This Moment of Science begins with a question sent in by Joe Frommer: If our skin cells constantly replace themselves, why are tattoos permanent?
You too may have wondered why we don't shed tattoos along with skin cells--especially if you have a tattoo you'd like to get rid of. Why can't you just shed that old snake tattoo?
To answer this question, we need to look first at the structure of skin. The top layer of skin, called the epidermis, is very thin--only 1/10 of a millimeter at most. This is where cells constantly divide from the base layer and move up to the surface to be sloughed off. Just below the epidermis is a second layer, the dermis, which doesn't undergo the same process of constant replacement.
A tattoo needle penetrates just a couple millimeters into the skin. But that's deep enough to leave a trail of ink all the way into the dermis. So even after all the superficial epidermal cells have sloughed off, tattoo ink remains in the dermis. A good tattoo ink won't dissolve in dermal fluid or cause an immune response. If the ink did cause an immune response, special white blood cells would gobble up and remove it, blurring the tattoo.
If you'd like to get rid of a tattoo, laser removal techniques are constantly improving. But they're still not perfect--it's possible to scar the skin or darken certain inks. So if you're not certain you want a green snake as a permanent part of your dermis, your best bet is to ponder it a little longer.