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.

Infusion of lamb carcasses with 0.3M CaClz resulted in acceleration of postmortem tenderization process. Control and treated animals had similar cathepsin B, H, and L activities. Various control groups had similar CDP-I, -II and inhibitor activities, whereas these were all decreased in CaClz infused animals. Hence, it was concluded that the activation of Ca?+-dependent proteases was responsible for the observed postmortem proteolysis and tendcrization and, it seems unlikely that activity of these cathepsins is related to postmortem tenderization under conditions used in this experiment.

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Clonally derived cultures of porcine skeletal muscle satellite cells were developed. The mitogenic effects of basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), insulin-like growth factors (IGF-I and -II), and platelet-derived growth factors (PDGF-AA and -BB) were examined. Individually, bFGF, IGFs, and PDGF-BB stimulated proliferation of porcine satellite cells grown in basal serum-free medium or Minimum Essential Medium containing 2% fetal bovine serum (MEM-2% FBS). EGF stimulated proliferation in MEM-2% FBS, but neither EGF nor PDGF-AA were mitogenic when added to serum-free medium. The interactions among bFGF, EGF, IGF-I, and PDGF-BB were examined in serum-free medium, using growth factor concentrations shown in dose-response experiments to induce maximal proliferative responses (10 ng/ml bFGF, EGF and PDGF-BB, and 50 ng/ml IGF-I). The combination of bFGF and IGF-I dramatically increased proliferation, and IGF-I also synergized with EGF to increase proliferation. EGF, IGF-I, and bFGF interacted with PDGF-BB to stimulate proliferation. With the exception of EGF and bFGF, combinations of two growth factors typically resulted in greater than additive responses. Simultaneous exposure of satellite cells to bFGF, PDGF-BB, EGF, and IGF-I produced a fivefold increase in DNA compared to cells grown in basal serum-free medium. Elimination of EGF did not reduce the mitogenic response, yet removal of IGF-I, bFGF, or PDGF-BB reduced proliferation by approximately 40, 20, and 10%, respectively. These mitogens are likely physiological regulators of porcine satellite cell activity.