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 P10 (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 subsarcolemmal 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-actinin 3 and nebulin expression between P10 and P15 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.
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