Practice question

Morphogenetic gradient experiments test dose-dependent fate induction rather than simple presence or absence of signal. In Xenopus, isolated animal cap ectoderm cultured with titrated recombinant Activin differentiates into ventral mesoderm at low dose, muscle at intermediate dose and organizer plus endoderm at high dose, recapitulating in vivo mesoderm patterning along vegetal-animal axis. This demonstrates diffusible factor encoding multiple fates by threshold via Smad2. Spemann organizer transplantation tests inductive capacity not quantitative gradient, CRISPR knockout removes gene but not gradient quantification, GFP tagging visualizes spindle not patterning. Activin dose-response remains classic quantitative proof for morphogen concept in vertebrates.