Transdifferentiation generally refers to a process whereby one mature epithelial cell phenotype converts into a different mature epithelial cell, with or without cell division. It is unclear whether transdifferentiation involves a transition state. Three types of EMT are recognized depending on the phenotype of the output cells. Type 1 EMT is seen when primitive epithelial cells transition into mesenchymal cells that form the diaspora of the basic body plan following gastrulation or neural crest migration. These mesenchymal cells either undergo MET to form secondary epithelial cells or apoptose. Type 2 EMT is seen when secondary epithelial cells or endothelial cells populate interstitial spaces with resident or inflammation-induced fibroblasts, the latter during persistent injury. Type 3 EMT is part of the metastatic process, whereby epithelial tumor cells leave a primary tumor nodule, migrate to a new tissue site, and reform as a secondary tumor nodule.