Bruce, California asked:
How do tattoos last for so long in the skin?
We put this to Neil Walker, consultant dermatologist in Oxford.
Tattoos by definition are permanent marks produced on the skin by the injection of the material by a puncturing. As a dermatologist, I see a variety of tattoos, not any of those which have been applied by a so-called tattoo artist, either a professional or amateur, sober or drunk. Occasionally, I see people who’ve had a black kind of tattoo, where a dye called PPD is used, which can cause nasty skin reactions.
Appropriately applied henna tattoo is not a tattoo at all. Rather, it’s a drying process using the paste to produce a design in the dead outer layers of the skin. The design fades as the skin regenerates and that is one of clues as to why tattoos applied by puncturing are permanent. Our skin is continually regenerating as the outer layers or epidermis grows from basal cells at the bottom to a dead hole-y layer at the top over a period of six to eight weeks.
Pigments implanted beneath the growing layer are in the dermis or supporting layer of the skin and are not removed by the natural process of skin turnover. The body recognizes pigment granules as foreign material and there are cells whose function is to remove such material by engulfing them and transporting it to the lymph glands. These cells are unable to engulf pigment granules every certain size and, therefore, the body seem to surround them at their microscopic level by a thin layer of fibrous or scar tissue. And they become permanently trapped in the dermis.
The removal process continues slowly and tattoos may fade to a degree over time with different colours fading at different rates depending on the particle size of the pigment. In summary, tattoos are permanent because pigment particles are injected under the growing layer of the skin and the body’s mechanisms for dealing with foreign materials can't remove the particles over a certain size.