A STUDY OF ALTERATIONS OF PLEURAL STRUCTURE BY GROWTH,DEVELOPMENT AND AGING.

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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 ultra-structural alterations during normal aging in the visceral pleura of male mice aged 1 day, 1 wk, 2 wks, 4 wks, 3 mos, 6 mos, 12 mos, 18 mos, 24 mos, and 30 mos, by both scanning and transmission electron microscopy. Surface microvilli (SMV) of mesothelial cells appeared sparse at day 1 (density=D : 18.2/10mum^2), then gradually increased in density and reached a plateau at approximately 18 mos (D : 331.9) when the mesothelial cells appeared bumpy. The length of the SMV changed in parallel with density (day 1=0.23 mm, 1 mos=2.27). After 24 mos, the SMV was partially loose (D=241.3), with an irregular and thin appearance (length : 2.35mum). The morphological properties of pleural connective tissues, including collagen, elastin, and fibroblasts, changed in parallel with the changes in mesothelial cells. Although SMV do not have any known functions in the pleura, they changed with age in a manner that likely corresponded to the changes in pleural connective tissue as well as structural changes in lung parenchyma during the aging process.