The mesothelium is a thin membrane that lines the body’s internal cavities and covers the organs within them. This lining is formed by a single layer of flattened cells, known as mesothelial cells, which creates a smooth, protective surface. The mesothelium acts as a barrier, helping to maintain the integrity of the internal environment and facilitating the proper movement of organs.
Where Mesothelial Tissue Resides
The mesothelium is found in the major serous cavities of the body. The cells form a monolayer that rests upon a thin basement membrane, which is supported by underlying connective tissue. This structure creates a non-adhesive lining for internal surfaces.
The three primary locations are the pleura, the peritoneum, and the pericardium, which collectively form the serous membranes. The pleura lines the chest cavity and covers the lungs, while the pericardium forms the sac around the heart. The peritoneum lines the abdominal cavity and envelops the abdominal organs, such as the intestines and liver.
In each cavity, the mesothelium is divided into the visceral layer, which directly covers the internal organs, and the parietal layer, which lines the cavity wall. These two layers are separated by a potential space containing a small amount of fluid. The physical structure of these cells is typically flat and irregular, referred to as simple squamous epithelium.
Essential Functions of the Mesothelium
The mesothelium’s primary job is to provide a slippery, protective surface, ensuring that organs can move without causing friction or damage. Mesothelial cells achieve this by secreting a lubricating fluid known as serous fluid into the cavity space. This fluid contains proteins and glycosaminoglycans that lower the friction between the visceral and parietal layers, allowing for smooth movement during activities like breathing and digestion.
This membrane also plays a significant role in regulating fluid dynamics and transport across the cavity lining. It acts as a semipermeable membrane, controlling the exchange of solutes and fluids between the cavity and the underlying tissues. This transport function is supported by the presence of microvilli on the cell surface, which trap the serous fluid and contribute to the slick surface.
The mesothelium is actively involved in local immune defense and signaling within the body cavities. The cells can synthesize and release pro-inflammatory cytokines and growth factors to initiate an immune response or tissue repair when needed.
The layer helps prevent the formation of internal adhesions. By maintaining a non-adhesive surface, the mesothelium ensures that the heart, lungs, and abdominal organs can constantly shift and slide past one another freely. The membrane also contributes to wound healing and coagulation processes, helping to repair damage to the serous surfaces.
Conditions Affecting Mesothelial Layers
Damage, irritation, or transformation of the mesothelial layers can lead to health issues, the most serious being malignant mesothelioma. This is a rare and aggressive cancer that originates in the mesothelial cells. It is strongly linked to the inhalation or ingestion of asbestos fibers, which embed in the tissue and cause chronic irritation and DNA damage.
Malignant mesothelioma is most commonly found in the pleura, where it is known as pleural mesothelioma and accounts for up to 90% of cases. The second most frequent location is the peritoneum, referred to as peritoneal mesothelioma. Rarer forms can affect the pericardium around the heart or the tunica vaginalis around the testes.
Inflammatory conditions frequently involve the mesothelium. Pleurisy, for example, is the inflammation of the pleura, typically causing sharp chest pain when breathing due to the irritated layers rubbing against each other. Similarly, peritonitis is the inflammation of the peritoneum, often caused by infection or injury that introduces bacteria into the abdominal cavity.
When the mesothelium is damaged, the body attempts to repair the tissue, but this process can sometimes result in the formation of scar tissue. Adhesions can restrict organ movement and lead to complications like chronic pain or bowel obstruction in the abdominal cavity.