Timing and sequence of the events in the development of extraocular muscles in staged human embryos: ultrastructural and histochemical study.

The ultrastructure and the appearance of glycogen were studied in the extraocular muscles of 14 externally normal human embryos (Carnegie stages 13-21). At stage 16, myofibrils with an immature Z line and glycogen granules appeared in the cytoplasm of the myoblast. The myoblasts came into cluster at stage 18, and fusion between the myotubes was observed at stage 20. At this stage, an M line appeared in the myofibrils. At stage 21, an A band with a Z line and an H band with an M line were observed, the sarcoplasmic reticulum appeared in the cytoplasm of the muscle fibers and glycogen increased in volume in the cytoplasm. In the previous study, we showed that the muscle-specific isoenzymes, such as creatine kinase, beta-enolase and glycogen phosphorylase, appeared from stage 18 to 20 in the extraocular muscles. The previous findings and the present results suggest that the fusion of the muscle cells occurs in the period when some molecular markers of muscle differentiation are expressed in vivo.