Why are fetal muscles slow?

Abstract

Differentiating fast and slow mammalian muscles contract slowly at birth and increase their speed during the 1st few weeks of life. Only small proportions of slow myosin light chains are found in early developing muscles and the fast type of light chains predominate. Differentiating muscle contains unique, embryonic forms of myosin which may partially determine the early slow responses. Additional reasons for these slow twitch times are suggested. Most skeletal muscles are initially formed from a small population of primary generation cells which are innervated by pioneering axons early in myogenesis. Subsequently, numerous secondary generation cells develop along the walls of primary myotubes, then separate and become independent units of contraction. Using affinity-purified antibodies to fast and slow [rat] myosin, it was found that most primary myotubes react with anti-slow myosin and are destined to become slow, type I fibers. Secondary generation cells stain exclusively with anti-fast myosin and develop into type II, fast fibers. Primary myotubes may constitute the fundamental motor units of the developing neuromuscular system. They may be responsible for early slow movements. Secondary generation cells become organized into large, fast motor units later in development, eclipsing the original slow response.