Distribution and ultrastructure of the stomata connecting the pleural cavity with lymphatics in the rat costal pleura.

We investigated the detailed distribution and ultrastructure of the stomata connecting the pleural cavity and the lymphatics in the rat costal pleura by scanning electron, transmission electron and light microscopy. The mesothelial cells lining the costal pleura appeared as both flattened and thick cell bodies. The thick cells possessed more rough endoplasmic reticula, Golgi complexes, mitochondria, and free ribosomes than the flattened cells. The thick cells were distributed in the intercostal regions each cephalic to the junction of the costal cartilage and bone, and in the band-like regions along the cephalic and caudal sides of each rib in the lateral and dorsal thoracic walls. In the regions lined with thick cells, there were stomata [12.9 +/- 10.3 microns2 (mean +/- SD) in area] consisting of prolongations of thick mesothelial cells and funnel-like projections of lymphatic endothelial cells that came up along the rims of the pores (5.9 +/- 3.2 microns2 in average area) in the submesothelial collagen fiber network. At the stomata, the basal lamina of the mesothelium was continuous with that of the endothelium. The mesothelial cells forming the stomata were mostly in close contact with the endothelial cells, but some gaps also existed between them. Valve-like endothelial flaps were frequently observed wherever endothelial cells constituting the stomata merged into the submesothelial lymphatics. Also present were lymphatic bulges that were either in close contact with the base of the thick mesothelial cells or exposed through the mesothelial pores. The lymphatic network was especially well developed in the submesothelial layer at and around the thick-cell regions. The initial lymphatics drained into the intercostal collecting lymphatics, which in turn led into either the parasternal or paravertebral lymphatic trunk. Our results suggest that the stomata play a major role in absorbing fluids and particulates in the pleural cavity. The thick mesothelial cells appear to secrete chemotactic substances to the endothelial cells. Understanding the heterogeneous distribution of the stomata could prove to be important clinically in inflammatory diseases and tumors in the chest.