Practice question

Primitive streak extension requires coordinated convergence-extension driven by planar cell polarity Wnt pathway activating Dishevelled, RhoA and JNK, orienting mediolateral intercalation of epiblast cells toward posterior midline. Simultaneously FGF signaling through FGFR1 and MAPK ERK maintains Brachyury expression, cell motility and EMT via Snail2. Inhibition of FGFR or Wnt5a/Wnt8c perturbs streak elongation, causes premature regression or randomization. BMP inhibition anteriorly via Cerberus confines streak posteriorly. This integrated Wnt-PCP and FGF module conserved across vertebrates regulates axis formation and cell rearrangements during gastrulation.