The process of malignancy emergence is associated with the acquisition of the capacity to invade other tissues. Several different biological processes have been described as involved in this process. Specifically, epithelial mesenchymal transition (EMT), a mechanism associated with embryogenesis and wound repair but also with mobility acquisition, is one of the concerned processes. In EMT an epithelial cell loses its epithelial characteristics, its junctions with neighbor cells and with the basal lamina and acquires mobility and mesenchymal characteristics. Also, factors of the tumor microenvironment have been described as involved. Tumor presence triggers a response in the surrounding tissue known as reactive stromal. It shows particular characteristics similar to those found in wound healing stroma: an increase of the fibroblast number and enhancing of the capillary density. The notable difference is the chronicity in the tumoral process. Of a high relevance seems to be the role of activated macrophages with a characteristic phenotype. Finally, cancer associated fibroblasts (CAF) are a type of cells found in tumors, developed from local tissue or possibly from bone marrow. CAF characteristically show a distinct morphology and secrete a high number of metalloproteases allowing tumoral cells advance through the tissue. Additionally, CAF have a direct effect on the survival of the epithelial cells. The three processes are interrelated and metastasis is probably caused by the effect of all of them and probably by other additional factors.
