Mesothelial cells form a single thin layer that wraps the organs in your chest and belly and lines the walls around them. Their best-known job is keeping those organs slippery, so the lungs, heart and gut can move without sticking.1PubMed. Mesothelial cells: their structure, function and role in serosal repair The lining was long thought to be little more than a slick wrapper, but it turns out to be an active tissue that moves fluid, joins immune defense and repairs itself.2Elsevier / PubMed Central. Mesothelial cells and peritoneal homeostasis That is why the same cells turn up in fluid reports, surgery recovery, dialysis care and asbestos-related disease.
Where Mesothelial Cells Are Found
Most animals, people included, have closed body cavities lined by a layer just one cell thick.3PubMed. Mesothelial-mesenchymal transitions in embryogenesis In the chest, this lining is the pleura. It covers each lung and lines the inside of the ribcage. In the abdomen, it is the peritoneum, which covers the stomach, intestines and liver and lines the belly wall. Around the heart it forms the pericardium, and the layer on the heart’s own surface is called the epicardium.4PubMed Central. The Role of the Epicardium During Heart Development and Repair
Each lining has two sides, one stuck to the organ and one to the wall, with a thin film of fluid between them. The inner and outer layers are really one continuous sheet folding back on itself. In health, the space between holds only a thin slick of fluid. Doctors call a buildup there an effusion, such as a pleural effusion around the lungs.
Under a microscope, the free surface of the cells is crowded with long, slender, finger-like projections.5Wiley Online Library (The Anatomical Record). The ultrastructural morphology and possible functional significance of mesothelial microvilli They greatly increase surface area and carry a sugary coating that likely helps lubricate the surface and stop neighboring surfaces from sticking.6Wiley Online Library (The Anatomical Record). The ultrastructural morphology and possible functional significance of mesothelial microvilli
Keeping Organs Moving Without Sticking
The lining’s first-known role is providing a slippery, non-adhesive, protective surface so organs can move inside their cavities.7PubMed. Mesothelial cells: their structure, function and role in serosal repair Your lungs can expand with each breath and your gut can shift during digestion without rubbing raw. The cells also release substances that both lay down and clear away fibrin, the sticky protein of clots.8PubMed. Mesothelial cells: their structure, function and role in serosal repair That means they help decide whether two touching surfaces stay separate or knit together.
When the gliding fails, you feel it. An inflamed chest lining causes pleurisy, a sharp pain when you breathe in. An inflamed heart sac lining causes pericarditis, often chest pain that eases when you lean forward. Both feel worse with movement because the surfaces rub instead of slide.
The non-stick role matters most after injury. Any cut, scrape or surgery strips the slick lining in places, and exposed tissue tends to stick to whatever it touches. Mesothelial cells release enzymes that dissolve the leaked fibrin before it hardens. When this clearing falls behind, the fibrin can become a scaffold for scar tissue, as described in the adhesions section below.
Moving Fluid In and Out of Body Spaces
The chest lining is not a passive wall. Its cells are metabolically active and can transport dissolved substances, including carrying protein across in tiny bubbles.9PubMed. Physiology and pathophysiology of pleural fluid turnover Fluid leaves the chest space through pressure differences, through these cell processes and through lymph drainage.10PubMed. Physiology and pathophysiology of pleural fluid turnover On the outer chest wall, tiny openings drain particles and cells into lymph vessels, though they are not thought to be the main route for removing fluid itself.11PubMed. Physiology and pathophysiology of pleural fluid turnover In the belly, fluid reaches drainage channels under the diaphragm through gaps between mesothelial cells.12PubMed Central. The role of the diaphragm in lymphatic absorption from the peritoneal cavity
Normally a small amount of fluid is made and cleared continuously, so the amount stays tiny. Fluid builds up when more enters a space than the lining and lymph can clear. Heart failure, infection, liver disease and cancer can all tip that balance, and blocked lymph drainage, as with some cancers, slows clearance further. Doctors treat an effusion as a clue that needs explaining. Draining the fluid with a needle relieves breathlessness, but the fluid often returns unless the underlying cause is treated.
Peritoneal dialysis works for the same reason. The belly lining is a large surface that fluid and waste can cross, so dialysis fluid placed in the belly draws waste out of the blood through it.
Helping the Immune System Fight Infection
Mesothelial cells help guide the body’s inflammation and immune response after infection, injury or cancer.13PubMed. Mesothelial cells regulate immune responses in health and disease: role for immunotherapy in malignant mesothelioma They carry sensors that recognize germs and release signals that call in other immune cells.14PubMed Central. Pleural mesothelial cells in pleural and lung diseases That helps defend against bacterial infections in the chest, including tuberculosis.15PubMed Central. Pleural mesothelial cells in pleural and lung diseases They also help steer white blood cells toward inflamed areas.16PubMed. Mesothelial cells: their structure, function and role in serosal repair Infected chest or belly fluid fills with immune cells quickly because the cells start the alarm rather than doing all the fighting themselves.
They can also eat invaders. Human belly-lining cells can swallow bacteria such as E. coli and show pieces of them to other immune cells.17PubMed. Human Peritoneal Mesothelial Cells Display Phagocytic and Antigen-Presenting Functions to Contribute to Intraperitoneal Immunity In a patient with a severe infection, chest fluid showed mesothelial cells actively swallowing white blood cells.18ASH Image Bank. Differentiation of mesothelial cells into macrophage phagocytic cells in a patient with clinical sepsis In lab studies, chest-lining cells from rats swallowed asbestos fibers.19American Journal of Pathology. Phagocytosis of chrysotile fibers by pleural mesothelial cells in culture
Their role is balancing inflammation, not only boosting it. Lab-grown cells that were infected with the COVID virus mostly released calming, anti-inflammatory signals, while also making enzymes that reshape the surrounding tissue.
What It Means When a Report Mentions Mesothelial Cells
Mesothelial cells shed into the fluid of body spaces, so they routinely appear in drained chest or belly fluid. After lung surgery, activated mesothelial cells in chest fluid peaked around day 3.20The FASEB Journal. Activated mesothelial cells in pleural fluid after lung resection and transplantation Their numbers rise with healing and irritation, not only disease. Cells described as “reactive mesothelial cells” (enlarged, activated cells) often reflect inflammation, infection or recent surgery. Reactive cells can sometimes look worrying under the microscope, which is why stain tests exist.
Pathologists use stains that light up mesothelial markers such as calretinin, WT-1, D2-40 and CK5/6.21PubMed Central. Application of immunohistochemistry in diagnosis and management of malignant mesothelioma They pair these with markers for other cancer types, such as MOC-31 and claudin-4, using at least two of each.22PubMed Central. Application of immunohistochemistry in diagnosis and management of malignant mesothelioma No single marker is perfect: about 8% of pancreatic cancers stained for calretinin.23PubMed Central. Expression of Calretinin, Marker of Mesothelial Differentiation, in Pancreatic Ductal Adenocarcinoma: A Potential Diagnostic Pitfall None of those pancreatic cancers stained for WT-1, which is why a panel is used.24PubMed Central. Expression of Calretinin, Marker of Mesothelial Differentiation, in Pancreatic Ductal Adenocarcinoma: A Potential Diagnostic Pitfall
A few questions are worth bringing to your doctor. Ask whether the cells were called benign, reactive, atypical or malignant, and what likely caused the fluid in the first place. If the word is “atypical,” ask whether stains, a repeat sample or a biopsy are planned. Mention any past asbestos exposure, even decades ago.
Healing After Injury and Surgery Adhesions
Unlike skin, the lining doesn’t heal only by cells creeping in from the wound edge, so even large injuries can heal over evenly.25PubMed. Evidence for incorporation of free-floating mesothelial cells as a mechanism of serosal healing Free-floating mesothelial cells land on the injured surface, multiply and rebuild the lining.26PubMed. Evidence for incorporation of free-floating mesothelial cells as a mechanism of serosal healing In animal studies, these cells settled on wounds 3 to 8 days after injury.27PubMed. Evidence for incorporation of free-floating mesothelial cells as a mechanism of serosal healing After lung surgery, chest fluid can hold enough active cells to cover several square centimeters of bare lining.28The FASEB Journal. Activated mesothelial cells in pleural fluid after lung resection and transplantation
When fibrin isn’t cleared and scar tissue forms instead, the result is adhesions: bands that join organs or tissues that should slide apart, common after belly surgery. Whether they form depends partly on the balance between a substance that breaks down fibrin and one that blocks that breakdown.29PubMed. Preservation of peritoneal fibrinolysis owing to decreased transcription of plasminogen activator inhibitor-1 in peritoneal mesothelial cells suppresses postoperative adhesion formation in laparoscopic surgery In mice, injured mesothelial cells and immune cells fed a loop that raised the blocking substance and further stalled fibrin clearing.30PubMed. Plasminogen activator inhibitor-1 regulates macrophage-dependent postoperative adhesion by enhancing EGF-HER1 signaling in mice Under certain injury signals, mesothelial cells can also turn into scar-forming cells.31PubMed. Pathobiological mechanisms of peritoneal adhesions: The mesenchymal transition of rat peritoneal mesothelial cells induced by TGF-β1 and IL-6 requires activation of Erk1/2 and Smad2 linker region phosphorylation
Many adhesions cause no symptoms at all. Some cause long-term belly pain or, rarely, a bowel blockage needing urgent care. Keyhole surgery tends to cause fewer adhesions than open surgery.32PubMed. Preservation of peritoneal fibrinolysis owing to decreased transcription of plasminogen activator inhibitor-1 in peritoneal mesothelial cells suppresses postoperative adhesion formation in laparoscopic surgery Sudden belly swelling, vomiting and no bowel movements after past surgery need prompt medical attention.
Researchers are testing delivery of lab-grown mesothelial cells to speed lining repair.33PubMed. The potential of mesothelial cells in tissue engineering and regenerative medicine applications In rats, mesothelial cell sheets on a gel prevented adhesions that formed in all untreated animals.34PubMed. Mesothelial cell sheets cultured on fibrin gel prevent adhesion formation in an intestinal hernia model These remain experimental, not routine treatment.
Asbestos and Mesothelioma
Mesothelioma is a cancer that starts in mesothelial cells, most often in the chest lining. Asbestos can transform human mesothelial cells into cancer cells.35PubMed Central. Asbestos induces mesothelial cell transformation via HMGB1-driven autophagy Fibers lodge in tissue and keep the lining in chronic inflammation.36PubMed Central. How asbestos and other fibers cause mesothelioma Inflamed immune cells release reactive molecules that can damage DNA.37PubMed Central. How asbestos and other fibers cause mesothelioma Asbestos kills most cells it touches, and the danger comes from the few that survive with DNA damage.38PubMed Central. Asbestos induces mesothelial cell transformation via HMGB1-driven autophagy A protein released near the fibers helps those damaged cells survive.39PubMed Central. Asbestos induces mesothelial cell transformation via HMGB1-driven autophagy Injury, inflammation and regrowth then repeat over a long delay before cancer appears.40PubMed Central. New insights into understanding the mechanisms, pathogenesis, and management of malignant mesotheliomas
That delay is long: mesothelioma typically appears 20 to 50 years after exposure. Higher and longer exposure raises risk, but most exposed people never develop it. Former construction, shipyard, insulation and plumbing work are common sources. If you have breathlessness, chest pain or fluid around a lung, tell your doctor about any past exposure.
Asbestos can also cause other chest-lining problems. It can cause patches of noncancerous thickening, which reports may call pleural plaques, and recurring fluid around the lungs. Plaques show past exposure but are not cancer.
Mesothelial Cells and Peritoneal Dialysis
Standard heat-sterilized glucose dialysis fluids contain breakdown products called GDPs.41PubMed. Glucose degradation products and the peritoneal mesothelium These slow mesothelial cell growth in a dose-dependent way,42PubMed. Glucose degradation products and the peritoneal mesothelium and they may explain why the cells disappear during long-term treatment.43PubMed. Glucose degradation products in peritoneal dialysis fluids may have both local and systemic effects: a study of residual fluid and mesothelial cells With newer pH-neutral, low-GDP fluids, the cells stayed more viable and kept growing better.44PubMed. Peritoneal dialysis with solutions low in glucose degradation products is associated with improved biocompatibility profile towards peritoneal mesothelial cells These fluids may help protect the lining over years of dialysis.45PubMed. Peritoneal dialysis with solutions low in glucose degradation products is associated with improved biocompatibility profile towards peritoneal mesothelial cells
With long-term dialysis, the lining can become thinner and lose its mesothelial cells. It may also become scarred and thickened, which slows how well waste and fluid cross it. This is one reason peritoneal dialysis sometimes has to be stopped and another treatment used.
Lining health affects how long peritoneal dialysis keeps working, so it is worth asking your kidney team which dialysis solution you use and whether low-GDP options suit you. Report cloudy fluid or belly pain quickly, since either can signal infection.
Building Blood Vessels and Repairing the Heart
Mesothelial cells do more than line cavities. In development, they give rise to muscle cells and support cells of internal organs.46PubMed Central. Identification and prospective isolation of a mesothelial precursor lineage giving rise to smooth muscle cells & fibroblasts for mammalian internal organs, and their vasculature
In the developing lung, they help form muscle in blood vessel walls.47PubMed Central. Mesothelium contributes to vascular smooth muscle and mesenchyme during lung development The heart’s lining, the epicardium, supplies cells and signals for building the heart and its blood vessels.48PubMed Central. The Role of the Epicardium During Heart Development and Repair In zebrafish, which regrow heart muscle, the epicardium is essential for regeneration.49PubMed Central. The epicardium as a hub for heart regeneration In mammals, it mostly takes part in scarring after heart injury.50PubMed Central. The epicardium as a hub for heart regeneration
Researchers care because understanding the epicardium may help heart repair after heart attacks. This research is early and not yet a treatment.