Embryologically, mesothelial cells originate from the mesoderm, and cells gradually differentiate from round or cuboidal cells to elongated cells lining the celomic cavity. However, they share many properties of epithelial cells, such as apical/basal polarity, basement membrane adhesion, junctional complexes, and surface microvilli. Their dual phenotypic features affect the function of these cells, determining how they respond to environmental changes.
Mesothelial cells form a monolayer of cobblestone-like cells in the peritoneal, pleural, and pericardial cavities and most internal organs. This monolayer, called the mesothelium, is a barrier and first line of defense against microorganisms and invading tumor cells. Following injury or exposure to foreign organisms or tumor cells, mesothelial cells initiate defense mechanisms, including inflammatory and immune responses. Mesothelial cells also provide a slippery non-adhesive surface to facilitate free movement of internal organs. When the integrity of the mesothelium is lost, a rapid healing response begins, with complete repair within a few days. If this repair mechanism is hindered in any way, pathologic changes can occur to the serosa, with potentially severe clinical manifestations such as fibrosis, adhesion formation, endometriosis, cancer, and metastases.