Morphology Classification and Immunoreactivity

Oligodendrocytes are small cells with a round or oval, relatively dense nucleus and a small rim of cytoplasm with a cell diameter of 6-8 |im. Oligodendrocytes possess ramified processes that can be demonstrated by silver techniques. They also exhibit a structural polymorphism that reflects differences in function. Three to five types of oligodendrocyte can be distinguished based on localization, which may also be related to differences in function.

1. Perineuronal oligodendrocytes (gray matter oligodendrocytes - Fig. 3.5a, b). This type of oligo-dendrocyte is commonly located near the larger pyramidal cells of the cerebral cortex and the nerve cells of the basal ganglia. Perineuronal oli-godendrocytes are considered to be analogous to capsule cells of the dorsal root ganglia. They constitute 51% of the total perineuronal glial population (Bunge 1968).

2. Perivascular oligodendrocytes (gray matter oligodendrocytes). Fairly numerous, these cells are seen mainly around cerebral cortical vessels.

3. Interfascicular oligodendrocytes (white matter oligodendrocytes - Fig. 3.3c, d). This is by far the most common type of cell in the white matter. These have processes that run parallel to nerve fibers and partly or completely encircle nerve fibers; the cell nuclei are disposed in rows. In the corpus callosum of the rat 69.8% of glial cells in these rows are oligodendrocytes (Mori and Leblond 1970).

4. Szuchet (1995) describes a fourth type of oli-godendroglia according to Rio Hortega (1956) which is associated with large axons and located near the entrance of nerve roots into the CNS.

5. An oligodendrocyte precursor cell is described as a fifth type, which comprises 5-8% of the glial cell population in the CNS (Levine et al. 2001). These cells have small cell bodies with multiple branched processes. In gray matter, the processes tend to be oriented radially, in white matter they are more longitudinally arranged and aligned with nerve fibers. Under normal circumstances the processes are in contact with synapses and nodes of Ranvier, a possible indication that these structures play a regulatory role in the axonal impulse conduction.

6. Meanwhile the oligodendrocyte has served as a model for lineage development in part due to the identification of specific additional phenotypic

Fig. 3.5a-d. Oligodendrocytes. a, b Perineuronal oligodendro- c, d interfascicular oligodendrocytes = white matter oligodendrocytes = gray matter oligodendrocytes (a carbonic anhydrase II = cytes (c CA II; d silver stain; magnification c X500, d Xl,000) CA II; magnification X500; b silver stain; magnification Xl,000);

Fig. 3.5a-d. Oligodendrocytes. a, b Perineuronal oligodendro- c, d interfascicular oligodendrocytes = white matter oligodendrocytes = gray matter oligodendrocytes (a carbonic anhydrase II = cytes (c CA II; d silver stain; magnification c X500, d Xl,000) CA II; magnification X500; b silver stain; magnification Xl,000);

stages during maturation (Grinspan 2002). The result is the identification of numerous signaling molecules inducing oligodendrocyte development.

Oligodendrocytes can be demonstrated by the silver technique and by antibodies to myelin basic protein (MBP), myelin oligodendrocyte glycoprotein, platelet-derived growth factor, galactocerebroside, or to proteolipid protein (Levine et al. 2001; for review, see Friedman et al. 1989). Also specific for oligodendro-cytes are the monoclonal antibodies Rip (Friedman et al. 1989), Otex 1 (Mori de Moro et al. 1990), and CC-1 (Bath et al. 1996). In addition, oligodendrocy-tes have been found to selectively express carbonic anhydrase II (CA II) (Cammer et al. 1977; Ghandour et al. 1980), gluthathione-S-transferase, isoenzyme pi (Tansey and Crammer 1991), and cell-surface sphingolipids, such as galactocerebroside (Raff et al. 1978).

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