Does a regenerating, neuroendocrine axon, which normally terminates near blood vessels, express neurovascular specificity? The intact neurosecretory axons (NSA) of the basal hypothalamus end near fenestrated blood vessels (FBV) of the pituitary gland's neural lobe. FBV are permeable to the peptide hormones released by the NSA. Will mature, regenerating NSA associate more closely with the FBV of grafted neural lobe than the FBV of other endocrine tissues, such as the pineal gland and adrenal medulla? Pieces, about 0.5mm3, of these 3 tissues were placed bilaterally in the retrochiasmatic area where the NSA are bundled and where there are no FBV. Two and 4 weeks later, the brains were processed for immunohistochemical localization of NSA with a monoclonal antibody to vasopressin neurophysin and for electron microscopy. A few regenerating NSA entered grafted pineal and adrenal medulla. Some of these NSA came to lie within about 0.5-2.0 um of the capillary wall. The NSA - FBV distance was the criterion for NSA - FBV association because there is no synaptoid or other membrane specialization to mark the association. Considerably more (0.05 greater than p) NSA entered the neural lobe grafts at both 2 and 4 weeks than grew into the pineal and adrenal medulla tissue. We conclude that (a) mature, regenerating NSA are capable of forming close associations with FBV other than their normal targets, and (b) neural lobe tissue, most likely its pituicyte component, has a markedly tropic effect on these axons.
