Myonuclei Are Retained In Human Gastrocnemius Muscle After Achilles Tendon Rupture

It is well established that the addition of myonuclei acquired from satellite cells are important for the adaptation process in human skeletal muscle when subjected to resistance training. However, it is debated whether myonuclei are lost or retained during periods of disuse and whether the myonuclear content reflects the largest historical size of a muscle fiber. There is strong evidence that myonuclei are not lost in mice muscles during periods of disuse, but this remains to be confirmed in humans. PURPOSE: The purpose of the present study was to examine how the myonuclei content in the gastrocnemius muscle is affected by a period of complete immobilization because of an Achilles tendon rupture. METHODS: Biopsies were obtained from six patients as soon as possible after the injury (average 3 days) and after 6 weeks of immobilization. The injured leg was immobilized first by a plaster cast (~4 weeks) and thereafter by an orthosis (~2 weeks). Histochemistry was used to assess cross sectional area (CSA) and myonuclear content in type I and II muscle fibers. RESULTS: The CSA remained unchanged in both type I (Pre: 4250 ± 580μm2, Post: 3860 ± 470μm2) and type II (Pre: 4430 ± 800μm2, Post: 4240 ± 920μm2) muscle fibers. The myonuclear content in type I fibers did not change (Pre: 2.2 ± 0.3, Post: 2.5 ± 0.4), whereas there was a trend for an increased (~30%) myonuclear content in type II fibers (Pre: 2.1 ± 0.4, Post: 2.7 ± 0.5, p = 0.06), resulting in a trend for a reduction (~25%) in nuclear domain (myonuclei per μm2 CSA) in type II fibers (Pre: 2130 ± 530, Post: 1600 ± 260, p = 0.08). CONCLUSIONS: Six weeks of immobilization after an Achilles tendon rupture did not reduce the number of myonuclei in the gastrocnemius muscle. Consequently, humans, just like rodents, does not seem to lose myonuclei during periods of disuse which supports the theory that there is a relationship between the myonuclear content and the hypertrophic history of a muscle fiber.