Classification and development of myofiber types in the superior oblique extraocular muscle of chicken.

Precise control of contractile force of extraocular muscles is required for appropriate movements and alignment of the eyes. It is unclear how such precise regulation of contractile force is achieved during development and maturation. By using the posthatch chicken as a model, we describe and quantify critical parameters of the developing superior oblique extraocular muscle from hatching to 16 weeks of age, including contractile force, muscle mass, myofiber diameters, classification of fiber types, and distribution and quantification of mitochondria. Analysis at the light- and electron microscopic levels 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 muscle mass gradually increase and stabilize at approximately 11 weeks. Tetanic tension continues to increase between 11 and 16 weeks. Myofiber diameters in both the orbital and global layer increase from hatching to six weeks, and then stabilize, whereas the myofiber number is constant after hatching. This finding suggests that muscle mass increases during late maturation due to increasing fiber length rather than fiber diameter. Quantitative ultrastructural analysis reveals continuing changes in the composition of the four muscle fiber types, suggesting ongoing fiber type conversion or differential replacement of myofiber types. Muscle fiber composition continues to change into late juvenile and adult age. Our study provides evidence for gradual, incremental, and continuing changes in avian myofiber composition and function that is similar to postnatal oculomotor maturation in visually oriented mammals such as kitten.