Quantitative analysis of the oligodendrocyte population revealed ongoing partial regeneration during and after chronic demyelination (Fig. 2) . The oligodendrocyte density was lowest after the initial 3 weeks of cuprizone in the acute phase. After 12 weeks of continuous cuprizone, the oligodendrocyte density continued to be severely reduced relative to values from nontreated mice. Ongoing partial regeneration during chronic demyelination was indicated by the fact that the oligodendrocyte density after 12 weeks was higher than after the initial 3 weeks. After ending the chronic cuprizone treatment, this oligodendrocyte regeneration improved during the recovery period. Interestingly, as shown in Figure 1 , remyelination did not progress similarly during this 6-week recovery period. The PLP mRNA in situ hybridization used to identify oligodendrocytes should detect both premyelinating and myelinating oligodendrocytes because PLP transcription precedes myelin formation (25) . These results indicate that after chronic demyelination, a proportion of premyelinating oligodendrocytes that are generated during the recovery period may fail to differentiate into myelinating oligodendrocytes.