Distribution and differentiation of A2B5+ glial precursors in the developing rat spinal cord

In many regions of the rat central nervous system, oligodendrocytes develop from migratory A2B5+ precursor cells. In the rat spinal cord, during early embryonic development the capacity for oligodendrogenesis appears to be restricted to ventral regions of the spinal cord, while cultures of postnatal rat spinal cord contain a distinct population of A2B5+ astrocyte precursors. To determine if, as in other regions of the CNS, spinal cord A2B5+ cells give rise directly to oligodendrocytes and astrocytes, the initial distribution, and subsequent dispersion, proliferation, and differentiation of spinal cord A2B5+ cells have been examined in both explant and dissociated cell cultures. Spinal cord oligodendrocytes develop from A2B5+ cells. At E14, A2B5+ cells are restricted to ventral regions of the spinal cord and as development proceeds they become more uniformly distributed throughout the spinal cord. In explant cultures, greater than 95% of the explants that contain oligodendrocytes also contain A2B5+ cells and a proportion of mature oligodendrocytes retain detectable A2B5 immunoreactivity briefly on their surface. The maturation of spinal cord oligodendrocyte precursors occurs in a number of distinct stages characterized by the expression of O4 immunoreactivity, which first appears at E16, and GC immunoreactivity, which first appears at E18. As spinal cord oligodendrocyte precursors acquire O4 immunoreactivity they appear to lose the ability to proliferate in response to PDGF but retain the ability to proliferate in response to bFGF, suggesting that the control of proliferation of oligodendrocyte precursors is, in part, dependent on their maturational state. In the presence of high serum, spinal cord A2B5+ cells fail to develop in isolated E14 dorsal spinal cord cultures, while in ventral cultures they subsequently differentiate into A2B5+ astrocytes suggesting that A2B5+ astrocyte precursors are also initially ventrally located. Unlike oligodendrocyte differentiation, however, the differentiation of spinal cord A2B5+ cells into astrocytes is delayed in early embryonic-derived cultures compared to those from older animals. These observations suggest that local influences may regulate the timing of spinal cord A2B5+ astrocyte development, but not spinal cord oligodendrocyte development.