Practice question

In Xenopus embryos, bone morphogenetic protein 4 produced by presumptive lens ectoderm and surrounding tissue promotes lens formation independent of optic vesicle at early stages. BMP4 suppresses neural fate, maintains Pax6 and Sox2 in anterior ectoderm, and induces L-Maf and delta-crystallin expression. Retinoic acid patterns retina, Wnt5a regulates morphogenesis, JAK-STAT not lens inductive. Overexpression of BMP4 in competent ectoderm causes ectopic lens differentiation, Noggin inhibition blocks lens, paralleling chick and mouse data where BMP4 from optic vesicle serves as early paracrine cue driving lens placode specification and crystallin production and growth.