giantpredatorymollusk asked: If every cell in our bodies is replaced in seven years, how do tattoos stay on?
RIGHT!? Isn’t that FREAKING COOL! Your skin cells only last for a few weeks, and yet your tattoo lasts YOUR WHOLE LIFE!
WELL! The outer layer of our skin is made of collagen, a flexible but durable protein. This is constantly sloughing off and being replaced from below as cells die leaving only their collagen-filled shells behind.
But when you get a tattoo, the needle punches past the outer layers of skin, doing quite a bit of damage. The result is that scar collagen forms around the dye that’s been placed in the dermis. Scar collagen, unlike skin collagen, doesn’t replace (which is why scars last forever.) The particles of ink are too large for white blood cells to surround them and carry them off to lymph nodes so they just sit there, surrounded by small amounts of scar collagen outside of your skin cells…pretty much forever.
This is the key to tattoo removal, by the way. Lasers are used to break the ink into smaller bits, so your white blood cells can take care of them.