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Human Extraocular Muscles: Unique Pattern of Myosin Heavy Chain Expression during Myotube Formation

¹ From the Department of Integrative Medical Biology, Section of Anatomy, Umeå University; and the ² Department of Musculoskeletal Research, National Institute for Working Life, Umeå, Sweden; and the ³ Department of Physiology and Institute for Biomedical Research, University of Sydney, New South Wales, Australia.

PURPOSE. To study the myosin heavy chain composition of the human extraocular muscles (EOMs) during development.

METHODS. EOMs from human fetuses of 8 to 22 weeks of gestation were studied with immunocytochemistry and gel electrophoresis. Antibodies specific against nine isoforms of myosin heavy chain (MyHC) were used in serial frozen sections.

RESULTS. The developing EOMs had a delayed time course of myotube formation and a unique composition and distribution of MyHCs compared with human limb skeletal muscle. The primary myotubes coexpressed two developmental isoforms of MyHCI from the earliest stages. The third developmental MyHCI delineated the future orbital layer at 10 to 12 weeks of gestation. MyHC-slow tonic also appeared early, whereas MyHC -cardiac and MyHC-extraocular, important components of adult EOM, were never detected at the gestational ages studied.

CONCLUSIONS. The developmental features of the EOMs differed significantly from those reported for limb muscles of the corresponding ages. It is clear that the knowledge of limb muscle development does not fully apply to more specialized muscles, such as the eye muscles. The extreme complexity displayed by the EOMs probably reflects their distinct embryonic origin, innervation, and regulatory program of myogenesis.

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