

Notes: Summary Pulse labelling with tritiated thymidine was used to determine the time of the final division of the neuroblasts which subsequently form rat lumbar dorsal root ganglion neurones. The final division occurred during a 4 day period, the maximum frequency being on day 12 of gestation. Separation of the ganglion cells into large light neurones and small dark neurones showed that the large light neurones were formed earlier than the small dark neurones. In both cases the final divisions occurred over a period of 3–4 days, but the peak rate of formation of large neurones was on day 12, and that of the small neurones was on day 13. Low power electron micrographs were used to measure mean cell diameter throughout development from day 11 of gestation until a postnatal age of 225 days. A marked increase in cell diameter occurred on day 15–15.5, about 3 days after the final cell divisions of the majority of the cells. The rate of growth increased just before birth, but no increase in mean cell diameter was found between day 21 of gestation and the third day postnatal. The growth was again rapid after this period until a plateau in cell diameter was reached about 33 days after birth.

Notes: Summary The innervation of the pituitary gland of Carassius auratus was studied by light and electron microscopy under various physiological and experimental conditions to investigate whether or not neurosecretory fibres play a role in regulating pars distalis function. Two types of neurosecretory fibre (Type A and Type B) were distinguished. Prolactin, ACTH and TSH1 cells were innervated by Type B fibre terminals separated from the endocrine cells by a continuous basal lamina (“indirect contacts”). Gonadotropic, STH and TSH2 cells were innervated by Type A as well as Type B neurosecretory fibres, mostly without an intervening basal lamina (“direct contacts”). The assessment of the amount of neurosecretory granules and microvesicles in nerve terminals during the pre-spawning, spawning and postspawning seasons and following the administration of Oestradiol, Thyroxine, Thiourea and Metopiron respectively revealed convincing evidence for a participation in pars distalis control for Type A and Type B fibres innervating gonadotropic cells and STH cells and Type B fibres innervating TSH2 and ACTH cells. Immediately after spawning both nerve fibre types innervating gonadotropic cells and Type A fibres innervating STH cells showed a striking decrease in the amount of dense core vesicles. During the spawning season nerve fibres innervating somatotropic cells, TSH2 cells and ACTH cells also undergo changes suggesting that prior to spawning major changes in the endocrine system of the goldfish take place.—These results point to a dual control, by peptides and amines, of teleost pars distalis function.

Notes: Summary Anlagen of the pars distalis (Rathke's pouch), adjacent ectoderm, endoderm, infundibulum and mesenchyme left in situ were put in 3% glutaraldehyde between 1–3 pm and fixed overnight. Epon sections of this material from six White Leghorn, Gallus gallus, embryos fixed at each stage, and of two control partes distales from laying hens of the same flock were examined. At stage 17 (∼12 hours after formation) Rathke's pouch cells were stratified, uninnervated, non-vascularized and stellate, with high nucleo-cytoplasmic ratios and few organelles. Except for lipid inclusions, pouch cells did not appear appreciably different by stage 27, either regionally within the pouch wall or from the adjacent epithelioid cells. Apparent major changes indicative of cytoplasmic maturation by stage 27 included: reduction in number of polysomes; appearance of single-membraned, dumb-bell shaped to rounded, dense granula which were usually in basal position but also in areas of the Golgi apparatus; greater prominence of rough endoplasmic reticulum with cisternae containing dense material, and of the Golgi apparatus, notably with variously dense and/or coated vesicles and material; reduction in lipid inclusions by stage 24. Non-terminating axons in the infundibulum were first seen at stage 27. We found no convincing evidence for any possible morphogenetic or other relationship between the pars distalis and surrounding tissues. “Melanophore-stimulating hormone-activity”, reported to appear on day 5 of incubation, “stored thyroid-stimulating hormone” or pars distalis hormone granula or activities could not be identified. Rathke's pouch and other epithelioid structures may have been secreting, taking up nutrients and/or differentiating, activities which could be important for yolk sac development, nutrition and/or cytodifferentiation, respectively.

Notes: Summary The glandulomuscular cells of Hydra are located exclusively in the basal disk. They are derived from epithelio-muscular cells which have been forced proximally. Light and electron microscopical studies show that prior to their destruction and elimination, the transformed epithelio-muscular cells (i.e. the glandulomuscular cells) undergo certain striking morphological and physiological changes. Golgi complexes and elements of rough E. R. increase remarkably in activity, and individually or jointly produce at least six types of morphologically different droplets. One additional type of droplet is thought to originate from neighboring digestive cells. Although the chemical nature of the individual droplets is uncertain, it is known that some are Alcian blue and PAS positive and contain hyaluronic acid. These evidences suggest the presence of an acid mucopolysaccharide material, the adhesive agent which attaches the animal to a substrate. The myonemes contain thick (200 Å in diameter) and thin (60 Å in diameter) filaments as in epithelio-muscular cells. There are also filaments of intermediate sizes and large fibers (770 Å in diameter). The myonemes are oriented radially with respect to the aboral pore and therefore in addition to contributing to the contraction and relaxation of the body column, they apparently regulate the opening and closing of the aboral pore. Although there is no evidence to substantiate the mechanism for transformation of epithelio-muscular cells to glandulomuscular cells as well as cell death of the latter cell types, these problems are discussed briefly.

Notes: Summary In addition to glandulomuscular cells, three other cell types are found in the basal disk of Hydra. These are interstitial cells, cnidoblasts and nerve cells. Although only a few of the two former cell types are recognized in this region, the observations from this study refute previous statements to the contrary. Nerve cells are concentrated in the basal disk where they form a type of “network” system, due to the presence of bipolar, tripolar and multipolar cells. In some specimens, they assume a circular arrangement around the base of the polyp. Using morphological criteria for identifying the three types of epidermal nerve cells (neurosensory, neurosecretory and ganglionic) in other body regions, only neurosensory and neurosecretory cells are observed in the basal disk. These are indistinguishable ultrastructurally from their respective counterparts in other regions. It is possible that ganglionic cells are also present in the basal disk, but there may be few such cells. It is suggested that the three cell types originate from the budding region and these precursor cells are then forced proximally. Interstitial cells, escaping their differentiative function, do not develop apparently into other cell types of the basal disk. Cnidoblasts contain normal nematocysts but their functional ability is uncertain. Neurosensory and neurosecretory cells arise directly and independently from interstitial cells in the budding region, as evidenced by the appearance of immature nerve cells in the peduncle and their absence in the basal disk. Although viable cells may be discarded from the basal disk, it is believed that most cells die in situ and are then eliminated. The possible role of nerve cells is discussed briefly.

Notes: Summary The proximal collaterals of the dorsal giant fibres of the earthworm were traced through serial sections from the cell bodies to the giant axons. Their structure and synaptic connections were examined. There are chemical as well as electrical synapses. Their fine structure is compared to that of other known invertebrate and vertebrate synapses. Both giant fibre systems have efferent chemical connections with thin postsynaptic arborizations which probably belong to ventral cord motoneurons. Moreover the median giant axon is connected by an electrical synapse with the giant interneurons. The lateral giant collaterals on the contrary receive many afferences through chemical synapses which were partly identified as sensory fibers from the epidermis, multisegmental axons from the main fibre bundles or giant interneurones. Other afferences probably come from unisegmental interneurones. In addition both lateral giant axons form an electrical chiasma synapse with special membrane folds.

Notes: Summary The intestinal caeca of the intraredial cercaria of Neophasis lageniformis differ in several aspects from previous descriptions of the intestinal caeca of other digenetic trematodes. The caeca are lined with a single-layered, non-syncytial epithelium with lamellate projections into the lumen. The cells are rich in glycogen particles and contain abundant granular endoplasmic reticulum. The apical cell membrane, which displays a unit membrane structure, is always covered by an external membrane of the same thickness as a unit membrane but showing no double-layered substructure. The external membrane, which is not equivalent to a glycocalyx, may continue from one cell to another. Apically on the projections the external membranes are apparently pushed out forming large, extracellular whorls, which fill up the caecal lumen. In the lumen there are also found crystal-like structures and lipid-like bodies which are suspended in a granular substance. The possible origin and function of the extracellular bodies are discussed.

Notes: Summary The thick rigid chorion of the egg of Triatoma secreted by the follicle cells shows two porous layers: an aerial layer in the exochorion, an alveolar one in the endochorion. The anterior part of the eggshell is closed up by an operculum which is heaved up by the hatching larva. The operculum has no alveolar layer. The air enters through the numerous holes of the shell surface into the aerial layer and through the micropyles into the alveolar layer. The egg has no respiratory plastron. The follicle cells produce also a vitelline envelope whose structure shows a rapid condensation at fertilization time. During its development the embryo secretes two layers: serosal and embryonic cuticle. At high humidities, at low temperatures the egg is able to increase its weight during the early stages of embryogenesis, and this increase stops when the serosal cuticle is secreted. In a dry atmosphere the egg loses water but can develop if the temperature is higher than 20°C. The little permeability of the egg is related to the structure of its envelopes. The chorion and the vitelline envelope prevent the water from getting out of the egg. The serosal cuticle seems to be opposed to the penetration of the water into the egg. The role of the embryonic cuticle is probably limited in the transit of water.

Notes: Summary The adrenergic and cholinergic innervation to the rat iris has been studied at a light and electron microscopic level. Catecholamine fluorescence histochemistry showed adrenergic nerves to be present in both the dilatator and the constrictor pupillae regions. At a fine structural level the terminal innervation of the iris was studied and criteria for the differentiation between presumptive adrenergic and presumptive cholinergic axon terminals were examined. To aid this examination presumptive adrenergic axons were either labelled with the “false” adrenergic transmitter, 5-hydroxydopamine, or chemical sympathectomy performed using 6-hydroxydopamine. The value of using acetylcholinesterase staining as a marker for cholinergic nerve terminals was also studied. Results showed a mixed adrenergic/cholinergic innervation to the dilatator pupillae. In the constrictor pupillae an exclusively cholinergic innervation was found although adrenergic and cholinergic nerves were found supplying the blood vessels and at the dilatator-constrictor interface. These findings are discussed with regard to innervation-function relationships in the iris.

Notes: Summary The olfactory mucosa of frog has been studied at an ultrastructural level to confirm previous light microscope observations in regard to the presence, in the sensory epithelium, of nerve fibres not belonging to the first cranial nerve proper. It has been observed that both myelinated and unmyelinated nerve fibres are present in the lamina propria and that eventually these fibres terminate inside the epithelium. Unmyelinated fibres usually contain dark core vesicles and similar content is seen in their intraepithelium terminals. Terminals containing only clear vesicles are also observed in the epithelium and they are believed to represent the terminals of the myelinated fibres. The significance of these ultrastructural findings is discussed in view of their functional meaning.

Notes: Summary In the median eminence of the newt a medial region and two lateral regions are described. In cross section, the medial region appears to be made up of 1) an outer or glandular zone (Zone I) containing aldehyde-thionine-positive and negative nerve fibres and blood capillaries. Nerve fibres appear aligned in palisade array along the capillaries. 2) An inner zone (Zone II) made up of a) a layer of aldehyde-thionine-positive nerve fibres (fibrous layer) belonging to the preoptic hypophyseal tract and b) a layer of ependymal cells lining the infundibular lumen and reaching the blood vessels with their long processes. The lateral regions display a less pronounced stratification and aldehyde-thionine positive nerve fibres are nearly absent. A slender lamina (ependymal border) containing mainly aldehyde-thionine-positive nerve fibres and ependymal cells connects the median eminence to the pars nervosa. At the ultrastructural level, in the outer zone of the medial region at least 4 types of nerve fibres and nerve endings are identified: Type I nerve fibres containing granular vesicles of 700–1000 Å and clear vesicles (250–400 Å). Type II nerve fibres containing granular vesicles and polymorphous granules of 900–1300 Å and clear vesicles (250–400 Å). Type III nerve fibres containing dense granules of 1200–2000 Å and clear vesicles of 250–400 Å. Type IV nerve fibres containing only clear vesicles of 250–400 Å. In the inner zone too, all these nerve fiber types are found among ependymal cells, while the fibrous layer consists of nerve fibres containing granules of 1200–2000 Å in diameter. In the lateral regions Type I, Type II and Type IV nerve fibres and their respective perivascular terminals are found; axons containing dense granules (1200–2000 Å) are scanty. In these regions typical synapses between Type I nerve fibres and processes rich in microtubules are visible. The classification and functional significance of nerve fibres in the median eminence are still unsolved, but it may be assumed that nerve fibres of the medial region belong to both the preoptic hypophyseal and tubero hypophyseal tract, while the lateral regions are characterized by nerve fibres of the tubero hypophyseal tract. Peculiar specializations of the ependymal cells in the median eminence of the newt are also discussed.

Notes: Summary The nervous system of juvenile and adult Ceriantheopsis americanus has been examined with the electron microscope. The nervous system is exclusively ectodermal, forming a plexus of fibres lying between the epithelium and the muscle layer. The plexus consists of three types of nerve fibre that vary in diameter from 0.1 to 20 μ, and a fourth type of fibre of uncertain nature. The reticulum, previously thought to be the ectodermal nervous system, is redescribed as the peduncles of epithelial supporting cells. Intraneural and neuromuscular synapses are described. About 97% of the interneural are polarized. The unpolarized synapses are a variety not previously described, consisting of a series of polarized zones, for which the name multipolarized synapse is proposed. Structurally, the interneural synapses are more complex than those previously seen in the Cnidaria. Presynaptic projections, periodic cleft densities, and a postsynaptic web are described. By contrast, the neuromuscular synapses bear no membrane specializations. On the basis of observed synaptic interactions of its components a preliminary model of the functional organization of the nervous system is proposed.

Notes: Summary The Octopus iris is composed of five different layers: A, the external epithelium; B, the chromatophore layer; C, the iridocyte layer; D, the layer of muscles and collagen strands; E, the pigment epithelium. The nerves innervating the sphincter and the chromatophore muscles are identified and their neuromuscular junction is described. The motor endings of chromatophore nerves have an additional ending in presynaptic position which probably functions as a modifier of neuromuscular transmission. The chromatophores are naked and exhibit a tubular channel system between plasmalemma and pigment container which looks similar to the T-system of muscle cells.

Notes: Summary The outgrowth of newborn rat cerebellum cultivated in the Maximow assembly was studied from the time of explanation until the end of the 1st week “in vitro”. Dynamic characteristics of migrating cells were investigated with time lapse microcinematography and with phase and interference contrast. Three types of cells were identified: bipolar fusiform cells, fibroblasts and macrophages. The ultrastructure of these cells as seen with scanning and transmission electron microscopes is described. Bipolar cells display the same characteristics as those of migrating granule cells “in vivo”. Differentiation of these cells towards neuronal types becomes evident at the end of the first week “in vitro”. Axonal as well as dendritic growth cones were examined, and their ultrastructural characteristics described. Attention is focussed into the abundance of smooth endoplasmic reticulum, which in supposed active growth has the form of thin cisternae, and, at rest or in retraction takes the form of large vesicles. In the latter case, filopodia disappear, and the tips of their processes acquire a bulbous shape.

Notes: Summary In the tortoise Testudo graeca, the lizards Lacerta dugesi and Lacerta pityusensis, and the snake Natrix natrix, the innervation of the testicular interstitial tissue was studied by light and electron microscopy, the acetylcholinesterase (ache) technique, the Falck-Hillarp method for the detection of catecholamines, and the application of 6-hydroxydopamine. The intertubular spaces of the reptilian testes studied contain adrenergic nerve fibers the amount and distribution of which varies considerably both in various species and in various stages of the reproduction cycle. Nerve fibers do not enter the seminiferous epithelium. Fluorescence microscopy of the lizard testis reveals catecholaminergic varicosities which are mainly arranged around blood vessels, but do not show obvious connexions to Leydig cells. Ache-positive fibers are equally distributed in lizard testes surrounding each seminiferous tubule. In Natrix natrix ache-positive fibers are irregularly spread among groups of tubules, without showing a definite relation to Leydig cells either. By electron microscopy bundles of unmyelinated axons and axon terminals can be more easily detected in the testes of immature animals than in adult. Terminals of nerve fibers containing small (400–500 Å in diameter) and large (800–1400 Å) dense-cored vesicles and sometimes small clear vesicles establish contacts with Leydig cells. Three types of contact are described. 1. “Contacts” par distance at a distance of about 2000 Å and basal lamina interposed; 2. membranous contacts having a 200 Å gap only between axolemma and Leydig cell plasmalemma; 3. invaginations of terminals into Leydig cell perikarya. The latter may exhibit surface specialisations, which strongly resemble postsynaptic membrane thickenings. Experiments using 6-hydroxydopamine underline the adrenergic character of testicular nerve fibers, which can be regarded as another example of non-cholinergic, ache-positive neurons. In the testis of the immature tortoise profiles of axons occur which probably represent purinergic, ache-positive neurons.

Notes: Summary Synaptic junctions in intact rat cerebral cortex have been examined following glutaraldehyde fixation and phosphotungstic acid (PTA) staining. In the presynaptic ending the network has a hexagonal arrangement, while the dense projections are regularly placed along the presynaptic membrane. Cleft densities occupy the intracleft region. The postsynaptic thickening extends uninterrupted along the length of the junction. Qualitatively, the majority of junctions fall into the ‘discontinuous-continuous’ category, in which the internal coat of the presynaptic membrane together with its associated dense projections is discontinuous along the length of the junction, whereas the postsynaptic thickening is continuous. By contrast, a small number of junctions are ‘continuous-continuous’. In an attempt to analyze the junctions quantitatively, nine indices were measured. Histograms of the size distributions of seven of these appear to be bimodal, and from this it is concluded that two junction populations may be distinguishable on quantitative grounds. It is also shown that the distance separating dense projections at the presynaptic membrane is of the order of 10–15 nm. This surprisingly low value has consequences for current ideas on the relationship between synaptic vesicles and dense projections, and these are discussed at length.

Notes: Summary Working with glutaraldehyde fixed, PTA stained rat cerebral cortex, the authors compared the ultrastructural features of synaptosomes with those of intact synaptic junctions. In general there is close correspondence between the two, although the cleft densities and postsynaptic focal densities of synaptosomes show a greater degree of focalization than their counterparts in synaptic junctions. The dense projections have similar profiles in both preparations, but are more difficult to distinguish clearly in synaptosomes on account of the closer packing of the presynaptic network around their apices. The limiting membrane of the presynaptic terminal is usually visible in synaptosomes, but not in synaptic junctions. Comparing the preparations quantitatively reinforces the qualitative findings, and points to their overall similarity. However a number of the indices in synaptosomes are significantly smaller than the corresponding ones in synaptic junctions, and this points to the operation of a shrinkage factor during fractionation procedures. This is confined to the pre- and post-synaptic components and does not affect the intervening contact region. Histograms of the size distributions of the indices are similar to those obtained for intact synaptic junctions, the majority displaying two peaks. It is concluded that synaptosomes accurately reproduce the major ultrastructural features of synaptic junctions.

Notes: Summary The white shell glands of Artemia salina have been investigated. Our results, compared to those obtained in the brown-coloured shell glands, occuring within the same species, reveal differences not only in the aspect of the secretory granules but also in the structure of the nucleus and the cytoplasm. These differences between the two types of glands appear to be more striking than a simple variation in the quantity of secretion or in the pigmentation of the gland. As the brown glands are supposed to contribute to the formation of the egg shells in oviparous animals, the secretion of white glands could favour the development of nauplii in ovoviviparous animals.

Notes: Summary Terminal axons emerging from the inner plexiform layer of the cat retina reach the wall of the arteria centralis retinae, as revealed by electron microscopy. Numerous unusually large dense core vesicles (about 1000 Å in diameter), of different electron densities, occur in the varicosities of these axons. These observations may be compatible with the idea of an innervation of the central artery of the retina which is non-autonomic, possibly intrinsic in nature.

Notes: Summary Neural elements within the parenchyma of the sebaceous gland have not been reported previously. Nerve endings have been observed only in the connective tissue surrounding the gland or in close association with the undifferentiated basal cells. In this study, electron microscopy revealed the possible presence of nerve endings (or terminal portions of neural elements) in the suprabasal level of functional sebaceous glands of pinnae of white rats. Morphologically, there are two distinct types of nerve endings. Type 1 is bordered by a membrane of relatively irregular contour and contains a single mitochondrion, various-sized vesicles, numerous microtubules, fine neurofilament-like fibrils, and occasional ribosome-like granules. Type II is also bordered by a membrane, but its contour was relatively smooth and rounded. Moreover, Type II contains many mitochondria, varying in size, density, and the arrangement of cristae. While ribosome-like granules are scattered throughout the structure in relative abundance, there are scarcely any fine neurofilament-like fibrils or microtubules. Whether these two structures are sensory or autonomic fibers could not be determined by electron microscopic examination.

Notes: Summary A light and electron microscope study of secondary lamellae of trout developing at 10° C is described. Collagen is secreted by mesenchyme cells in the connective tissue of the developing gill filament. This becomes enclosed in infoldings around the peripheries of mesenchyme cells. These cells become aligned in a single plane within folds of basement membrane and epithelium. The basement membrane of opposite sides of the fold becomes connected by a thick layer of collagen. Blood spaces form around the margin of the fold, connecting afferent and efferent filament blood vessels. Endothelial granules form in cells lining the outer border of the marginal channel when blood flow begins. Rows of pillar cells separate from the proximal layer of mesenchyme cells as the secondary lamellae develops further. New secondary lamellae are added at the filament tips. At 28 days, the basement membrane consists of 2 layers, a fine fibrous layer and 5–15 orthogonally arranged layers of collagen fibres. By 31 days, the collagen is arranged at random and the layer is thinner. A clear layer is also present by 67 days. A multilayered epithelium containing mucous and chloride cells is present at 28 days. By 102 days, the secondary lamellae are covered by 2 layers of epithelial cells only. Chloride cells are present in much greater numbers in developing gills than in the adult.

Notes: Summary The rostro-caudal gradient of differentiation found in vertebrate embryos has been utilized to examine the sequence of synaptic junction development in the spinal cord of Xenopus laevis at a late embryonic stage. Uniform samples were taken at various points along the cord of a stage 27 embryo and examined in the electron microscope. The general ultrastructure of the cord demonstrated the rostro-caudal gradient of development. The sequence of synaptic junction development was like that in the cervical region (Hayes and Roberts, 1973). “Membrane-vesicle clusters” and “immature” synaptic junctions were found most caudally followed by synaptic junctions, first with cleft and subsynaptic membrane density, then with only cleft density and finally, most rostrally, with cleft, subsynaptic membrane, and subsynaptic cytoplasmic density. Mature synaptic junctions were found in increasing numbers from the mid to anterior trunk cord and could mediate alternating trunk flexions made by the embryos at this stage of development. “Membrane-vesicle clusters” were found near processes containing irregular vesicles and also near membrane outlines. These may be signs of dendritic growth. “Membrane-vesicle clusters” were also found in varicosities, facing the space around the spinal cord and in nerve fibres peripherally between the skin and myotomes. This suggests an association of early stages in synaptogenesis with axon growth. This and other possible inferences about axon and dendrite growth in relation to synaptogenesis are discussed.

Notes: Summary In several adult specimens of Mugil capito caught in a hypersaline lagoon, an intra-neurohypophyseal channel system lined with coronet cells has been observed. Coronet cells are normally found only in the saccus vasculosus, and in modern Teleostei, this structure is always completely separated from the neurohypophysis. The penetration of coronet cells into the neural lobe, may be an adaptive phenomenon possibly connected with an osmo-protective role of these cells.

Notes: Summary The innervation of the brown, red and yellow chromatophore muscle cells in Loligo vulgaris shows quantitative and qualitative differences. The nerve bundles regularly contain axons with electronlucent vesicles of about 300 Å diameter, and occasionally axons with dense core vesicles of about 600 Å diameter. The myofibrils of the contractile cortex show a staggered arrangement and are wound in a spiral with respect to the axis of the muscle cell. In the axial sarcoplasm there is additionally a bundle of thin filaments of about 70 Å thickness. The bundle is orientated in parallel with the longitudinal axis of the muscle cell. Its function may be to maintain tonic contractions.

Notes: Summary The ultrastructure of the innervation of the anterior cerebral artery of the rat was studied in control animals and in animals after superior cervical ganglionectomy. Fluorescence histochemistry shows a periarterial network of intensely fluorescent fibers which are divided into two groups, adventitial and periadventitial. The fluorescence begins to decrease 26 hours after, and completely disappears about 32 hours after, ganglionectomy. Fine structural changes are first observed 18 hours after ganglionectomy, when the axoplasm of degenerating axons becomes electron dense. This density gradually increases up to about 32 hours. By 32 hours most axons with disintegrating axolemmas become inclusion bodies of the Schwann cells. At this stage, synaptic vesicles can still be distinguished as less dense areas, but the membrane structures of synaptic vesicles and mitochondria are difficult to recognize. The degenerating axons are gradually absorbed and by 38 hours dense, residual bodies are observed in the Schwann cells. Generally speaking, the degeneration occurs first in the adventitial fibers and then in the periadventitial fibers. The transient appearance of small, granular vesicles is noticed in axon terminals about 18 hours after denervation, although very few small, granular vesicles are seen in control tissue or at later stages of degeneration.

Notes: Summary The ultrastructure of layers I and II of the motor cerebral cortex of rat brain has been studied at birth, 4, 7 and 14 days postnatal and in the adult. Compared with the adult, neonatal rat motor cortex exhibited a large extracellular space which decreases with increasing age. At all stages studied the neurons were seen to contain the organelles usually found in adult neurons. Growth cones were present in decreasing numbers up to 14 days old. Synapses were detectable at birth and there was an obvious increase in their number throughout the postnatal development. At the earliest stages studied there was a lack of specialization characteristic of the adult. Many synapses were either avesicular or relatively so and lacked the high degree of modification of adult pre- and postsynaptic membranes. By 7 days after birth many synapses existed which in all morphological respects resembled those of the adult, and by 14 days, the majority were of the adult type. These findings, particularly with reference to the postnatal development of synapses, have been discussed in relation to the known electrophysiological findings.

Notes: Summary Nerve fibers were demonstrated in the glycogen body of the avian lumbar spinal cord by using silver-impregnation and fluorescence microscopic (Falck-Hillarp) techniques. These nerve fibers originated from nuclear areas in lateral juxtaposition with the glycogen body. The fluorescence of the nuclear area was strongest near the polar regions of the glycogen body. It was suggested that the aminergic neurons of the avian lumbar spinal cord may influence the glycogen body. The avian glycogen body was compared with other storage sites of glycogen within the central nervous system.

Notes: Summary By means of electron microscopy “light” and “dark” pinealocytes can be distinguished in the guinea-pig pineal gland. Glial cells are rare. In the “light” pinealocyte. the most frequent cell type, some “vesicle-crowned rodlets” (VCR) show circumscribed thickenings. From these structures “vesicle-crowned balls” (VCB) have to be clearly distinguished. Furthermore “cylinders” occur, which, it is suggested, are precursors of VCB. “Dark” pinealocytes characterized by chromatin-rich nuclei and electron-dense cytoplasm are rare and contain fewer vesicles, VCR, VCB and “cylinders” than “light” pinealocytes. Numerous non-myelinated nerve fibres are situated within perivascular spaces, a few also in the parenchyma. Synapses between nerve fibres and pinealocytes were not observed. In the pineal gland of pregnant guinea-pigs the following changes can be observed in the second half of gestation. The “light” cells show many nuclear indentations and an increase of “active” zones, mitochondria, smooth ER, agranular vesicles, VCR, VCB, and “cylinders” respectively. The “dark” cells increase in number. After birth these changes reverse to normal within one week. Constant darkness leads to an activation of the “light” cells accompanied by an increase of the VCR and to an increase in number of the “dark” cells. Under constant illumination the “light” cells show a decrease of their organelles and a strong increase of the VCR. After 70 days the VCR also show a change in shape. Following reserpine treatment the VCR decrease in number and show signs of degeneration. It is discussed that the VCR function as pre- or postsynaptic structures and that they are involved either in transmitting impulses from nerve fibres to pinealocytes or from one pinealocyte to the other.

Notes: Summary Retinae from two- and three-day-old rats were explanted in plasma clots and grown in vitro with the flying coverslip method. After seven to seventeen days in culture, the retinal tissue was fixed with aldehydes and osmium tetroxide and embedded for examination with the electron microscope. Study of cross sections (perpendicular to the coverslip) revealed a histotypic pattern of organization, especially in the thicker regions of the explants. Layering of cells quite similar to that in the intact retina was seen to develop from the relatively primitive, explanted retinal epithelium. However, each layer contained fewer cells than its counterpart in vivo. All major neuronal cell types were distinguished by their location and cytological characteristics. Development of the saccules of sensory cell outer segments was observed to occur in vitro by an infolding of the plasma membrane. Synaptic ribbon complexes developed to the mature form in the outer plexiform layers, while conventional synapses were numerous in the inner plexiform layers. Synaptic ribbons were also seen in bipolar cell axons in the inner plexiform layers. Amacrine and ganglion cells in these regions were relatively sparse. A survey of posterior regions of noncultured three-day-old rat retinae showed no synapses of any sort; therefore the synapses in the cultures formed in vitro. The retina is recommended for studies of synaptogenesis in tissue culture, for it offers several advantages over expiants from other areas of the neuraxis, including a clear layering pattern, many identifiable cell processes with characteristic synaptic relationships between them, and a well-defined sequence of developmental events.

Notes: Summary This investigation is concerned with pineal organs of human embryos 60 to 150 days old. At every stage central nerve fibres enter the pineal organ by way of the habenular commissure, but are restricted to the pineal's proximal part. On about the 60th day of the development the sympathetic nervus conarii grows into the distal pole of the pineal organ from a dorso-caudal direction and plays the predominant part in the innervation of the pineal organ. After penetrating, it soon branches out and forms a network in the pineal tissue. Much later, not until the 5th embryonic month, sympathetic nerves appear accompanying the supplying vessels in the perivascular spaces. After a short time these nerves pierce the outer limiting basement membrane and penetrate the parenchyma. Towards the end of the 5th embryonic month the axons of the sympathetic nerves form varicosities containing clear and dense core vesicles. At this point large amounts of laminated granules appear primarily in cell processes, probably of pinealocytes. Isolated granules also occur in the varicosities of axons. The granules encountered here are most likely secretory granules.

Notes: Summary Nervous (sensory) pathways of the pineal complex were studied in Lampetra planeri. The following observations were made: (1) The pineal tract, containing more than 600 unmyelinated fibres, connects directly or indirectly to the posterior commissure. (2) Connections of the nervous region (NR) adjoining the parapinéal organ (“ganglion parapinéales” of Tretjakoff or “division of the left habenular ganglion” of Studnička) have been re-examined; our study shows that the rostral division of NR is connected to the parapineal organ by a short tract and to the left habenular ganglion (LHG) by a nervous layer (NL). The NR, NL, and LHG contain the same following elements: (1) A collection of typical neurones (which are rare in the NL) in an abundant meshwork of nerve fibres. (2) At least two main types of axons: (a) Type 1 axons, containing few granules (1,000–1,300 Å diameter). (b) Type 2 axons, containing numerous granules (900–2,500 Å diameter), and resembling neurosecretory axons. Type 2 axons are not observed in the pineal and parapinéal organs. (3) Axo-dendritic and axo-axonic(?) synapses. Our study concludes that NR and NL represent the rostral part of LHG; nevertheless, NR and NL may contain the parapinéal tract. The problem of possible incorporation of epithalamic elements (e.g., elements of the habenular ganglia) into the parapinéal and pineal organs in other animals has been considered phylogenetically.

Notes: Summary Umbilical vessels of guinea-pig fetuses were studied shortly before birth. In all umbilical cords investigated an innervation of the umbilical vessels is lacking. The intrafetal parts of the umbilical vessels on the other hand are richly innervated. A marked difference in the amount of nerve fibres and the pattern of innervation is found between artery and vein. The artery is supplied by a dense nerve plexus which spins around the media and which originates from nerve bundles within the outer adventitial layers. The comparatively scanty innervation of the vein exhibits a more coarsely meshed net pattern. The nerve bundles in the vein exhibit a close affinity to the vasa vasorum. Number and type of the close contacts between the muscle cells are different in the various sections of the umbilical vessels. Similar to the distribution of nerves they are almost absent in the vessels of the umbilical cord, numerously, however, in the intrafetal parts. Contrary to the innervation, the close contacts in the vein are developed more numerously and more broadly than in the corresponding artery.

Notes: Summary The vessels of the glomera that lie between the aorta and the pulmonary trunk, on the anterior and posterior wall of the pulmonary trunk, as well as the so-called glomus pulmonale of the cat have been injected with gelatine-india ink mixture. The paraganglia mentioned above are associated with the coronary circulation as chemoreceptors and are designated as the glomera coronaria. The investigations demonstrate a morphologic similarity with the glomus caroticum. Section of the vagus causes a degeneration of the synaptic structures on the glomus cells as well as changes in the cells themselves.

Notes: Summary The development of adrenergic nerves to the anterior eye segment was studied in human and guinea-pig embryos. Adrenergic terminals had already appeared in the earliest human embryos available (4–6 cm). They first appeared mainly in nerve trunks in the primitive chorioid, especially in the region of the developing ciliary body. Adrenergic nerves then grow into different structures of the eye as these develop, but typical terminals in contact with effector cells appeared late during the development, about the 25–30 cm stage. No adrenergic nerves were observed in the chamber angle. Corneal adrenergic nerves (also intraepithelial terminals) appeared much more frequently in embryos than in adults. No adrenergic neurons were observed in the retina. In the guinea-pig, the first adrenergic fibres were observed at about gestation day 35. The general principle of the development was very similar to that of the humans. At gestation day 45 to 50, the supply of adrenergic fibres was essentially that of the adult animal, except that the corneal adrenergic fibres were increasing until just before birth and that the adrenergic terminals of the chamber angle appeared shortly before term.

Notes: Summary The intrinsic innervation of the anterior byssal retractor muscle in Mytilus edulis L. has been demonstrated using Maillet's osmium-zinc iodide technique (ZIO). Grouped and isolated osmiophilic cells, thought to be neurons, and nerve fibers forming neuromuscular contacts and nerve endings, have been observed. When the period of fixation is not overly prolonged, glio-interstitial cells may also be distinguished.

Notes: Summary Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were employed to study the innervation and vascular supply of crayfish skeletal muscle. Blood vessels and nerve terminals identified by TEM were often closely associated. Synaptic regions of the nerve terminals were always located under sarcolemma and contained both dense-cored and agranular synaptic vesicles. Axo-axonal synapses of several different types were observed. Blood vessels consisted of several “vessel cells” or “supporting cells” enclosing a lumen, which was connected to the exterior by fine channels between the “supporting cells”. SEM of whole freeze-dried muscles revealed two types of ramifying structure, which often ran in parallel over the muscle surface. One, identified as nerve, was more cylindrical and had a smoother surface than the other, which was identified as blood vessel. Fine nerve branches disappeared under the sarcolemma, probably near synaptic regions, but synapses could not be seen. Blood vessels also had fine terminations which merged into the sarcolemma.

Notes: Summary The chromaffin tissue in the ganglion cervicale uteri of mice was investigated in different functional stages of their reproductive system by light and electron microscopy. The observations correspond to the findings obtained in extra-adrenal chromaffin tissue localized in other regions. 1. The chromaffin tissue consists of granule containing paraganglionic cells and of surrounding cells that resemble Schwann cells. 2. Occasionally the paraganglionic cells possess cilia. 3. The granules in the cytoplasm of paraganglionic cells are very electron dense and are enveloped by a membrane that forms a vesicle with a diameter either corresponding to that of the granule or considerably larger than the latter. 4. Cells with little granules have a better developed rough endoplasmic reticulum than cells with a great number of granules. 5. The paraganglionic cells have no special relationships to blood vessels. 6. The paraganglionic cells are innervated by synapses that possess synaptic vesicles and by synapses that show accumulations of dense core vesicles. The paraganglionic cells seem to be under the control of cholinergic and adrenergic innervation. 7. Differences of ultrastructure of the paraganglionic cells were seen only between adult and infantile animals. The latter have a greater number of cells containing only a small quantity of granules. 8. The findings suggest the hypothesis of a local endocrine influence of the paraganglionic cells on the neurons of the ganglion cervicale uteri by release of catecholamines.

Notes: Summary Light and electron microscopic observations on the ampullary organs of Polyodon spathula (Chondrostei, Osteichthyes) reveal a sensory epithelium similar to that found in the Lorenzinian ampulla, an electroreceptor found in marine Elasmobranchs. The sensory cells have a very small luminal part provided with a cilium. They are innervated by many nerve endings. Each nerve fibre apparently makes synaptic contact with several sensory cells. The synaptic structure in the sensory cell is composed of a flat sheet, the outermost part of which is surrounded by 3 or 4 annuli of densely staining material. The sheet extends into a protrusion of the sensory cell, and there is a corresponding invagination in the nerve terminal. The conclusion that these organs are electroreceptors, is supported by the finding that the fish responds to the introduction of an iron tube in the aquarium, whereas a wooden rod introduced in the same way causes no response.

Notes: Summary The ultrastructural characteristics of the neuromuscular junctions were studied in the neurogenic heart of Limulus polyphemus. Several types were encountered. The first type consisted of nerve terminals which synapsed along the borders of the myocardial fibers, embedded just under the sarcolemma. A second type of terminal was ensheathed in glial cells, synapsing on the outer sarcolemmal membrane. The third, and most prevalent type of junction consisted of terminals which synapsed with arms of granular sarcoplasm, remote from the fibrillar portion of the muscle fibers. Junctional complexes of the third type were often observed near intercalated discs and were often formed by several axons synapsing with arms of sarcoplasm from several muscle fibers. The results are discussed in relation to the previously reported electrophysiological characteristics of the neuromuscular junctions.

Notes: Summary The ultrastructure of the pars intermedia (PI) of the normal VII +/+ and hereditary nephrogenic diabetes inspidus DI Os/+ mice has been studied with particular reference to the morphology of the glandular cells and their innervation. Four types of cells were observed in both the genotypes of mice, 1) the light glandular cell, 2) the dark cell, 3) a type of cell similar to ependymal cells and 4) a small percentage of typical ACTH cells, observed mostly on the PI border of the cleft and rarely in the centre of PI. The predominant light glandular cells contain mainly two types of membrane bound granules: 1) electron dense core granules, which measure 1500–2500 Å and 2) electron lucent vesicles, which measure 3000–4000 Å in diameter. Granules of intermediate size with various density are also present in both types of mice. The electron dense core granules are predominant in DI Os/+ mice, whereas, electron lucent vesicles are predominant in the normal VII +/+ mice. Similar uniform size membrane bound electron dense granules have been observed in ACTH cells of PI and pars distalis. From earlier experimental evidences and the present observations, it is concluded that the dense core granules in PI may be synthesizing ACTH or ACTH-like substance. It is also discussed that these dense core granules may further mature and give rise to MSH in the form of electron lucent vesicles. If it is so, PI light glandular cells may have dual functions, of producing MSH and ACTH. One of the functions of ependymal-like cells, may be the transport of PI secretion. Three types of nerve endings are observed throughout the PI, making synaptic contact with the predominant cell type. The innervation is more in DI Os/+ mice than in normal mice. The classification of these nerves is according to Bargmann and co-workers 1) peptidergic neurosecretory fibers, contain mainly membrane bound dense core granules, measuring 1200 to 1800 Å, and are the classic neurosecretory granules; 2) adrenergic fibers, measuring 700–900 Å; 3) cholinergic fibers, measuring 300–400 Å. Adrenergic and cholinergic fibers are more towards the hypophysial cleft. The increased innervation, the synaptic contact, the extremely hypertrophied PI and the greater activity of its light glandular cells in the DI Os/+ mice show the PI is under the influence of the nervous system.

Notes: Summary The innervation of the salt gland of the goose, the duck and the swan was investigated by means of electron microscopy. Axonal swellings were observed in relationship to secretory cells as well as to central duct cells. The terminals contain synaptic and densecored vesicles. There are no specialized pre- and postsynaptic membranes.

Notes: Summary In order to induce a modification of the plasmatic electrolyte composition, newts (Triturus cristatus) are placed into a potassium chloride solution (KCl 5‰), while controls are kept in tap water. All animals are previously hypophysectomized. 8 days after the beginning of the experiment, the interrenal cells of the hypophysectomized newts kept in KCl solution exhibit a very strong steroidogenic activity, as evidenced by a striking lipid depletion, an increase in the number of mitochondria and a considerable development of the agranular tubular endoplasmic reticulum. On the other hand, the Δ5-3β-hydroxysteroid dehydrogenase reaction is notably intensified. All the cells of the interrenal tissue are affected by the stimulatory effect of the KCl solution. Control animals show only reduced steroidogenic activity of their interrenal cells, as demonstrated both by the morphology of these cells and the weak Δ5-3β-hydroxysteroid dehydrogenase reaction. The mechanisms involved in the stimulatory effect on steroidogenesis of the variation of the electrolyte composition of the plasma are discussed.

Notes: Summary The ‘homing’ process of lymphoid cells in the developing palatine tonsil of the rabbit was investigated by culturing isolated embryonic tonsils in diffusion chambers on the chick chorioallantoic membrane. An analysis of the origin and fate of lymphoid cells in cultured tonsils from subsequent stages of development strongly supported the existence of the ‘homing’ mechanism for lymphoid cells, postulated on morphological data. From the 19th day of gestation there is an influx of motile lymphoid (stem) cells into the tonsillar primordium, reaching a peak between the 22nd and 24th day. Of these cells a large proportion is immobilized by mesenchymal cells and becomes sessile lymphoid cells which remain firmly attached to mesenchymal cells during culture. After the 24th day practically all sessile lymphoid cells lose their contact with the mesenchymal cells and migrate to the bottom of the diffusion chamber. This different behaviour of sessile lymphoid cells in cultured tonsils of later developmental stages may imply a functional differentiation of these cells. By comparing the fate of morphologically differentiated lymphoid cells in cultured palatine tonsils and thymus of rabbit embryos, the different nature of tonsil and thymus lymphocytes could be demonstrated.

Notes: Summary The eighth retinular cell (R 8) of Grapsus lacks cytoplasmic pigment granules and basically resembles those previously known in the ghost crab Ocypode and the mysid Praunus. Distally located, R 8 comprises four lobes inserted between the outer ends of the seven regular retinular cells (R 1–R 7). A thin cytoplasmic bridge connects these lobes. One lobe adjacent to R 1 contains the nucleus of R 8 and gives rise proximally to the cell's axon. The short distal eighth rhabdomere consists of microvilli (mvl) protruding axially from all four lobes. Similar R 8's were found also in two other crab families and in two other genera of mysids. In Grapsus the eighth rhabdomere is extraordinary in possessing mvl oriented in two orthogonal directions parallel to the mvl of R 1–R 7. The distal 20% of the rhabdom consists of mvl originating exclusively from R 8. These appear in somewhat irregular bands and are alternately oriented parallel to the animal's vertical or horizontal axis. More proximally the retinula contains eleven sectors but the rhabdom still comprises bands of alternating mvl with those from R 8 joined respectively by the rhabdomeres of R 1, 4, and 5 (horizontal) and R 2, 3, 6 and 7 (vertical). The rest of the rhabdom shows typical decapod organization with seven interdigitating rhabdomeres.

Notes: Summary A light and electron microscopic study was made of the structure of the gill arch, filament and secondary lamella of Salmo gairdneri R. Blood pathways through the gill were traced from serial histological sections, and from the examination of ink perfused tissue and perspex casts formed following resin injection of the circulatory system. The epithelium covering the gill consists of unspecialized, dark, chloride and mucous cells. The distribution of specialized cells appears to be related to gill function. The basement membrane underlying the epithelium consists of three layers, the inner collagen layer being continuous with the connective tissue core of the gills. Blood supply to the secondary lamellar respiratory surface is via branchial, filament and secondary lamellar arteries. Blood spaces of the secondary lamellae are delimited by pillar cells containing what appears to be contractile material. The marginal channel of each lamella is bounded distally by cells of endothelial origin. A network of lymph spaces within the filaments connects with efferent branchial arteries. Nutritionary capillaries within the filaments connect with afferent branchial arteries. No shunts between afferent and efferent filament arteries were found. Data from this study and previous physiological and histopathological studies suggest a mechanism for the control of blood flow to suit the respiratory requirements of the fish. This mechanism involves a system of recruitment of additional respiratory units and changes in overall blood flow patterns.

Notes: Summary Morphological evidence is presented supporting the possibility of basal secretion into hypendymal capillaries of the adult rabbit subcommissural organ (SCO). The synthetic apparatus of the SCO cell is described as well as the heterogeneous granules and vesicles which are concentrated in the basal processes bordering a widened perivascular space. The origin of the electron dense granules, of which two fairly distinct subgroups are found, is discussed. A binding of secretory sacs to the lateral plasma membrane is seen. The possibility of a lateral secretion is supported by the presence of a system of extracellular channels between SCO cells which are filled with a flocculent material resembling that of the secretory sacs. Nerve perikarya which are separated from the SCO by only a few glial fibers are demonstrated. Synapses are described in nerve fascicles bordering on the hypendymal capillaries. The possibility of an innervation of the hypendymal region is discussed as well as possible nervous connections with the pineal gland.

Notes: Summary Histological and fine structural studies of adrenal cortices were performed on male Sprague-Dawley rats which had been given intravenous injections of prostaglandin E1 or E2. It was found that there were increased numbers of intracellular lipid droplets, free ribonucleoprotein particles, cholesterol ester clefts and coated vesicles of both the small and large varieties. A reorganization of the internal mitochondrial membranes and the appearance of protrusions of parenchymal cytoplasm into the sinusoidal lumina accompanied by vasodilation were also observed. These alterations are not typical of those observed following exogenous ACTH administration and are therefore considered to be prostaglandin-induced.

Notes: Summary The salivary glands of the moth, Manduca sexta (Insecta: Sphingidae), are unlike most other salivary glands in that they are innervated from one source only. Vital staining of nerves with methylene-blue reveals numerous fine nerves extending to the glands from the oesophageal nerve, a part of the stomatogastric or visceral nervous system. Light and electron microscopy confirm that only the fluid-secreting cells, confined to a discrete region in these glands, are innervated. Axons with or without glial wrappings are found in intercellular spaces between fluid-secreting cells. Axons lacking a glial sheath contain, after glutaraldehyde-osmium tetroxide fixation, large granular and small agranular vesicles. In nerve endings in glands fixed with permanganate these smaller vesicles are granular, having the electron-dense cores characteristic of monoamine-containing neurons. These nerve endings with “synaptoid areas” are in close (“direct”) contact with the fluid-secreting cells.

Notes: Summary The archigregarine, Selenidium, is parasitic in the digestive tracts of some polychaete worms. The trophozoites of Selenidium are worm-like in appearance, and are attached by one end to the intestinal epithelium of the host, while the remainder of the organism performs rhythmic bending movements, reminiscent of nematode worms. The trophozoites have a multilayered, longitudinally folded pellicle, and beneath this are longitudinally oriented microtubules, arranged in precise fashions. The arrangements of both the pellicle, and the microtubules change during the bending movements. Furthermore, if trophozoites are treated with the drug colchicine, the sub-pellicular microtubules are destroyed, the patterns of pellicular folding are altered, and the trophozoites cease to move. The contribution of the pellicle and the microtubules to the characteristic movements of the trophozoites is evaluated and discussed.

Notes: Summary The somatic muscles of the tardigrades Macrobiotus hufelandi and Milnesium tardigradum consist of elongated cells which are structurally independent of each other and insert in the body wall. The contractile part of the muscle cells contains thick and thin myofilaments. The thin filaments run together in typical dense bodies. In relaxed muscles clear A-, I- and H-zones are formed, and the dense bodies are often accumulated in groups. In ribbon like I-zones they are sometimes arranged side by side in rows and resemble fragmented Z-rods. The sarcoplasmic reticulum is localized primarily in the small region between the contractile apparatus and sarcolemma, and is represented by a system of smooth, flat, reticulated cisternae. The contractile apparatus of the somatic muscles is compared to that of smooth and obliquely striated muscles of several invertebrates. On the basis of certain distinguishing features of the contractile part of somatic muscles of tardigrades, it is suggested that these muscles should be referred to as a type intermediate between smooth and obliquely striated muscles.

Notes: Summary The vestibular fibres of ammocoetes of Lampetra planeri form a peculiar type of synaptic contact with the large nerve cells of the vestibular nuclei. In fact, a large expansion of the fibre is enveloped by the nerve cell cytoplasm, which makes a sort of spoon or actual channel by sealing its own plasma membrane with adhesion plaques around the fibre. In the whole area of juxtapposition between the fibre and the cell membranes, three different types of specialized contacts occur: desmosomoid junctions, which are the most numerous; zones having the characteristics of chemically active synapses on account of the presence of vesicles clustered on the presynaptic side; gap-junctions, where electrical transmission might occur. A puzzling feature is the presence of an enormous number of presynaptic vesicles located in the central part of the axoplasm of the fibre expansion.

Notes: Summary Electron microscope studies of axons, distributing singly or in small bundles in the human ventricular and atrial myocardium, indicate a few per-cent of the axon profiles to be significantly large in diameter (1.5–3.0 μ). They are characteristically packed with a profuse number of mitochondria along with large granular vesicles, glycogen rosettes, lysosomic bodies; and some of them terminate on a “specific terminal cell” (Knoche and Schmitt). These mitochondria-rich, large axons are assumed to be terminal portions of the cardiac afferents. About half of the axons encountered in the ventricle and 2/3 in the atrium are non-vesiculated, usually less than 0.5 μ. in diameter. The varicosities containing numerous vesicles are mostly 0.5–1.5 μ in diameter and are assumed to be terminal portions of the cardiac efferents. The ratio between the number of axon profiles containing small granular vesicles and that of axon profiles containing agranular vesicles without small granular vesicles is 2∶1 in the ventricular myocardium and 1∶1.7 in the atrial myocardium.

Notes: Summary The fibre anatomy of the glomeruli neuropil of the antennal lobes (sensory centres) of Locusta migratoria has been analysed by electron microscopy. The glomeruli consist of interneuronal and sensory elements of the antennal nerve, forming a complicated mesh-work of fine fibres. The great majority of nervous fibres is filled with clear synaptic vesicles, which stain with zinc-iodide-osmic-acid (Akert and Sandri, 1968), Two types of synaptic junctions are found, differing especially in shape and size of their synaptic gap. Both types show a clear morphological polarity, so that the distinction of pre- and postsynaptic fibres is possible. Inter- and intracellular paramembraneous synaptic material can be demonstrated by the bismuth iodide impregnation of Pfenninger et al. (1969) This method, showing the same effect as in vertebrate nervous systems, seems to be very adequate for the identification of synaptic structures and synaptic distribution. Normally presynaptic structures are situated in angles of nerve fibres. Although degeneration experiments were made, the synaptic interaction of the various fibre types could only partially be explained. Sensory fibres interact with intemeurons, which show pre- and postsynaptic zones. They seem to be responsible for the integration of different glomeruli. There is some evidence that sensory fibres have synaptic connections, too. Postsynaptic profiles with several synaptic contacts and free of vesicles are very rare, whereas “axo-axonic” contacts are found quite often. These glomeruli with their great number of vesicle-filled profiles differ from corpora pedunculata calyx glomeruli (see Lamparter et al. 1969).

Notes: Summary The hilus cells in the ovary of the pig have been investigated with the electron microscope. These elements are identical with Leydig cells. The hilus cells contain an abundant agranular endoplasmic reticulum, which is either organized as a tubular network or as an onion like system of closely packed flattened cisternae sometimes filling up half of a cell and often being concentrated around lipid droplets and pigment granules. The mitochondria have cristae, tubuli and sacculi and a matrix of variable electron density. The golgi cisternae are poorly developed. The cells have centrioles, and there are hints of an amitotic cell division. The cytoplasm of the hilus cells contains lipid droplets, which have a diameter up to 5 μm. The pigment granules are extremely polymorphic, nearly always surrounded by a single membrane. They possibly may be derived from lipid droplets and mitochondria. Single axons of the hilus nerve occasionally penetrate the basement membrane and come into close contact with the hilus cells. Typical synapses were not observed. The results are discussed on the background of light microscopical, biochemical and pathological findings and compared with those obtained on Leydig cells of different species. Various cell images are interpreted to represent progressive stages of maturation.

Notes: Summary By combining the Golgi and the electronmicroscope techniques it has been possible to identify accurately the system of centrifugal fibers which arborizes in the lamina of muscoid flies forming the so-called nervous bags. Each of them originates from a single fiber entering the lamina at the site in which the second order and the long visual fibers leave it. This single fiber represents the peripheral portion of a T-shaped trunk stemming from a small neuronal body located in the external region of the medulla. The central branch terminates within the first synaptic field of this visual center. After entering the lamina the centrifugal fiber ramifies profusely and its branches can be seen climbing and synapsing on the surface of the photoreceptor axon endings. The synaptic loci show characteristic synaptic ribbons located within the nervous bag fibers. This fact suggests that direction of conduction is from the medulla to the lamina. This study has also revealed that the intramedullar terminals of the centrifugal fibers establish intimate contacts with one of the two second order fiber endings.

Notes: Summary Quantitation of synapses at different postnatal ages has been undertaken in the cerebral cortex of the rat. In this study axial ratios of presynaptic bags, proportion of cortex occupied by presynaptic bags and numbers of synapses per unit volume of cortex have been estimated. Observations on synaptic vesicle packing densities have also been made. Synaptic bags become increasingly spherical up to 7 days of age and become more elongated thereafter. The proportion of cortex occupied by presynaptic bags increases rapidly up to 7 days of age and then at a decelerated rate up to maturity. The number of synapses per unit volume increases slowly over the first four days after which there is a rapid increase to 14 days, followed by a decelerated rate. The average presynaptic bag shows marked changes in volume with increasing age which indicate the probability of two stages of synaptic development. This two stage development is further reflected in the estimates on vesicle packing densities. The implications of the results are discussed in relationship to changes in functional activity of the cortex during postnatal development.

Notes: Summary Synaptosomes derived from 2–21 days postnatal rat cerebral cortex have been examined following glutaraldehyde fixation and block PTA staining, with the aim of investigating the maturation of the paramembranous densities at the contact region between the pre- and postsynaptic components. The internal coats of pre- and postsynaptic membranes first appear as undifferentiated plaque-like thickenings, which gradually develop into, or are replaced by, dense projections and postsynaptic focal densities respectively. Both sets of densities pass through an interconnected phase before starting to emerge as discrete entities at 5–7 days. The external coats of the pre- and postsynaptic membranes coalesce to form a plate-like structure which breaks down during development to form the cleft densities or transverse bars of the adult contact region. Although for the first few days of postnatal development only one type of synaptosome can be identified, from 5 days onwards two types corresponding to types A and B of adult life become recognizable. Increase in height of the dense projections has been correlated with increase in the number of synaptic vesicles per synaptosome during postnatal development, indicating that the synaptic vesicles may play a role in the formation and maturation of dense projections. The possible importance of other factors in this process is also discussed.

Notes: Summary The rabbit SA-node was outlined electrophysiologically and its adrenergic and cholinergic innervation patterns were studied with the electron microscope. Differentiation between adrenergic and cholinergic terminals was achieved by fixation of the specimens in KMnO4 which produces dense-cored synaptic vesicles in adrenergic terminals, whereas synaptic vesicles in cholinergic terminals are empty. It was found that adrenergic and cholinergic nerve terminals often come in close apposition to each other, the distance between adjoining membranes being in the order of 250 Å. At times, faint membrane thickenings could be seen in these places. The available pharmacological, physiological and morphological evidence leaves little room for doubt that cholinergic terminal fibers can influence the adrenergic ones. From mainly morphological evidence it is also postulated that adrenergic terminals influence cholinergic terminals.

Notes: Summary The median eminence and the pars nervosa of Rana esculenta have a different structure. The median eminence has 2 different zones: the outer zone situated near the pars distalis and the inner zone under the ependyme. In the outer zone there are, according to the size and the shape of the granules, 5 types of nerve terminals. 1. Endings containing spherical fine dense granules of 800 to 1000 Å in diameter; 2. Endings with spherical granules from 1000 to 1200 Å in diameter; 3. Endings with granules of irregular shape which are bigger than the former (1200 to 1600 Å); 4. Endings with spherical dense granules of about 1200 to 1800 Å in diameter; 5. A few endings containing only clear vesicles. Type 3 and type 4 endings are probably neurosecretory. The inner zone contains numerous neurosecretory fibres. They are of two types: one with big granules (1600–2400 Å), the second with smaller granules (1300–2000 Å). Non-neurosecretory fibres have also been observed. The pars nervosa contains two principal types of neurosecretory fibres: one with dense granules of 1600 to 2400 Å in diameter, the other with lighter granules of about 1300 to 2000 Å. In the external zone lining the pars intermedia, aminergic fibres with fine granules have been observed.

Notes: Summary Two neuron types are distinguished by electron microscopy in the lateral geniculate nucleus (LGN) of the monkey-a large cell (P cell) interpreted as a geniculostriate relay cell, and a small cell (I cell) interpreted as an inhibitory interneuron. The I cell, distinguished by its small size, infolded nucleus, small mitochondria, cilium and small granular bodies, forms about 10% of the total neuron population. It could not be determined whether this cell has an axon, but its dendrites, which contain aggregates of flattened vesicles, are thought to form a proportion of the “F processes”, profiles which are post-synaptic to the retinal (RLP) axons and presynaptic to the dendrites of the P cells. The small dark (RSD) axon terminals of unknown origin contact the dendrites of both cell types. After eye enucleation the P cells of the affected laminae of the LGN shrink and partially withdraw their dendrites from the neuropil. By 29 months' survival, they have only a narrow cytoplasmic rim around the nucleus. A necrotic process also occurs, affecting fine dendrites by 22 days and large profiles by 45 days, but it is not clear whether whole cells are destroyed by this process. At 45 days the I cells are commonly seen to form somatodendritic synapses. The appearance of these synapses is interpreted as the result of a withdrawal to the soma of the presynaptic dendrites. It is concluded that the I cells are probably inhibitory interneurons subject to excitation and presynaptic inhibition by the RLP and RSD axons, and a diagram is presented to demonstrate the possible significance of these connections for the transmission of information through the LGN.

Notes: Summary Light- and electron-microscopic observations were made on the alpha, beta and delta cells of the pancreatic islets of the domestic fowl and on their cytologic changes following the administration of glucose, glucagon, insulin, alloxan and carbutamide. 1. Under the light and electron microscope, two types of islets are recognized: (1) the “alpha” islet composed of alpha and delta cells and (2) the “beta” islet made up of beta and delta cells. 2. Alpha cells are large, round or columnar in shape, and contain less-developed cellular organelles and characteristic alpha granules. These dense, spherical granules are surrounded by a single smooth membrane and the matrix, with high magnification, exhibits a glomerular structure. 3. Beta cells are round, oval, or irregular in shape, and contain more or less developed cellular organelles and characteristic beta granules. These granules are polymorphic and consist of three main types; needle or bar-shaped, spherical, and ring-shaped; they are enclosed by a smooth membrane. 4. Delta cells are characterized by the presence of less dense spherical granules (diameter about 500 mμ) that are partially surrounded by an indistinct membrane. They are considered to be an independent type of cell of unknown function. 5. “Acinar-islet cells” with intermediate endocrine and exocrine morphology, are observed between the endocrine and exocrine cells along the periphery of the islets. The presence of occasional fragments of plasma membrane suggests that the cytoplasm of acinar and islet cells may intermingle. 6. The alpha cells exhibit margination of granules and release of granules by emiocytosis, in hypoglycemia induced by the administration of insulin or carbutamide. 7. After administration of glucose, glucagon or carbutamide, beta cells exhibit vacuolation and release of granules by intracytoplasmic dissolution of the specific needle- or bar-shaped granules followed by diacrine passage through the plasma membrane. On the other hand, after the administration of glucose, glucagon or carbutamide, Gomori-positive, dense, small-cored granules occur in the periphery of cytoplasm along the capillary with indications of release by emiocytosis. 8. Delta cells increase remarkably and extrude granules by emiocytosis, after the administration of glucagon, alloxan or carbutamide. 9. Administration of carbutamide stimulates the secretory activity of beta cells, as indicated by the diacrine feature of specific beta granules and the emiocytosis of small-cored granules. Carbutamide also causes an increase of immature non-granular cells in the alpha and beta islets.

Notes: Summary Electron microscopic studies of neural processes in the cerebellum, optic tectum, and cerebral hemisphere of the frog reveal a distinctive system of SER cisternae lying at intervals (commonly 1–2 μm apart) perpendicular to the long axis of axons and dendrites, interconnected by tubular, longitudinally orientated SER elements, and in direct continuity with the outer membrane of mitochondria. The transverse cisternae are fenestrated, with a single mierotubule (or rarely, two) passing through the centre of each 50–75 nm fenestration. Extensions of the SER-microtubule complex may be located parasynaptically in axon terminals and dendrites. The SER of dendritic spines also appears to be continuous with the fenestrated cisternae. Possible roles for the specialized SER (particularly of the parasynaptic extensions), such as calcium ion sequestration and ATP or monoamine oxidase transport, are discussed.

Notes: Summary The innervation of the portal vein of the white rat was examined with light-, fluorescence-, and electronmicroscopic techniques. The results are as follows: 1. Paraldehyde treated vein preparations (Falck's fluorescence method) demonstrate a predominantly longitudinally orientated external nerve plexus, being situated on the outermost muscle cells. Near the liver the nerve net is characterized by broad meshes, near the intestinal tract by narrow ones. The circular subendothelial inner plexus originates in the outer plexus of the intestinal vascular bed. 2. Nerve bundles in the fibrous adventitia were demonstrated with Gomori's Acethylcholinesterase method. In other respects, the enzyme activity was only observed in the intercellular spaces of the muscle layer. The electronmicroscopic demonstration of Acetylcholinesterase (Karnovsky's method) further showed that the enzyme activity is restricted to the muscle cell membrane. 3. The electronmicroscopic examination verified the results obtained with fluorescence microscopic techniques. a) In the proximity of the liver, only isolated nerve bundles occur on the outer muscle layer. The individual nerves are entirely surrounded by Schwann cells. Only a few of the axons in the vicinity to the muscle cell have agranular vesicles. Evaginations of the vesicular axons occur infrequently. Their distance from the muscle cell amounts to 1000–2000 Å. b) In more than one thousand thin sections, no axons were found inside the thick muscular layer. c) Subendothelial axons (inner plexus) are either partially or totally evaginated from the Schwann cells. They are densely filled with empty vesicles (350–650 Å) and contain a few dense core vesicles of 800–1600 Å in diameter. Synaptic endings were not observed. d) A dense collection of vesicle-containing axons, that were partially in their entirety and partially only from the muscle cell proximal side evaginated from the Schwann cells, were observed in the single muscle layer at the junction of the superior mesenteric and the portal vein. From these bundles, smaller bundles and individual axons pass between the muscle cells and reach the endothelium. Typical synapses were not observed. No vesiclecontaining axon was nearer than 1000 Å to the muscle cell. 4. Those axons possessing vesicles and being evaginated are considered to be vegetative conducting pathways. The excitation of the effector structures by transmitter substances is discussed in connection with the post mortem autonomic vascular contractility.

Notes: Summary 1. There is a gradual proximo-distal increase in the thickness of the muscle coat of the human ductuli efferentes, duetus epididymidis and ductus deferens. Circularly arranged smooth muscle bundles predominate in the ductuli efferentes and ductus epididymidis of the caput section. Scanty strands of longitudinally and obliquely oriented smooth muscle bundles form an additional, incomplete outer muscle layer around the ductus epididymidis of the corpus. Small smooth muscle-like cells constitute the muscle elements of the upper sections of the excretory ducts (from the ductuli efferentes to the midcauda). At the transition of the corpus and cauda epididymidis ordinary large smooth muscle cells join the small contractile cells to form—in more distal sections of the cauda—a composed, thick subepithelial muscle coat. In most distal portions of the cauda, the two-layered muscle coat of the ductus epididymidis is transformed into a three-layered coat, a pattern of construction which is retained in the vas deferens. 2. Electron microscopically, three types of contractile cells are distinguished in the human ductuli efferentes and ductus epididymidis: a) contractile cells of medium transparency containing exclusively thin myofilaments (60 Å in diameter), b) dark contractile cells containing bundles of thin myofilaments (60 Å in diameter) and single coarse filaments (140 Å in diameter), c) light contractile cells with loosely dispersed, interweaving thin and thick myofilaments. Commutual diameter changes at regular intervals are seen in individual myofilaments, giving the impression of structural periodicity not unlike that of filaments of striated muscle. Ordinary smooth muscle cells of the cauda epididymidis and vas deferens are characterized by uniformly sized, closely packed but evenly distributed thin myofilaments with numerous dense patches. 3. Fluorescence microscopy performed on formaldehyde treated freeze dried tissues reveals that the contractile cells of the ductuli efferentes in man and monkey receive a low number of single adrenergic terminal fibres penetrating the depth of the muscle coat. The adrenergic innervation of the ductus epididymidis is restricted to small peritubular nerve fascicles running contiguous to the most superficially located bundles of smooth muscle-like cells. The adrenergic ground plexus is rather wide-meshed in the proximal cauda, becomes increasingly dense in more distal cauda sections and in initial, funicular portions of the vas deferens, and reaches maximum density in abdominal parts of the ductus. Perivascular and adventitial adrenergic plexuses are well developed at arteries of the caput and corpus epididymidis in man, monkey, rabbit, guinea-pig and rat. 4. Electron microscopically, noradrenergic nerves have been identified by the presence of small granular vesicles in preterminal varicose axon dilatations. Nerve fibre swellings filled with small empty spherical vesicles have been considered to belong to “cholinergic” neurons whereas occasional varicosities equipped with some large membrane bound granules and abundant mitochondria may represent local expansions of sensory axons. 5. Neuromuscular relationships in the upper sections of excretory ducts comprise adrenergic synapses by distance (more than 1000 Å), and a few intimate, ensheated close contacts, whereas the main type of contact of nerves to ordinary smooth muscle cells in the lower duct section is by means of close but not intimate approach (500–2000 Å). 6. Adrenergic synapses in the ductus epididymidis and ductus deferens of the monkey resemble—what concerns their morphology, relationship to effectors and distribution pattern—those of man. 7. In accordance with the total number of vascular and non-vascular adrenergic nerves, visualized by fluorescence microscopy, the amount of noradrenaline varied considerably in different sections of the human male internal genital organs: The lowest amounts were estimated in the testis (0.12±0.03 μg/g). Medium to high concentrations were detected in various sections of the caput and corpus epididymidis (ductuli efferentes 0.60±0.09 μg/g; ductuli efferentes and caput 0.72±0.13 μg/g; corpus epididymidis 1.04±0.25 μg/g; proximal cauda 0.95±0.17 μg/g; distal cauda 0.97±0.19 μg/g). The highest noradrenaline content was found in the human vas deferens (prox. vas deferens 1.11±0.21 μg/g; interm. vas deferens 1.20±0.42 μg/g; distal portion 1.43±0.39 μg/g). 8. For comparison, the noradrenaline content of the testis and epididymis of the rhesus monkey, the epididymis of the rabbit and the vas deferens of the rabbit, mouse, guinea-pig and rat has been determined. 9. Adrenaline of exogenous origin was detected in the vas deferens, cauda epididymidis and plexus pampiniformis of two cases who received this catecholamine as part of the local anaesthetic drug mixture. Due to methodological reasons, the presence of small amounts of adrenaline of endogenous source in adrenergic nerves of the human and monkey internal male genital organs cannot be excluded. 10. The differences in motility behaviour of the ductus epididymidis (spontaneous, rhythmic contractions) and ductus deferens (absence of any spontaneous movements under conditions at rest) in vivo and in vitro have been correlated with the occurrence of specialized contractile cells in the upper segment (ductuli efferentes, ductus epididymidis of the caput, corpus and initial cauda) and ordinary large smooth muscle cells in the lower segment (ductus epididymidis of the distal cauda and the vas deferens) and furthermore correlated with differences in the pattern of the adrenergic innervation; the concept is advanced that progressive cytological differentiation of smooth muscle cells and the development of a dense direct adrenergic innervation suppresses autocontractility and, that the reverse condition may favour spontaneous motility of smooth muscle elements.

Notes: Summary The glandula infraorbitalis buccalis in rabbits represents the type of a tubuloacinar mucous secreting gland. The secretory product of gland cells is electron lucent. Extrusion generally is characterized by the formation of an opening at the cell surface and the discharge of the secretory material. Intercellular canaliculi are absent. Mitochondria and arrays of granular endoplasmic reticulum are more numerous than in other mucous glands of rabbits. Intercalated ducts are short and connect the tubules with intralobular ducts. Striated ducts are absent in the infraorbital gland. Nerve terminals occur in some instances between gland cells and between glandular and myoepithelial cells.