If they say that all human cells replace themselves every 7 years or so, why can scars remain on your body for the entire life?
because scar tissue is not the same *type* of tissue as surrounding tissue. So even if the scar tissue is replaced, it’s replaced with the tissue making it up, which does not look the same as the other tissue around it.
For example if you get a small cut the tissue on either side of the cut divides and meets in the middle to repair the cut.
With a larger cut, it might take too long to do this – we need it closed ASAP. So you have tissue from the sides coming together but *also* tissue coming up from below in the middle and all three meet. But now the middle looks like the “below” tissue because that’s what it’s made from
Unfortunately that piece of information is wrong. All human cells do not replace themselves every 7 years, otherwise we won’t have diseases like Alzheimer’s which are caused by neurons getting damaged over time. Some cell types can replace themselves, such as skin, some can’t, like neurons.
Specific to scarring, the cells that heal the damaged area are not skin, but rather from fibrous tissue. Because of this, even after the scar heals, it remains that way, and future injuries and healings to the scar may also not make it go away.
Edit: replaced neurodegenerative with “neurons getting damaged over time” to make it Eli5 compatible
Edit 2: broke the single sentence second paragraph into two sentences for readability
scar tissue is shaped differently on a microscopic level that the rest of your skin. it replaces in roughly the same shape but the edges do shrink over time as more and more of the tissue corrects itself with each iteration, its just hard to notice because it takes such a considerable amount of time. i used to have 4 thick meaty scars down my back from when i was lil but they dont exist anymore
Because scars formed not only by cells, but also by fibrosis tissue, which is mostly formed by proteins between cells. Also, 7 years cycle is myth. Some cells live for days (like sperm), some for weeks (external layers of skin), some for months (red blood cells), some – for an entire life (neurons).