Myofibril

Myofibrils (obsolete term: sarcostyles) are cylindrical organelles, found within muscle cellss. They are bundles of filaments that run from one end of the cell to the other and are attached to the cell surface membrane at each end. The filaments of myofibrils, myofilaments, consist of 2 types, thick and thin. Thin filaments consist primarily of the protein, actin; thick filaments consist primarily of the protein, myosin. In striated muscle, such as skeletal and cardiac muscle, the actin and myosin filaments each have a specific and constant length on the order of a few micrometers, far less than the length of the elongated muscle cell (a few millimeters in the case of human skeletal muscle cells). The filaments are organized into repeated subunits along the length of the myofibril. These subunits are called sarcomeres. The muscle cell is nearly filled with myofibrils running parallel to each other on the long axis of the cell. The sarcomeric subunits of one myofibril are in nearly perfect alignment with those of the myofibrils next to it. This alignment gives rise to certain optical properties which cause the cell to appear striped or striated. In smooth muscle cells, this alignment is absent. Hence there are no apparent striations and the cells are called "smooth".

The photo below is a high power picture (electron micrograph) of a small region of a skeletal muscle cell. The long axis of the cell is indicated by the direction of the RED double arrow. That arrow begins and ends on the boundaries of a sarcomere. The GREEN arrows demarcate the width of 2 adjacent, parallel myofibrils. The LIGHT BLUE arrow, labeled TK, indicates the length of a portion of a sarcomere made up of thick filaments, which can be seen running in the long axis of the myofibril. The thin filaments extend from the dark boundary of a sarcomere into the region occupied by the thick filaments. They interdigitate with the thick filaments there. In this photo, thin filaments do not show up well. They are in the white or clear band between the sarcomere boundary and the region occupied by the thick filaments, but continue into the region where you can see the thick filaments.

[(Where should this be placed?) An A band is a dark region of striations in a myofibril.]