Practice question

Migration of epiblast through streak is active EMT driven by FGF signaling. FGF8 secreted by streak cells activates FGFR1 on neighboring epiblast via MAPK and PI3K pathways upregulating Snail2 repressing E-cadherin, permitting ingression. After EMT FGF acts as chemorepellent guiding mesoderm laterally. Inhibition with SU5402 or dominant-negative FGFR blocks ingression causing epiblast accumulation. Noggin, chordin and goosecoid dorsalize after ingression but do not regulate migration machinery itself. Therefore migration through primitive streak directly regulated by FGF8 acting as both EMT inducer and repellent gradient. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.