Morphology and Subtypes

As a whole, oligodendrocytes are very refractive to stains. In vivo, the structure of the oligodendrocyte cell body is shrouded to some extent by the vast networks of myelin it produces as well as by its close association with neighboring axons. In fact, a cell may be identified as an oligodendrocyte based on its continuity with the outermost layer of the myelin sheath. The cells are morphologically diverse and can be classified into four subtypes. Type 1 oligodendrocytes have spherical or slightly polygonal cell bodies. They have several thin processes that emerge from the cell body in the direction of nerve fibers. This subtype can be found in the forebrain, cerebellum, and spinal cord, and these oligodendrocytes are usually arranged around blood vessels, neurons, and fiber tracts. Type 2 oligodendro-cytes are polygonal or cuboid in shape, with fewer and thicker processes than those of the Type 1 subgroup. These cells are only found in the white matter and are closely associated with nerve fibers. Type 3 oligoden-drocytes have even fewer processes directed toward nerve fibers and are found in the cerebral and cerebellar peduncles, medulla oblongata, and the spinal cord. Finally, type 4 oligodendrocytes are found near the entrance of nerve roots into the CNS and in association with large axons. These categories are arbitrary in nature because oligodendrocytes rarely fit perfectly into any one subgroup. Oligodendroglial cells may also be classified as interfascicular, perivas-cular, or perineuronal satellite cells, depending on their location.

Although the gross structure of oligodendrocytes varies widely, little difference in the fine structure has been observed in the CNS. The oligodendroglial cell body and the associated myelin membrane are enriched with sphyngoglycolipids such as galactosylcer-amide and its sulfated form, sulfatide. Immunocytochemical staining of the cell body in vivo may prove difficult because the vast network of myelin results in poor antibody penetration. Furthermore, the cell surface marker repertoire changes with different stages of differentiation. However, in vitro, isolated oligodendrocytes can be visualized by immunofluor-escent staining, as shown in Fig. 3. Similarly, intracellular injection of dyes has revealed much about the ultrastructure of oligodendrocytes. The cytoplasm of the cells has well-developed Golgi apparati and abundant ribosomes, both free and bound to an extensive endoplasmic reticulum. This is to be expected since high-scale myelin protein synthesis and transport are the two main functions of this cell type. The greater density of the cytoplasm and nucleus, as well as the absence of glycogen granules or bundles of specific intermediate filaments, can distinguish oligodendrocytes from astrocytes. Oligodendrocytes do not contain intermediate filaments in the cytoplasm but, rather, actin microfilaments and a high content of microtubules, particularly in the processes. By electron microscopy, oligodendrocytes can be further subdivided into light, medium, and dark, distinguishable by decreasing size and increasing cytoplasmic density. It has been proposed that these structural differences are correlated to functional differences as well. For example, the light oligodendrocytes appear to be highly involved in the production of myelin, whereas the dark oligodendrocytes may be involved with myelin maintenance.

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