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.
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The authors are indebted to Mr. G. Gendusa and P. Balbi for technical assistance.
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Fasolo, A., Franzoni, M. & Mazzi, V. The neurohypophysis of the crested newt. Z. Zellforsch 141, 203–221 (1973). https://doi.org/10.1007/BF00311354
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DOI: https://doi.org/10.1007/BF00311354