Regeneration of cat posterior temporalis muscle in culture

Alteration in myosatellite cell commitment with muscle maturation

Myosatellite cells are myoblasts found between the basal lamina and sarcolemma of myofibers of postnatal mice. The extent to which these cells are programmed, upon differentiation, to express isoforms of contractile protein genes specific to the type of fiber with which they are associated has been evaluated in vitro using myosatellite cells derived from the soleus and the extensor digitorum longus muscles (EDL) of 4-day-old and adult transgenic mice, which express nuclear localizing beta-galactosidase (nlsbeta-gal) under the control of the promoter and 3' enhancer of the gene encoding fast myosin light chain 3F (MLC3F) (Kelly et al. [1995] J. Cell Biol. 129:383-396). Cultures were allowed to differentiate either as myocytes (mononucleated cells), to prevent possible modification of the myosatellite phenotype by other myonuclei in mosaic myotubes, or as myotubes. Transgene expression was age related, with 90% and 70% of the myocytes derived from the neonatal EDL and soleus muscles (muscles that had not yet achieved their mature phenotype), respectively, having nuclei encoding beta-gal; 61% and 32% of the myocyte nuclei derived from myosatellite cells of the adult EDL (a fast muscle) and the adult soleus muscle (a mixed muscle containing many slow myofibers), respectively, expressed this transgene. Because myosatellite cells found in adult muscles are the progeny of those found in the neonate, an alteration of myosatellite cell commitment to express this transgene occurs with muscle maturation. Because expression of the transgene in neonatal and adult muscle in vivo reflects the expression of the endogenous MLC3F gene (Kelly et al. [1995] J. Cell Biol. 129:383-396), it is likely that expression of the transgene by differentiated myosatellite cells reflects the extent of commitment of these cells to produce MLC3F. A hypothesis is presented that MLC3F is widely expressed in developing muscles but eliminated in myofibers that undergo maturation toward a slower phenotype.

Expression of superfast myosin in aneural regenerates of cat jaw muscle

We investigated whether innervation is necessary for the expression of superfast myosin in regenerating cat jaw-closing muscle. Strips of jaw muscle were permitted to regenerate bilaterally in the beds of a fast limb muscle with innervation on one side being prevented surgically. Immunocytochemical analyses using anti-myosin heavy chain antibodies were done at various times postoperatively, the latest being after 78 days. We found little difference between innervated and uninnervated regenerates up to 27 days postoperatively. All regenerating myotubes expressed fetal myosin. In addition, some myotubes stained for superfast or slow myosin, while others stained for both superfast and slow myosins. Subsequently, uninnervated myotubes became atrophic but continued to express fetal, slow, and superfast myosins while innervated myofibers suppressed fetal and slow myosin expression. These results are consistent with the notion that satellite cells of jaw-closing muscles are committed to express superfast myosin during myogenesis, and that the expression of this program is independent of innervation.