Mesothelial cells are specialized epithelial cells that form a delicate lining for the major internal cavities of the body. These cells create a protective surface that covers both the cavity walls and the organs housed within them. The primary tissue layer they form is known as the mesothelium, which acts as a barrier and a functional interface. Mesothelial cells are dynamic participants in maintaining the stability and function of the internal environment.
Cellular Identity and Structure
Mesothelial cells are generally categorized as simple squamous epithelial cells, meaning they are flattened and arranged in a single layer, forming a pavement-like sheet on a supporting membrane. This monolayer of cells rests upon a thin basal lamina, which separates the mesothelium from the underlying connective tissue. While typically flattened, these cells can sometimes take on a cuboidal shape, particularly in areas of injury or specific anatomical locations like the diaphragm.
The surface of each cell facing the internal cavity is covered with numerous microvilli. These microvilli significantly increase the surface area of the mesothelium. This extensive surface area is coated with a hydrophilic protective layer known as the glycocalyx, which is composed largely of glycosaminoglycans like hyaluronan. The combination of the microvilli and the glycocalyx is fundamental to the cell’s main mechanical function.
Mesothelial cells possess both epithelial and mesenchymal characteristics, which grants them a unique level of plasticity. They are held together by junctions that provide barrier function, similar to other epithelial cells. However, their mesodermal origin and ability to differentiate into other cell types, like myofibroblasts during injury, highlight their distinct nature from typical epithelial lining cells.
Anatomical Placement within the Body
Mesothelial cells line the serous membranes, which are thin, double-layered linings found in three main closed cavities of the body. The layer covering the cavity wall is called the parietal layer, and the layer covering the surface of the internal organs is the visceral layer. The small space between these two layers is the serous cavity, which contains a small amount of fluid.
The three primary locations for mesothelial cells are the pleura, the pericardium, and the peritoneum. The pleura lines the chest cavity and covers the lungs, creating the pleural cavity. The pericardium surrounds the heart, forming the pericardial cavity. The peritoneum is the largest serous membrane, lining the abdominal cavity and covering the abdominal organs.
The cells are also found in other specific locations, such as the tunica vaginalis, which lines the testes. The sheer size of the peritoneum means that the mesothelial cells lining it cover a surface area approximately equivalent to the skin.
Essential Physiological Roles
The mesothelium provides lubrication to minimize friction between moving organs. The microvilli secrete a serous fluid into the cavity space, which contains a mixture of hyaluronic acid, glycoproteins, and phospholipids. This fluid layer ensures that organs like the lungs, heart, and intestines can glide smoothly against the cavity walls and each other during normal functions, such as breathing and digestion.
Mesothelial cells also regulate fluid transport and maintain homeostasis within the serous cavities. The mesothelium acts as a semi-permeable membrane, controlling the movement of fluid, electrolytes, and solutes between the cavity and the underlying tissues. They possess various transport systems, including ion channels and aquaporins, that govern the balance between fluid secretion and re-absorption.
The mesothelium is a component of the body’s local immune surveillance and defense system. As a first line of defense, mesothelial cells respond rapidly to injury or infection by producing and releasing inflammatory mediators, such as cytokines like TNF-alpha and IL-1beta. They can interact with immune cells and participate in antigen presentation, helping to trigger and regulate the local inflammatory response.
These cells are involved in tissue repair and wound healing following injury. When the mesothelium is damaged, the cells mobilize to regenerate the protective layer, often by migrating from the edges of the wound or by using free-floating mesothelial cells in the serous fluid. They also contribute to the balance of procoagulant and fibrinolytic activities, necessary for managing the formation and breakdown of fibrin clots during healing.
Clinical Significance and Pathology
Mesothelial cells are the source of a rare but aggressive form of cancer known as malignant mesothelioma. This neoplasm arises directly from the mesothelium, most commonly in the pleura, accounting for the majority of cases, but also in the peritoneum. The development of mesothelioma is strongly linked to previous exposure to asbestos fibers, which, when inhaled or ingested, can become lodged in the mesothelium and trigger genetic damage.
Mesothelial cells are central to the development of inflammation in the serous cavities, known as serositis. Examples include peritonitis (inflammation of the peritoneum) and pleuritis (inflammation of the pleura). In these conditions, the mesothelial cells contribute to the pathology by producing inflammatory cytokines and altering their permeability in response to infection or trauma.
A pathological consequence of mesothelial injury is the formation of adhesions, which are bands of scar tissue that connect organs to each other or to the cavity wall. If healing is impaired, mesothelial cells can undergo a transformation, leading to the excessive production of extracellular matrix components and fibrin. These fibrous adhesions can impede the normal movement of internal organs, leading to chronic pain and bowel obstruction.