Electronmicroscopic observations on the visceral and parietal rat's pleura after contralateral pneumonectomy.

The mechanism of compensatory growth and healing of the pleura remains unresolved. Contralateral visceral and parietal (diaphragmatic and costal) pleura were investigated by transmission electron microscopy, following an experimental pneumonectomy (EP). Fifteen young-adult Wistar rats were divided into three groups and with survival times of 1, 5 and 8 days respectively after EP. Three sham-operated (thoracic cavity opened and closed) and three unoperated rats served as controls. One day following EP the superficial mesothelial cells have more microvilli and microvesicles, but a lower number of specialized contacts. Multiplication of extravasal cells leads to an increase of the thickness of the layer over the basal lamina and of the submesothelial layer. Five days after EP the superficial cells show a stratified arrangement in longer sectors of both pleural sheets. Along with typical mesothelial cells there are three new populations of cells: (1) with an abundant granular endoplasmic reticulum and secretory granules, (2) with fibroblast-like characteristics and (3) with a more extensive lysosomal system. The submesothelial layer is thickened due to newly formed blood vessels and collagen bundles. Eight days after EP the mesothelial cells build multi-row arrangement sectors and surround intercellular dilatations covered with microvilli. 'Activated' high mesothelial cells characterize the monolayer sectors. The submesothelial layer remains thicker due to larger collagen bundles and elastic fibers. The changes in the mesothelium and in the connective tissue layer suggest the existence of two periods. The first one is characterized by different mesothelial cell populations, new vasculogenesis and starting of fibrillogenesis. In the second period there are 'activated' mesothelial cells, pleural villi, groups of lymphatic lacunae and significant fibrillogenesis.