There are 28 distinct types of MMPs in vertebrates, in which 23 are expressed in human tissue. They can be expressed in many different types of cells. Classical MMPs (MMP -1, -2, -3, -7, -9, -11) have been identified in endometrium by in situ hybridization and immunohistochemistry. New members of the MMP family (MMP–19 and -26) are in general found in both epithelial cells and in epithelial tumor cells. It has been suggested that MMP-26 could have an important role in ECM remodeling that is associated with angiogenesis and rapid epithelial growth. MMP-14, -15,-16, -17, -24, and -25 in specific are all activated intracellularly and are typically expressed on the cell surface. MMP-12 in specific are expressed mainly in macrophages, and are also found in osteoclasts and hypertrophic chondrocytes. Overall, MMPs are secreted by connective tissue, pro-inflammatory and uteroplacental cells such as endothelial cells, osteoblasts, vascular smooth muscle, macrophages and neutrophils.