Fig. 6. BMP signaling regulates many aspects of neural tube patterning. (A) BMP signaling regulates homeobox gene expression to define dorsal and intermediate cell fates. In cooperation with SHH signaling, BMPs set the expression domain boundaries of Pax6 and Pax7. The border between dorsal and intermediate cell fates, marked by the dorsal border of high level Pax6 expression is refined by the BMP-mediated activation of Msx1, which represses Dbx2 expression. (B) BMP signaling regulates the dorsal expression of bHLH proteins along a gradient of activity. bHLH protein expression boundaries are set by thresholds of BMP signaling. bHLH expression domains give rise to a limited number of types of terminally differentiated neurons. (C) BMP signaling promotes a diversity of intermediate cell fates. BMP regulation of Pax7 sets a dorsal limit on the generation of Evx1-expressing neurons. BMP regulation of the dorsal border of Dbx1-expressing cells may help divide the Pax2⁺, Lim1/2⁺ cells into two distinct progenitor populations.