An Overview of Mesothelium Structure and Function

Purpose and Role in Disease

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The mesothelium is the layer of tissues (epithelium) that surrounds the organs of the chest (pleura and pericardium), abdominal cavity (peritoneum and mesentery), and pelvis (including the tunica vaginalis that surrounds the testes). It functions to protect internal structures and aid in movement and breathing. A number of medical problems may involve these membranes such as pleural and pericardial effusions, adhesions, and a type of cancer known as mesothelioma.

Structure

Mesothelial cells begin as the mesoderm during development (the lungs derive from endoderm) and apparently play an important part in the development of the lung.

The mesothelium is a single, continuous layer of epithelial cells that is divided into three primary regions:

  • The pleura are the two membranes that form a lining around the lungs. This, in turn, is divided into the visceral pleura which lies directly over the lungs, and the parietal pleura, which is the outer layer of the lung lining.
  • The pericardium refers to the mesothelium that forms a lining around the heart and the mediastinum, the area in the chest between the lungs.
  • The peritoneum includes the layers of mesothelium lining the abdominal cavity. This region that includes the mesentery is also divided into two layers. The visceral peritoneum lines the internal organs, and the parietal peritoneum, which lines the abdominal and pelvic walls.
  • The tunica vaginalis is the layers of mesothelium lining the testes in males.

Mesothelium vs. Endothelium

The other type of tissue that lines some body surfaces is the endothelium, derived from the ectoderm and endoderm during development. As mesothelial cells line the regions noted above (serosal membranes), epithelial cells line blood vessels and lymphatic vessels in the body.

Function

It was once thought that the only purpose of the mesothelium was to provide a slippery, non-sticky surface in order to protect the organs of the chest and abdominal cavity during movement and breathing.

We are now learning that, similar to the tonsils and appendix, the mesothelium has other important functions as well, and functions as a dynamic membrane rather than simply being a border that allows smooth movement. These include:

  • The transport and movement of fluids and matter across the membrane from the outer aspects of the chest and abdominal cavities to the internal organs. In regulating these activities, the mesothelial tissues may play an important role in homeostasis (balance) of the body.
  • Coagulation (blood clotting)
  • Healing
  • Immune function: The mesothelium appears to have a protective role both against infection as well as the spread of tumors. Pleural mesothelial cells secrete substances that are active against bacteria, including those responsible for tuberculosis.
  • Protection against tumors: Mesothelial cells appear to secrete substances that inhibit the growth of digestive tract tumors. These substances appear to inhibit the ability of cancer cells to "stick" to the mesothelium, and as such, decrease the spread or metastasis of some tumors.

Medical Conditions 

There are a number of diseases that can affect the mesothelioma, or affect other organs of the body when the mesothelioma is damaged. some of these include:

Mesothelioma: Mesothelioma is a type of cancer that begins in the cells of the mesothelium. These cancers may occur anywhere that mesothelium is present, such as the lining of the lungs (pleural mesothelioma), the lining of the abdomen (peritoneal mesothelioma), or pelvis. This cancer is most common in people who have been exposed to asbestos and continues to increase in incidence worldwide.

Symptoms of mesothelioma may include a cough, difficulty swallowing, abdominal pain and bloating, and other symptoms depending upon the location of the tumor. A surgical procedure called pleurodesis (removal of the pleura) is sometimes performed to remove the tumor, though the prognosis is usually poor by the time this cancer is diagnosed.

Adhesions: Adhesions (bands of scar tissue) are a common complication of abdominal surgery, and, due to obstructions caused by adhesions, are a common reason why abdominal surgery is needed. Scar tissue involving the mesothelium in the abdominal cavity may create bands of tissue, which in turn can trap the bowel and cause obstruction. (You can picture this as taking a piece of gum that has been chewed, stretching it around in the abdominal cavity, and then having it harden.)

Symptoms of adhesions often include abdominal pain (that can be severe), cramping, nausea and vomiting, and bloating. When adhesions are mild (not causing a complete obstruction) people may suffer from chronic intermittent episodes of abdominal pain, especially after eating large meals. There is research in progress looking at ways to reduce the development of adhesions after surgery.

Mesothelium also surrounds the male and female reproductive organs, and adhesions in these regions are an important cause of infertility.

Fibrosis: Fibrosis is a word used to describe scar tissue that often forms after an episode of inflammation. Researchers are currently investigating the role which the pleural mesothelium may play in fibrosis, particularly conditions such as idiopathic pulmonary fibrosis.

Pleural Effusions: Pleural effusions are a build-up of fluid in the cavity between the two layers of the pleura (the pleural mesothelium), and is influenced by substances secreted by pleural mesothelial cells. There are many possible causes of pleural effusions. When cancer cells are present in the fluid between the pleura, it is referred to as a malignant pleural effusion.

Pericardial Effusions: Pericardial effusions are similar to pleural effusions, but rather are a build-up of fluid between the two layers of tissue lining the heart. There are a number of causes, including some infections, chest surgery, and cancers such as lung cancer. When a pericardial effusion develops slowly, it may cause few symptoms and only be detected on imaging studies. If the effusion is large or develops rapidly, in contrast, it can result in a life-threatening condition known as cardiac tamponade, as the fluid in this area can severely restrict the motion of the heart.

A Word From Verywell

The mesothelium is an important structure lining the chest, abdomen, and pelvis, and serves not only to lubricate movements of organs in these regions, but has important functions in fluid transport, blood clotting, and in resistance to infections and the spread of cancers. While the mesothelium can aid in controlling tumors, it is also particularly sensitive to asbestos, which can lead to mesothelioma in people who are exposed.

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