Neurosecretory axon regeneration into intrahypothalamic neural lobe allografts: neurophysin immunohistochemistry and fine structure.

Neural lobe allografts placed stereotactically into the hypothalamo-neurohypophysial tract between the paraventricular and supraoptic nuclei were investigated between 5 and 70 days post-transplantation (dpt). They contained temporally increasing numbers of neurophysin-positive axons. At the fine structural level, endogenous neurosecretory axons had virtually disappeared from successful (vascularized) grafts by 5 dpt. At this time, single host neurosecretory axons and especially numerous growth cones were associated with pituicyte processes and/or scalloped basal lamina scaffolds. Axon terminals containing neurosecretory granulated vesicles and microvesicles were present only occasionally at 5 dpt but became much more numerous subsequently. These terminals were associated with pituicytes and abutted the parenchymal basal lamina of pericapillary connective tissue spaces. In addition, beginning at 10 dpt, neurolemmocyte-like cells were associated with neurosecretory axons. At 70 dpt, the fine structural characteristics of grafted neural lobes were virtually indistinguishable from those of intact controls, except for the presence of occasional areas of more extensive connective tissue, nonfenestrated capillaries and neurolemmocyte-like cells.