Practice question

Primitive streak regression defines elongation of body axis as Hensen's node moves posteriorly leaving behind notochord and somitic mesoderm. Regression controlled by balance between FGF4/8 maintaining posterior stem zone and retinoic acid anteriorly, but Sonic Hedgehog from axial mesoderm plays key inductive role downregulating FGF and Wnt3a in streak, reducing EMT and cell ingression, promoting neural elongation. Inhibition of Shh with cyclopamine retains streak, overexpression accelerates regression. Therefore regression influenced by Sonic Hedgehog signaling coordinating axis extension with neural patterning rather than retinoic acid or BMP alone.