epithelial cell mesothelial cell mesothelial cell A flat, squamous-like epithelial cell of mesodermal origin. It forms the mesothelium, which lines the body's serous cavities including the pleural, peritoneal, and pericardial spaces. This cell plays a crucial role in synthesizing and secreting lubricants, such as glycosaminoglycans and surfactants, which minimize friction between adjacent tissues during movement.