Practice question

Ectodermal competence to become epidermis requires active BMP4/BMP7 signaling through Alk3/6 receptors phosphorylating Smad1/5/8, inducing epidermal transcription factors Msx1 and epidermal keratin via Dlx and AP2. Organizer releases Noggin, Chordin, Follistatin and Cerberus that sequester BMPs extracellularly, preventing BMP receptor activation in dorsal ectoderm. Under BMP inhibition, Smad1 inactive, p63 suppressed, allowing neural genes Sox3, Sox2 expression. Thus neural induction occurs essentially by derepressing default neural fate through blocking BMP signaling. Addition of BMP converts neural plate back to epidermis, confirming inhibition model.