Oligodendrocytes resemble astrocytes and may be related to them, but they have fewer processes, which branch infrequently (Figs. 15, Ol and 21D). The soma is small and the nucleus distinctive. It is much smaller than the large, pale nucleus in astrocytes: round, heterochromatic, and deeply staining. The dense cytoplasm also stains darkly because as it is rich in rER and free ribosomes, with a conspicuous Golgi complex and many mitochondria. This dusky quality is perhaps the most characteristic feature, as striking in the EM as in dye-stained sections. Oligodendrocytes are usually the darkest cells around.

Interfascicular oligodendrocytes make and maintain central myelin (see preceding discussion). In white matter, they lie in rows between bundles of axons (Fig. 15, Ol). In gray matter, satellite oligodendrocytes closely associate with nerve cell bodies. The relationship is unclear. EM shows a smooth zone ofapposition between glial cell and neuron, with nothing to suggest what transpires between them. Oligodendrocytes in culture show shallow rhythmic pulsations. The significance of such activity is not known.

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