Head induction in the chick by primitive endoderm of mammalian, but not avian origin

Different types of endoderm, including primitive, definitive and mesendoder m, play a role in the induction and patterning of the vertebrate head. We h ave studied the formation of the anterior neural plate in chick embryos usi ng the homeobox gene GANF as a marker. GANF is first expressed after mesend oderm ingression from Hensen's node. We found that, after transplantation, neither the avian hypoblast nor the anterior definitive endoderm is capable of GANF induction, whereas the mesendoderm (young head process, prechordal plate) exhibits a strong inductive potential. GANF induction cannot be sep arated from the formation of a proper neural plate, which requires an intac t lower layer and the presence of the prechordal mesendoderm, It is inhibit ed by BMP4 and promoted by the presence of the BMP antagonist Noggin, In or der to investigate the inductive potential of the mammalian visceral endode rm, we used rabbit embryos which, in contrast to mouse embryos, allow the m orphological recognition of the prospective anterior pole in the living, pr e-primitive-streak embryo. The anterior visceral endoderm from such rabbit embryos induced neuralization and independent, ectopic GANF expression doma ins in the area pellucida or the area opaca of chick hosts, Thus, the signa ls for head induction reside in the anterior visceral endoderm of mammals w hereas, in birds and amphibia, they reside in the prechordal mesendoderm, i ndicating a heterochronic shift of the head inductive capacity during the e volution of mammalia.