Practice question

In normal gastrula, dorsal blastopore lip secretes noggin that antagonizes BMP signaling creating low BMP zone permitting dorsal mesoderm and neural ectoderm formation. Ventral marginal zone normally receives high BMP specifying ventral mesoderm and epidermis. Ectopic noggin mRNA injected ventrally diffuses extracellularly binding BMPs and preventing receptor engagement, generating BMP-free niche ventrally. Neighboring cells respond by expressing organizer genes and self-organizing into second axis with notochord, somites and neural tube. This phenocopies Spemann graft proving single BMP antagonist mimics organizer transplantation. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.