Ki-67 immunostaining (Fig. 7C ) indicated that the density of cycling cells in the corpus callosum was less than 20% of the density of OP cells, indicating a relatively low level of ongoing proliferation. Cells with nuclear Ki-67 immunoreactivity were often observed as doublets, as is appropriate for confirming detection of a cycling population. The density of cells immunolabeled for Ki-67 nuclear antigen increased in chronically lesioned corpus callosum with a return to nonlesioned levels during the recovery phase. The density of endogenous cells that were proliferating in the corpus callosum of FGF2 j/j mice was similar to the values for FGF2 +/+ mice (Fig. 7B ) and for C57Bl/6 mice (Fig. 3) . The similarity of FGF2 j/j mice and FGF2 +/+ mice in our analysis of OP cells and cycling cells indicates that the dramatic differences observed in oligodendrocytes and remyelination may result from a permissive effect of FGF2 removal on OP differentiation into remyelinating oligodendrocytes.