This is what the myotubes of regenerating mammalian skeletal (striated) muscle look like under the microscope. They closely resemble the myotubes found in embryonic muscle. 
Notice the numerous, closely packed and centrally positioned nuclei. In adult mammalian muscle fibers (fiber = cell in this context) the nuclei are situated at the periphery, just under what is called the sarcolemma -- a strongly reinforced, highly specialized membrane-shell on the cell boundary. The nuclei migrate to the periphery as differentiation progresses.
Myofibrils, the cellular organs that house the contractile apparatus, although not visible in this particular section, are present at this stage (6-day); they contain the proteins directly responsible for muscle's lengthening and shortening; and, with better optics, exhibit the cross-striations (fig. 5) from which skeletal muscle takes its alternative names, "striated" and "striped" muscle. Anatomists use "skeletal muscle" to avoid ambiguity: Muscle of the heart wall (cardiac) also happens to be striped.
Myotubes virtually burst upon the microscopic scene on the fourth day after injury; the synthesis of actomyosin, the contractile protein supercomplex in the myofibril, is vigorous (biochemically) during day five. Sensivity to the antibiotic actinomycin D (an indicator of transcription) is greatest during the third day; i. e., in the interval just prior to the first wave of myotubes.
Pietsch and McCollister (unpublished) tracked the arrival on the micrscopic scene of typical myotubes at one-hour intervals after 90 hours, post-injury; they detected a few typical myotubes (in contradistinction to nonlinear syncytia) at 94 hours; the structures in question became numerous at 96 hours and their numbers seemed to plateau between then and 98 hours. Subsequent to the initial waves of myotubes, the structures arrived on the scene in spurts. The data in question were not quantified nor subjected to statistical analysis, and therefore not previously published. However, one thing is quite clear: in the early rounds of muscle regeneration the several events (replication, transcription and translation) leading up to the first wave of myotubes are highly synchronized; it would be very easy, therefore, to miss a critical moment in the design of an experiment . Some published data (see Pietsch and McCollister, 1965) suggested that the synchrony during the first waves exceeds 90 percent; that it decays after the advent of the first round of events, and that subsequent waves overlap to make later regeneration appear random.
ADULT HUMAN SKELETAL (STRIATED) MUSCLE
