The relative contribution of increases in fiber area to stretch-induced muscle enlargement was evaluated in the slow tonic fibers of the anterior latissimus dorsi of adult Japanese quails. A weight corresponding to 10% of the bird's body mass was attached to one wing. Thirty days of stretch in 34 birds averaged 171.8 +/- 13.5% increase in muscle mass and 23.5 +/- 0.8% increase in muscle fiber length. The volume density of noncontractile tissue increased in middle and distal regions of stretch-enlarged muscles. Mean fiber cross-sectional area increased 56.7 +/- 12.3% in the midregion of stretched muscles. Further analysis indicated slow beta-fiber hypertrophy occurred in proximal, middle, and distal regions; however, fast alpha-type fiber hypertrophy was limited to middle regions of stretched muscles. Stretched muscles had a significant increase in the frequency of slow beta-fibers that were less than 500 microns 2 in all regions and fast alpha-type fibers in middle and distal regions. Total fiber number was determined after nitric acid digestion of connective tissue in 10 birds. Fiber number increased 51.8 +/- 19.4% in stretched muscle. These results are the first to clearly show that muscle fiber proliferation contributes substantially to adult skeletal muscle stretch-induced enlargement, although we do not know whether the responses of the slow tonic anterior latissimus dorsi might be similar or different from mammalian twitch muscle.
The contributions of fiber hypertrophy and new fiber formation to the onset of stretch-induced muscle enlargement were evaluated in the anterior latissimus dorsi (ALD) of adult Japanese quails, because it was not known whether the mechanisms which initiate new fiber formation were dependent on first achieving significant fiber hypertrophy. A weight corresponding to 10% of the bird's body mass was attached to one wing, and eight birds were killed after each day during the first week of stretch. Muscle mass was significantly increased after 48 h of stretch; however, the elevation in nonmuscle tissue accounted for this increase. Muscle mass corrected for non-muscle tissue was significantly greater than the intra-animal control by the fourth day of stretch. Mean fiber cross-sectional area did not change during days 0-6, but cross-sectional area was 30.0 +/- 17.2% greater than the intra-animal control areas at day 7. Fiber number determined after nitric acid digestion of connective tissue was 27.1 +/- 5.8% greater than the intra-animal control at days 5-7 of stretch, but the number of fibers in the control muscles at days 5 and 6 were lower than at day 0. Thus new fiber formation was not preceded by significant fiber hypertrophy. These results fail to support a mechanism for new fiber formation which involves fiber splitting from hypertrophied myofibers during the first week of stretch.
Satellite cell activity was examined in the stretch-enlarge anterior latissimus dorsi muscle (ALD) of the adult quail. Thirty-seven birds had a weight equal to 10% of their body mass attached to one wing while the contralateral wing served as an intra-animal control. At various time intervals after application of the wing weight (from 1 to 30 days), the birds were injected with tritiated thymidine and killed 1 h later. Stretched muscle length was greater by day 1 and mass by day 3 when compared with the contralateral muscle. Satellite cells actively synthesizing DNA were quantitated in fiber segments of the control and stretched ALD. A minimum of 1,500 muscle nuclei (satellite cell nuclei and myonuclei) were counted in each muscle. Labeling in stretched muscle was expressed by the percent labeled nuclei per total nuclei counted. Satellite cell labeling was initiated by day 1, peaked between days 3 and 7, and was not statistically different from control values at day 30. These results demonstrate that satellite cells are induced to enter the cell cycle in the stretch-enlarged ALD muscle from the adult quail, and the peak of proliferative activity is within the first week of stretch.
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