Practice question

Xenopus animal cap assays revealed BMP4 as critical paracrine inducer for lens. Optic vesicle and surrounding mesenchyme express BMP4 and BMP7 that diffuse to competent ectoderm, activating SMAD1/5/8 phosphorylation and complex formation with SMAD4, which cooperates with Pax6 and Sox2 to open delta1-crystallin loci. Inhibition of BMP with Noggin or chordin blocks lens marker expression in co-cultures, while recombinant BMP4 restores lens placode formation in absence of optic vesicle. Wnt and Hedgehog antagonize this process, thus BMP provides instructive cue for Xenopus lens specification, distinct from FGF's later role.