Vive la difference

Abstract

Epithelial to mesenchymal transition (EMT) is a term used to describe cellular and molecular changes involved when a group of cells switch from an epithelial type to a mesenchymal type of organization. Based on their biological context, EMTs have been loosely categorized into three types, namely the developmental (type 1), fibrosis (type 2) and cancer (type 3) EMTs. Type 1 EMTs occur during normal embryonic development, prominent examples of which are EMT events during gastrulation and neural crest cell formation. Although interesting to developmental biologists in their own right, cellular and molecular studies of type 1 EMTs are considered to be fundamental for understanding pathological (type 2 and type 3) EMTs because they happen in physiological conditions and because pathological EMTs are often regarded as mis-regulated reactivation of developmental EMT programs. This important conceptual framework, however, does not mean that phenotypic and mechanistic variations in diverse EMTs can be similarly categorized. Remarkable differences in tissue organizational changes and in molecular regulation of these changes are recognized not only among different types of EMTs, but often among EMTs within each of the three types. This is in somewhat perplexing contrast to the understanding of a limited number of cellular modifications needed to constitute an EMT and to the increasing evidence of a conserved pool of molecular players involved in diverse EMTs. One may argue that the differences reflect a current lack of understanding of the complete molecular repertoire, and that one can expect a “unifying” molecular picture of EMTs in general as more EMT genes are being discovered in various model systems. This may well be true. However, a given EMT event often takes place in a dynamic context and has its unique tissue environment which is hard to generalize. Recognizing these unique features may therefore be equally important for understanding its regulation. Three articles in this issue offer an insightful look at the complexity of type 1 EMTs. Ohta et al. describes the developmental context of the gastrulation EMT in birds. Although known to developmental biologists, researchers in the cell biology Vive la difference Letter from the Guest Editor