A scar forms when skin repairs a wound with tissue different from the original.

The longer version

Wound healing replaces damaged tissue in two different ways depending on depth. Shallow injuries confined to the outer skin layer can be rebuilt from cells that survive at the base, restoring something close to the original structure.

Deeper damage destroys the hair follicles and glands that would normally regenerate that layer, so the body fills the gap quickly with collagen laid down by fibroblasts. The result is a patch that lacks hair, sweat glands and the normal pattern of ridges, and whose fibres run in a single direction rather than the woven arrangement of ordinary skin. Over time it can soften and fade, though the underlying structure never fully matches the surrounding skin.

Where this entry sits

These facts look at how the human body works in ordinary moments, from breathing and blinking to balance and sleep. They focus on anatomy, senses, and small reactions that happen constantly without any conscious effort. The full list sits on the Human Body page, where every entry is listed in order.