Mesothelium

What is the mesothelium?

The mesothelium is a one-cell thick membranous tissue that lines the three serous cavities of the body: the pericardial, pleural, and peritoneal. It has essentially protective functions, but it also intervenes in the movement of the organs contained in these cavities.

The serous cavities of the body, also known as coelomic cavities, are the cavities that contain the main organs, generally called “soft organs”. The coelom can be defined as the general cavity of the body, which opens to the outside only through the mouth and anus.

Benign mesothelial cells. Source: Nephron, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

The pericardial cavity is the one that contains or surrounds the heart; the pleural cavity, on the other hand, is the one that surrounds the lungs and the peritoneal cavity, finally, it is the one that encloses the organs found in our abdomen, ie the stomach, intestines, liver, pancreas, kidneys , etc.

In addition to functioning as a protective, limiting or covering layer, the mesothelium has other homeostatic functions, among which the formation of a lubricated, friction-free surface that facilitates the movement of organs within the three mentioned cavities stands out.

Likewise, this tissue has a lot to do with the mechanisms of transcellular exchange, with the development of the heart, lungs and intestines, etc.

Furthermore, the mesothelium is found around the female and male reproductive structures, where it performs the same protective functions as in the three serous cavities.

Structure and functions of the mesothelium

The mesothelium, as discussed above, is a membranous structure – a tissue – one cell thick. It has important functions in protecting the vital organs of the human body, as it provides a slippery and non-sticky surface “within” which the organs have a certain freedom of movement.

The lungs, contained in the pleural cavity, for example, can easily expand during inhalation and contract during exhalation, without their associated mesothelium preventing them; in the same way that the intestines, contained in the visceral cavity, can “swell” and “deflate” rhythmically (peristalsis) during digestion.

Illustration of a mesothelial cell

The cells that make up the mesothelial tissue are called mesothelial cells and, in addition to forming this “membrane”, they have other functions:

  • They participate in the transport of fluids and cells to and from the serous cavities.
  • They are involved in the presentation of antigens for the components of the immune system responsible for their recognition.
  • They play an important role in the processes of inflammation and tissue repair.
  • They also participate in blood clotting and fibrinolysis.
  • They can take place in the adhesion of tumor cells during the formation of certain tumors.

Mesothelial cells are derived from the mesoderm, which is the middle embryonic layer of the blastoderm (the other two being the ectoderm and the endoderm) and the tissue they form – the mesothelium – is a slowly regenerating tissue.

They are generally hexagonal cells, although some have interdigitations or projections on the periphery. They present a large number of gap junctions , which are sites of contact and communication in the membranes between adjacent cell bodies.

Mesothelial cells also exhibit apical-basolateral polarity, significant cell-cell adhesion, and a basement membrane.

Some authors point out that these cells present microvilli on their luminal face, that is, on the surface that they expose to the organs that cover the mesothelial tissue, but their distribution may vary.

The structure and main characteristics of mesothelium, despite delimiting different cavities, are relatively equivalent throughout the entire body.

Embryology

The mesothelial layer that lines the serous cavities of our body begins to form during the third week of embryonic development, precisely when the cells of the mesodermal layer of the embryo begin to segment during gastrulation.

The mesoderm, as mentioned moments ago, is the middle layer of the blastoderm, which is defined as the membrane that forms after fertilization of the ovum during sexual reproduction, which delimits the blastocyst, the hollow sphere that characterizes the first stages. of animal embryonic development.

At the same time that the division or segmentation of the mesodermal cells occurs, fissures are formed in the lateral region of the mesoderm and, eventually, these fissures or fissures end up forming a “U” -shaped structure known as the intraembryonic coelom.

The “arms” of the “U” shaped cavity remain on the lateral plate of the mesoderm and “meet” in the midline, which corresponds to the region of the mesoderm that will later give rise to the cardiac tissue.

The cells that define the primitive coelomic tissue and its derivatives, which correspond to the pericardial, pleural and peritoneal cavities, are those that form the mesothelium.

At the beginning of embryonic development, these cells have a cuboidal appearance, forming a pseudostratified epithelium, but later they go through three sequential developmental events:

  • Formation of a basal lamina that separates the mesothelium from the underlying tissue.
  • Appearance of intercellular unions or gap junctions.
  • Change in cell shape from cuboidal to squamous.

Parietal mesothelium and visceral mesothelium

During the definition of the coelomic cavities or subdivisions in the embryo, that is, during the separation and / or definition of the pleural, cardiac and visceral cavities, the coelomic epithelium that lines the internal walls of said cavities and that derives from the mesoderm is called parietal mesothelium.

However, during organogenesis, some derivatives of the parietal mesothelium eventually cover developing organs, forming a distinctive tissue known as the visceral mesothelium.

There is a small space between the parietal mesothelium and the visceral mesothelium, which is commonly filled with a serous fluid rich in very diverse molecules: immunoglobulins, proteins of the complement system, lysozyme and other proteins that participate in the protection against bacterial infections and that they decrease friction, allowing some movement of the organs enclosed by these tissues.

Diseases or pathologies

Like any tissue of the human body, the mesothelium can present abnormalities related to defects during its establishment in embryonic development, inflammations, tumors, infections and other types of important pathologies.

Serous adhesions, for example, are very important from a clinical point of view, since they usually restrict the movement of organs within the pleural and abdominal cavities, which is of utmost importance for their function.

The appearance of these adhesions may be due to injury or damage to the mesothelium caused during surgical procedures, which prevent the mesothelial cells from fulfilling one of their normal functions related to fibrinolysis.

Metaplasias or changes in the differentiation of the cells that compose it, as well as in their ability to adapt in response to different injuries is another of the most common conditions related to the mesothelium.

Cytology of a pleural mesothelioma caused by prolonged exposure to asbestos (Source: Robertolyra, via Wikimedia Commons)

Mesotheliomas are tumors, benign or malignant, that are caused by an unbridled proliferation of mesothelial cells. These can occur in any of the three serous cavities, so the symptoms will depend on those organs that see their function affected.

References

  1. Hiriart, E., Deepe, R., & Wessels, A. (2019). Mesothelium and malignant mesothelioma. Journal of developmental biology, 7 (2), 7.
  2. Jones, JS (Ed.). (2012). Pathology of the mesothelium. Springer Science & Business Media.
  3. Lewis, WH (1923). Mesenchyme and mesothelium. The Journal of experimental medicine, 38 (3), 257-262.
  4. Mutsaers, SE (2004). The mesothelial cell. The international journal of biochemistry & cell biology, 36 (1), 9-16.
  5. Whitaker, D., Papadimitriou, JM, & Walters, MI (1982). The mesothelium: its fibrinolytic properties. The Journal of Pathology, 136 (4), 291-299.
  6. Whitaker, D., Papadimitriou, JM, & Walters, MN (1982). The mesothelium and its reactions: a review. CRC Critical reviews in Toxicology, 10 (2), 81-144.

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