What are myofibrils primarily composed of?

Myofibrils are primarily composed of thick and thin filaments. These filaments are essential for muscle contraction and are organized into repeating units called sarcomeres, which are the functional contractile units of muscles. The thick filaments are primarily made up of myosin, while the thin filaments consist mainly of actin, along with regulatory proteins like troponin and tropomyosin. This arrangement allows for the sliding filament mechanism of muscle contraction, where the interaction between thick and thin filaments enables muscle fibers to shorten and generate force.

The other choices, while containing components relevant to muscle and connective tissue, do not accurately describe the primary composition of myofibrils. Collagen and elastin are components of connective tissue, while skeletal fibers refer more broadly to the muscle fibers that encompass myofibrils rather than constituting them directly.