Myofibrils

Myofibrils are the contractile units of muscle cells and are responsible for muscle contraction. They are composed of repeating units called sarcomeres, which give them a striped appearance. These alternating dark and light bands on myofibrils are responsible for the striated (striped) appearance of skeletal muscle tissue.

Sarcomeres

Sarcomeres are the functional units of myofibrils and are responsible for muscle contraction. They are composed of thick and thin filaments that slide past each other during muscle contraction. These filaments are arranged in a repeating pattern, which gives rise to the alternating dark and light bands seen on myofibrils.

Dark and Light Bands

The dark bands on myofibrils are called A bands, and they contain thick filaments made up of the protein myosin. The A band appears dark because the thick filaments overlap with thin filaments from neighboring sarcomeres.

The light bands on myofibrils are called I bands, and they contain thin filaments made up of the protein actin. The I band appears light because it contains only thin filaments and does not overlap with thick filaments from neighboring sarcomeres.

Striated Appearance

The alternating dark and light bands on myofibrils create a striated (striped) appearance when observed under a microscope. This striation is a characteristic feature of skeletal muscle tissue. The arrangement of thick and thin filaments within sarcomeres gives rise to this distinct pattern.

Other Structures

- Fascicles: Fascicles are bundles of muscle fibers within a muscle. They do not have the alternating dark and light bands seen on myofibrils.
- Sarcolemma: Sarcolemma is the cell membrane of muscle fibers. It does not show the alternating dark and light bands.
- Fascia: Fascia is a connective tissue that surrounds and separates muscles, but it does not have the alternating dark and light bands seen on myofibrils.

Therefore, the correct answer is option 'D' - Myofibrils, as they are the structures that exhibit the alternating dark and light bands characteristic of striated muscle tissue.