The doped mice got a boost in muscle mass and muscle nuclei. To test this idea, his lab gave some mice testosterone, and left others untreated. "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. Gundersen thinks the results contradict the conventional wisdom that nuclei disappear when muscles atrophy. But those extra nuclei stuck around, even as the muscle shrank by half. Unsurprisingly, testosterone boosted nuclei number. Later, after stopping the testosterone, he could watch what happened as those muscles atrophied. Gundersen could then track the nuclei over time as he induced muscle growth by giving the mice testosterone, a steroid hormone. The stain spreads throughout the muscle cells, illuminating their nuclei. The researchers injected a stain into muscle cells that mice use to flex their toes. Gundersen and colleagues developed another method that zoomed in on individual muscle cells. 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. "It can be very difficult to distinguish between muscle nuclei from other nuclei," says Gundersen. 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. Muscle cells gain this flexibility by breaking the biological norm of one nucleus to a cell some muscle cells house thousands of nuclei. Muscle cells can be sculpted into many forms and can stretch to volumes 100,000 times larger than a normal cell. One thing is for sure: Muscles need to be versatile to meet animals' needs to move. But some scientists caution against extrapolating too far from these studies into humans while conflicting evidence exists. This work could affect public health policy and anti-doping efforts in sports, says Lawrence Schwartz, a biologist at the University of Massachusetts, Amherst who wrote the review. Rather than dying as muscles lose mass, nuclei added during muscle growth persist and could give older muscles an edge in regaining fitness later on, new research suggests. But scientists haven't been able to pin down how that would actually work.Ī growing body of research reviewed Friday in the journal Frontiers in Physiology suggests that muscle nuclei - the factories that power new muscle growth - may be the answer. Muscle physiology lore has long held that it is easier to regain muscle mass in once-fit muscles than build it anew, especially as we age. ![]() Skeletal muscle cells from a rabbit were stained with fluorescent markers to highlight cell nuclei (blue) and proteins in the cytoskeleton (red and green).ĭaniel Schroen, Cell Applications Inc./Science SourceĬan muscles remember their younger, fitter selves?
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