Ultrastructural demonstration of neurohaemal contacts in the internal zone of the median eminence of the Mongolian gerbil (Meriones unguiculatus): correlation with synaptophysin immunohistochemistry

Summary Electron microscopy of the median eminence (ME) of the Mongolan gerbil (Meriones unguiculatus) revealed that, unlike most other mammalian species, abundant neurohaemal contacts were present not only in the external zone (EZ), but also in the internal zone (IZ) up to the subependymal layer. In the IZ, nerve terminals with dense core vesicles and/or small clear vesicles abutted on the outer basal lamina of the perivascular space of portal capillaries, alternating with tanycyte processes. In addition to these neurohaemal contacts, several layers of vesicle-filled varicosities surrounded the portal vasculature. An analysis of serial thin sections showed that the latter varicosities could also reach the perivascular basal lamina or contact it through small extensions in other planes of section. Apparently at least some of the nerve terminals making neurohaemal contacts were en passant in nature. A correlative investigation of synaptophysin (a major integral membrane protein of small synaptic vesicles) immunoreactivity at the light microscopical level demonstrated a conspicuously dense immunostaining around portal capillaries in both EZ and IZ of the proximal and distal ME (neural stalk). Since this perivascular accumulation of immunoreactivity coincides precisely with the ultrastructural accumulation of vesicle-filled axons which establish numerous neurohaemal contacts, it is concluded that this pattern of synaptophysin immunostaining indicates sites of neurohaemal contacts at the light microscopical level. During postnatal development, the perivascular concentration of synaptophysin immunoreactivity in the IZ appeared concomitantly with the early postnatal expansion of long portal capillary loops into the IZ. By direct electron-microscopical demonstration and indirect immunohistochemical evidence, the present study of the gerbil ME reveals that the whole extent of portal capillaries up to the subependymal layer constitutes an area for numerous neurohaemal contacts. Hence, the common view that neurohaemal contacts are restricted to the EZ of the mammalian ME is not generally valid.