Head-organizing activities of endodermal tissues in vertebrates.

The embryonic organizer represents the major regulatory centre for the establishment of the body axes during gastrulation. Here, we discuss the endodermal contributions to the organizer of amphibia, birds and mammals. We differentiate between the definitive, prospective liver endoderm, and the primitive, prospective extraembryonic endoderm, the latter addressed as the hypoblast in birds and the visceral endoderm in mammals. We further discuss the role of the prechordal plate, a mesendodermal tissue underlying the prospective forebrain. Our conclusion points out the similarity of the amphibian and the avian organizer, with a concentration of inductive potentials in time and space. On the other hand, we discuss the unique feature of mammals, that have shifted certain aspects of the head organizer into the anterior visceral endoderm.