Histochemical profiles of fibers in the rat tibialis anterior muscle during early postnatal development.

The enzyme activities of intra- and extrafusal fibers in the tibialis anterior muscle of rats during postnatal development have been investigated. Muscle fibers 1 day after birth showed a uniform reaction for adenosine triphosphatase (ATPase), succinate dehydrogenase (SDH), and alpha-glycerophosphate dehydrogenase (alpha-GPD) activities. Fast-twitch (F) and slow-twitch (S) fibers with ATPase activity were found at 9 and 11 days. Thereafter, the type shift of muscle fibers from S to F was observed in the deep and middle portions. Fast-twitch oxidative glycolytic (FOG), fast-twitch glycolytic (FG), and slow-twitch oxidative (SO) fibers with ATPase, SDH, and alpha-GPD activities were found at 15 (the superficial portion) and 17 days (the deep and middle portions). The histochemical differentiation of intrafusal muscle fibers (7 and 9 days) was found earlier than that of extrafusal muscle fibers.