Plasticity in the in vitro neurohypophysis: effects of osmotic changes on pituicytes.

The ultrastructure of rat neurohypophyses incubated in vitro was studied to investigate secretion-related changes that may be intrinsic to the isolated neurohypophysis and to establish the morphological integrity of this preparation. Neurohypophyses were incubated for 2 h in an in vitro chamber in medium of low (290 mosM/kg), normal (310 mosM/kg), or high (340 mosM/kg) osmolality. Subsequent morphometric analyses at the ultrastructural level revealed that the number of axons completely enclosed by pituicyte cytoplasm was inversely related and the amount of neuro-vascular contact was directly related to the osmolality of the medium. These results mimic those found in preparations from in vivo experiments with hydrated and dehydrated rats. The number of pituicyte liposomes did not vary consistently with osmolality. We conclude that: dynamic interactions between pituicytes and neurosecretory axons may be stimulated, at least in part, by the immediately surrounding milieu; pituicytes may have an active role as modulators of hormone release; and the in vitro neurohypophysis provides a suitable model to study events intrinsic to this area.