Building Permission

When a muscle fibre, like the one shown, is injured, its associated stem cells, like the one coloured pink, are keen to start rebuilding. But, they must wait until immune cells have finished clearing up debris. In fact, the immune cells send ‘wait’ signals to the stem cells preventing their activity. Stem cells are impatient, though, and as soon as an injury occurs, they start producing and covering themselves with hyaluronic acid (coloured pale green and seen outlining the stem cell). This carbohydrate goo gradually thickens until it's dense enough to interrupt the immune cells’ signals, essentially giving the stem cells the go-ahead to start work. As humans age, production of hyaluronic acid diminishes, which may explain why injuries in older people take so long to repair. Without a thick gooey barrier to block the immune signals, the stem cells are forced to wait, their building permit denied.

Written by Ruth Williams

Ruth Williams

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Ruth is a freelance science journalist based in the US. She's a regular correspondent for The Scientist, and her work has appeared in The Lancet, Nature, Scientific American Mind, BBC Focus and elsewhere. Twitter @rooph

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• Image from work by Kiran Nakka and colleagues
• Sprott Center for Stem Cell Research, Regenerative Medicine Program, Ottawa Hospital Research Institute, Ottawa, ON, Canada
• Image copyright held by the original authors
• Research published in Science, August 2022