In vitro analysis of the oligodendrocyte lineage in mice during demyelination and remyelination

We then examined the relative abundances of the different 5 and 7) . The relative proportion of type 2 astrocytes (04+ GFAP+ G C -) increased between 1 and 3 div, even though the cultures were maintained in defined medium with only 0.5 % FBS (Figs. 5 and 7). Interestingly, in cultures of neonatal rat optic nerve, 10% FBS is required to induce development of a substantial number of type 2 astrocyteg (Raft et al., 1983) . In cultures of demyel~ated spinal cord tissue (5 wpi), oligodendrocytes were the predominant cell type after 1 div (Fig. 5) . However, a larger percentage of the O-2A population was composed of type 2 astroeytes in these cultures as compared with cultures of control tissue. By 3 div, type 2 astrocytes became even more prevalent than oligodendrocytes. At each time point, O-2A progenitor cells represented ,,020% of the O-2A lineage population. A remarkable feature in cultures from demyelinated tissue was the presence of mixed oligodendrocyte-astrocyte phenotype cells, which were rarely seen in control cultures (Fig. 5) . These cells expressed GC on their surface and contained intracellular filaments immunostained with GFAP (Fig. 8) . Such mixed phenotype cells (04+ GC+ GFAP+) were found consistently using two different antibodies to GC in combination with two different antibodies to GFAP (see Materials and Methods). Certain growth factors have been shown to have an effect on the fate of O-2A lineage cells: IGF-I promotes oligodendrocyte development (McMorris et al., 1986) , whereas bFGF inhibits myelin gene expression (McKirmon and Dubois-Dalcq, 1990 ). Therefore, we examined whether treatment with such growth factors from 1-3 div would influence the antigenic phenotypes expressed by O-2A lineage cells isolated during the course of demyelination and remyelination (Fig. 9 ). In cultures of demyelinating/remyelinating tissue exogenous IGF-I increased the proportion of oligodendrocytes relative to type 2 astrocytes, whereas bFGF decreased the relative number of oligodendrocytes. PDGF did not markedly alter the ratio of type 2 astrocytes to oligodendrocytes, in agreement with the observation that PDGF allows timely differentiation of oligodendroeytes and type 2 astroeytes in cultures of developing CNS tissue Raft et al., 1988) .