As many as one in three people have tattoos—but outside of professional tattoo artists, it's unlikely that a lot of us actually know how they work. In a new video from the Institute of Human Anatomy, Jonathan Bennion explains exactly what happens to the skin when it is being tattooed, why the body doesn't reject the foreign ink, and why most tattoos fade with time.
The skin is part of the body's integumentary system, consisting of three layers, each of which are made from completely different tissue. There's the epidermis on the top, the dermis in the middle, and then the hypodermis, also known as the subcutaneous layer. "The nature of the tissue will definitely influence how ink is deposited in that layer, how permanent the ink is in that layer, and even how the tattoo works," says Bennion.
When it comes to the epidermis, a paper-thin layer made from epithelial tissue, tattooists have to go deeper. That's because the cells closest to the skin's surface flatten and die out to provide a barrier against friction, and are ultimately shed. The human body can shed up to a million of these dead cells in a single day. "Some of the ink, as it gets injected into the skin, does get deposited in this layer," says Bennion. "Eventually, any ink that's in there is going to flake off... So the next layer is extremely important to tattooing."
The thicker dermal layer is made from dense irregular connective tissue, and contains sweat glands, blood vessels, nerve endings, and fibrocyte cells which produce collagen—it is this collagen which gives the dermis its strength and elasticity to be pulled in all directions.
"When ink is being deposited into the dermal layer, a lot of that ink is going to get suspended in that collagen matrix, and some of it will even be taken up by those fibrocytes," says Bennion. "But that's not the only thing that's going on here. During tattooing, a needle is puncturing the skin at anywhere from 50 to 3,000 times per minute. That's creating trauma as well as injecting a foreign substance into the skin. Usually when our body deals with trauma or a foreign substance gets introduced, our immune system has something to say about that, and the inflammatory process is activated."
Inflammation is where the body deploys white blood cells to the point of injury. White blood cells called macrophages, which engulf foreign bodies like viruses and bacteria, then try to engulf the ink and break it down. "However, it's used to breaking down and digesting biological substances," adds Bennion. "Ink can include dye, plastics, and even solids that our white blood cells are not equipped to break down. We think that instead of being able to break it down because they can't, it's more like 'let's isolate this and contain it and not let this ink go to other areas of the body.'"
Bennion goes on to explain that when these macrophages die, they release the ink, and a new macrophage comes along and engulfs it—or at least, most of it, with stray ink particles being taken away into the lymphatic system. And this ongoing engulf-and-release process over a person's lifetime is actually the basis of one current working theory as to why tattoos tend to fade over the years.
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