Reduced OL numbers in E18.5 in Olig1 null embryos. The levels of Mbp (C,G) and Plp (D,H) expression were decreased in Olig1 null spinal cords compared to corresponding controls (A , E and B , F respectively). Differentiating OLs were quantified based on Mbp and Plp expression (I). Three sections per embryo (n = 3) were counted and the data presented as mean ± s.e.m. p values (p < 0.05) were calculated by Student’s t-test. Scale bar: 80 μm.