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The Development of Longitudinal Variation of Myosin Isoforms in the Orbital Fibers of Extraocular Muscles of Rats

¹From the Department of Cell and Developmental Biology, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania; the ²Department of Neurology, Case Western Reserve University and University Hospitals of Cleveland, Cleveland, Ohio; and the ³Department of Physiology, Institute for Biomedical Research, University of Sydney, Australia.

PURPOSE. To examine the appearance of longitudinal variation of extraocular and embryonic myosin heavy chain (MyHC) isoforms during the development of orbital singly innervated fibers of rat extraocular muscles (EOMs).

METHODS. EOMs were dissected from rat pups of various ages and stained with isoform-specific monoclonal antibodies to the embryonic and extraocular MyHC isoforms and to neurofilaments, as well as with labeled -bungarotoxin. The orbital layers of whole muscles were examined by confocal microscopy. RNase protection assays for the embryonic (Myh3) and extraocular (Myh13) MyHC isoform mRNAs were also performed.

RESULTS. At 10 days postpartum, the EOM MyHC RNA was first detected by RNase protection assay. At 11 days postpartum, the extraocular isoform was detected in the orbital fibers as two thin stripes just proximal and distal to the neuromuscular junction (NMJ). Over the next few weeks, the area occupied by the extraocular isoform increased to include the entire central region of the orbital fibers at and surrounding the NMJ. At the same time, the embryonic isoform became excluded from the region of the NMJ.

CONCLUSIONS. The orbital layer fibers of rat EOMs contain a longitudinal variation in MyHC isoforms not seen in other skeletal muscles. Development of this longitudinal variation begins as a late event postpartum; and the first appearance of it may be closely linked to neural contact. This targeting of MyHC isoforms to distinct domains is unique to EOMs.

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