Evidence for dynamic interactions between pituicytes and neurosecretory axons in the rat

Several ultrastructural features of pituicytes as well as their relationship with neurosecretory axons were measured in neural lobes from rats either: non-deprived; water-deprived for 4, 12, or 24 h; or rehydrated for 12 or 24 h after an initial 24 h dehydration. A significant increase in pituicyte lipid bodies occurs with as little as 4 h of water deprivation. This is a time when several dehydration-related changes are known to occur in the somata of those neurosecretory cells sending axons to the neural lobe. Thus, increases in pituicyte lipid bodies may be related to the initiation of hormone release. In the neural lobes of water replete, non-deprived rats, pituicytes were found to contain neurosecretory axons completely enclosed by their cytoplasm. The number of these per unit area of cytoplasm decreased by 64% with 24 h of water deprivation and returned to normal levels with 24 h of rehydration. Since few of the enclosed axons were found to be degenerating, enclosure is unlikely to represent the early stages of phagocytosis.These findings suggest that during periods of little hormone secretion the neurosecretory axons are enclosed by the glial cells, but are released under conditions of increased hormone demand. This may provide the morphological framework for pituicyte involvement in inhibition and facilitation of hormone release from the neurohypophysis.