Transected axons of adult hypothalamo-neurohypophysial neurons regenerate along tanycytic processes.

In the intact hypothalamo-neurohypophysial system, oxytocinergic or vasopressinergic neurons project their axons throughout the internal layer of the median eminence towards the blood vessels of the hypophysial neural lobe. When transected at the level of the median eminence, these axons undergo massive sprouting towards the external layer of the organ and the underlying perivascular region containing hypophysial portal vessels. The present study was designed to explore the possible roles of median eminence glial cells in such a reorganization of transected neurohypophysial axons. The relationships between regenerating axons and glial cells were studied by laser scanning confocal microscopy and electron microscopy on vibratome sections immunostained with specific antibodies against neurohypophysial peptides and/or against glial markers. All along the intact median eminence, two main types of glial cells were identified: (1) tanycytes immunoreactive to vimentin and slightly immunoreactive to glial fibrillary acidic protein, and (2) classical astrocytes immunoreactive to glial fibrillary acidic protein but vimentin-negative. In the intact median eminence, neurohypophysial axons were associated with astrocytic processes located in the internal layer. After a lesion of the hypophysial stalk, peptidergic regenerating axonal sprouts were found to project massively towards the external layer and to penetrate the underlying perivascular region in close association with tanycytic-like processes immunoreactive to both vimentin and to glial fibrillary acidic protein. In contrast, regenerating sprouts were absent from those regions of the lesioned median eminence containing astrocytic processes immunoreactive to glial fibrillary acidic protein but vimentin-negative. When fixed lesioned median eminences were treated by placing crystals of the lipophilic dye DiI on their ventricular surface, regenerating axons were found to be closely associated with DiI-labelled tanycytic-like end feet terminating in the external layer, and with connected thin processes projecting through the external vascular region. These data indicate that in the median eminence of the adult rat, lesioned neurohypophysial axons regenerate in close association with tanycytic processes.