For the purpose of studying the aging changes of macromolecular synthesis in the tracheal epithelial cells of experimental animals, we studied 10 groups of aging mice during development and aging from fetal day 19 to postnatal month 24. They were injected with 3H-uridine, a precursor for RNA synthesis, sacrificed and the tracheal tissues were taken out, fixed and processed for light and electron microscopic radioautography. On many radioautograms the localization of silver grains demonstrating RNA synthesis in tracheal epithelial cells in respective aging groups were analyzed qualitatively and quantitatively. The number of mitochondria per cell, the number of labeled mitochondria with silver grains and the number of silver grains in each cell in respective aging groups were analyzed quantitatively in relation to the aging of animals. The results revealed that the RNA synthetic activity as expressed by the incorporations of RNA precursor, i.e., the number of silver grains in cell nuclei, cell organelles, changed due to the aging of animals. The number of mitochondria, the number of labeled mitochondria and the mitochondrial labeling index labeled with silver grains were counted in each tracheal epithelial cell. It was demonstrated that the number of mitochondria increased from embryonic day 19 to postnatal newborn day 1, 3, 9, 14, adult month 1, 2, reaching the maximum and kept plateau, while the number of labeled mitochondria increased from embryonic day 19 to postnatal adult month 2, reaching the maximum but the labeling indices showing RNA synthesis increased from embryonic day 19 to postnatal newborn day 14 and adult month 1 and decreased to year 2, indicating the aging changes. Based upon our findings, available literature on macromolecular synthesis in mitochondria of various cells are reviewed
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