Table 2.

Progenitors in cultures differentiated for 5 days

Genotype	Number of precursors counted per field	Percentage of cells that are precursors
+/+	1.0 ± 0.3*	15 ± 5**
−/−	10.5 ± 3.9*	62 ± 8**

The majority of p27 −/− cells in cultures differentiated for 5 days are oligodendrocyte precursors. Cultures were maintained in serum-free differentiation medium for 5 days, and the number of process-bearing cells was quantitated on each coverslip (n = 17 cultures from 8 p27 +/+ mice; n = 15 cultures from 6 p27 −/− mice). Progenitors were defined by the presence of simple short processes and NG2 immunoreactivity (Figs. 4 and 5). The raw numbers of progenitors counted per field are shown in the first column. Percentages were calculated by normalizing the number of progenitors to the total number of process-bearing cells and are shown in the third column. The results are expressed as mean ± s.e.m. The P value indicates statistical significance using Student’s t-test. (*) P = 0.02; (**) P = 0.0001.