The neurohypophysial axons in normal and dehydrated rats: an electron microscopic study including ZIO-staining method.

The neurohypophysial axons from normal rats and those dehydrated for 4 and 12 days were investigated by means of conventional glutaraldehyde fixation and the zinc-iodide-osmium (ZIO) staining technique. Conventionally fixed axons from dehydrated rats revealed depletion of dense core of neurosecretory granules, more frequent occurrence of microvesicles, proliferation of smooth endoplasmic reticulum or accumulation of lysosomes. It has been disclosed that the ZIO reaction product has the ability to stain microvesicles (normal rats) and smooth endoplasmic reticulum, lysosomes, some neurosecretory granules and some parts of stromal inclusion at the release site (dehydrated rats). Quantitative data are also presented. They indicate a significantly decreased number of both neurosecretory granules (NSG) and ZIO-positive microvesicles (MV) in dehydrated rats. Equally, the value of the median of the MV/NSG ratio was significantly increased from 1.38 in normal rats to 5.50 and 7.50 in rats dehydrated for 4 and 12 days, respectively. Thus, the possible involvement of ZIO-positive microvesicles in a process associated with neurosecretion is suggested. While in normal rats a correlation was found between the number of neurosecretory granules and the area of sectioned profiles of the axons, no evidence was obtained for it in 12-day dehydrated rats. Conversely, no evidence for a relationship between the axon area and the number of ZIO-positive microvesicles was obtained in normal rats, while in rats dehydrated for 12 days a correlation was found.