The ultrastructural characterization of macroglial cell types



In the prelaminar region of adult cat optic nerve, silver impregnation revealed a macroglial cell population consisting solely of small fibrous astrocytes. Electron microscopically the cells were characterized by abundant processes containing closely packed filaments and occasional glycogen granules; their perikarya contained few organelles, glycogen particles were conspicuous but not abundant, and their cytoplasmic and nucleoplasmic matrices were of low density.

The laminar region contained typical fibrous astrocytes. These cells were characterized by similar processes and matrices but their perikarya contained filaments and were richer in organelles whose orientation was primarily radial.

The postlaminar region contained both astrocytes and oligodendrocytes. The latter were identified as cells characterized by the absence of filaments and glycogen, the presence of canaliculi, dense cytoplasmic and nucleoplasmic matrices and perikarya very rich in organelles, whose orientation was primarily circumferential.

The presence of filaments and/or glycogen on the one hand, and of dense matrices on the other hand, seemed to be mutually exclusive and provided diagnostic criteria for astrocytes and oligodendrocytes respectively. Bivalent forms were not seen. Cells usually identified as microglia were not seen.

The relationship between astrocytes and finely myelinated nerve fibers in the lamina cribrosa indicates a sheath-supporting role. Astrocytes are probably generally concerned with the isolation and insulation of neurons and their processes. Oligodendrocytes may be concerned with the energetic support of neurons and their processes.