Skeletal myogenesis is a precise procedure marked by specific changes in muscle cell morphology and cytoarchitecture. Cessation of proliferation by skeletal muscle precursor cells (myoblasts) coincides with the induction of fusion to form multinucleated myotubes and the initiation of differentiation, the process through which sarcomeres are formed. Concurrently, there is a distinct upregulation in expression of muscle-specific isoforms and an extreme downregulation of non-muscle-specific cytoskeletal isoforms. The sarcomere is the contractile unit of the cell and is comprised of a number of different proteins aggregated and aligned in very ordered arrays along the myotube. It is this rigorously controlled alignment that gives striated muscle its characteristic "striped" appearance. Previous studies, conducted predominantly in cardiac muscle, propose models for the development of the sarcomere that attribute little of the differentiative process to the myoblast morphology and cytoskeletal arrangement. In this study, perturbation of myoblast morphology and cytoskeletal arrangement by transfection with nonmuscle actin genes in the mouse skeletal muscle cell line C2 resulted in myotubes of both varied morphology and sarcomeric structure. The results presented herein not only provide novel insights into the formation of the sarcomere in skeletal muscle, but also suggest a role for myoblast morphology and cytoskeletal structure in the subsequent differentiation of the myotube. (C) 2003 Elsevier Inc. All rights reserved.