The fact that skin can scar is puzzling. Does it forget how to grow back correctly? Does the DNA forget? Do the SDopers know the inside scoop on scarring?
- Jinx
I don’t know if this helps any (if at all – probably not) … maybe more of a rider on your OP … but skin isn’t the only thing that scars. I thought any kind of tissue can scar. I know that I’ve heard of people having scarring on the lungs, and brain (but that could be a different thing). I think scarring (bruising – is there a difference?) can be the cause of paralysis in people with spinal cord injuries.
One thing I do know: scar tissue is not as “good” as skin. Once skin becomes scar tissue, it isn’t as resilient (?) as the normal skin used to be.
I think it has something to do with the fact that scar tissue is undifferentiated cells…sort of generic cells. In higher mammals after a certain age the specific cells are no longer available to allow tissues to regenerate properly…so they scar instead, sort of patching damage with generic patches.
Or something. I could of course be totally wrong here. 
-XT
I’m 90% talking through my hat here, but let me take a shot.
Normally, skin grows in layers, with new cells growing in the deep layers as older layers move up toward the surface. The cells in the upper layer die, becoming the water-resistant outer layer of skin.
If you injure your skin, your body quickly grows in scar tissue (which, as xtisme says, is undifferentiated skin cells) in order to restore the barrier between it and the nasty, germy outside world as quickly as possible.
Depending on how deeply into the skin the injury went, and and how extensive the injury was, over time, the normal layers of skin will grow in from under and around the scar, and scar tissue will be replaced by normal skin.
Well, according to te book, “An Introduction to Tissue Biomaterial Interactions”, scar tissue forms, in part, because,
Basically, when you get wounded, the surronding tissue doesn’t grow back the way it grew the first time. Many, many things happen, but in an effort to simplify things, we won’t discuss them all here. But basically, when a wound starts healing, one of the first things that’s done by all the cells, proteins, etc… in the body is to build an extracellular matrix. Without this matrix, there would be nothing for the new cells to hold on to, as well as the fact that the matrix, in addition to the type of cells, determines the properties of the tissue being formed. A skin matrix is much different than a muscle matrix.
One way to explain it is that the body gets overzealous in making the matrix. It wants to be damn sure that the wound heals, so it basically puts more of what is really needed in an effort to make the wound heal better. So, as a result, you have a scar.
Scars are still an area that have some mystery about them. Why do some people have more noticable scars? How come, sometimes, you can remove a scar by wounding itm and have it grow back to not be a scar? (of course, it could also grow back to be a worse scar.)
(I am not a doctor, just a biomedical engineer. I’ve left my answer fairly vague in the opes of not getting anything wrong. Maybe someone more qualified (QtM, I’m talkin’ to you) can explain things further.)