A quantitative study of satellite cells and myonuclei in stretched avian slow tonic muscle

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Abstract
Satellite cell frequency was assessed in control and stretched anterior latissimus dorsi (ALD) muscles of adult quail. A weight equal to 10% of body mass was attached to one wing for time intervals ranging from 1–30 days. The contralateral ALD served as the intra‐animal control. Satellite cell frequency, expressed as a percentage of total myofiber nuclei within the basal lamina, was determined in eight control and stretched ALD muscles, that had been weighted for 5, 7, or 10 days. Satellite cell frequency was determined in 584 control and 473 stretched fibers and was not different in the control or stretched ALD muscles (15.6 ± 2.3%, 16.7 ± 6.1%, respectively). The number of myofiber nuclei (myonuclei and satellite cell nuclei) was examined in whole fiber segments from control and stretched ALD muscles of 27 adult quail. Nuclear frequency was determined in 500 control and 1,200 stretched fiber segments. Fiber volume was calculated from fiber length and diameter measurements. Nuclear number normalized to 10,000 μm3 fiber volume was correlated to fiber cross‐sectional area (P < 0.0001). Fibers with cross‐sectional areas less than 500 μm2 had a greater nuclear to fiber volume ratio compared to fibers with areas greater than 500 μm2. The nuclear‐to‐cytoplasmic ratio was not constant in smaller fibers. Nuclear density decreased as fiber cross‐sectional area increased up to 500 μm2. Fibers with cross‐sectional areas greater than 500 μm2 demonstrated a constant nuclear‐to‐cytoplasmic ratio. The results demonstrate that absolute nuclear number increased to maintain a constant nuclear‐to‐cytoplasmic ratio in the stretched hypertrophied fibers. Daughter cells originating from activated satellite cells may have contributed to the myonuclear population to maintain a constant nuclear‐to‐cytoplasmic ratio in the hypertrophied fibers of the adult quail.