Bright field images of Erbb3+/- (A,B) and Erbb3-/- (C,D) NC cultures grown in the absence (A,C) or presence (B,D) of NRG1. Cells were cultured for two weeks after neural tube isolation, replated and DAPI stained one day later. Although differentiated MCs (dark cells) were detected in both genotypes, pigmented MCs from Erbb3 mutant cultures (C) were darker. When exogenous NRG1 was added to Erbb3+/- NC cultures (B), a dramatic reduction in pigmented MCs was observed. No such difference was noted when Erbb3 mutant cultures (D) were grown with NRG1, suggesting that activation of NRG1/ERBB3 signaling inhibits MC differentiation.