These data demonstrate that significant changes occur in the EOMs from P10 to P15 and suggest that visual stimulation may play a role in the signals that regulate both nmMyH IIB and EO-MyHC developmental transitions. The pronounced distinctions of the orbital and global layers occurring by P15 establish the adult morphologic phenotype of the muscle.

Extraocular muscles are a unique subset of striated muscles. During postnatal development, the extraocular muscles undergo a number of myosin isoform transitions that occur between postnatal day 10 (P10) and P15. These include: 1) loss of embryonic myosin from the global layer resulting in the expression restricted to the orbital layer; 2) the onset of expression of extraocular myosin and the putative tonic myosin (myh 7b/14); and 3) the redistribution of nonmuscle myosin IIB from a subsarcolemma position to a sarcomeric distribution in the slow fibers of the global layer. For this study, we examined the postnatal appearance and distribution of α-actinin, tropomyosin, and nebulin isoforms during postnatal development of the rat extraocular muscles. Although sarcomeric α-actinin is detectable from birth, α-actinin 3 appears around P15. Both tropomyosin-1 and -2 are present from birth in the same distribution as in the adult animal. The expression of nebulin was monitored by gel electrophoresis and western blots. At P5–10, nebulin exhibits a lower molecular mass than observed P15 and later during postnatal development. The changes in α-actinin3 and nebulin expression between P10 and 15 coincide with transitions in myosin isoforms as detailed above. These data point to P10–P15 as the critical period for the maturation of the extraocular muscles, coinciding with eyelid opening.