If all human cells replace themselves every 7 years, why can scars remain on you body your entire life?
In: Biology
Because the scar tissue is getting replaced with newer scar tissue. That’s also why scars can fade and shrink over the years; the surrounding skin cells can sort of fill in some of where a new scar had originally been.
But your body isn’t reproducing your whole body again in 7 years, like a new car model. It’s constantly replacing cells, ~~b~~millions are being made every second. And they’re replacing it with the same thing that’s already there, so if it’s a scar, it’s going to mostly stay a scar.
They don’t. You might generate a number of new cells equal to the number of cells that make up a whole body every ~7 years, but the vast, vast majority of that is high-turnover cells like skin, GI lining, RBCs, neutrophils and so on, while some barely or don’t replicate at all.
Also, scars are like a quick patch-up job. As far as evolution is concerned, it’s a very efficient solution to major damage, so has stuck around.
1. Not all cells replace themselves every 7 years–some do it much more frequently (e.g. skin cells can last for a week or two, and the lining of your intestines may last a week or less), others are essentially the same cells your entire lifetime (e.g. neurons and parts of your eye). The 7 years thing is a myth.
2. Scar tissue is the result of your body not having true regenerative ability. In a lot of cases, what you have at the end of development is the “best” most complete x you will ever have, and healing is as much about *quickly* patching over wounds to keep you healthy and alive as it is about the much slower, more expensive, and difficult process or replacing what was lost properly. Often, how cells get in that location and know what to do *comes* from developmental processes and related signaling pathways–wound recovery is a different situation, and your cells don’t always intuitively know exactly what was lost. They lack that bigger picture and complete awareness. So, wound recovery via patching is a necessary step, hence, scar tissue and incomplete healing, which can disrupt delicate parts of the body or result in accumulating (permanent) loss of function in time.
The cells replace those cells that die. The new cells take the place of the dead cell. Since the old cell was a scar cell the new cell will also be a scar cell. The new cells therefore form the scar just like the old cells did. It only gets a tiny bit smaller over time as the scar cells at the outermost edges might get replaced by the proper skin cell instead of a new scar cell. In addition the 7 years is an average for how long our cells last. Some cells are replaced every few weeks and some are never replaced.
There’s some great explanations of the questions, but I’d like to add that afaik scars are constantly being maintained by the body. Some infections (scurvy is one) can cause scars to fail and open the old wound due to lack of resources in the body, particularly collagen which is what scars are built from.
You could ask why you still have arms or a head, too. Or why you can’t regrow a lost arm. It’s because your body doesn’t just create a whole new body, it maintains and replaces the structures that are already there with new cells that are the same type as the old cells were.
Even though individual cells within the skin periodically die and are replaced with new cells, the scar collagen remains. The only time when wounds will heal without producing scars is during the fetal stage of life, when the skin produces fetal collagen, a protein that is different from adult collagen,
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