Practice question

Primitive streak appears as posterior-to-anterior linear thickening where epiblast cells converge, undergo epithelial-mesenchymal transition mediated by Snail2 and E-cadherin loss, and ingress. Anterior tip defines future head and Hensen's node, posterior end tail bud, thereby visibly establishing anterior-posterior polarity long before somites appear. Molecularly Brachyury, Wnt3a and Nodal show graded posterior-high expression. Left-right asymmetric genes Shh, FGF8, Nodal later expressed around node use streak as midline reference. Regression of streak after ingression deposits notochord and paraxial mesoderm along AP axis, coordinating body elongation and axis patterning.