Muscle differentiation and growth are accomplished by two fundamental sets of cellular processes, protein accretion and cell proliferation. This review is restricted to a discussion of the role of cell proliferation in the growth of muscle during prenatal and postnatal life. More specifically, the discussion is divided into three topic areas, which include the role of muscle precursor cell proliferation in prenatal and postnatal development and a review of factors that may be regulating the proliferation of myogenic cells. The proliferation of embryonic myogenic cells and their differentiation into multinucleated fibers, as well as the proliferation of myogenic ceils in postnatal muscle (satellite cells) are major factors determining the ultimate mass of muscle that can be produced by an animal.
Paired sides from U.S. Choice grade beef were aged immediately after slaughter at 2 and 16°C. Samples were removed from longissimus and semitendinosus at slaughter and at 1, 3 and 7 days postmortem for ATPase assay, phase microscopy, shear and organoleptic evaluation. Rib steaks from sides aged at 16°C for 1‐day postmortem were as tender as steaks from sides aged at 2°C for 7 days postmortem. Flavor development of rib steaks also was more rapid at 16°C than at 2°C. Tenderness of semitendinosus steaks was improved by aging sides at 16°C; the difference in improvement of tenderness of semitendinosus, however, was not as great between 2°nd 16° as it was for rib steaks. Ca++, Mg++ and EGTA‐modified ATPase activity of myofibrils from both muscles increased with postmortem time, with myofibrils from muscles held at 16°C having slightly higher ATPase activity than myofibrils from muscles held at 2° Increased EGTA‐modified ATPase activity was indicative of loss of calcium sensitivity of the myofibril. Sarcomeres of myofibrils from longissimus were longer at 1‐day postmortem than those from at‐death longissimus and they remained essentially unchanged during the remainder of postmortem aging; however, tenderness improved at 16°C for 1 day and at 2°C for 3 days. Also greater fragmentation of myofibrils from longissimus postmortem aged at 16°C for 1 day and at 2°C for 3 days was observed, suggesting that the rate of myofibril fragmentation is an important factor in tenderization.
Tryptophan loading of rats resulted in a continuous non-linear uptake of L-tryptophan from plasma into the brain. The optimum tryptophan load for increasing cerebral 5-hydroxytryptamine (5-HT) level was 25 mg/kg. Above this, there was a gradual decrease both in the levels and synthesis of 5-HT and 5hydroxyindoleacetic acid (5-HIAA) as assessed from simultaneous intraperitoneal or intraventricular injections of ~['~Cltryptophan. A 5-10 fold increase in cerebral tryptophan produced a limited stimulation of 5-HT synthesis. When the cerebral tryptophan level reached 1 x lo-' M, substrate inhibition in uiuo of the tryptophan monooxygenase (tryptophan-5-hydroxylase) but not of the indoleamine-2,3-dioxygenase occurred. Cerebral synthesis of kynurenine increased linearly with increasing tryptophan load. At a plasma ratio of 50:l tryptophan to kynurenine, tryptophan loading interfered with the entry of peripheral kynurenine. Tryptophan loading also increased the efflux of 5-hydroxyindoles from the brain. One hour after intraperitoneal injection of L-kynurenine sulfate (5 mg/kg) into rats, there was a shift in the plasma ratio of Ltryptophan to L-kynurenine to 4:l. In these rats, a 20% reduction of cerebral tryptophan was noted.
Viable, mononucleated ceils were liberated from minced hind limb skeletal muscle of mice by incubation with .1% pronase at 37 C for 1 hour. Mice immediately after birth, at the most rapid growth stage and near maximum body weight were utilized as the source of skeletal muscle. Cell suspensions were placed in cell culture under conditions that permitted evaluation of several general properties of the cell population. The total number of viable cells/ gram of muscle decreased sharply during growth to a minimum value of approximately 2 x 104 cells/gram. The composition of this total cell population was evaluated in cell culture by cloning the cells at low density. Each colony of cells originating from a single cell was evaluated microscopically after 3 weeks in culture to determine if it was myogenic as evidenced by the presence of multinucleated myotubes. The percentage of myogenic clones was 26.2%, 23.7% and 17.0% for neonatal, rapidly growing and young adult mice muscle, respectively. Minimum values for the quantity of satellite cells/gram of muscle and the number of satellite cells/hind limb were calculated at each of the three ages. Both the concentration of satellite cells and the total quantity of satellite cells in Michigan Agricultural Experiment Station Journal Article No. 8298. hind limb muscle declined during growth. The combination of cell culture and cloning is useful for obtaining a measure of changes in viable satellite cell number under different conditions; however, a number of limitations are inherent and are emphasized. (
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