PHENOTYPE OF ADULT-MOUSE MUSCLE MYOBLASTS REFLECTS THEIR FIBER-TYPE OF ORIGIN

Phenotypic diversity among mature skeletal muscle fibers originates fr om muscle progenitor cells, primary and secondary myoblasts, each of w hich is intrinsically committed to express a characteristic complement of developmentally regulated myosin heavy chain genes when differenti ated. Similarly, postnatal muscle myoblasts, the satellite cells nestl ing beneath basement membranes of mature skeletal muscle fibers, have been shown to exhibit diversity, related to whether the muscle in whic h they reside is of a slow, fast or superfast type. Here we analyzed t his association in more detail, evaluating the myosin heavy chain gene expression in immature muscle fibers (myotubes) formed in vitro from satellite cells extracted from isolated, living, single muscle-fibers of mature murine muscle. We identified a population of satellite cells that form myotubes expressing type I(slow) myosin heavy chain and fou nd this population to be preferentially associated with individual slo w muscle-fibers. These results not only confirm diversity among mammal ian satellite cells, but also demonstrate that the phenotype of satell ite cells is indicative of the type of fiber from which they derive.