Population of muscle fibers during postnatal development in the rat hindlimb.

Changes in the numbers of muscle fibers during the postnatal development (1, 7, 14 and 21 days old) of rat EDL and SOL muscles were studied. Quantitative analysis of fibers was performed using cryosectioned specimens. The 1 micrometer-thick sections were taken from epon-embedded blocks of these muscle, and light micrographs were obtained. Thereafter, the same 1 micrometer-thick specimens were embedded again for EM studies. Many small cells, which could not be confirmed as myofibers at the light microscopic level, were identified as skeletal muscle fibers with certainty by electron microscopy. The population of muscle fibers apparently increased by ca. 45% in the 7-day-old rat and by ca. 67% in the 21-day-old rat, when compared with 1-day-old rat EDL and SOL muscles. This number increased gradually during postnatal development over several weeks. It was suggested that a small cell (myofiber) adjacent to a large myofiber with adherent junctions and with a common basement membrane develops into a real muscle fiber.