Tattoos are pretty much permanent. Their ink can live on in the skin for decades. But biologists have been curious about how tattoos manage that. As inked skin cells die, the trash-collecting cells — part of the body’s immune system — should remove the ink along with the dead skin cells. Now French researchers think they know why that isn’t happening.
The body's trash collectors pick up the ink, then remain in place, holding the ink there. When these immune cells die, they release the ink, only for new immune cells to slurp it up. Throughout, the tattoo’s image remains the same.
There have been other ideas for why tattoos are permanent. Some researchers have proposed that the ink stays in connective cells. Others think it remains in immune cells that simply live a long time. Sandrine Henri, who works in France at the Immunology Center of Marseille-Luminy, was skeptical.
She wanted to probe the role of immune cells known as macrophages (MAK-roh-fayj-es). “Macrophages will scavenge everything. That’s their job,” Henri says. “If they could do their job properly, tattoo ink would be removed rapidly.” But the ink wasn’t. She decided to investigate why.
She and her colleagues tattooed the tails of mice with green ink. Then they watched to see how macrophages in the skin responded.
Like Roomba vacuums, macrophages suck up cellular debris in the body. And here, they gobbled up the ink. But they did not go on to digest and remove it. Instead, the immune cells held onto the ink until they were killed by researchers.
Some 90 days later, new macrophages had moved in and reabsorbed the ink. And throughout, there was no difference in the look of the tattoo.
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Henri’s team described its findings March 6 in the Journal of Experimental Medicine. Her team now proposes that macrophages likely explain how tattoos persist in people, too.
But they can’t be sure — yet, says Desmond Tobin. A mouse study simply won’t settle that, says this skin expert in England at the University of Bradford. Macrophages may live longer in people than in mice, he says. Long-lived macrophages, for instance, might preserve tattoos in humans. If true, they wouldn’t need to adopt some ink-release-and-recapture tactic.
Whatever the final answer, the new findings might still help improve tattoo removal, Henri’s team says. Tattoos are usually removed with lasers. Combining lasers with some treatment that also focuses on getting rid of nearby skin macrophages might make that removal more effective, the researchers say.
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biology The study of living things. The scientists who study them are known as biologists.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
colleague Someone who works with another; a co-worker or team member.
debris Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.
digest (noun: digestion) To break down food into simple compounds that the body can absorb and use for growth. Some sewage-treatment plants harness microbes to digest — or degrade — wastes so that the breakdown products can be recycled for use elsewhere in the environment.
generation A group of individuals (in any species) born at about the same time or that are regarded as a single group. Your parents belong to one generation of your family, for example, and your grandparents to another. Similarly, you and everyone within a few years of your age across the planet are referred to as belonging to a particular generation of humans.
immune (adj.) Having to do with the immunity. (v.) Able to ward off a particular infection. Alternatively, this term can be used to mean an organism shows no impacts from exposure to a particular poison or process. More generally, the term may signal that something cannot be hurt by a particular drug, disease or chemical.
immune system The collection of cells and their responses that help the body fight off infections and deal with foreign substances that may provoke allergies.
immunology The field of biomedicine that deals with the immune system. A doctor or scientist who works in that field is known as an immunologist.
laser A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.
macrophage A type of white blood cell dispatched by the immune system. Like janitors of the body, they gobble up germs, wastes and debris for disposal. These cells also stimulate other immune cells by exposing them to small bits of the invaders.
scavenge To collect something useful from what had been discarded as waste or trash.
skeptical Not easily convinced; having doubts or reservations.
Journal: A. Baranska et al. Unveiling skin macrophage dynamics explains both tattoo persistence and strenuous removal. Journal of Experimental Medicine. Published online March 6, 2018. doi: 10.1084/jem.20171608.