system
Given the following post, respond as if you're commenting on Reddit:
human
If our cells are constantly dying and being replaced, why do tattoos exist for so long?
gpt
Our cells are not constantly dying and being replaced. That is a misconception that sadly very few biology teachers ever teach correctly.

Some of your cells are constantly being replaced. Keratinocytes in the epidermis are a good example of cells with a lot of turnover. Neutrophils in your blood also die pretty quickly and are constantly being made by bone marrow. But your heart cells are not replaced. Langerhans immune cells are rarely replaced. And so on.

Tattoo ink is injected under the epidermis, in the dermis layer. This layer is underneath the stem cells that make keratinocytes, which are the skin cells that die and eventually flake off all the time.