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L. K. McLoon, L. V. Thompson, F. Pedrosa-Domellöf; A Continuum of Fiber Types in Extraocular Muscle. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4491.
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The extraocular muscles (EOM) are extremely complex relative to their myosin heavy chain isoform (MyHC) expression patterns. We examined single isolated myofibers for their ability to generate force and then examined each fiber for its MyHC content. In addition, normal EOM muscles were serially sectioned, and co-expression patterns for MyHC isoforms were determined.
Single myofibers were isolated from rectus muscles of adult rabbits, permeabilized, and mounted into an in vitro force assessment apparatus. Single fiber diameter, force, force/cross-sectional area, and unloaded shortening velocity were determined. Subsequently, each fiber was prepared for gel electrophoresis. MyHC isoform content was determined on SDS-PAGE, and the relative content of each MyHC isoform was analyzed using densitometry, thus allowing for the determination of contractile properties and MyHC protein analysis for each fiber. A second set of adult rabbit EOM muscles were serially sectioned and analyzed for co-expression patterns of MyHC isoforms. Human muscles were also similarly examined in tissue sections.
In contrast to limb skeletal muscle fibers, there was no correlation between force and individual fiber diameter nor between shortening velocity and fiber diameter. Based on gel electrophoresis, individual myofibers expressed from one to five MyHC isoforms. There appeared to be a continuum of forces and shortening velocities generated by individual myofibers as well as a continuum of MyHC isoform expression patterns in the 200 myofibers examined. Analysis of serial sections showed that single fibers from adult rabbits EOM could express up to 5 different MyHC isoforms in rabbits and up to 4 in human EOM.
There is great heterogeneity in MyHC content in adult EOM, with a continuum of myofibers, presumably reflecting the presence of very small motor units and adaptation to the finest motor control. Based on other studies from our laboratories demonstrating the short timetable needed for isoform switching in the EOM, we suggest that the EOM have a high capacity to adapting and modifying their MyHC isoform composition in response to altered demands of eye movements.
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