OL numbers recover by P3. At P3, the numbers of Mbp-and Plp-expressing cells in Olig1 null spinal cords (C,D and G,H) were indistinguishable from controls (A,B and E,F respectively). Differentiating OLs were quantified based on Mbp and Plp expression (I). Three sections per mouse (n = 3) were counted and the data displayed as mean ± s.e.m. p values (p < 0.05) were calculated by Student’s t-test. Scale bar: 100 μm.