Practice question

Early Xenopus animal caps isolated before gastrulation cultured in simple saline express neural markers NCAM, Sox2 and neurofilament spontaneously without added growth factors, indicating intrinsic tendency. In intact embryo BMP4 from ventral mesoderm and ectoderm actively suppresses neural fate inducing epidermal keratin via Smad1, Dlx3 and p63. Organizer transplantation provides BMP antagonists removing suppression. Thus default or ground state of competent ectoderm is neural tissue, epidermal fate requires active BMP induction. This default model revolutionized understanding of neural induction as disinhibition rather than instructive activation, concept conserved across vertebrates.