Fig. 5. M-lines are expressed early during in vivo development and continuously in organotypic culture of extraocular muscle (EOM). The presence or absence of a structural M-line was evaluated in EOMs of rats between the ages of E17 and P45. A representative electron photomicrograph illustrating the presence of an M-line in E17 eye muscle (A) is shown here. M-line morphology was also assessed in EOM grown in organotypic co-culture with oculomotor motoneurons after 20 days (B), 28 days (C) and 35 days (D) in vitro. M-lines were present in newborn and prenatal EOM and in organotypic co-cultures at all stages. M-lines are indicated by arrows or by the letter M; arrowheads denote Z-lines.