system
Given the following post, respond as if you're commenting on Reddit:
human
Skin cells constantly regenerate. So how/why is it possible to have scars?
gpt
The process of wound healing involves more than just the replication of epithelial (skin) cells. Skin is composed of both epithelial cells and an extracellular, collagen-containing, connective tissue that is laid down in a meshwork pattern when skin develops naturally. In a wound, both the cellular and connective tissue components must be regenerated. 

Scars form primarily due to the regeneration of the connective tissue. The first reason for this is that the body relies more on the regeneration of connective tissue rather than cells to fill in the hole left by the wound. This is because it is quicker to synthesize a lot of collagen than it is replicate a lot of cells. This makes the healed region more fibrous than cellular. In fact, in larger scars, the central regions may be entirely connective tissue, devoid of actual epithelial cells. Second, unlike your pre-wound connective tissue which is arranged in a meshwork, the collagen in a healing wound is laid down in a single orientation which leads to the tougher feeling, but ultimately weaker scarred tissue.