M-Line

The M-line is a protein-rich structure at the center of a sarcomere, the repeating contractile unit of striated skeletal and cardiac muscle. It lies in the middle of the A band, within the central region occupied by thick, myosin-containing filaments. Its main structural role is to link and organize neighboring thick filaments, helping maintain their orderly arrangement within the sarcomere .^1,2

Proteins of the myomesin family are major structural components of this region. Myomesin molecules interact with myosin and with titin, the giant protein that extends from the Z-disc toward the center of the sarcomere. Through these connections, the M-line is integrated with both the thick-filament lattice and the structural framework of the sarcomere .²

The terms M-line and M-band are sometimes used broadly for the same central sarcomeric region, although they can describe different levels of structure. Electron microscopy shows that the wider M-band can contain several narrow, electron-dense M-lines, including a central M1 line and additional lines on either side. Their prominence and protein composition can vary among muscle types .²

The M-line is distinct from the Z-disc. Both help organize sarcomeric filaments, but the M-line is located at the sarcomere center and primarily organizes thick filaments, whereas Z-discs form the sarcomere boundaries and anchor thin, actin-containing filaments .^1,2