Distribution of mesothelial stomata in the rat

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Abstract
In 15 rats the morphology and distribution of stomata and mesothelial cells of the diaphragm and of different serosal areas were studied by light and electron microscopy. Cell surface patterns, identified by SEM, varied according to location. Mesothelial cells of the diaphragmatic peritoneum were either cuboidal or flattened. Stomata were found only among cuboidal cells and were consistently and exclusively observed on the muscular part of the diaphragm, predominantly on the peritoneal surface. Stomata appeared oval or rounded, 4–10 μm in diameter. When India ink was introduced into the peritoneal cavity as a tracer, carbon particles were rapidly removed through stomata into the subjacent lymphatic lacunae of the diaphragm. Mesothelium overlying the diaphragm differed morphologically from that over the remaining peritoneal and pleural cavities. Mesothelium of the latter formed a continuous sheet of flattened cells whose surfaces were studded with numerous microvilli. Stomata were absent. It is suggested that stomata are exclusive to the diaphragm and may serve as the main drainage channels for fluid and particles from the peritoneal cavity. This has important clinical implications. Thus stomata could represent a liability by providing a route for the escape of tumor cells and pathogens from the peritoneal cavity. However, they may represent an asset by providing an accessible route for blood transfusion and for peritoneal dialysis in treating chronic renal failure.