[A morphological study to elucidate the differences in visceral pleura in young and old mice]

The pleura is not only a mechanical envelope for the lung but also represents a crossroad for the exchange of cells and fluids. We studied ultrastructural differences in the visceral pleura of male mice in a young age group (aged 1 day, 1 week (wk), 2 wks) and an old age group (10 months (mos), 16 mos, 18 mos, 28 mos, and 30 mos), by both scanning and transmission electron microscopy. Surface microvilli (SMV) of mesothelial cells appeared to be sparse at day 1, then gradually increased in density and reached a plateau at approximately 10 mos. The length of the SMV changed in parallel with density which was confirmed by a morphometric study. After 28 mos, the SMV was partially loose, with an irregular and thin appearance. The morphological properties of pleural connective tissues, including collagen, elastin, and fibroblasts, changed in morphology with the changes in mesothelial cells; the elastic layer beneath mesothelial cells formed at 2wks and was maintained until 28 mos; collagen bundles increased in volume density throughout life, whereas cellular components including mesothelial cells, fibroblasts, and alveolar epithelial cells became atrophic with aging. From these observations, we concluded that although SMV do not have any known functions in the pleura, they change with age in a manner that likely corresponds to the changes in pleural connective tissue as well as structural changes in lung parenchyma.