Regional variations in glia and neuropil within the hypothalamic ventromedial nucleus

The normal fine structure of glia and neuropil in the various regional subdivisions of the hypothalamic ventromedial nucleus in the adult male albino rat is described in this report. Although most HVM astrocytes throughout the nucleus appear ordinary in cytology, certain astrocytes found in the posterior ventrolateral subdivision of the nucleus appear somewhat reactive, in that they contain numerous thick fascicles of gliofibrils and pleomorphic dense bodies. The processes of these reactive astrocytes elaborate multiple, concentric lamellae which encapsulate small, round, pyknotic masses (probably degenerate axonal elements). These degenerate profiles, which are greatly outnumbered by normal HVM boutons, may represent synaptic contacts that deteriorate spontaneously in the normal adult HVM. Other signs of spontaneous degeneration occurring within this neuropil include the occasional presence of large masses of necrotic debris which appear engulfed by microglia. These findings suggest the presence in the normal adult HVM neuropil of a low-grade degenerative process with attendant gliosis, which is topographically centered about the posterior ventrolateral region of the nucleus. Regions of neuropil containing degenerate boutons also contain altered neuronal processes, some of which may be growth cones. The topographic proximity of the degenerating boutons to possible signs of axonal or dendritic regeneration indicates that certain synaptic circuits in the normal adult HVM may be plastic, and subject to spontaneous remodelling.