Practice question

Lineage analysis of Hensen's node using quail-chick chimeras reveals median pit cells ingress after streak regression to form notochordal rod extending caudally, expressing Brachyury, FoxA2 and Sonic hedgehog providing signals for ventral neural tube and somite patterning. Lateral and rostral node cells contribute prechordal mesendoderm underlying forebrain, expressing Goosecoid, Otx2 and Hex, crucial for head induction and forebrain specification by inhibiting Wnt and BMP. Together axial progenitors provide structural scaffold and signaling center for body plan, not limb or epidermal derivatives.