Muscles May Preserve a Shortcut To Restore Lost Strength

New research reviewed in the journal Frontiers in Physiology suggests that muscle nuclei -- the factories that power new muscle growth -- could give older muscles an edge in regaining fitness later on. 'Muscles need to be versatile to meet animals' needs to move,' reports NPR. 'Muscle cells can be sculpted into many forms and can stretch to volumes 100,000 times larger than a normal cell. Muscle cells gain this flexibility by breaking the biological norm of one nucleus to a cell; some muscle cells house thousands of nuclei. In mammals, these extra nuclei come from stem cells called satellite cells that surround the muscle. When demands on the muscle increase, these satellite cells fuse with muscle cells, combining their nuclei and paving the way for more muscle.' From the report: Physiologists had thought that a single nucleus supported a certain volume of cell. As a muscle cell grew, it needed more nuclei to support that extra volume. But as a muscle shrinks from lack of use, it gets rid of those unnecessary extra nuclei. This view found support in studies that found nuclei were scrapped as muscles atrophied. But [Kristian Gundersen, a muscle biologist from the University of Oslo] and [Lawrence Schwartz, a biologist at the University of Massachusetts] say those experiments overlooked what was really happening.

Take a cross section of muscle tissue and you'll find a sort of marbled mishmash of muscle cells surrounded by numerous other cell types, such as satellite cells and fibroblasts. Researchers could have been measuring the death of cells that support muscle and incorrectly inferred that muscle cells lose their nuclei, according to Gundersen and Schwartz. Gundersen and colleagues developed another method that zoomed in on individual muscle cells. The researchers injected a stain into muscle cells that mice use to flex their toes. The stain spreads throughout the muscle cells, illuminating their nuclei. Gundersen could then track the nuclei over time as he induced muscle growth by giving the mice testosterone, a steroid hormone. Later, after stopping the testosterone, he could watch what happened as those muscles atrophied. Unsurprisingly, testosterone boosted nuclei number. But those extra nuclei stuck around, even as the muscle shrank by half. Gundersen thinks the results contradict the dogma that nuclei disappear when muscles atrophy. 'Nuclei are lost by cell death,' he says, 'just not the actual muscle nuclei that confer strength.' What's more, he says these retained extra nuclei might explain how a muscle remembers its past fitness.

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