The mesothelium is a membrane that forms the lining of several body cavities: the pleura (chest), the peritoneum (abdominal cavity including the mesentery) and pericardium (heart sac). Also mesothelial cells surrounding the male internal reproductive organs (testicles vaginal testis) and covers the internal reproductive organs of women (the tunica serosa uteri). mesothelium covering of the internal organs is called visceral mesothelium, whereas the layer which covers the walls of the body is called the parietal mesothelium and mesothelial cyst.