The epithelial-mesenchymal transition, or EMT, is an important though not fully understood process in embryonic development and in cancer biology. In the epithelial-mesenchymal transition, epithelial cells, which tend to be tightly-packed cells with limited mobility that line body cavities, begin to resemble mesenchymal cells by becoming more mobile and leaving the epithelium. Mesenchymal cells are stem cells that can differentiate into different types of cells; this can be beneficial during embryonic development or harmful in organisms with cancer. The opposite of EMT is the mesenchymal-epithelial transition, or MET.
There are three main traits that characterize the epithelial-mesenchymal transition: Cell adhesion decreases, causing epithelial cells to separate from the epithelium, or the tissue composed of epithelial cells; The expression of E-cadherin, a gene that suppresses cancer, is significantly reduced; and cells previously bound to the epithelium become much more mobile and may move throughout the organism. An examination of cells before and after the epithelial-mesenchymal transition reveals a change from structured, ordered, and closely-packed cells to unstructured cells spread about with no particular organization.
The epithelial-mesenchymal transition is thought to be an important part of embryonic development. The epithelial cells in the epithelium must separate and function as stem cells in order for proper cellular growth to occur, leading to correct development of the organism's body. The epithelial-mesenchymal transition is often necessary for the development of different tissue types in the growing embryo.
In humans, the epithelial-mesenchymal transition is notable for its role in cancer. This is particularly true in solid-tumor cancers, such as pancreatic, colorectal, and hepatocellular cancers. There are many genetic pathways related to the regulation of cellular proliferation and growth that are known or believed to induce EMT in some cases. If the expression of these genes is improperly regulated, E-cadherin, the tumor suppressor associated with EMT, may be inhibited substantially, thereby promoting the formation of tumors. A variety of other proteins, such as collagen and some transcription factors, have been shown to induce EMT.
The epithelial-mesenchymal transition increases the ability of cancerous cells to metastasize, or spread throughout the body. Epithelial cells exhibit low mobility and tend to adhere to the cells around them. Cells with mesenchymal traits, on the other hand, are much more mobile and tend not to adhere to surrounding cells to a significant degree. As such, they are able to spread through the blood much more readily, spreading the cancer to other parts of the body. EMT is one of the processes, then, that causes many forms of cancer to be invasive.