The mechanisms of metastasis are still not fully understood. It is a very complex process, which is determined by the tumor cells themselves, but also influenced by the immune system and the surrounding tissue. Most malignant tumors are in contrast to intact tissues not stabilized by a connective tissue stroma, so that the loose tumor cells are a more or less undifferentiated group of cells. The fact that tumor cells can detach from the cell structure is probably due to the fact that the number of adhesion molecules on the cell membrane is reduced. Individual tumor cells or tumor cell groups can penetrate the surrounding tissue, and thus reach the blood- or lymphatic vessels.