Insulin-like growth factor I stimulates myoblast expansion and myofiber development in the limb

Insulin-like growth factor I (IGF-I) is expressed in the anterior and posterior mesodermal cells of the developing limb. However, a definite role for IGF-I during early limb organogenesis is unknown. To determine the inherent participation of IGF-I during limb organ development, a retroviral delivery system (RCAS) was used to overexpress IGF-I throughout the developing hind limb of stage 24 chicken embryos. The area of the belly of the external gastrocnemius muscle in the IGF-I infected limb was an average of 160, 90, 70, and 80% larger than the contralateral control muscle belly, 4, 5, 6, and 7 days postinjection, respectively (all differences P < 0.01). In comparison to the contralateral control muscles, there were a significantly greater number of muscle fibers in the IGF-I infected muscles (P < 0.05), confirming that the majority of IGF-I-mediated muscle enlargement was due to an increase in total fiber numbers (hyperplasia). Four days postinjection, there was a 32% increase in myoblast to myofiber ratio in the muscle of injected limbs compared with the muscle in the contralateral noninjected control limbs (P < 0.05). This result demonstrates that IGF-I acts to expand the undifferentiated myoblast population, and as a result, more myofibers subsequently develop, and the muscles expressing ectopic IGF-I are enlarged by means of hyperplasia. There was no difference in tibiotarsus and fibula length or diameter between the IGF-I injected and control limb, suggesting that ectopic IGF-I expression within the mesoderm was not a nonspecific growth stimulant of all tissues of the developing limb, but specifically enhanced skeletal muscle development and growth. Ectopic IGF-I expression had no significant effect on myostatin mRNA concentrations. Our results support a model where mesodermally expressed IGF-I acts to regulate the number of primary myofibers, and, therefore, size of skeletal muscles, which form during the initial events of limb myogenesis.