Fig. 1. Expression of slow MyHC2 in slow MA and fast PM muscle fibers. ED13 myoblasts from MA and PM muscles were incubated for 7 days. On the third day of incubation, some of the cultures received ED5 spinal cord explants to provide innervation. On day 4 of incubation, medium in some of the cultures was supplemented with 100 µM ryanodine. Muscle fibers were immunostained with F59 and S58 mAbs to detect fast MyHCs and slow MyHC2, respectively, followed by fluorochrome conjugated secondary antibodies. Slow MyHC2 was detected in innervated MA muscle fibers and in innervated PM muscle fibers cultured in medium containing 100 µM ryanodine.