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Review
Mixing it up: the biological significance of hybrid skeletal muscle fibers
Scott Medler
Journal of Experimental Biology 2019 222: jeb200832 doi: 10.1242/jeb.200832 Published 29 November 2019
Scott Medler
Biology Department, State University of New York at Fredonia, Fredonia, NY 14063, USA
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  • For correspondence: scott.medler@fredonia.edu
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ABSTRACT

Skeletal muscle fibers are classified according to the myosin heavy chain (MHC) isoforms and other myofibrillar proteins expressed within these cells. In addition to ‘pure’ fibers expressing single MHC isoforms, many fibers are ‘hybrids’ that co-express two or more different isoforms of MHC or other myofibrillar proteins. Although hybrid fibers have been recognized by muscle biologists for more than three decades, uncertainty persists about their prevalence in normal muscles, their role in fiber-type transitions, and what they might tell us about fiber-type regulation at the cellular and molecular levels. This Review summarizes current knowledge on the relative abundance of hybrid fibers in a variety of muscles from different species. Data from more than 150 muscles from 39 species demonstrate that hybrid fibers are common, frequently representing 25% or more of the fibers in normal muscles. Hybrid fibers appear to have two main roles: (1) they function as intermediates during the fiber-type transitions associated with skeletal muscle development, adaptation to exercise and aging; and (2) they provide a functional continuum of fiber phenotypes, as they possess physiological properties that are intermediate to those of pure fiber types. One aspect of hybrid fibers that is not widely recognized is that fiber-type asymmetries – such as dramatic differences in the MHC composition along the length of single fibers – appear to be a common aspect of many fibers. The final section of this Review examines the possible role of differential activities of nuclei in different myonuclear domains in establishing fiber-type asymmetries.

FOOTNOTES

  • Competing interests

    The author declares no competing or financial interests.

  • Supplementary information

    Supplementary information available online at http://jeb.biologists.org/lookup/doi/10.1242/jeb.200832.supplemental

  • © 2019. Published by The Company of Biologists Ltd
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Keywords

  • Hybrid muscle fiber
  • Muscle plasticity
  • Skeletal muscle

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Review
Mixing it up: the biological significance of hybrid skeletal muscle fibers
Scott Medler
Journal of Experimental Biology 2019 222: jeb200832 doi: 10.1242/jeb.200832 Published 29 November 2019
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Review
Mixing it up: the biological significance of hybrid skeletal muscle fibers
Scott Medler
Journal of Experimental Biology 2019 222: jeb200832 doi: 10.1242/jeb.200832 Published 29 November 2019

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Article navigation

  • Top
  • Article
    • ABSTRACT
    • Introduction
    • The prevalence of hybrid fibers
    • Parameters used to classify fiber types
    • Role of hybrid fibers in fiber-type transitions
    • Hybrid fibers as points along a continuum
    • Fiber-type asymmetries
    • Myonuclear domains and fiber-type asymmetries
    • Conclusions and future directions
    • Acknowledgements
    • FOOTNOTES
    • References
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