Early endoderm development in vertebrates: lineage differentiation and morphogenetic function.

Gastrulation of the vertebrate embryo culminates in the formation of three primary germ layers: ectoderm, mesoderm and endoderm. The endoderm contributes to the lining of the gut and the associated organs. New components of the molecular pathway for endoderm specification have been identified in the zebrafish and Xenopus. In the mouse, the activity of orthologous factors is involved with the allocation and differentiation of the definitive endoderm. Morphogenetic interactions between the endoderm and the other germ layer derivatives are critical for the morphogenesis of head structures and organogenesis of gut derivatives.