Heads up to Bmp inhibition

According to the much-debated default model of neural induction, ectodermal cells in early embryos form neural tissue unless instructed by bone morphogenetic proteins (Bmps), in particular, to do otherwise. In early Xenopus embryos, a dorsoventral-patterning, Bmp morphogenetic gradient is believed to be established by two centres: the ventral gastrula centre, a source of Bmp signalling molecules; and the dorsal Spemann organizer, a source of secreted Bmp antagonists. On p. 3381, Reversade et al. now report that in the absence of the dorsal organizing centre, knocking down Bmp2, Bmp4 and Bmp7 is sufficient to cause extensive CNS and head induction in embryos that would otherwise not develop neural tissue. Knocking down any one Bmp mildly dorsalized embryonic explants, while knocking down all three compromised trunk and tail development, and strongly dorsalized embyos. The authors conclude from their findings that Bmp inhibition is sufficient for neural induction in vivo, thus supporting the default model, and that brain formation does not require Spemann organizer signals in the absence of ventral Bmps.