Fig. 8. BMPs have diverse functions for progenitor and postmitotic neurons in the dorsal spinal cord during development. (A) Graded signalling from the RP-derived BMPs is sufficient to induce the dI1, dI2 and dI3 cell fates. BMPRIA and BMPRIB have a shared redundant activity mediating dorsal neural cell fate specification, presumably acting through the Smad transcriptional regulator. (B) Subsequently, BMP heterodimers act as a diffusible chemorepellent to direct (dI1) commissural axons away from the RP. This activity is predominantly mediated by BMPRIB, which acts through an as yet unknown second messenger intermediate to locally reorganize the cytoskeleton.