Mesothelium

MESOTHELIUM, the name currently given to the single-layered flat epithelium covering the serous membranes of the abdominal and thoracic cavities, on the basis that it arises from the mesodermal sheets limiting the primary body cavity. Previously it was commonly classified as endothelium, being identified with the flat epithelium lining the vascular system and derived from mesenchyme; now the name endothelium is reserved only for the latter. On preparations stained by ordinary methods, the boundaries between mesothelial cells are not visible, only the oval-shaped nuclei, thin and flat, with very fine chromatin grains and small nucleoli, are distinctly prominent. To reveal the boundaries, the mesothelium must be treated with silver nitrate, whereupon they appear as black, winding lines. The mesothelial cells have an irregular polygonal shape, with an average diameter of 22 μ (Kolliker); they vary depending on the degree of stretching of the serous membrane. The thickness of the cells is insignificant, the thickest being found on the abdominal surface of the diaphragm and on the surface of the spleen; on the diaphragm, there are also islands of small cells directly adjacent to lymphatic vessels—these are the sites of most active fluid absorption from the peritoneal cavity (Ranvier's wells). The fine structure of the mesothelial cell is as follows. On the surface of the cell is a thin cuticular covering, on which a brush of thin and short hairs can be detected, clearly visible in lower vertebrates. The cuticles of adjacent cells fit closely together; between them silver is deposited during impregnation. In the protoplasm, mitochondria have been described in considerable quantity and a sphere with a centrosome, located in a depression on the upper surface of the nucleus. The protoplasmic parts of the cells also adjoin each other, but between them slits can easily form; the existence of preformed intercellular bridges (Ranvier, Kolossov) has recently been questioned. Mesothelial cells are capable of absorbing acid vital dyes in very small amounts in the form of grains arranged around the nucleus; they can allow particles of ink to pass through their bodies. Under the influence of various irritations, the cells easily change; bright grains appear in their protoplasm, which, judging by their staining with Sudan, are of a fatty nature; the cells contract so that slits form between them, they become rounded and are easily desquamated. Such detached cells are found in serous exudates. In inflammatory processes, transformation of mesothelial cells into mobile macrophages as well as into fibroblasts has been described, which then participate in adhesive inflammation; the possibility of reverse transformation of fibroblasts into covering cells is also assumed. Maximov's experiments with the culture of serous membranes in vitro show that, on the one hand, mesothelial cells can give rise to typical cuboidal epithelium, and on the other to spindle-shaped fibroblasts; transition into macrophages was not observed. In general, views on the nature and potentialities of the mesothelium are by no means established.

V. Karpov. Mesothelioma, a tumor of the mesothelium, synonyms: primary cancer, malignant epithelioma of the peritoneum, pleura; former name—endothelioma of the peritoneum, pleura. The neoplasm rarely has the appearance of a node growing in some place of the serous cavity; much more often the tumor very quickly spreads over the serous covering of the peritoneum, pleura in the form of a flat, slightly warty whitish mass, simulating as if a thickening of the serous covering over a considerable extent. Sometimes the tumor grows in the form of a large number of small nodules scattered over the serous covering. Microscopically, mesothelioma has the appearance of a solid cancer of small cells, more rarely—adenocarcinoma of cuboidal or cylindrical cells. A very rare and at the same time controversial form in terms of belonging to mesotheliomas is the colloid or mucoid form of these neoplasms. The diagnosis of mesothelioma at autopsy can only be made after excluding the spread over the serous cavity of a primary cancer of any organ, primary endothelioma of the lymphatic vessels, and in cases of mucoid forms of peritoneal mesothelioma—false myxoma of the abdominal cavity.