EPITHELIOMESENCHYMAL TRANSFORMATION DURING FORMATION OF THE MESODERM IN THE MAMMALIAN EMBRYO

The earliest example of epithelio-mesenchymal transformation during em bryonic development is the generation of the third germ layer, the mes oderm, from the epiblast (or primitive ectoderm), which marks the begi nning of gastrulation. Although it has been regarded as most likely th at the principles of this transformation in invertebrates and lower ve rtebrates also apply to amniotes, morphological and molecular details of mesoderm formation in birds and, in particular, in mammals, which m ay support this assumption, have only recently been clarified. This ch apter thus brings together the light- and electron-microscopical morph ology of epithelio-mesenchymal transformation during initial mesoderm formation in the mammalian embryo. Cellular differentiation during thi s 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 change s and which links the promoting and inhibiting influences of regulator y factors to some of the changes observed during epithelio-mesenchymal transformation.