Myofribril

Myofibril is a cylindrical organelle, found within muscle cells. It is any of the threadlike fibrils that make up the contractile part of a striated muscle fiber. It is also called sarcostyle. It is a bundle of actomyosin filaments that run from one end of the cell to the other and are attached to the cell surface membrane at each end. Actomyosin motors are important in muscle contraction as well as other processes like retraction of membrane blebs, filiopod retraction, and uropodium advancement.

The myofibrils have alternate light and dark bands, which contain protein filaments responsible for the muscle's contractile ability and give the muscle its typical striped appearance under the microscope. The filaments of myofibrils, myofilaments, consist of two types, thick and thin.

Thin filaments consist primarily of the protein actin, coiled with nebulin filaments. Thick filaments consist primarily of the protein myosin, held in place by titin filaments. The protein complex composed of actin and myosin is sometimes referred to as actomyosin.