What structures primarily compose muscle cells?

Muscle cells, also known as myocytes, are primarily composed of myofibrils. Myofibrils are long, thread-like structures that run the length of the muscle cell and play a crucial role in muscle contraction. They are organized into units called sarcomeres, which are the basic functional units of muscle tissue. Sarcomeres contain various proteins, including actin and myosin, that interact to produce the force required for muscle contraction.

The presence of myofibrils made up of sarcomeres is essential for the contractile function of muscle cells, distinguishing them from other cell types that do not have this specialized structure. Moreover, this arrangement allows for the striated appearance of skeletal and cardiac muscle under a microscope, highlighting the organized nature of these muscle tissues.