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Myostatin (GDF-8) inhibits the activation, proliferation, and differentiation of myogenic satellite cells. The relative importance of this growth factor is demonstrated in myostatin-null mice and cattle possessing defective myostatin genes. These defects result in greatly enhanced musculature. In the present study, the effect of myostatin on chicken myogenic satellite cells derived from two different skeletal muscles was studied. The effect of anti-myostatin antibodies on cellular responses was also examined. Satellite cells isolated from the pectoralis major (PM) muscles were more responsive to the proliferation depressing effects of myostatin compared to cells from the biceps femoris (BF; P or=0.05). Myostatin administered to proliferating cells depressed the synthesis of decorin (P or= 0.05). Administration of anti-myostatin antibodies to proliferating cultures increased cell proliferation by 6-7% over 3 days. There was no effect on differentiation of either PM or BF cells. The present study demonstrates that there are differences in the responsiveness to myostatin of chicken satellite cells derived from different muscles. Evidence is also given to support the role of endogenous myostatin in autocrine regulation of muscle growth.

Although satellite cell differentiation is involved in postnatal myogenesis from growth to posttrauma regeneration, the early stages of this process remain unclear. This study investigated pHuDes-nls-lacZ transgene activity, as revealed by X-gal staining and the accumulation of MyoD, myogenin, endogenous desmin, and myosin, in order to determine whether satellite cells share the same activation program during growth and regeneration. After birth, skeletal myonuclei in which myogenin expression was limited were briefly characterized by transgene activity. Satellite cells were only evidenced by MyoD and slow myosin accumulation, but failed to initiate transgene expression. After freeze trauma, satellite cell activation led to MyoD, myogenin, and desmin expression. Subsequently, when myosin expression occurred, transgene activation was apparent in regenerating structures, with more intense X-gal staining in mononucleated cells than regenerating myotubes. After the second week posttrauma, only desmin and myogenin expression were maintained in regenerating structures. In culture, the behavior of satellite cells showed that desmin expression was committed before transgene activation occurred, i.e., concurrently with MyoD, myogenin, myosin expression, and the first fusion events. Quantitative analysis confirmed the discrepancy between endogenous desmin and transgene expression and demonstrated the close correlation between transgene activation and the fusion index. Our results strongly suggest that satellite cells promote distinct pathways of myogenic response during growth and regeneration., (Copyright 1998 Academic Press.)