Mesothelium

Mesothelium is a specialized layer of simple squamous cells that forms the lining of the body's serous cavities and covers the external surfaces of many thoracic and abdominal organs. It lines the pleura around the lungs, the peritoneum lining the abdominal cavity, and the pericardium around the heart. The mesothelium is arranged as parietal mesothelium lining the cavity walls and visceral mesothelium covering the organs, with a thin film of serous fluid between the two layers to reduce friction during organ movement.

Origin and function: Mesothelial cells arise from the mesoderm during embryonic development. In the adult, the mesothelium provides lubrication through serous fluid, acts as a protective barrier, and participates in immune responses and fluid transport. The tissue is supported by a submesothelial connective tissue layer and can respond to injury with adaptive proliferation and repair.

Clinical relevance: Malignant tumors arising from mesothelial cells, called mesotheliomas, most often involve the pleura but can affect the peritoneum or other serous surfaces. They are strongly associated with asbestos exposure in many cases. Diagnosis relies on histopathology and immunohistochemistry, utilizing markers that support mesothelial origin (for example, calretinin or WT-1) and help distinguish mesothelioma from adenocarcinomas. Inflammatory or reactive changes of mesothelium can occur with infection, irritation, or injury, sometimes leading to effusions or thickening of serous membranes.