Fig. 6. Extraocular muscle (EOM) M-line expression is not preserved in dark-reared rats. Dark rearing is known to alter eye movements and to delay development of EOM-specific traits. We raised newborn rats either in complete darkness (DR) or in a 12 h:12 h light:dark cycle (con). M-line morphology was evaluated by electron microscopy. At P14 (A) and P56 (C), EOMs of normally reared rats lack M-lines. M-lines were not preserved in rats dark reared for 14 days (B) or 56 days (D).