Generation of the germ layers along the animal-vegetal axis in Xenopus laevis

After completion of gastrulation, typical vertebrate embryos consist of thr ee cell sheets, called germ layers. The outer layer, the ectoderm, which pr oduces the cells of the epidermis and the nervous system; the inner layer, the endoderm, producing the lining of the digestive tube and its associated organs (pancreas, liver, lungs etc.) and the middle layer, the mesoderm, w hich gives rise to several organs (heart, kidney, gonads), connective tissu es (bone, muscles, tendons, blood vessels), and blood cells. The formation of the germ layers is one of the earliest embryonic events to subdivide mul ticellular embryos into a few compartments. In Xenopus laevis, the spatial domains of three germ layers are largely separated along the animal-vegetal axis even before gastrulation; ectoderm in the animal pole region; mesoder m in the equatorial region and endoderm in the vegetal pole region. In this review, we summarise the recent advances in our understanding of the forma tion of the germ layers in Xenopus laevis.