Developmental aspects of the innervation of skeletal muscle fibers in the chick embryo.

The M. complexus in the chick, commonly called the hatching muscle, undergoes conspicuous growth during the latter stages of embryonic development. Myogenesis of this muscle was compared to that of M. biceps femoris with regard to development of types of muscle fiber and their innervation. In both muscles beta fibers are of relatively uniform size and show little growth in diameter between 12 days of development and hatching; alpha fibers develop continuously and display a wide range of diameters at all stages. Initial thickenings on the sarcolemma of beta fibers where axons are closely approximate were first observed at 10 days of development in both muscles. In both muscles beta fibers are innervated prior to alpha fibers. Terminal axon networks bridge intercellular spaces and contact beta fibers in different myogenic clusters. alpha fibers that develop on the surface membrane of beta fibers exhibit focal thickenings of the membrane and some cell projections that are directed toward axon-beta fiber contacts. These changes occurred only in alpha fibers of M. complexus. At 14 days of embryogenesis, the processes of synaptogenesis and of myelin formation are less advanced in M. biceps femoris than in M. complexus. At this stage alpha fibers were observed to be innervated in M. complexus, but not yet in M. biceps femoris. Each beta fiber was observed to be encircled by several preterminal axons. It is concluded that the earlier development of M. complexus is correlated with an equally early development of nerve-muscle interactions.