What structure in skeletal muscle is responsible for the appearance of striations?

The appearance of striations in skeletal muscle is primarily due to the organization of sarcomeres. Sarcomeres are the basic contractile units of muscle fibers and consist of alternating thick and thin filaments, which are made up of the proteins myosin and actin, respectively. This organized structure creates a banding pattern of light and dark regions when viewed under a microscope, leading to the characteristic striations seen in skeletal muscle.

The arrangement of myofibrils, which are composed of many sarcomeres lined up in series, contributes to the overall striated appearance but does not independently cause the striations. While muscle fibers (the cells themselves) encompass many myofibrils and organize them into a functional unit, the distinct banding pattern stems from the internal organization of their sarcomeres. Fascicles are larger bundles of muscle fibers, which do not directly influence the striated pattern observed at the microscopic level. Hence, it is the sarcomeres that are crucial for the visible striations in skeletal muscle.