ULTRASTRUCTURE OF DEVELOPING MUSCLE IN THE UPPER LIMBS OF THE HUMAN EMBRYO AND FETUS

Background: The ultrastructure of the myogenesis, which proceeds along with the appearance of muscle-specific proteins and isozymes, has not been fully described in the upper limb of staged human embryos. Metho ds: Eight human embryos (Carnegie stage 14-22) and two fetuses (11 and 12 weeks of gestation) were fixed with 5% glutaraldehyde, 4% paraform aldehyde, and 0.2% picric acid in 0.1 M phosphate buffer, pH 7.2. The upper limbs were dissected out and processed for transmission electron microscopy, and sections of the biceps brachii muscle were cut and ex amined. Results: At stage 14, the myoblasts were loosely scattered in the ventral proximal region of the upper limb bud and had a small amou nt of cytoplasm with a few intracellular organelles. At stage 16, the myoblasts were spindle shaped and oriented parallel to the axis of the upper limb bud. These cells had irregularly shaped nuclei with promin ent nucleoli, rough endoplasmic reticulum (ER), and mitochondria, but no myofilaments were observed. At stages 17-19, rough ER, free ribosom es, and mitochondria increased in number and thick and thin filaments with faint Z-lines appeared in the peripheral cytoplasm of the myotube . The plasma membranes of some neighboring myotubes were continuous, s uggesting that these cells were in the initial stages of the fusion pr ocess. At stage 22, the striated pattern of the myofilaments became ev ident and tubular structures appeared around them and near the plasma membrane. In the fetus at the 11th week, the basal lamina began to sur round the myotubes, and T-tubules with sarcoplasmic reticulum were obs erved. Dyads and triads were observed in the myotube of the 12th week fetus. Conclusion: These findings suggest that rapid myogenesis occurs during the late embryonic period in human upper limbs and that the ul trastructural characteristics of mature myotubes are established durin g the early fetal period. (C) 1995 Wiley-Liss, Inc.