Practice question

Primitive streak initiation is regulated by posterior marginal zone signaling center at posterior border of area pellucida, specifically Koller's sickle region. Koller's sickle thickened epiblast expresses Vg1/GDF1 and Nodal inducing adjacent epiblast to express Brachyury and undergo epithelial-mesenchymal transition forming streak. Transplantation of Koller's sickle to anterior marginal zone induces ectopic streak. Area opaca remains extraembryonic, Hensen's node appears later as anterior tip, area pellucida is competent tissue. Thus initiation occurs at Koller's sickle homologous to amphibian Nieuwkoop center signaling domain. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.