Microscopic Study of the Lungs of Sprague-Dawley Rats from Various Ages of Postnatal and Aging Periods

Physiological changes in postnatal and aging lung are associated with a variety of microscopic changes in the lung, especially the alveolar lung tissue, both in the interstitial and epithelial component. Interstitial tissue of the lung will increase in thickness that is supposed to be due to changes in fiber composition, particularly collagen. However, the exact changes are still under debate and the underlying process is still unclear. The epithelial component that experiences changes is type II alveolar cells or pneumocyte II (surfactant producing cells). The ratio of pneumocyte II against pneumocyte I is expected to decline with age. This decrease will certainly affect their function in maintaining pulmonary surfactant supply. To maintain normal vital functions and synthesis of surfactant, lung tissue is also dependent on the availability of glucose because glucose is the fundamental building blocks of glycerol backbone of surfactant. In the aging process, accumulation of glycogen in the brain, skeletal muscle and kidney has been reported, so glycogen is supposed as a marker of cell aging (senescence cells). Nevertheless, studies on glycogen accumulation in lung tissue in aging have not been reported. This study aims to observe the microscopic changes in lung tissue, both in interstitial tissue and alveolar epithelial cell (pneumocyte II) in postnatal and aging Sprague-Dawley rats. This was an observational analytical study to determine the correlation between age and the amount of interstitial collagen fibers, the thickness of the interstitial tissue, glycoproteins and glycogen accumulation in the interstitial component, the ratio of pneumocyte II and I and accumulation of glycogen in pneumocyte II. Correlation tests were performed with Spearman correlation test (p≥0.05) using SPSS 17 for Windows. The study was conducted on Sprague-Dawley rats aged 2 days (n = 6), 4 days (n = 6), 10 days (n = 6), 16 days (n = 6), 3-4 months (n = 6) and more than 12 months (n = 6). Lung tissue was cut with a thickness of 5 µm and stained with Mason’s trichrome to visualize collagen and with periodic acid Schiff to visualize glycoproteins and glycogen. There was a low correlation between age and proportion of interstitial tissue (r = 0.291), the accumulation of glycoproteins in the interstitial (r = 0.300) and accumulation of glycogen in pneumocyte II (r = 266). There was a moderate correlation between age and the amount of interstitial collagen fibers (r = 0.687), but there was no correlation between age and pneumocyte II/I ratio (r = 0.123). With increasing age, the interstitial lung tissue would increase in thickness, collagen fibers and glycoproteins and glycogen accumulation. While in the lung epithelium, no evidence of decline in the ratio of pneumocyte II/I, but there is an increasing accumulation of glycogen in pneumocyte II.