Dominant-negative BMP-4 receptor affects normal neuronal differentiation in dissociated cell culture. A, Cells infected with DAP virus bear neurite trees typical of normal differentiating neurons. When cultured in the presence of BMP-4 (30 ng/ml), neurites elaborate more extensively. B, Cells infected with the DEL virus do not extend neurites, even after the application of BMP-4 (30 ng/ml). The quantitative analysis shown inB indicates that the distribution of neurite lengths shifts after BMP-4 treatment for 24 hr. C, BDNF treatment of DEL-infected precursors induces an increase in neurite length indicating that the truncated receptor introduced by DEL is not nonspecifically blocking differentiation. Scale bar, 20 μm.