Original Article: Electron Microscopic Radioautographic Studies on the RNA Synthesis in the Colonic Epithelial Cells of Aging Mice

For the purpose of studying the aging changes of macromolecular synthesis in the colonic cells of experimental animals, we studied 10 groups of aging mice during aging from fetal day 19 to postnatal month 24. They were injected with 3 H-uridine, a precursor for RNA synthesis, sacrificed and the colonic 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 colonic epithelial cells in respective aging groups were analyzed qualitatively. The number of mitochondria per cell, the number of labeled mitochondria with silver grains in each cell in respective aging groups were analyzed qualitatively and quantitatively in relation to the aging of animals. The results revealed that the RNA synthesis as expressed by the number of silver grains in cell nuclei, cell organelles, changed with the aging of animals. It was demonstrated that the number of mitochondria increased from embryonic day 19 to postnatal newborn day 1, 3, 7, 14, adult month 1 and 2, reaching the maximum, then decreased to senile year 1 to 2. On the other hand, the number of labeled mitochondria showing RNA synthesis at various ages increased from embryonic day 19 to postnatal newborn day 1, 3, 7, 14, adult month 1 and 2, reaching the maximum and decreased slightly to senile month 6 to senile year 1 and 2. To the contrary, the labeling index increased from embryonic day 19 to postnatal day 1, 2 and 3, then decreased to day 14 and month 1, and increased again to month 6, 12 and month 24, indicating the aging changes. These results demonstrated that intramitochondrial RNA synthesis in the colonic epithelial cells increased, then decreased and again increased due to aging of individual animals depending upon the cellular activities at respective aging stages. Based upon our findings, available literatures on macromolecular synthesis in mitochondria of various cells are reviewed.