Simple squamous epithelium of mesodermal origin which lines serous membranes. Examples: mesothelium of pleura, mesothelium of peritoneum[FMA]. Wikipedia: The mesothelium is a membrane that forms the lining of several body cavities: the pleura (thoracal cavity), peritoneum (abdominal cavity including the mesentery) and pericardium (heart sac). Mesothelial tissue also surrounds the male internal reproductive organs (the tunica vaginalis testis) and covers the internal reproductive organs of women (the tunica serosa uteri).