Normal tissues consist of parenchymal cells and stromal cells. The parenchymal cells are the functional units of an organ, whereas the stromal cells provide the structure of the organ and secrete extracellular matrix as supportive, connective tissue.[3] In normal epithelial tissues, epithelial cells, or parenchymal cells of epithelia, are highly organized, polar cells.[5] These cells are separated from stromal cells by a basement membrane that prevents these cell populations from mixing.[5] A mixture of these cell types is recognized, normally, as a wound, as in the example of a cut to the skin.[6] Metastasis is an example of a disease state in which a breach of the basement membrane barrier occurs.[7]