Epithelio-mesenchymal transformation during formation of the mesoderm in the mammalian embryo.

The earliest example of epithelio-mesenchymal transformation during embryonic development is the generation of the third germ layer, the mesoderm, from the epiblast (or primitive ectoderm), which marks the beginning of gastrulation. Although it has been regarded as most likely that the principles of this transformation in invertebrates and lower vertebrates also apply to amniotes, morphological and molecular details of mesoderm formation in birds and, in particular, in mammals, which may support this assumption, have only recently been clarified. This chapter thus brings together the light- and electron-microscopical morphology of epithelio-mesenchymal transformation during initial mesoderm formation in the mammalian embryo. Cellular differentiation during this process with regard to the cytoskeleton, cell adhesion molecules and the extracellular matrix are also covered as are cell kinetic studies and the candidate growth factors and genes most likely to be involved in the regulation of mesoderm formation in mammals. Finally, a model is presented which summarizes these morphological and molecular changes and which links the promoting and inhibiting influences of regulatory factors to some of the changes observed during epithelio-mesenchymal transformation.