Skeletal muscle cells grown in tissue culture have been used over the past 70 years to study the processes of muscle differentiation and development and their regulation by the nervous system. Although in vitro studies have advanced our understanding of these processes, many important in vivo developmental influences are lost when cells are put into a static, artificial in vitro environment. For instance, uninnervated skeletal myotubes differentiated in vitro are of an embryonic type and never go on to develop into fibers with adult characteristics (e.g. slow or fast fiber types). The purpose of this project is to develop an entirely new in vitro system to grow skeletal muscle cells under conditions more comparable to those found in vivo. Passive and active tensions are known to be important in vivo for skeletal muscle organogenesis and healthy growth during embyrogenesis and postnatal development. We will develop an in vitro system which places developing skeletal muscle cells under different patterns of passive tension, simulating those which occur in vivo. We will study the effects of this activity on (1) muscle morphogenesis by ultrastructural microscopic techniques; (2) muscle growth characteristics determined by protein and DNA accumulation, rates of total protein and myosin heavy chain synthesis and degradation, and specific proteinase (cathepsin) activities; and (3) myosin light and heavy chain gene expression using specific monoclonal antibodies. These studies should advance our knowledge of the forces which regulate skeletal muscle differentiation and growth, and better delineate the mechanism by which the nervous system regulates skeletal muscle fiber properties. They may also increase our understanding of the developmental defects which occur in congenital limb malformations and the muscular dystrophies.