If skin replenishes itself, why do we have scars?

Scar from a 1mm melanoma by Lee Jordan, on FlickrWhen I was five years old, on a school trip to the zoo, I put my foot down a rabbit hole. To my horror, when it came out my knee was covered in blood. Despite my protestations that my leg was split open, the teacher insisted we carry on. Today – many, many years later – I still bear the mark. Scars last for life, but why?

Skin is a waterproof and bacteria-proof barrier that acts as a well-fitting bag for the organs inside. Under the microscope, a slice through my knee (ouch) would show the skin is made up of layers. The outermost layer is dead cells that create the barrier. Beneath that is a layer of living cells which, when they die, become part of the top layer. It is these living cells that replenish themselves continuously and after an injury this layer reforms.

But to understand how scarring happens, we need to look deeper. The layer of tissue beneath provides an anchor-point, not only for the cells above but also for blood vessels and nerves. This lower tier tissue contains many proteins, some of which are elastic and some like rods. They are woven together in a regular pattern and give skin both its strength and its flexibility.

When an injury breaks this layer, the chance of scarring depends on how severely the organisation of the rod-like proteins is disturbed. If a cut is small then much of the structural array is still there. This framework acts as a template when the skin is repaired – meaning the protein pattern is preserved and the repair is clean. In a large injury, new skin proteins are laid down haphazardly because the framework (which would act as a guide) has gone. The result is a bumpy, less elastic, base layer for the cells – which we see as a scar.

Stitching a wound brings the structural proteins back close together and helps to keep disruption to a minimum – so stitching reduces scarring. For this reason, on behalf of my blemished knee, I would like to take this opportunity to say to Miss Nonsense-You’ll-Be-Fine, “I told you I needed a doctor…. ”

Question from @alisouthsea via twitter

Answer by Julie Webb

Article by Julie Webb

May 1, 2013

Dr Julie Webb is a freelance science communicator, who contributes to MRC's biomedical picture of the day. You can read her blog here.

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