The histochemistry of very small muscle fibres in growing skeletal muscles.

Serial sections of biceps femoris muscles from 10 rapidly growing pigs were reacted for succinate dehydrogenase (SDH) and myofibrillar adenosine triphosphatase (ATPase) and were stained with silver to delineate the endomysial boundaries of their muscle fibres. The histochemistry of very small fibres (less than 0.001 mm2) was similar to that of surrounding fibres with a normal diameter. Of the small fibres, 71.5% had strong ATPase, 27.5% had weak ATPase, 22% had strong SDH, 23.8% had intermediate SDH and 54.1% had weak SDH reactions. Corresponding values for surrounding fibres with a normal diameter were 87.9% with strong ATPase, 11.8% with weak ATPase, 35.1% with strong SDH, 14.5% with intermediate SDH, and 50.5% with weak SDH reactions. An appreciable number of small fibres were histochemically unrelated to any of their surrounding fibres: 11.0% for ATPase, 12.8% for SDH, and 5.5% for both ATPase and SDH. The cross-sectional shapes of small fibres were similar to those of their surrounding fibres. It was concluded that these small fibres were probably the tapered ends of intrafascicularly terminating muscle fibres rather than new muscle fibres formed by splitting.