Electron microscopy of oligodendrocytes in normal rat cerebrum

Abstract

Electron microscopic examination of rat cerebral gray and white matter following formaldehyde or glutaraldehyde vascular perfusion reveals distinctive morphological features for identification of oligodendrocytes. These elements are situated as satellites of neurons (perikarya, dendrites and axons), and display a wide range of nuclear and cytoplasmic densities which tend to parallel each other. Cytoplasmic density appears to be an uncertain criterion for neuroglial identification, but oligodendrocytes are recognizable by the following distinctive features: large quantities of free ribosomes or ribosomal rosettes, numerous nuclear pores and light patches of otherwise dense chromatin adjacent to the pores, extensive cytoplasmic microtubules, absence of broad processes seen in other cerebral elements but presence of numerous fine processes which can occasionally be traced to form the outer mesaxon of myelin sheaths, absence of glycogen and fine gliofibrils uniquely present in astrocytes, and the usual presence of thin rims of perinuclear cytoplasm frequently associated with large dilatations of the nuclear cleft. Arguments are adduced for rejecting the identification of “dark” glial elements as microglia. The relation of oligodendrocytes to myelin and the functional significance of the wide spectrum in cytoplasmic density are considered in relation to the synthetic demands of adjacent neurons.