Ultrastructural study on the pleural stomata in human.

The ultrastructure of human diaphragmatic, thoracic wall and visceral pleura was studied by scanning and transmission electron microscope. Human pleural stomata, which are usually round or oval in shape with about 6.2 microns in diameter, were present on the diaphragmatic pleura. They were never found on the thoracic or visceral wall. Majority of human pleural stomata were quite deep, forming channels which seemed to connect pleural cavity with underlying lymphatic lacunae. Some of them occurred as shallow pits exposing components of the underlying connective tissue. In some areas of the diaphragmatic pleura the pleural stomata were furnished with great microvilli of mesothelial cells surrounding their openings. These microvilli were always longer and had denser network of filaments in comparison with other on the surface of mesothelial cell. In this way the mesothelial cells formed valve-like cytoplasmic processes into the channels. The results of the present study suggested that the pleural stomata, as permanent structures, provided a rapid removal of fluid, particles and cells from the pleural cavity into the diaphragmatic lymphatics.