M line appearance in developing striated muscle.

Myofilament assembly occurs in a definite sequence. Myofibrils first appear within embryonic myotomes as non-striated, linear arrangements of parallel thick and thin myofilaments (crude sarcomeres) with periodic dense cross bands (Z lines). In the center of sarcomeres within these early myofibrils, faint M lines are often detected. In older embryos, after typical cross striations became apparent, the M lines can be detected bisecting each A band.