Changes in creatine kinase and its isoenzymes in human fetal muscle during development.

The total creatine kinase activity and its isoenzyme patterns were investigated in the skeletal muscle of 87 fetuses, of which 80 were presumed normal, 5 were anencephalic and 2 were "at risk" for Duchenne muscular dystrophy (DMD). No differences, either in total enzyme activity or in the isoenzyme distributions, were found between the anencephalic fetuses or those at risk for DMD, when compared to normal fetuses of similar gestation. Creatine kinase activity was found to rise steadily throughout embryonic life. During fetal development, the isoenzyme pattern in skeletal muscle was observed to change from the initial prevalence of the brain (BB) type, to the predominance of the muscle (MM) form. The most pronounced change occurred between the 6th and the 16th week of gestation, a period characterized by the rapid fusion of myoblasts to form myotubes and the concomitant production of myofibrils. It is proposed that there is a close association between the creatine kinase isoenzymes spectrum and the stage of muscle development.