Why do scars remain if the body replaces its cells every 7 years?

Answer 1

It's because the repair process works from the bottom up

A couple of sites agree as to why scars are permanent, but here's the clearest explanation I found:

Unfortunately, many times when our skin is wounded, the cut isn't clean, the healing conditions are not ideal and we end up with a scar. "Evolution has selected for scarring," says Dr. John Newman, a cosmetic surgeon and researcher at the Laser Center of Virginia in Virginia Beach. "Scarring is the result of a system that has learned to respond extremely quickly to a wound."

When the skin is split open, the body immediately starts to pull the edges of the wound together. It does this by growing epithelial tissue over the open wound at the rate of approximately 1 millimeter every 24 hours. Fibroblasts (special collagen-producing cells found in the dermis, the second layer of skin) then rush into the wounded tissue and start laying down collagen to strengthen the skin.

Normally, fibroblasts produce an organized lattice of collagen that is very strong, but when in emergency-response mode, they respond by laying down collagen in a very unorganized, haphazard way. "It is kind of like nailing down a crisscross of two-by-fours over a hole in a deck," says Newman. "It seals the hole, but it doesn't look very nice."

He explains that "a scar is actually a bunch of unorganized collagen in the dermal layer of the skin." Over time, the tissue will try to reorganize, and the scar may appear to soften, but the skin may never completely return to its original state - particularly if the cut extends beyond the epidermis, the skin's outer layer.

HOW REGENERATION WORKS

Skin cells regenerate from the bottom up, with a turnover time of roughly one month. "You can think of the epidermis (the top layer of skin that you see) as a staircase," says Newman, "where the bottom stair is the base of the epidermis and the top stair is exposed to the air." New epidermal cells are formed by cell division at the bottom of the staircase. To make room for these new cells, the older epidermal cells are pushed upward toward the top of the staircase. The oldest cells die and fall off.

The genetic material, or DNA, within any cell does not generally change, so the cells that are dividing on the bottom stairs are transmitting the same genetic characteristics from one generation of cells to the next. Unfortunately, if these characteristics are of scar tissue, that is what will be regenerated. And regardless of how much epithelial regeneration occurs above it, the scar can still show.

Answer 2

Scars form when the body creates collagen fibers to quickly repair damaged skin. While our cells do regenerate over time, the healing process of scars is different, leading to the scar tissue remaining even as cells regenerate.