Practice question

Large micromeres formed at vegetal pole inherit nuclear beta-catenin and express Pmar1-HesC circuit driving autonomous specification toward skeletogenesis. They activate gene regulatory network including alx1, ets1, tel, tbr, dri, producing primary mesenchyme cells. These cells undergo epithelial-mesenchymal transition, ingress into blastocoel, migrate along blastocoel wall to form ring pattern, fuse into syncytium, and deposit calcium carbonate rods forming larval skeleton spicules. Isolated micromeres cultured in vitro differentiate into spicules without signals from other cells, demonstrating autonomous mesodermal skeleton fate rather than ectoderm or gut endoderm, essential for pluteus morphology and support.