Comparative rates of decline in the primary cloning efficiencies of smooth muscle cells from the aging thoracic aorta of two murine species of contrasting maximum life span potentials.

Primary cloning assays of thoracic aortic smooth muscle cells from an F1 hybrid strain of Mus musculus demonstrated linear regressions of replicative potentials as functions of donor age (6-30 months), with regression coefficients, in two independent cohorts, of -1.69 +/- 0.27 (SE) and -1.88 +/- 0.19 (SE) clones per milligram wet weight of intima-media per month and correlation coefficients of -0.83 and -0.92 (P less than 0.001). Secondary cloning (dilute plating from first passages) also demonstrated a high negative correlation (r = -0.90) between donor age and cloning efficiency, thus implicating intrinsic differences in cell populations. Comparable primary cloning assays on the aortas of aging cohorts of Peromyscus leucopus, a murine species with a maximum life span potential approximately twice that of Mus musculus, yielded about twice the number of clonable smooth muscle cells per unit weight; the rate of decline with age was slightly but significantly greater (P less than 0.01). Electron-microscopic studies revealed cellular alterations confined to the first subintimal layer of aortas from mice (Mus musculus) 18 months and older.