Clinical History:
- This previously healthy 35-year-old patient presented with proximal upper and lower extremity weakness and myalgias. Laboratory studies showed normal CPK. The patient’s home medication is negative for myotoxic medications.
- Muscle fiber typing was performed with dual immunohistochemical staining for fast and slow myosin heavy chains (MHCf-brown / MHCs-red).
What do the fibers showing intermediate staining (reddish-brown) represent?
Answer:
- Hybrid myofibers: muscle fibers that show an admixture of myosin heavy chain fast (type 2 myofiber: fast twitch / glycolytic) and slow isoforms (type 1 myofiber: slow twitch / oxidative) type immunostaining.
- These are felt to represent intermediate forms of myofibers transitioning from one myofiber type to another during muscle development, adaptation to exercise and aging. This would provide a spectrum of muscle fiber capability, and may vary along the length of a single muscle fiber.
Reference(s) / Additional Reading:
- Schiaffino S. Muscle fiber type diversity revealed by anti-myosin heavy chain antibodies. FEBS J. 2018 Oct;285(20):3688-3694. doi: 10.1111/febs.14502. Epub 2018 May 24. PMID: 29761627.
- Medler S. Mixing it up: the biological significance of hybrid skeletal muscle fibers. J Exp Biol. 2019 Nov 29;222(Pt 23):jeb200832. doi: 10.1242/jeb.200832. PMID: 31784473.
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