March 9 (UPI) -- Researchers in France have finally figured out why tattoos are permanent, rather than fading away as skin cells die.
Scientists at Aix Marseille University found that dead cells release tattoo ink into neighboring cells, which allows them to remain permanently visible, according to a study published Tuesday in the Journal of Experimental Medicine.
With the new understanding, researchers hope the findings can lead to better ways to remove unwanted tattoos.
About 45 million Americans have at least one tattoo, but 17 percent said they have some regret after getting them, according to Statistics Brain Research. Eleven percent said they were in the process of removing one, or that it had already been done.
Originally, the belief was tattoos stained fibroblast cells in the dermal layer of the skin. But more recently, researchers have suggested that macrophages, which are specialized immune cells that reside in the dermis, are attracted to the wound inflicted by the tattoo needle. They take up the tattoo pigment and move it to other cells.
"We think that, when tattoo pigment-laden macrophages die during the course of adult life, neighboring macrophages recapture the released pigments and insure in a dynamic manner the stable appearance and long-term persistence of tattoos," Sandrine Henri of Marseille's Center of Immunoly said in a Rockefeller University Press release.
Henri and his study leader Bernard Malissen don't have tattoos. The New York Times reported they were examining skin cells in black mice for another project but they noticed macrophages scavenging the melanin were released by dying, pigment-making cells. They wondered if the same process occurred in tattoos.
The researchers developed a genetically engineered mouse that allowed them to kill the macrophages in the dermis and other tissues.
Then, over a few weeks, these cells were replaced by new macrophages derived from precursor cells known as monocytes.
The dermal macrophages were the only cell type to take up pigment when the mice's tails were tattooed. The tattoo remained the same as the macrophages were killed off, and then they dispersed the pigments into surrounding areas.
Even when the macrophages were killed off at one time, this process continued.
When the researchers transferred a piece of tattooed skin from one mouse to another, they found, after six weeks, most of the pigment-carrying macrophages were derived from the recipient animal rather than the donor.
With these findings, researchers believe tattoos can be removed by laser pulses that cause skin cells to die and release their pigment. Then they are transported from the skin and into the body's lymphatic system.
"Tattoo removal can be likely improved by combining laser surgery with the transient ablation of the macrophages present in the tattoo area," Malissen said. "As a result, the fragmented pigment particles generated using laser pulses will not be immediately recaptured, a condition increasing the probability of having them drained away via the lymphatic vessels."
The authors speculate that the laser removal could take as many as 20 treatments.