The cells of the myotome, the mononucleated myoblasts, elongate in a direction parallel to the long axis of the embryo (see Plate 1-17) and undergo repeated mitotic divisions, subsequently fusing with each other to form syncytia. Each syncytium becomes a tube with continuous cytoplasm, and the numerous nuclei within it are centrally located. The process is similar to the formation of the tubular cardiac muscle fiber except that in the latter each centrally located nucleus is within a separate cell.

The syncytial myotubes of skeletal muscle become muscle fibers as myofilaments of actin and myosin are laid down within the cytoplasm. The thin actin and the thick myosin polypeptide myofilaments become strung out parallel to the long axis of the fiber and are arranged in a side-by-side, interdigitating relationship so that they can slide past each other to cause muscle contraction. The cross-banded, or striated, appearance of skeletal muscle at the microscopic level reflects this relationship between the two types of submicroscopic filaments. The myofilaments group together into numerous longitudinal bundles known as myofibrils, which occupy the bulk of the fiber; the nuclei and nearly all the mitochondria are relocated to the periphery, where they are in contact with the outer membrane of the fiber, the sarcolemma (see Plate 1-17).

The myotube stage of fiber formation begins at about the fifth week. Subsequent generations of myotubes develop from the persisting population of myoblasts found in close relationship to muscle fibers. The nuclei of the muscle fibers themselves, once in place, do not divide mitotically or amitotically; consequently, in order to increase their number, the incorporation of new myoblasts into the syncytia is required, especially when the fibers grow in length.

The growth of skeletal muscles is the result of an increase in both the number of muscle fibers and the size of the individual fibers. The greatest increase in the number of fibers occurs before birth, after which time both the number and size of the fibers increase. In the male, there is a fourteen-fold increase in fiber number from 2 months to age 16, with a rapid spurt at age 2, and a maximum rate of increase from ages 10 to 16, during which time the fibers double in number. There is also a steady linear increase in the size of muscle fibers from infancy to adolescence and beyond in the male. In the female, the increase in fiber number is more linear than in the male, with an overall tenfold postnatal increase. However, in the female, the increase in fiber size is more rapid than in the male after age 3½, reaching a plateau at age 10½. After age 14½, fiber size in males exceeds that in females. Fiber numbers increase steadily in both sexes up to about age 50, after which there is a steady decline.

Muscle fibers are very fine threads, up to 30 cm in length but less than 0.1 mm in width, which contract to about 57% of their resting length. Only the largest muscle fibers in an adult would be visible to the naked eye if they could be individually excised. Muscles will develop completely in the absence of an innervation that is due to a congenital nervous system abnormality. Thus, nerves do not supply a necessary organizing stimulus, and gross muscle morphogenesis will go to completion with function never having occurred. However, a muscle that never had a nerve supply does not attain its full differentiation at the fiber level and disappears with time.