There’s that saying that you replace all the cells in your body every seven years. Cells reproduce by dividing. Since we don't double in size regularly, presumably cells are also dying and getting cleared away at a similar rate.

So, how does tattoo ink remain in the skin, practically unchanged, for decades? If the cells containing ink are dividing and dying, shouldn't the ink become diluted across cells and eventually cleared away with dead cells?

I noticed I was confused here. Noticing confusion is great for building better, more accurate models of the world, so I went down a rabbit hole to figure it out. (This concludes the tenuous link to productivity – the rest of this post is pure nerd sniping!)

Was my model of how cells divide accurate?

We know the answer thanks, at least in part, to bombs. Apparently Cold War nuclear tests temporarily altered the ratio of carbon isotopes being fixed into new cells. In a rare case of beneficial serendipity, the radiation gave researchers the chance to measure when specific cells were added to the body. They could even identify the rate at which slow-changing cells, like skeletal and heart cells, are replaced.

According to this fascinating bio-numbers website, it appears that most cells in the body are replaced in under a year. Fat, heart, nervous system, skeleton, eye lens, and (in women) egg cells are the only ones that replace themselves less frequently than a year (or not at all, as far as we can currently measure).

Therefore, skin cells should divide and dead cells be cleared out in under a year, not the decades that tattoos would suggest. It doesn't seem that the ink getting into deep tissue skin cells would change that, since nearly all the cells in the body are regularly replaced.

Still confused, I googled how tattoos work. Ignoring the citation-less pages insisting ink lasts because it gets trapped in a deep skin layer, basically all of the other results cite a 2018 study where scientists tattooed mice tails.

Studying the green tattoos, the scientists identified one type of cell that “eats” the ink: dermal macrophages. Macrophages are a type of white blood cell that engulfs and digests foreign particles as an immune response, but they apparently can’t break the ink down. Thus, each macrophage releases the ink when it dies, only for another macrophage to recapture the ink. Successive generations of these immune cells just hold onto the tattoo ink for years.

Even newer studies suggest that other cells also contain tattoo ink. The main second type are fibroblasts, which can become phagocytic when inflammation occurs – in other words, they are probably following the same process as the macrophages. I’m still confused by traces of ink in skin cells. They seem to be a minority (though I didn’t find good numbers on this), so perhaps the answer is simply that a few stray traces of ink get passed down through successive skin cells despite the majority being eliminated with dead cells. 

Anyway, the answer to my confusion is that tattoos don’t rely on skin cells to survive. Rather, your immune system is just throwing the ink in a perpetually revolving prison.