The relation of age to the incorporation of tritiated thymidine into the thymus of Fischer rats

The influence of age on the wound healing of experimental myocardial infarction in rats

In 6-weeks-old, 5-months-old and 21-months-old rats myocardial infarction was induced by coronary artery ligature. After performing the ligature the animals were administered 3H-thymidine, 3H-proline or 35S-sulphate at different times. The following parameters were determined: number of DNA- and tropocollagen-synthesizing connective tissue cells at the infarction border and at the infarction site; mean silver grain density above the nuclei or cells; duration of a cycle; number of mitoses, and incorporation of radioactive sulphate at the infarction site. In addition, the labelling and mitotic indices as well as the percentage of the standard deviation from the mean values were estimated. The following results were obtained: 1. The rate of granulation tissue formation in the necrotic zone is determined by the mitotic activity of the cells. With advancing age the cell cycles are being prolonged which results in retardation of wound healing. 2. The disturbed DNA-replication in old age is not associated with a time shift in the occurrence of the mitotic and labelling peaks. 3. With advancing age the number of fibroblasts synthesizing collagen precursors decreases. There exists no age-dependence of the 3H-proline incorporation rate, of the intracellular transport, of the synthesis of collagen precursor and of the release of labelled tropocollagen. In all age-groups under study these processes last approximately 4 hours. 4. Collagen fibre formation is accompanied by an increased synthesis of acid mucopolysaccharides. In infarction callosities the content of acid mucopolysaccharides mostly is constant. 5. The proliferating endothelial cells have a pronounced metabolic activity and a markedly short generation time.