Differences in normal structure and reaction to adjuvant between the costal and the visceral pleura

Visceral and costal pleurae of the rabbit were studied by light and electron microscopy in the normal state, after India ink injection into the pleural cavity and in pleuritis induced by intrapleurally injected adjuvant. In normal rabbits, the mesothelial cells in the visceral pleura had more numerous microvilli and pinocytotic vesicles than in the costal pleura. The surface coat on the microvilli was much thicker in the former than in the latter. Intrapleurally injected India ink particles were phagocytosed by mesothelial cells and macrophages. The phagocytotic activity was more prominent on the costal side than on the visceral side. In rabbits with adjuvant induced pleuritis, the visceral pleura, but not the costal pleura, showed mushroom-like projections on the pleural surface which were composed of a fibrin mass mixed with phagocytotic macrophages and covered by proliferative mesothelial cells. These hitherto poorly known structures, which were formed, though less conspicuously, also after India ink administration, seemed to play an important role in the initial protection of the lung against foreign body invasions from the pleural cavity. With the progress of pleuritis, the mushroom-like projections changed into patchy granulation tissues which were well localized and did not exceed the elastic layer. In the costal pleura, apparently due to a poorer protective mechanism than in the visceral pleura, granulation tissue was formed much more extensively and diffusely. No signs of transformation of mesothelial cells into fibroblasts were recognized. Fat-containing cells closely resembling the Ito cells in the liver occurred in the submesothelial and subpleural layers of the visceral pleura.