Ultrastructural observations on the human visceral pleura.

Visceral pleura with underlying lung parenchyma was obtained following thoracotomy carried out in 21 patients of both sexes, aged 27-56 years. The samples were taken from unaltered lung regions, and were studied using transmission electron microscopy (TEM). The main components of the visceral pleural tissue complex, mesothelium, basal lamina (BL), elastic membrane (EM), and submesothelial connective tissue layer, are described, with special reference to their relations with the corresponding structures of the lung interstitium. These elements have significant quantitative regional diversities but can be recognized in all investigated cases, and show constant structural uniformity. Two basic mesothelial cell types, squamous (flat) and cuboidal (high) are discerned, based on the differences in size, shape, cell organelles and membrane specialization. The cuboidal mesothelial cells are especially characteristic for the visceral pleura. In addition, an intermediate mesothelial cell that apparently represents a stage in the development of a squamous to a cubiodal mesothelial cell is fairly common. For more rarely, degenerative mesothelial cells are found.