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The cell cycle time of satellite cells in growing rats was determined to be approximately 32 hr, with an S-phase of 14 hr. The estimated cycle time was the same for satellite cells in both oxidative soleus and glycolytic EDL muscles and is consistent with the rate at which myonuclei are produced during growth. Continuous infusion of bromodeoxyuridine (BrdU) was used to determine if all satellite cells had the same cycle time in vivo. Approximately 80% of the satellite cell population was readily labeled over the first 5 days of continuous infusion. Remaining satellite cells accumulated label at a much slower rate and were still not completely saturated after an additional 9 days of infusion. Only a small portion of the cells labeled with BrdU during the first 5 days could be labeled with a second label ([3H]thymidine) during tandem continuous infusion experiments, suggesting that they pass through a limited number of mitotic divisions prior to fusion. These results suggest that satellite cells in growing oxidative and glycolytic skeletal muscles can be subdivided into two distinct compartments. About 80% divide with a 32-hr cell cycle duration and are responsible chiefly for providing myonuclei to growing fibers. The remaining 20% of the cells divide more slowly, probably because the cells enter a G(0)-phase between mitotic divisions. These reserve cells, through asymmetric divisions, may generate the myonuclei-producing satellite cell population. Proliferative potential for regeneration and adaptive responses is likely located in this reserve population.

The absolute number of satellite cells (SC) in young and adult rat skeletal muscle was estimated by correlating data from light and electron microscopic quantitation of myofiber nuclei (myonuclei and satellite cell nuclei) with biochemical assays for determining total muscle DNA content. Expressed both as a proportion of total myofiber nuclei and as absolute numbers, satellite cells are more numerous in the predominantly oxidative soleus muscle than in the mixed glycolytic/oxidative extensor digitorum longus (EDL) muscle at 1, 12, and 24 months of age. Satellite cell proportions decline gradually in both muscles with advancing age, but absolute numbers increase significantly in the soleus between 1 and 12 months, while EDL exhibits a continuous significant decline in satellite cells between each of the ages examined.

Normal adult mouse tibialis anterior muscles were perfused continuously with 3H-thymidine for nine days. Quantitation of the satellite cell population in these muscles reveals that not only is the frequency of satellite cell nuclei low, but that those present are mitotically quiescent.