Practice question

High Activin concentration hyperactivates Smad2/FAST/FoxH1 pathway leading to induction of organizer-specific transcription factors goosecoid, chordin, noggin and Cerberus while repressing ventral genes like Ventx. Caps exposed to 50 ng/ml Activin elongate and exhibit dorsal lip behavior forming notochord and head mesoderm, functioning as secondary organizer capable of inducing complete ectopic axis upon transplantation into ventral side. Low dose induces Xbra and ventral mesoderm and blood. Epidermal differentiation occurs without Activin, apoptosis not specific response, neural tube induction requires BMP antagonism plus FGF. Hence high Activin specifies Spemann organizer identity via threshold gene activation and cell movement.