Myosin heavy chain expression in mouse extraocular muscle: more complex than expected.

PURPOSE: To characterize the expression patterns of myosin heavy chain (MyHC) isoforms in mouse extraocular muscles (EOMs) during postnatal development.
METHODS: MyHC isoform expression in mouse EOMs from postnatal day (P)0 to 3 months was evaluated by quantitative polymerase chair reaction (qPCR) and immunohistochemistry. The longitudinal and cross-sectional distribution of each MyHC isoform and coexpression of certain isoforms in single muscle fibers was determined by single, double, and triple immunohistochemistry.
RESULTS: MyHC isoform expression in postnatal EOMs followed the developmental rules observed in other skeletal muscles; however, important exceptions were found. First, developmental isoforms were retained in the orbital layer of the adult EOMs. Second, expression of emb-MyHC, neo-MyHC, and 2A-MyHC was restricted to the orbital layer and that of 2B-MyHC to the global layer. Third, although slow-MyHC and 2B-MyHC did not exhibit obvious longitudinal variations, emb-MyHC, neo-MyHC, and 2A-MyHC were more abundant distally and were excluded from the innervational zone, whereas eom-MyHC complemented their expression and was more abundant in the mid-belly region in both the orbital and global layers. Fourth, coexpression of MyHC isoforms in single global layer fibers was rare, but it was common among the orbital layer fibers.
CONCLUSIONS: MyHC isoforms have complex expression patterns, exhibiting not only longitudinal and cross-sectional variation of each isoform, but also of coexpression in single fibers. The highly heterogeneous MyHC expression reflects the complex contractile profiles of EOMs, which in turn are a function of the requirements of eye movements, which range from extremely fast saccades to sustained position, each with a need for precise coordination of each eye.