Practice question

Koller's sickle is thickened crescent of epiblast cells plus polyingressing mesenchymal cells at posterior edge of area pellucida, expressing Vg1/GDF1, Nodal and FGF4. Experimental fate mapping shows it acts as avian Nieuwkoop-like center, secreting TGF-beta signals inducing adjacent epiblast to express Brachyury and form primitive streak. Posterior marginal zone overlying sickle provides Wnt8c permissive signal. Together they organize streak initiation, maintaining high beta-catenin and Nodal activity posteriorly. Removal or anterior transplantation abolishes or relocates streak, confirming Koller's sickle as essential streak initiator for gastrulation.