Correlated muscle and nerve development in the bullfrog cutaneous pectoris

The development of the cutaneous pectoris muscle was studied and compared with the differentiation of its peripheral nerve in bullfrog (Rana catesbeiana) tadpoles and frogs by light and electron microscopic techniques. This muscle preparation was chosen for this study because it possesses a number of advantages for (and has become a model system for) the study of correlated nerve-muscle development. At the earliest stage examined (stage XI) the presumptive muscle did not contain any contractile or morphologically distinguishable myotubes, but was contacted by the well-defined cutaneous pectoris nerve trunk. Myotubes were present at stage XII, the same time that nerve-associated acetylcholine receptor aggregations and nerve-evoked muscle contractions were first observed. The adult number of axons was present in the cutaneous pectoris nerve at stage XII, but no axons were myelinated. Gradually thereafter, the number of muscle fibers increased and the cutaneous pectoris axons became myelinated. By stages XX and XXI, but prior to metamorphic climax (stage XXV), the adult numbers of cutaneous pectoris muscle fibers and myelinated and unmyelinated nerve fibers were present. These numbers did not change significantly between stages XX and XXI, through metamorphosis, and in the adult, even during the period of the most rapid loss of multiple innervation in the first 2 weeks after metamorphosis. These results show that the nerve was present and in contact with the cutaneous pectoris muscle from the earliest stages of development prior to muscle differentiation, at a time when the muscle was a disorganized mass of undifferentiated cells. Such early contact suggests that the nerve may have a significant influence on muscle maturation.