[Myelinized oxytocinergic neurosecretory fibers in the region of the median eminence of the rat].

The hypothalamo-neurohypophysial tract of the rat passes the zona interna of the median eminence. The oxytocinergic fibres of this tract were demonstrated by their yellow secondary fluorescence using the pseudoisocyanine-technique. In the same area the staining for myelin sheats gives positive results. The electron microscopical analysis has shown, that nerve fibres containing neurophysin vesicles in the hypothalamo-neurophysial tract can be myelinated partially. The myelinated part of the fibre tract is localized between the caudal end of the optic chiasma and the proximal part of the infundibular stalk. In the infundibulum and in the neurohypophysis myelin sheats are lacking, but the preterminal part of the paraventriculo-neurohypophysical tract is myelinated partially. Concerning the functional significance of the myelin sheats of oxytocinergic fibres it is assumed that in the region of the median eminence a release of neurohormones is prevented.