DIFFERENTIATION OF RED AND WHITE FIBERS IN MUSCLE FROM FETAL, NEONATAL AND INFANT RHESUS MONKEYS

The hydroxyproline (collagen) content of muscle in terms of wet weight was lowest at 90 days gestational age (rhesus monkey) and increased until birth. Histologic techniques also demonstrated a progressive increase in staining density for connective tissue with increasing fetal age. However, when expressed in terms of nitrogen content of muscle, the hydroxyproline concentrations were similar from the 90-day fetal through the 2-6-week infant series, and were consistently higher in rapidly growing as compared to adult muscle. The succinic dehydrogenase activity (soleus), on the basis of wet weight or of nitrogen, was lowest in the 90-day fetal series and reached a plateau by 120 days. The histochemical data indicated an increase in staining density for succinic dehydrogenase in muscle as the fetus matured. By 90 days gestational age, the hydroxyproline concentration of red muscle was lower as compared to that of white muscle, and this difference was also apparent histologically. The succinic dehydrogenase activity was higher in red as compared to white muscle at all ages, and histochemical differentiation was apparent at 120 days fetal age.