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As skeletal muscle mass and strength decline with age, known as sarcopenia, the satellite cell population decreases in absolute number as well as a percentage of nuclei in skeletal muscle tissue. The rate of growth rapidly decelerates in late stages of adulthood as adipose tissue gradually accumulates more fat when energy intake exceeds expenditure in the body.
Biology for bodybuilders full#
Skeletal muscle growth occurs dramatically during adolescence, along with physical skeletal development for the full functional capacity of musculoskeletal system. 45 Extrinsic cues have been shown to regulate satellite cell activation, including hepatocyte growth factor and neuronal nitric oxide synthase, fibroblast growth factors (FGFs), and FGF receptor 1. 43,44 Intrinsic activation of satellite cells is controlled by proximal signals from the myofiber microenvironment, inflammatory responses, and microvasculature. 42 Myogenin and MRF4 are expressed in cells when they start differentiation. In mouse skeletal muscles, activated satellite cells (satellite cells entering the cell cycle) first express either Myf5 or MyoD following the coexpression of Myf5 and MyoD. Quiescent satellite cells do not express MRFs. The MRF expressions during satellite cell activation, proliferation, and differentiation are analogous to their expressions manifested during the embryonic formation of myofiber. A reserve pool of satellite cell is retained for future use after muscle regeneration by the asymmetrical division of satellite cells, in which one daughter cell returns to quiescence and replenish the satellite cell pool, while the other daughter cell enters the differentiation to form myofiber. Muscle satellite cell is functional as adult stem cell. 34 Satellite cells are normally in quiescent state in adult muscle, become activated to proliferate in response to injury, and give rise to more satellite cells and contribute to muscle growth or regeneration. In a healthy adult, satellite cells represent ~ 2–7% of nuclei within skeletal muscle. The contribution of satellite cells to muscle growth or buildup decreases as animals and humans age. Muscle satellite cells can account for about 30% of the nuclei associated with muscle growth during early postnatal growth. The source of nuclei is strongly believed to come from muscle satellite cells, which are quiescent mononucleated cells, located between the sarcolemma and basement membrane of muscle fibers. By adding nuclei to the existing myofiber, the cell will maintain appropriate nuclear to cytoplasmic ratio for efficient protein synthesis and utilization. One of the main purposes of the recruitment of new nuclei is to maintain constant demand of protein synthesis. Muscle growth is characterized by continuous recruitment of nuclei to the existing fiber through the period of postnatal growth. Muscle fiber is a unique cell with multiple nuclei. This relationship of muscle fiber number and growth potential has been demonstrated in the enlarged muscles of double-muscled cattle, in genetically different sizes of animals, and in runts as compared with normal pigs. 40,41 Due to the limitation of fiber size increase, the growth potential for skeletal muscle is virtually determined by the number of fibers established at around the time of birth.
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39 Others argued that the increase in fiber number during the first days of postnatal life was a result of maturation and elongation of the existing myotubes rather than due to a production of new fibers. For example, the increase in muscle fiber number was observed shortly after birth in rodents.
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35–38 However, other studies indicate that limited extent of muscle cell proliferation occurs after birth in some species. 35 For example, there is no significant change in postnatal fiber number in mice, rat, pig, and cattle. After birth, the number of skeletal muscle fiber in most mammals and avian species does not increase due to the completion of embryonic proliferation of skeletal muscle cells. The enlargement of muscle tissue during postnatal growth is believed to be the consequence of increase in satellite cell activities, which fuse to adjacent muscle fibers to increase their sizes. The muscle fiber is formed by myoblast fusion, which needs continuous supply of nuclei.
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Postnatal muscle growth mainly results from muscle hypertrophy through the increase in muscle fiber length and girth. The number of muscle fiber is determined before birth. Muscle growth is determined by an increase in muscle fiber number (hyperplasia) as well as an increase in muscle fiber size (hypertrophy). Jinzeng Yang, in Progress in Molecular Biology and Translational Science, 2014 5 Postnatal Development of Skeletal Muscle