Practice question

Koller's sickle is thickened posterior marginal zone crescent at edge of area pellucida rich in cells expressing Brachyury, Nodal and Vg1. Fate mapping shows its cells contribute to posterior amnion and extraembryonic mesoderm but critically its signaling determines posterior identity of embryo, inducing primitive streak at its location. Thus region becomes future caudal end before streak extends. Area pellucida becomes central embryo, Hensen's node anterior tip, area opaca extraembryonic. Therefore Koller's sickle marks presumptive posterior end establishing axis polarity before gastrulation movements initiate. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.