Regeneration of neural lobe-like neurovascular contact regions in explanted neural lobes placed in the hypothalamo-neurohypophysial tract in the lateral retrochiasmatic area.

Neural lobes that had been explanted 30 days earlier were transplanted into retrochiasmatic lesions of the hypothalamo-neurohypophysial tract 15 days before being observed by electron microscopy and neurophysin immunohistochemistry. Neurovascular contact regions consisting of microvascular networks surrounded by neurophysin-immunoreactive terminals developed in 86% of the grafted explants. The fine structure of such regions resembled that of the neural lobe, with palisades of neurosecretory axon terminals abutting the basal laminae associated with the microvessels and plexuses of neurosecretory axons occupying the spaces within the vascular network. Both continuous and fenestrated blood vessels were present. Lamellopodia from glial cells partially ensheathed both axons and terminals, and sometimes separated the terminals from the perivascular basal lamina. Profiles in which neurosecretory granulated vesicles were depleted and many microvesicles were present were interpreted as terminals from which hormones had been released. No regeneration occurred into explants that had been cryotreated to kill their pituicytes and other cells before transplantation. These observations demonstrate that neurosecretory axons served in the hypothalamus can regenerate to form a new neural lobe-like structure when an appropriate microenvironment is available, and that neural lobes explanted 21 days earlier retain the elements required to supply that microenvironment. They also provide evidence that viable pituicytes are essential for regeneration of neurosecretory axons and terminals into transplanted explants.