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CA McMullen, FH Andrade; Missing Cytoskeletal Elements in Rat Extraocular Muscle . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1457.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: M-lines are structural landmarks in cardiac and skeletal muscles that provide stability to the sarcomeres and anchoring sites for certain metabolic enzymes. Although the M-lines are particularly well developed in active, fast contracting muscles, they appear to be absent in rat extraocular muscles (EOMs). In consequence, we tested the hypothesis that rat EOMs do not express myomesin 1 and 2, the main cytoskeletal elements of M-lines in other skeletal muscles. Methods: Perfusion-fixed EOMs and diaphragm bundles, and fresh EOMs, diaphragm bundles and whole lateral omohyoid muscles were isolated from 45-day old male Sprague-Dawley rats. Fixed muscles were postfixed in 1% osmium tetroxide, stained en bloc in uranyl acetate, dehydrated, and embedded in epoxy resin. Thin sections were used for transmission electron microscopy (EM). Fresh muscles were frozen, and 5 µm-thick sections were used for fluorescence microscopy with myomesin isoform-specific monoclonal antibodies. Results: EM clearly demonstrated M-lines in all diaphragm muscle fibers examined, but not in EOM fibers. Immunocytochemical localization of myomesin 1 and 2 revealed a stereotypical banding pattern in the diaphragm and lateral omohyoid muscle fibers, which was completely absent in the EOMs. Conclusion: These results support our original hypothesis that rat EOMs do not contain myomesin 1 or 2, and explain the lack of M-lines in these muscles. It is possible that rat EOMs may express alternative cytoskeletal proteins to maintain sarcomere integrity; however, the functional properties of these low-force producing muscles may not necessitate the added mechanical stability provided by the M-lines. Finally, the metabolic consequences of the lack of M-lines remain unknown.
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