EMT Concept and Examples from the Vertebrate Embryo

Epithelial-mesenchymal transformation (EMT) creates a family of invasive cell types from the relatively sedentary epithelial cells that line the surfaces of the body. The mesenchymal cell’s primary trait is that, unlike the epithelium of origin, it can invade extracellular matrix and migrate great distances in the embryo. It is a bipolar cell with a very active front end rich in filopodia that apparently drive motility by providing new actin cortex for the myosin endoplasm of the fibroblast to slide forward on. In this chapter, we describe the progression of EMTs as they actually occur in the vertebrate embryo from primitive streak stages to craniofacial remodeling stages in older embryos. We propose a mechanism of TGFβ driven LEF-1 transcription that may be responsible for most, if not all, embryonic EMTs that result in formation of fully active, invasive mesenchymal cells, and we emphasize the importance of studying physiologically relevant signal transduction pathways that lead to the acquisition of invasive motility in vivo, rather than pathways that give rise to nonmotile, stress-fiber rich cells in vitro.