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M.H. J. Wiesen, S. Bogdanovich, T.S. Khurana; Molecular and Ultrastructural Characterization of Adult Rat Extraocular Muscle (EOM) M–Lines . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4681.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Sarcomeres consisting of alternately placed A– and I–bands form fundamental structural & functional units of skeletal muscle. The M–line is an electron microscopic (EM) feature situated in the middle of the A–band. Confusion exists in literature regarding the presence of M–lines and expression levels of M–line components in EOM. To clarify this disparity, we performed a detailed molecular and EM characterization of M–Lines in rectus EOMs. Methods: Standard molecular methods (e.g. semi–quantitative and real–time RT–PCR) were used to analyze expression levels of mRNAs encoding M–line components. For EM, adequate fixation was ensured by cannulating and perfusing the left common carotid (for EOM) and common illiac for Tibialis anterior (TA). Analysis of M–lines in transverse sections allowed us to unambiguously score fibers in orbital and global layers separately. 900 fibers from 3 different rats were analyzed by EM. Morphometric analysis was used to validate structural findings. Results: mRNA expression levels of Myom1 (quantified by 3' primers which detect all Myom1 isoforms) and CK–MM showed a mild downregulation while Myom2 was highly downregulated in EOM compared to TA. Conversely, EH–Myom1 and CK–BB were upregulated in EOM. Using EM, M–lines were poorly defined or absent in the orbital layer (OL) fibers. In the global layer (GL) fibers containing well defined M–lines (morphologically indistinguishable from those in TA) were identified and documented. Conclusions: Overall, Myom 1 levels were mildly decreased in EOM compared to TA. Well–defined M–lines (similar to those seen in leg muscle) were observed in specific locales of the GL but not OL. These data are consistent with our profiling–based hypothesis (Budak et al. Physiol. Genomics 2004) that compartmentalization of OL and GL resulted from divergent evolution. (We thank Prof. Clara Franzini–Armstrong for invaluable guidance)
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