Some findings on the fine structure of the neurohypophysis in dehydrated and pitressin-treated mice.

The fine structure of the pituitary posterior lobe was observed using normal, dehydrated and Pitressin-treated mice. The normal posterior lobe of the mouse contains numerous neurosecretory granules of about 1,300-2,000 angstrom diameter in the nerve terminals. These granules are often found in contact with neurotubules running longitudinally in the axon. In the 2, 5 and 7 day-dehydrated animals the secretory granules are markedly decreased in number, while small vesicles of about 500 angstrom diameter are conspicuously increased in number in the nerve terminal. It is very difficult to obtain an electron microscopic figure of the exocytosis of the secretory granule. The cluster of small vesicles in the nerve terminal is sometimes surrounded by an electron dense membrane and gradually changed to the cytolysomes. These facts suggest that most of these vesicles are not synaptic vesicles containing acetylcholine but materials derived from the limiting membrane of the released granules and they are gradually disposed of by lysosomes. In Pitressin-treated animals (3 days, 200mU/day), an increase in number of the neurosecretory granules and an appearance of the disposal process of the excess neurosecretory granules are characteristic in the nerve terminal. Some granules are aggregated to become higher in electron density and surrounded by an electron dense membrane. They are positive for acid phosphatase reaction and are degenerated by an autolytic process. In both cases above, the pituicytes showed a marked reaction. The elements of rough endoplasmic reticulum and the Golgi apparatus become well developed and many large lysosomes are seen in the cytoplasm.