Purified OPCs cultured in proliferation- or differentiation-inducing media. OPCs purified from postnatal day-7 rats were cultured on glass coverslips for 4 d in defined, serum-free media that either contained saturating amount of mitogens (proliferation media [A,C]) or T3 with no mitogens (differentiation media [B,D]). Cells were imaged live on an inverted phase contrast microscope (A,B), or fixed, immunostained for expression of myelin basic protein (MBP, green), and mounted with DAPI to reveal nuclei (blue) (C,D).