Epithelial-mesenchymal Transition - The Concept

The Concept

EMT was first recognized as a feature of embryogenesis, which is vital for morphogenesis during embryonic development. In vertebrates, epithelium and mesenchyme are the basic tissue phenotypes. The EMT can be defined as a process that produces complete loss of epithelial traits by the former epithelial cells accompanied by total acquisition of mesenchymal characteristics, such as vimentin, myosin, invasive motility, and so on. Although EMT evolved among several other animals, the remarkable ability of developing embryos to change one tissue type to the other reached its peak in the vertebrates. EMT takes place during the construction of the vertebral column out of the extracellular matrix, which is to be synthesized by fibroblasts and osteoblasts that encircle the neural tube. The major source of these cells are sclerotome and somite mesenchyme as well as primitive streak. Mesenchymal morphology allows the cells to travel to specific targets in the embryo, where they differentiate and/or induce differentiation of other cells.

In the lower chordates, gastrulation is a totally epithelial event. The amphibians form a blastopore through which presumptive mesodermal and endodermal epithelia invaginate. Amphioxus forms an epithelial neural tube and dorsal notochord but does not have the EMT potential of the primitive streak. In higher chordates, the mesenchyme originated out of the primitive streak migrates anteriorly to form the somites and participate with neural crest mesenchyme in formation of the heart mesoderm. Mesenchymal cells from the primitive streak participate also in the formation of many epithelial mesodermal organs, such as notochord as well as somites. This process involves mesenchymal–epithelial transition.

One more source of embryonic mesenchyme is the dorsal neural tube epithelium. It forms the neural crest and considerable amounts of craniofacial crest mesenchyme which forms connective tissue that contributes to the head and face in the vertebrate.