Practice question

Posterior marginal zone is extraembryonic ring of cells between area opaca and area pellucida posteriorly expressing Vg1, Nodal, Wnt8c and Tbx6. It secretes signals inducing primitive streak and Koller's sickle in adjacent epiblast and can induce axis when grafted anteriorly, demonstrating inducing rather than responding character. Like amphibian Nieuwkoop center in dorsal vegetal cells, PMZ is vegetal signaling center containing Nodal-related determinants and requiring Wnt/beta-catenin input, inducing organizer fate in overlying marginal zone. Thus PMZ is avian equivalent of Nieuwkoop center rather than blastocoel or node.