This striking improvement in remyelination corresponded with a dramatic increase in oligodendrocyte repopulation of chronic lesions in FGF2 j/j mice, which was not found in FGF2 +/+ mice (Fig. 6) . Before cuprizone treatment, nonlesioned FGF2 +/+ and FGF2 j/j mice had a similar density of oligodendrocytes in the corpus callosum at 26 weeks of age, which corresponded with the longest cuprizone treatment and recovery protocol. In addition, the cuprizone-induced loss of oligodendrocytes in FGF2 +/+ and FGF2 j/j mice was similar at the end of the chronic cuprizone treatment. In conjunction with the quantitative myelin immunostaining analysis, these findings indicate that FGF2 +/+ and FGF2 j/j mice experienced a similar chronic disease severity. Importantly, during the 6-week recovery period after chronic demyelination, the oligodendrocyte density returned to nonlesioned values in the FGF2 j/j mice, but not in FGF2 +/+ mice. Analysis of an additional intermediate recovery period in FGF2 j/j mice confirmed this increased oligodendrocyte regeneration and demonstrated that this repopulation could occur within 3 weeks after removal of cuprizone from the diet.