The mesothelium is a specialized membrane that lines several of the body’s internal cavities and covers the outer surface of most of its organs. This tissue forms a continuous, thin, and protective layer that is fundamental to the normal movement and function of the structures it envelops. Understanding this membrane is a starting point for learning about how internal organs are protected and how certain medical conditions, including specific types of cancer, can develop.
Defining the Mesothelium and Its Locations
The mesothelium is composed of a single, continuous layer of flattened, specialized cells known as mesothelial cells. These cells are supported by a thin basement membrane and a layer of connective tissue. This structure, derived from the embryonic mesoderm, creates a smooth and slippery surface.
This tissue is organized into two distinct layers. The visceral layer is the portion of the mesothelium that directly covers the surface of the internal organs. The parietal layer lines the walls of the body cavities that contain these organs. Between these two layers is a very narrow, fluid-filled space.
One primary location of the mesothelium is the pleura, which is associated with the lungs. The visceral pleura clings to the surface of the lungs, while the parietal pleura lines the inside of the chest wall. The space between them, the pleural cavity, allows the lungs to expand and contract during breathing without resistance. This structure is often compared to a fist pushing into an underinflated balloon, where the fist represents the lung and the two layers of the balloon represent the visceral and parietal pleura.
A second major area is the peritoneum, the most extensive serous membrane in the body, which lines the abdominopelvic cavity. Its parietal layer lines the abdominal wall, and its visceral layer covers the exterior of organs such as the stomach, intestines, liver, and spleen. This arrangement helps to hold abdominal organs in place and allows them to slide past one another. The pericardium is the mesothelial membrane that encloses the heart in a sac, while the tunica vaginalis covers the testes in males.
The Protective and Functional Roles
A significant function of the mesothelium is providing lubrication for organ movement. The mesothelial cells secrete a thin substance called serous fluid, which fills the space between the visceral and parietal layers. This fluid acts as a lubricant, creating a smooth surface that reduces friction. This allows organs like the heart and lungs to move against adjacent structures without causing wear or damage.
The mesothelium also has a role in fluid and cellular transport. The surface of mesothelial cells is covered in microvilli, which are tiny projections that increase the surface area for secretion and absorption. These structures help manage the volume of serous fluid and allow for the movement of cells, like immune cells, across the cavity. This function supports the body’s response to inflammation and infection within these spaces.
The mesothelium contributes to tissue repair and inflammatory processes. When an injury occurs within a serous cavity, mesothelial cells can become activated. They participate in healing by helping to form blood clots and by releasing substances that attract immune cells to the site of damage. This response helps protect against invading microorganisms and manages the initial stages of tissue repair.
Conditions Affecting the Mesothelium
The most widely known condition involving this tissue is mesothelioma, a rare and aggressive form of cancer. This disease arises from the mesothelial cells and is primarily caused by exposure to asbestos fibers. When inhaled or ingested, these microscopic fibers can become lodged in the mesothelial linings of the lungs or abdomen. Over decades, the irritation from these fibers can cause chronic inflammation and genetic damage, leading to the transformation of mesothelial cells. It is distinct from lung cancer, which develops from the epithelial cells within the lungs, not the lining outside of them.
Inflammation of the mesothelium is a more common issue. When the pleura becomes inflamed, the condition is known as pleurisy, which can cause sharp chest pain, especially with breathing, as the two inflamed layers of the pleura rub against each other. Similarly, inflammation of the peritoneum is called peritonitis, a serious condition that can result from infection or injury in the abdomen.
Damage to the mesothelium from surgery, infection, or inflammation, can lead to adhesions. Adhesions are bands of scar tissue that can develop between the visceral and parietal mesothelial layers, causing them to stick together. This can restrict the normal movement of organs and, in the abdomen, may lead to complications like bowel obstruction. In some cases, benign, or non-cancerous, tumors and fluid-filled cysts can also develop from mesothelial tissue.