Architecture and function of the extravascular fluid pathway: special reference to the macula cribriformis in the diaphragm.

Silver staining, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been used to delineate the structure and function of the extravascular fluid pathway of the diaphragm of monkeys, rats, mice and rabbits. The submesothelial connective tissue was examined by SEM using the NaOH maceration method. A sieve-like structure, the macula cribriformis, was found in the submesothelial connective tissue on the peritoneal side of both the muscular and tendinous portions. The macula cribriformis, which consisted of densely-packed collagen fibrils, was oval, spindle and polygonal in shape and ranged in diameter from 20 to 300 microns with 5-300 foramina (3-15 microns). The macula cribriformis was located between peritoneal mesothelial cells with stomata and subperitoneal lymphatic capillaries. Intraperitoneally-injected latex particles were carried into the diaphragmatic lymphatic capillaries via the peritoneal stomata and the foramina of the macula cribriformis. These results support the hypothesis proposed by Kihara (Okajimas Fol Anat Jpn 28: 601-621, 1956) that the macula cribriformis is a pre-lymphocapillary pathway, one of the extravascular fluid pathways.