Ultrastructure and three-dimensional study of the lymphatic stomata in the costal pleura of the rabbit.

The aim of this report was to investigate the ultrastructure and three-dimensional organization of the pleural lymphatic stomata in the adult rabbit costal pleura by electron microscopy. A computer image processing system attached to the scanning electron microscopy was used to get statistical evaluation of the dimensions of pleural lymphatic stomata. Mesothelial cells were digested by 2 mol/L NaOH solution in order to expose the submesothelial connective tissue with macula cribriformis. Two kinds of mesothelial cells were observed on the costal pleura: the flattened cells and the cuboidal ones. Both had microvilli on their surface. Pleural lymphatic stomata were located only in the regions of cuboidal mesothelial cells. The average area of a stoma was 7.20 +/- 3.69 microm2 and their average density 121 +/- 72/mm2. Pleural cavity is connected with the lympo-vascular system by lymphatic stomata and the interstitial layer with a dense network of lamina cribriformis. The macula cribriformis (7-60 microm in diameter) were found in subpleural connective tissue below the cuboidal mesothelial cells. Consequently on cross-section, the pathway represents a channel consisting of the stoma, the connective tissue space, and the gap between endothelial cells of the lymphatics. Closed lymphatic stomata and the milky spots composed of macrophages could be observed on the costal pleura. Our results suggest that the pleural cavity is connected with the lymphatic capillaries through the lymphatic stomata and the subpleural channel. This is the only "highway" from the pleural cavity to the vessels. This pathway may be importantly involved in the material exchange and the immunity of the pleura cavity.