Presence of embryonic myosin in normal postural muscles of the adult rat

Indirect immunofluorescence was used to localize embryonic myosin heavy chains in soleus, adductor longus, tibialis anterior, plantaris, and extensor digitorum longus muscles of 6-month-old rats. A monoclonal antibody (2B6), specifically recognizing rat embryonic myosin, was applied to unfixed, transverse, frozen sections. The number of embryonic myosin-positive (EMP) extrafusal fibers was expressed as a percentage of the total number of fibers. EMP extrafusal fibers were only seen in the soleus and adductor longus muscles, both postural muscles. Approximately 1% of the soleus muscle fibers appeared positively stained for embryonic myosin. The majority of such fibers had a small diameter (<500 ), appeared intensely fluorescent, and typically contained central nuclei. Re-expression of embryonic myosin due to spontaneous fiber denervation is not a likely factor in this study, since alpha-bungarotoxin and N-CAM localization were restricted to the motor end-plate region of EMP fibers. Since embryonic myosin was shown to disappear in all normal-sized myofibers by 2 to 3 months of age, the results suggest that the EMP extrafusal fibers seen in postural muscles of 6 to 12-month-old animals are regenerating myofibers. We speculate that a small number of muscle fibers may be regenerating in normal, adult postural muscles, in response to fiber damage possibly caused by excessive recruitment or overloading.