what are the boundaries of each sarcomere?
<answer> Dark lines called Z-lines are the boundaries of each sarcomere. <context> Thin and thick filaments are organized into repeating units called sarcomeres. Dark lines called Z-lines mark off the boundaries of each sarcomere. Thin filaments attach to the Z-lines at both ends of the sarcomere, while thick filaments are unattached. Each myofibril contains thousands of sarcomeres.