An immunocytochemical marker for early type I muscle fibers in the developing rat hindlimb.

Muscle fibers develop sequentially from several generations of myotubes that express specific isoforms of myosin heavy chain (MHC). We observed that the chicken-derived monoclonal antibody (mAb) S46 binds to myotubes of the fetal rat hindlimb in a specific temporal and spatial pattern. To determine the type and fate of the S46-reactive myotubes, we immunoreacted sections of fetal, neonatal and postnatal hindlimb muscles to this antibody. The mAb S46 bound to a subpopulation of primary myotubes in the tibialis anterior, and to all primary and slow/fast secondary myotubes in the soleus muscle. The S46-reactive primary myotubes represented the oldest set of myotubes in the muscles. Reactivity to S46 was present from the earliest stages of muscle development, peaked in the late fetal period, and dissipated in the first postnatal week, suggesting that mAb S46 binds to a developmental form of slow myosin. The regional distribution of myotubes that bound S46 in fetal muscles was identical to the distribution of type I (slow-twitch) fibers in the adult, indicating that S46-reactive myotubes ultimately develop into type I extrafusal fibers. Thus, mAb S46 can be used as a marker for prospective type I extrafusal fibers in the rat hindlimb.