[Biochemical and histochemical study of type IIC muscle fiber]

Fiber type differentiation of human skeletal muscle starts at approximately 18-28 weeks of gestation, and normally completes at birth. Therefore type IIC muscle fibers are not visualized in the biopsied specimens except for some pathological conditions, such as Werdning-Hoffmann disease or Duchenne muscular dystrophy of which onsets are believed in the embryonal or neonatal period and some motor neuron disease. On the other hand in rat muscle fibers have not differentiated in the neonatal period, and most of them show reaction of type IIC muscle fiber. When we study the specificity of type IIC fiber, rat is the most useful animal. The present study aimed biochemical and histochemical analysis of type IIC fiber in soleus, gastrocnemius and extensor digitorum longus (EDL) at neonatal period, 10 days, 3 weeks, 4 weeks and adult. The ratios triglyceride to total protein were in soleus 0.038 (neonatal), 0.252 (10 days), 0.239 (3 weeks), 0.229 (4 weeks) and 0.145 (adults), in gastrocnemius 0.054, 0.241, 0.118, 0.077, and 0.078, in EDL 0.026, 0.094, 0.071, 0.071 and 0.071. Whereas, the ratios glycogen to total protein were in soleus 0.024 (neonatal), 0.017 (10 days), 0.22 (adults), in gastrocnemius 0.054, 0.025, 0.022, and in EDL 0.016, 0.011, 0.014. Also there was significant difference in the fatty acid composition of triglyceride between neonatal and 10 days in each muscle. These results suggested that the energy metabolism in neonatal muscle depends on anaerobic glycolysis rather than the oxidative phosphorylation and also suggested that, though histochemical characteristics remain the same, biochemical properties change during the course of muscle development.