When it comes to tattoos, one thing has always fascinated scientists: how does a tattoo manage to last so long when human skin cells die so frequently? A new study has shed some light on the issue.
The longevity of tattoos over time would be due not to the longevity of the cells containing the pigments, but to the recapture of the latter from the dying cells by their replacements. A process whose understanding could help to better get rid of unwanted inks.
At least 10% of the general population, or 25 to 35% of the population of young people and adults under 40 years of age, are reported to have a tattoo. If the tattoos do not change in appearance over time, it would be because the pigments are constantly transmitted to new cells when the first ones die, according to a study led by the French research institutions INSERM, the CNRS and the University Aix-Marseille grouped together at the Luminy Marseille Immunology Center. The study was published on March 6, 2018 in the journal Journal of Experimental Medicine.
Until now, it was thought that tattoos persisted because the pigments were stored in the connective tissue located in the dermis (under the epidermis), 1 to 2 millimeters deep, where there are cells called fibroblasts. However, researchers have found that the pigment is actually fully captured by macrophages, immune cells whose main ability is to engulf foreign substances to eliminate them or present them to the rest of the immune system. Each organ of the body has its own macrophages, called “residents”, and the skin is no exception. In principle, macrophages have the ability to migrate to the lymph nodes to activate other cells of immunity when there is inflammation, but this is not the case of dermal macrophages, which explains why pigments stay in place. However, while they do not move, macrophages have a lifespan limited to a few weeks: so how does a tattoo manage to last so long?
A “capture-release-recapture” cycle
A team led by Sandrine Henri and Bernard Malissen found the answer to the age old question. Using genetically modified mice so that their macrophages can be destroyed at the request that the team, the scientists were able to determine that the destroyed cells were replaced by new macrophages, formed from precursor cells present in the blood (monocytes). While dying, the macrophages released the tattoo pigments which, too big to evacuate, were recaptured by the newly formed macrophage.
“Using the ability to remove them, we have demonstrated that tattoo pigment particles can undergo successive cycles of capture-release-recapture without tattoo removal,” explain the authors in the publication. The conclusion was confirmed by a second experiment: by transferring a piece of tattooed skin from one mouse to another, the scientists found that 6 weeks later, most of the macrophages containing the pigments were those of the recipient mouse. “The long-term persistence of the tattoo is probably based on the renewal of macrophages rather than the longevity of macrophages,” the authors conclude.
Temporary removal of macrophages could speed up the erasure of tattoos
“It remains to be determined whether the model of capture-release-recapture of the tattoo pigments applies to humans,” commented the authors. If this is confirmed, these results could indeed improve tattoo erasure techniques, now based on laser pulses that cause the death of skin cells and the release and fragmentation of their pigments to be evacuated by the lymphatic vessels. “Tattoo removal via this laser technique can probably be improved by the temporary removal of macrophages present in the tattoo area,” say the researchers. “Thus fragmented pigment particles generated by laser pulses will not be immediately recaptured: this condition increases the likelihood of them being evacuated by the lymphatic vessels.”