Biological process in animal connective tissue The epithelialmesenchymal transition (EMT) is a process by which epithelial cells lose their cell polarity and cellcell adhesion, and gain migratory and invasive properties to become mesenchymal stem cells; these are multipotent stromal cells that can differentiate into a variety of cell types. EMT is essential for numerous developmental processes including mesoderm formation and neural tube formation. EMT has also been shown to occur in wound healing, in organ fibrosis and in the initiation of metastasis in cancer progression. Epithelialmesenchymal transition was first recognized as a feature of embryogenesis by Betty Hay in the 1980s.[1][2] EMT, and its reverse process, MET (mesenchymal-epithelial transition) are critical for development of many tissues and organs in the developing embryo, and numerous embryonic events such as gastrulation, neural crest formation, heart valve formation, secondary palate development, and myogenesis.[3] Epithelial and mesenchymal cells differ in phenotype as well as function, though both share inherent plasticity.[2] Epithelial cells are closely connected to each other by tight junctions, gap junctions and adherens junctions, have an apico-basal polarity, polarization of the actin cytoskeleton and are bound by a basal lamina at their basal surface. Mesenchymal cells, on the other hand, lack this polarization, have a spindle-shaped morphology and interact with each other only through focal points.[4] Epithelial cells express high levels of E-cadherin, whereas mesenchymal cells express those of N-cadherin, fibronectin and vimentin.