Practice question

Anterior endomesoderm consisting of pharyngeal endoderm and prechordal plate mesoderm involutes early and underlies anterior neuroectoderm. These cells secrete antagonists Cerberus, Dkk1 and Frzb inhibiting Wnt and BMP, plus IGF signals, creating permissive anterior condition suppressing caudalization. They induce Otx2, Six3 and BF1 expression specifying forebrain and midbrain identity. Posterior hindbrain and spinal cord require later chordamesoderm producing Wnt, FGF and retinoic acid to caudalize neural plate via Hox activation. Hence head organizer activity localized in pharyngeal endoderm and prechordal plate determines anterior brain regionalization.