Development of myofibrils in the gizzard of chicken embryos. Intracellular distribution of structural proteins and development of contractility.

The intracellular distributions of major muscle proteins, myosin, actin, tropomyosin, alpha-actinin, and desmin, in smooth muscle cells of chicken gizzard at various stages of embryogenesis were investigated by immunofluorescence-labeling of enzyme-dispersed cells cultured up to three hours. These muscle proteins, except some part of myosin, were organized into fibrous structures as soon as synthesis and accumulation of proteins started. As for myosin, a considerable amount of it was dispersed in soluble cytoplasm as well. On the other hand, Ca++-dependent contractility was detected with detergent-extracted myoblasts and glycerinated tissue from embryos older than 7 days. Although the nascent myofibrils bear a resemblance to "stress fibers," the former could be distinguished from the latter by their high stability in dispersed, spherical cells. The above findings, therefore, show that the synthesis of contractile proteins is followed by immediate assembly of them into functional myofibrils without undergoing any intermediate structure. Based on these findings, the mechanism of myofibril formation in developing smooth muscle cells is discussed.