Morphological and morphometric studies on the ultrastructural changes during the active release of neurosecretory substance from the neurohypophyseal nerve terminals in dehydrated rats.

Secretory nerve terminals in the posterior pituitaries of normal rats and animals dehydrated for 1, 3, 5, 7, and 9 days were observed with the transmission electron microscope. Neurosecretory granules were markedly decreased after dehydration and received the minimum number at 5 days. Small vesicles in the terminals may be classified into two types: one in aggregated vesicles about 53 nm in diameter and the other is dispersed vesicles 66 nm in diameter. The former vesicles are seen in the normal terminals and slightly decreased in number after dehydration, while the latter are hardly seen in normal pituitaries, but increase enormously due to dehydration at 3 days and thereafter. It may be conjectured that the former are the real synaptic vesicles probably containing acetylcholine, while the latter are the fragments of limiting membranes of the neurosecretory granules. Inversely to the slight decrease of dispersed vesicles at 7 and 9 days of dehydration, reticular tubules in the nerve terminals were strikingly increased in volume. Such a terminal reticulum may be formed by fusion of vesicles derived from fragmentation of the envelopes of neurosecretory granules. Finally the reticulum as well as dispersed vesicles may be destroyed by lysosomes.