Practice question

Noggin and Chordin are dorsal organizer-secreted BMP antagonists identified in Spemann organizer by expression cloning and biochemical purification. They bind BMP2, BMP4, BMP7 directly in extracellular space via cysteine-knot and von Willebrand domains with high nanomolar affinity, blocking engagement with BMP receptors ALK3, ALK6, BMPR2 and downstream Smad1/5/8 phosphorylation. This inhibition creates dorsal low BMP environment permitting neural induction from ectoderm and dorsal mesoderm specification of notochord. In organizer grafts, Noggin mRNA injection dorsalizes ventralized Xenopus embryos. They do not activate Hedgehog, Wnt, or degrade Shh; rather sequester BMP antagonizing ventral signals.