Postnatal muscle fiber assembly: localization of newly synthesized myofibrillar proteins.

Addition of newly synthesized contractile proteins to striated muscle during postnatal life was studied by labeling young rat diaphragms in vitro with (3)H-leucine and extracting the soluble proteins in glycerol. The grain distribution over glycerinated fibers in electron microscopic autoradiographs indicates that new myofibrillar proteins are added about the periphery of existing fibrils.