The only known function of oligodendrocytes is to myelinate axons in the vertebrate CNS. In this review, we consider how the number of oligodendrocytes is matched during development to the number and length of axons requiring myelination. Oligodendrocytes are postmitotic cells (Card and Pfeiffer, 1990; Hardy and Reynolds, 1991). They develop from proliferating precursor cells that migrate into developing white matter from germinal zones around the ventricles of the brain and the central canal of the spinal cord (Paterson et al., 1973; Raff et al., 1983; Small et al., 1987; Levine and Goldman, 1988; Hardy and Reynolds, 1991; Pringle et al., 1992; Grove et al., 1993). Thus the final number of oligodendrocytes in any region of the CNS will depend on the number of precursor cells that migrate into it, the numberoftimestheprecursorcellsdivide beforethey differentiate, and the number of oligodendrocytes and precursor cells that undergo normal cell death in the region (Barres et al., 1992a). As seems to be the case for animal ceils in general, the migration, proliferation, differentiation, and survival of an oligodendrocyte lineage cell all depend on an interplay between the intrinsic properties of the cell, which depend on the cell’s history, and extracellular signals produced by other cells.

We have studied oligodendrocyte development in the rat optic nerve, one of the simplest parts of the CNS. It contains two main cell types-astrocytes and oligodendrocytes-in addition to the axons of retinal ganglion cells. The astrocytes provide a structural framework for the nerve, whereas the oligodendrocytes myelinate the axons. Cultures of optic nerve cells generally contain two types of astrocytes: type-l astrocytes develop from optic stalk cells and type-2 astrocy-tes develop from oligodendrocyte precursor cells, which are therefore called O-2A progenitor cells (Raff et al., 1983), although there is still no convincing evidence that these cells develop in the normal CNS (Skoff, 1990; Fulton et al., 1992). Type-l astrocytes first appear in the nerve about a week before birth,