in vertebrates and man, the epithelial tissue that lines the serous membranes of the body cavity (peritoneum, pleura, pericardium).

Mesothelium is formed from mesoderm. It consists of a single layer of densely arranged flat polygonal cells. In lower vertebrates (cyclostomes, fish, amphibians), mesothelial cells are wholly or partly furnished with cilia. In mammals and man, the surfaces of the cells are covered with microvilli, which assist in the absorption and excretion of cavitary fluid. Mesothelium is highly sensitive to external influences. When the serous membranes are irritated and an inflammatory reaction results, the continuity of the mesothelial layer is interrupted, the cells are destroyed, and the underlying connective tissue is exposed. The connective tissue cells then penetrate the irritated area and phagocytize the dying tissue areas and, in septic inflammation, the bacteria. The connective tissue then proliferates, demarcating the focus of irritation and forming adhesions. Mesothelium grows over the adhesions, preventing them from further development and preventing visceral concrescence.