Epithelial-mesenchymal transition: the history of the concept, debatable issues

Abstract

   The paper discusses diverse aspects of the concept of epithelial-mesenchymal transition and compares the basics of this concept with the classical concepts of tissue biology. The idea of the epithelial-mesenchymal transition was first suggested by Elizabeth Hay (1927–2007) in 1968 based on the analysis of tridermogenesis in the development and growth of the avian embryo. In the late 80s – early 90s of the twentieth century, the concept of epithelial–mesenchymal transition won supporters-pathomorphologists in our country who applied the concept to explain mechanisms of multiple pathological processes. The controversial issue arises: to which extent the concept of epithelial-mesenchymal transition is validated. It should be noted that a number of its basic principles are open to question. Thus, it is hardly correct to consider the cells of the ectoderma as epithelial cells, since they do not express immunohistochemical markers of epithelial tissue cells. That is, cells with a true epithelial phenotype are not yet represented at this stage of embryogenesis. It should also be taken into account that all evidence for the epithelial-mesenchymal transition is based on indirect immunocytochemical findings (decreased expression of epithelial markers, increased expression of markers of mesenchymal tissue genesis). Moreover, saying about the “epithelial-mesenchymal transition” specialists mean cell populations and not specific cells, which is hardly the same. All the above allows arguing the existence of the epithelial-mesenchymal transition; this issue needs additional research and more fact-based support.