ALBERT

Notes: Summary 1. The fine structure of the neurohypophysis in normal, adrenaline-treated and pilocarpine-treated rabbits has been studied, using the electron microscope. The normal structure of the nerve terminal and pericapillary region is quite similar to that of the rat as described by Palay (1957) and Hartmann (1958). 2. Pituicytes are significantly different, however, and are classified into three types. The first one (Type A) is identical with the astrocyte of Farquhar and Hartmann (1957) and of Schultz, Maynard and Pease (1958). This type cell has a round or oval nucleus of low electron density and abundant watery cytoplasm with few formed elements. 3. The second type (Type B) is considered to be an atypical astrocyte. This type cell has a nucleus closely resembling that of Type A, but with abundant cytoplasm containing relatively numerous formed elements. The inverse relationship between relative numbers of Type A and Type B cells in a given gland suggests the possibility of one type being readily transformed into the other under normal conditions. The last type (Type C) has a dark, irregular nucleus and dense cytoplasm with many cell organelles. This cell is structurally identical with the microglia of Farquhar and Hartmann, and of Schultz, Maynard and Pease. A typical oligodendrocyte is not found. No secretory granules are present in pituicytes. The pituicytes are in contact with nerve fibers with a double membrane intervening, and sometimes enclose nerve fibers in invaginations of their cytoplasm. The cell bodies of the pituicytes are located in the parenchymatous region far from blood capillaries, and their processes, along with the nerve terminals, reach to the outer basement membrane of the pericapillary space. 4. During 1–2 hours after injection of adrenaline there was found to be a statistically significant increase in number and size of secretory granules, as well as structural changes of mitochondria in the nerve terminals, enlargement of the pericapillary space and endothelial alterations. These phenomena are considered to reflect a blocking of the release of secretory material after injection of adrenaline. 5. After injection of pilocarpine, a statistically significant decrease in number and size of neurosecretory granules, plus the appearance of many optically empty granules, structural changes of mitochondria and attenuation of the pericapillary space were observed. These phenomena are considered to be related to an acceleration of the release of secretory granules after injection of pilocarpine.