Final answer:
Z-discs are the dense structures that anchor thin filaments, while M lines hold together thick filaments in muscle fibers.
Step-by-step explanation:
Thick filaments and thin filaments are the two main types of filaments in muscle fibers. The dense structures that anchor thin filaments are called Z-discs. These Z-discs attach to the ends of the thin filaments and help maintain the structural arrangement of the myofibril. Additionally, M lines hold together the thick filaments at the center of the sarcomere.