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S. A. Croes, L. M. Baryshnikova, C. S. von Bartheld; Classification, Development and Maturation of Myofiber Types in Chicken Extraocular Muscles. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5278.
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Precise control of contractile force of extraocular muscles is required for appropriate movements and alignment of the eyes. It is not known how such precise regulation of contractile force is achieved and how this system matures. We hypothesize that motoneurons instruct the extraocular myofibers which they innervate and that they regulate their phenotype to adapt to the precise force requirements.
By using the posthatch chicken as a model system, we describe and quantify critical parameters of the developing superior oblique extraocular muscle from hatching to 4-5 months of age, including contractile force, muscle mass, myofiber number and diameters, classification of fiber types, and distribution and size of mitochondria.
Analysis at the light- and electron microscopic level shows that chicken myofiber types largely correspond to their mammalian counterparts, with four fiber types in the orbital and four types in the global layer. Twitch tension muscle force and mass gradually increase and stabilize at about 11 weeks. Tetanic tension still increases between 11 and 16 weeks. Myofiber diameters in both the orbital and global layer increase from hatching to 6 weeks, and then most myofiber diameters stabilize, while the myofiber number remains constant after hatching. This suggests that, during late maturation, muscle mass increases by increasing fiber length rather than fiber diameter. Our quantitative ultrastructural analysis confirms this trend and reveals substantial and protracted changes in percentages of the four muscle fiber types and mitochondrial parameters. These changes suggest myofiber type conversion and/or differential and ongoing replacement of myofibers.
The finding of continued changes in myofiber composition into adult ages in this highly visual and precocious species is as unexpected as it is intriguing. We conclude that the phase of oculomotor development and plasticity extends much longer than previously anticipated. These data provide a baseline for future examination to identify relevant signals and mechanisms that regulate extraocular muscle force and fiber composition needed for precise eye alignments.
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