the_original_Retro: Human cells can replace themselves, this is correct. But they need a scaffold to replace themselves ON for them to be in the right place. And the nature of that scaffold is why scars stick around forever.
Let’s compare our bodies to a multi-floor brick building that King Kong or Cloverfield or Godzilla or something punches a big chunk out of.
You have a couple choices to do something about that building before the weather gets in and wrecks it worse. But a feasible one of them isn’t a complete tear-down and rebuild using scaffolding and heavy construction to recreate the building properly. People have got to go on living in there and there’s not enough free spending money around to do it.
So you patch that hole as best you can and maybe brick up the opening, and that’s good enough for people to keep living in it. But it leaves a not-very-pretty gap in your building. It’s functional even if some of the electrical stuff or elevators don’t work due to the still missing area, and it looks ugly because you couldn’t quite get everything perfect without bringing in super-expensive heavy machinery and shutting everything down, and the bricks don’t match. So you’re left with a serviceable building with ugly spots that you can’t ever afford to make perfect-looking again.
Scarring’s the same. The body doesn’t have the ability to regenerate huge missing areas because it can’t create scaffolding once you’re out of the womb. All of the ‘heavy equipment’ necessary for it is no longer available. This wasn’t critical enough of a skill for us to evolve as a species because enough of us survived and had kids even without it to take over the world. So the body goes with a “walling off” strategy without coming with a bunch of perfectly set-up scaffolding to build new clean supporting structures for the new cells to grow back into their perfect original shape.
And those wall-offs are dead ‘hard’ tissue that is permanently set into their walled-off shape and can’t be replaced. Again, perfect-looking repairs weren’t necessary to the survival of our species so we didn’t evolve them.
PhyterNL: I’m pretty sure scars do change over time as cells are replaced. I have many scars from my youth that I can barely see anymore. But the whole reason a scar exists is that the underlying collagen fibers, the extracellular material that acts as connective tissue, has been damaged. Thus any new cell growth is going to reflect that with raised, bumpy or discolored skin. So a scar may fade but will never go away completely.
Madpenguin_CH: Your scar-tissue is replaced as well. Basically your body can simply “forget“ what is scar tissue and how it originally was supposed to look like.
Luckily we have developed a mechanism that reduces scar tissue back after it has done it’s job but it’s not perfect and since, most often, the consequence of this “failiure“ is only a minor optical flaw the evolutionary pressure to improve that process is rather low.
Possible causes for the system “failing“ are most likely quite diverse but what can be said is: Replacing cells does not automatically restore the information of how the cells were arranged.
JJ-CyberTonic: Some cells take up to 10 years. And the body can only replicate what is there, rather than restore it to it’s original state, if it could remember how it was before and recreate that we would never age!
GISP: The cells in the scars are replaced with by new “scar cells” and not by cells as they where before the damaga, as cells dosnt have a memory or know how its “surpose” to be.
If you weld a damaged car, and keep the maintancance up, and continue to repair it as it breaks, it dosnt fix the original damage, you just keep repairing. 🙂
Sidenote: Repairs going wrong = cancer.