Muscle Disease.

Skeletal muscle begins to develop in the mesodermic somites at 2-3 weeks' gestation. At 4-5 gestational weeks (GW), the myotomes develop as a syncytium of nuclei and cytoplasm. At 7-9 GW, the syncytium elongates, and cells fuse and form the primitive myotubes. These are multinucleated cells with nuclei situated centrally in a hollow tube of thin cytoplasm. This process continues, with the newest myotubes being the smallest. Actin and myosin filaments develop and form myofibrils in the myotubes. At 10 GW, the embryonic muscle is well formed into myotubes. The muscle fibers are larger, and striations are evident. At 12 GW, motor end plates and muscle spindles are developing. At 13 GW, the myofibrils become thicker in the older fibers, and new smaller myotubes continue to develop. At 15 GW, two populations of muscle fibers are present: (1) large, more mature fibers with peripheral nuclei and filled-in cytoplasm and (2) smaller, more immature myotubes. After 16 GW, all of the muscle fibers continue to increase in size, and the myotubes gradually disappear. However, the pattern of large and small muscle fibers persists into the postnatal period and into the second year of life. The [ABSTRACT FROM AUTHOR]