Practice question

Vertebrate embryos predominantly employ conditional specification, allowing extensive regulation and complex pattern formation through sequential inductive interactions. Mesoderm induction by Nodal and FGF in Xenopus marginal zone, neural induction by BMP antagonists like Noggin and Chordin from Spemann organizer, and limb bud patterning via reciprocal feedback between FGF10 and Sonic hedgehog all require continuous cell-cell communication. Fate maps remain broad early and sharpen through signaling. Consequently, isolated blastomeres from early cleavage stages retain ability to form multiple tissue types, and grafts alter fate to suit host, indicating fate determined extrinsically rather than through inherited determinants.