A study of the two-way transport of horseradish peroxidase across the visceral pleura

Electron microscopic alterations of the rat's pleura after experimental haemothorax

The initial signs of pleural reactivity and the subsequent mechanisms of pleural healing still remain unsolved. The visceral and parietal (costal and diaphragmatic) pleura were investigated following an experimental haemothorax (EH) by transmission electron microscopy. Young-adult Wistar rats were divided in five groups and survived 6 hours, 1, 3, 8 and 15 days respectively after EH. Six hours after EH the mesothelial cells had a more prominent lysosomal system and electron-dense material in the vesicles, as in the dilatated intercellular spaces. On the 1st day of the EH the mesothelial cytoplasm formed a thin interrupted band. The extravasal cells built multiple layers over the basal lamina, leading to a thicker submesothelial layer, occupying the superficial position toward the pleural cavity. The activated mesothelial cells covered both pleural sheets on the 3rd day after EH. Eight days after EH different membrane bodies, large apical evaginations, elastic-like formations, an extensive vesicular and cytofilamentous systems characterized the mesothelium. The wider elastic membrane showed thickenings, protrusions, bifurcations and double course. Fifteen days after EH larger zones in both pleural sheets displayed thinner basal lamina, remnants of elastic membrane and a thicker submesothelial layer. In conclusion, different newly formed structures (reversible and stable) retain the tendency of enlargement of the pleural surface in all investigated periods. Simultaneous intercellular and transcellular transport, as an increase of the lysosomal system characterize the passing of the electron-dense material through the mesothelium. The early period (until 3rd day after EH) is characterized by more prominent mesothelial changes, involving activated cells. The initiation of the late period (on the 8th day after EH) begins with the appearance of lamellar bodies and newly formed elastic membrane. The following late changes (on the 15th day after EH) concern predominantly the components of the connective tissue layer, such as collagen accumulations and blood capillaries. The present data suggest that the alterations over the entire pleura are irregular and asynchronous, showing significant morphological differences in both pleura sheets, some of them are diffuse in character, the final ones appear to be stable and ensure incomplete pleural restoration.

Stimulation of mesothelial cell proliferation by exudate macrophages enhances serosal wound healing in a murine model

Examination of thermally induced serosal lesions in mice displayed collections of inflammatory cells, predominantly macrophages, on and surrounding the wound within 48 hours of injury. Furthermore, by 2 days a large number of uninjured mesothelial cells adjacent to the wound were synthesizing DNA. From these findings, it was hypothesized that macrophages play a major role in serosal repair by stimulating mesothelial cell proliferation. Again, using a murine model of mesothelial regeneration, depletion of circulating monocytes significantly delayed serosal healing whereas addition of peritoneal exudate cells to the wound site 36 hours before injury increased the healing rate. In vivo assessment of mesothelial cell proliferation using tritiated thymidine incorporation and autoradiography demonstrated that peritoneal exudate cells stimulated mesothelial cell proliferation (12.44 +/- 1.63% labeling index, compared with controls in which medium only was used 4.48 +/- 0.71%). The mesothelial proliferation was predominantly because of macrophage-secreted products with molecular weights of 36 to 53 kd or 67 to 100 kd. These data support the hypothesis that macrophages play an important role in serosal healing by stimulating mesothelial cell proliferation.

Scanning and transmission electron microscopic study of visceral and parietal peritoneal regions in the rat

The visceral peritoneum of intraabdominal organs (spleen, stomach, liver, small intestine), omentum majus and the parietal peritoneum of the anterior abdominal wall and the diaphragm were studied in adult Wistar rats by combined scanning and transmission electron microscopy (SEM, TEM). In general, the peritoneal surface consisted of a mesothelium composed of cubic, flat or intermediate cell types delimited by a basal lamina. Cubic mesothelial cells predominated in parenchymal organs (spleen, liver) and were characterized by prominent and indentated nuclei, a cytoplasm richly supplied with organelles, a dense microvillous coat, basal invaginations and elaborate intercellular contacts. Flat mesothelial cells were observed in the intestinal, omental and parietal peritoneum (tendinous diaphragm, abdominal wall) and showed elongated nuclei, scant cytoplasm, a poorly developed organelle apparatus and sparsely distributed microvilli. An intermediate mesothelial cell type was described within the gastric peritoneum characterized by a central cytoplasmic protrusion at the nuclear region containing most of the cytoplasmic organelles and by thin finger-like cytoplasmic processes. The submesothelial connective tissue layer was composed of collagen fiber bundles, fibroblasts and free cells (macrophages, granulocytes, mast cells) and contained blood and lymphatic vessels. In the spleen, elastic fibers formed a membranous structure with intercalated smooth muscle cells. Mesothelial openings were observed as tunnel-like invaginations within the hepatic peritoneum and as clusters of peritoneal stomata within the parietal peritoneum of the anterior abdominal wall and the muscular diaphragm. The round or oval openings of the peritoneal stomata were frequently occluded by overlapping adjacent mesothelial cells and their microvillous coat or obstructed by cellular material. At the side of the peritoneal stomata the mesothelial cell layer was interrupted to allow a direct access to the underlying submesothelial lymphatic system. The mesothelium and lymphatic endothelium shared a common basal lamina. The endothelial cells were discontinuous and displayed valve-like plasmalemmatic interdigitations facilitating an intercellular transport of fluids and corpuscular elements from the peritoneal cavity to the submesothelial lymphatic lacunae. The findings underline the morphological heterogeneity of the peritoneum in visceral and parietal regions, suggesting different functional implications, and further support the presence of extra-diaphragmatic peritoneal stomata.