Tattoos last a lifetime. Now, experiments with tattooed mice might explain why: immune cells called macrophages ‘recycle’ dark pigments, keeping the ink in place.
Macrophages in the skin eat foreign material, including bits of tattoo ink, which the cells then store internally. Many scientists had thought that the immune cells’ extreme longevity explains why tattoos don’t disappear. But Sandrine Henri and Bernard Malissen of the Immunology Centre of Marseille-Luminy in France and their colleagues tested a different hypothesis. They tattooed bright-green bands on the tails of mice, and then, three weeks later, killed the macrophages that had ingested the ink. As the cells died, they released the pigments, which were readily taken up by neighbouring macrophages.
This pigment recycling probably helps tattoos to persist, the authors say, although they add that a few long-lived macrophages packed with pigments might contribute to tattoos’ permanence.