Practice question

Coordinated extension of primitive streak and polarized movement of epiblast cells requires Wnt planar cell polarity pathway including ligands Wnt5a, Wnt8c, receptors Frizzled, co-receptors Vangl2, Prickle and downstream RhoA, ROCK and JNK regulating cytoskeletal anisotropy and convergent extension. FGF8 expressed in streak regulates cell chemotaxis repelling ingressed mesoderm laterally, interacting with Wnt to orient streak. BMP signaling patterns dorso-ventral mesoderm but does not drive streak migration, Hedgehog later patterns neural tube, so Wnt PCP and FGF8 together regulate primitive streak migration and elongation. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.