Oligodendroglia

Oligodendroglia were first studied in detail and clearly identified as a separate type of glial cell by del Rio Hortega (1921, 1928). He classified oligodendroglia according to the branching pattern of their processes and recognized four types (Fig. 19). Type I cells, usually located in gray matter, had branches that radiated in all directions. Types II–IV were found in tracts and had a more parallel arrangement. Branches in type II were shorter and more numerous. There were fewer, longer processes in type III, and type IV cells had a single long branch extending from either pole. Del Rio Hortega’s illustrations [reprinted in Bunge (1968) and Wood and Bunge (1984)] clearly show the close relationships of myelin sheaths and oligodendroglial processes, and he thought that the main function of this glial cell was myelin formation and maintenance. His findings have been confirmed by many light and electron microscopic studies (Bunge 1968; Billings-Gagliardi et al. 1983; Puelles 1978; Spacek 1971; Stensaas and Stensaas 1968b; Wood and Bunge 1984). Early light microscopic observations also showed that in gray matter, oligodendroglia were found clustered around neuronal perikarya. These “satellite” oligodendroglia had fewer, thinner processes, did not have obvious connections to myelin sheaths and were thought to function as supporting cells for the neurons they surrounded. More recently, myelin basic protein (MBP) and myelin-associated glycoprotein (MAG) have been detected in some perineuronal oligodendrocytes in both developing and remyelinating CNS (Ludwin 1979; Luduena and Wessels 1973; Sternberger 1984). These observations show that some oligodendroglia, although next to neurons in a “satellite” position, have processes connected to myelin sheaths. Further studies of myelin constituent expression by oligodendroglia shown by electron microscopy to be perineuronal would be of interest, especially if the regions studies lacked myelin. This evidence would help determine if there is a subset of oligodendroglial cells analogous to the perineuronal satellite Schwann cells in dorsal root ganglia that does not form myelin and has different functional properties.