Microanatomy of the rat diaphragm with special reference to the lymphatics and mesothelial stomata

The three-dimensional microstructure of the rat diaphragm was studied in order to reveal morphological bases which permit peritoneal fluids to pass across the diaphragm to enter the pleural cavity. The methods used include scanning electron microscopy of either intact or alkali-treated tissues, enzyme-histochemistry, and confocal laser scanning microscopy (CLSM). The peritoneal and pleural surfaces of the diaphragm are covered with mesothelial cells studded with numerous microvilli. There are many round gaps between mesothelial cells on the peritoneal side of the diaphragm. The subperitoneal connective tissue contains voluminous, irregularly shaped lymphatics which extended many funnel-shaped projections of the endothelia towards the pored region of the mesothelium. On coming into contact with the mesothelium, many of the lymphatic projections are perforated at their ends, thus giving rise to stomata connecting the peritoneal cavity and lymphatic lumen. Some projections ended blindly while plugging the mesothelial pores, thereby making visible some intercellular gaps in this contact. The subperitoneal sheet of collagen fiber network possesses clusters of pores which tightly fit the passage of the lymphatic projections. CLSM of the diaphragm after intraperitoneal injection of FITC-dextran has demonstrated the tracer both in the lymphatic lumen and in the connective tissue spaces. The tracer has also been detected in the lymphatics located in the subpleural connective tissue space. These results indicate that peritoneal fluid is allowed to flow into the lymphatics directly through the stomata and indirectly through the intercellular gaps between endothelia and mesothelial cells, and then drain into the subpleural lymphatics. Discussions were made on the probable mechanisms by which a hydrothorax may occur during continuous ambulatory peritoneal dialysis.