What is the mesothelium?
The mesothelium is the epithelial lining of body cavities, also called serosal linings. There are three main cavities in the human body: abdominopelvic cavity, thoracic cavity, and peritoneal cavity.
These cavities contain visceral organs – abdominal and pelvic viscera, respiratory organs, and peritoneal organs. Mesothelial cells cover the serosal surfaces of viscera in these cavities and play a critical role in their protection against injury during daily activities.
What is a Mesothelial cell?
A mesothelial cell is a type of simple columnar epithelial cell which is characterized by a polygonal surface shape that closely resembles a cobblestone. These cells also possess microvilli that project from their apical surface and contain extensive intercellular junctions, which serve to facilitate the rapid transport of materials between adjacent mesothelial cells.
Additionally, mesothelial cells can be found in open circulation (marginal pool) within the body. The extensive intercellular junctions between mesothelial cells allow them to act as a single functional unit in regard to transport processes, while still maintaining their individual cellular identity.
Quick Answer: When were Mesothelial cells identified first?
Mesothelial cells were first identified in 1863 by Friedrich Sigmund Merkel based on their location at serosal surfaces and the presence of microvilli. The name “mesothelium” was first coined by anatomist George Huntington in 1885.
Three Types of Mesothelial cells
Mesothelial cells can be found in the cardiovascular system (heart), pleura (lining of lungs/abdominal cavity), peritoneum (lining of the abdominal cavity), and pericardium (lining of the heart). There are three types of mesothelial cells:
1. Surface epithelium where the apical border faces the lumen, is characterized by microvilli which are referred to as “brush border”.
2. Transitional epithelium which lines the urinary system, the reproductive system, and gastrointestinal tract.
3. Mesothelial cells line the solid organs which form the serosal surfaces of body cavities such as the pleura (lining of lungs/abdominal cavity), peritoneum (lining of the abdominal cavity), and pericardium (lining of the heart).
Mesothelium forms by the differentiation of mesenchyme into embryonic mesothelial cells. The embryonic mesothelium is derived from the lateral plate mesoderm of the trilaminar embryo and forms during gastrulation between days 19 and 24. This mesothelial layer initially lines the dorsal aspect of the primitive gut, where it will later develop into a portion of the serosal lining for abdominal organs. A second population of cells, derived from the splanchnic mesoderm, exists as a layer lining the ventral aspect of the primitive gut.
The embryonic mesothelium has been shown to differentiate into multiple cell types which present in distinct groupings. In addition to forming a simple epithelial layer as seen in the gastrointestinal tract and genitourinary tract, mesenchyme can also form smooth muscle cells – forming the tunica media layer of blood vessels and lymphatic vessels, as well as elastic fibers.
While this distinct embryonic mesothelium is no longer visible by the end of gastrulation (8th week), all derivatives of these cells continue to have a mesothelial phenotype. In addition to forming pleural and peritoneal surfaces, serosal mitral cells have been shown to form the alveoli of the lung in the developing embryo.
Mesothelial covers all visceral organs inside our body, but it can also be found in open circulation or marginal pools within the body. Cells of mesothelial origin in open circulation are also called “mesenchymal cells”.
During gastrulation, the lateral plate mesoderm is differentiated into two sections: parietal and visceral plates. The parietal plate is located at the surface of the embryo opposite to allantois. The visceral plate lies between the parietal plate and the endoderm.
The visceral plate is a source of mesenchyme, which will differentiate into three layers: parietal layer, intermediate layer, and visceral layer. The parietal layer will turn into the epithelial lining for the respiratory system, urinary system, and gastrointestinal system. The intermediate layer will become smooth muscle cells of blood vessels and lymphatic vessels. The visceral layer will generate endodermal, mesodermal, and ectodermal derivatives.
Mesothelial cells are simple squamous epithelial cells that cover the serosal surfaces of visceral organs inside our body to protect them from injuries during daily activities. They clinically manifest in the pleura (outer lining of lungs), peritoneum (lining of stomach, abdominal cavity), and 10% of the serosal lining in the heart. In pleura, mesothelial cells are also known as “pleural mesothelial cells” to differentiate from lung epithelial cells located inside the alveolar space.
Peritoneal surfaces have a simple squamous epithelium and it is also composed of mesothelial cells. Mesothelial cells cover serosal surfaces tightly, but there are specialized regions where they facilitate transportation between organs to maintain homeostasis. These areas allow for the free movement of fluid and solutes such as nutrients or wastes. There are three types of such regions:
- 1. Fenestrated capillaries, where a diaphragm is formed by two layers of mesothelial cells instead of the endothelium.
- 2. Inter-endothelial slits/pores, which are small gaps between endothelial cells in the heart and great vessels allowing the transport of molecules up to 1 kD.
- 3. Transmesothelial organ movements along the intestine, allowing for digestion and absorption of food.
What do You mean By Tight Junction?
Cell-cell junctions form unique regions of epithelium called “tight junctions” at their apical border. The tight junction is an important structure in cell adhesion, barrier formation, and paracellular transport.
The paracellular movement of molecules is regulated by tight junctions. When there is no pathological process, the opening between endothelial cells in capillaries and epithelium cells has a width of around 40 angstroms (400nm) allowing selective passage to small solutes. Larger molecules needed to be transported are transported through pores and diaphragms formed by mesothelial cells. This explains how lymphatic vessels can transport protein of up to 50kD.
Colinearity of Mesothelial Cells
Colinearity of mesothelial cells is achieved through intercellular connections called desmosomes which are plaques of proteins between adjacent cells. These proteins are transmembrane proteins called cadherins that allow for adhesion to occur by binding to the CD domains (Canonical, in beta-propeller; Noncanonical, in Declining Strand and Ig C2 type 1 motifs).
Mesothelial cells can also be attached through ‘hemidesmosomes’ that consist of integrins and transmembrane proteins called “fibronectin”. Focal adhesion complexes (FAC), which is a group of proteins including integrins and “focal adhesion kinase” (FAK) that binds to the ECM, also allow for mesothelial cells to attach to other cells or extracellular matrix.
Cardiac mesothelial cells are very different from other mesothelial cells. They have large nuclei with prominent nucleoli and they form cardiac sphincters which regulate blood flow between atria and ventricles.
Conclusion: What is Mesothelium?
The mesothelium is one of several types of epithelium, including surface epithelium that covers the body surface or lining of cavities, transitional epithelium that lines hollow organs, and endothelium which is a layer of flat cells that line the internal surfaces of blood vessels and heart.
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