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Treebeard's Stumper Answer
29 January 1999

Prune Fingers

Nothing relieves the pains of a long bike ride like a good soak in the hot tub. I can usually tell when I've had enough because my fingers and toes get wrinkled, like prunes. But why does this happen? Real prunes are wrinkled because they shrink as they dry. But I'm in hot water up to my neck in the hot tub, so that can't be the reason. Why do my hands and feet get wrinkled (or pruned?), but not the rest of me? Does salt water or temperature make a difference? Duty calls. It's time to fire up the hot tub and do some research!

Fingers and toes get wrinkled like prunes after a good soak. I thought of salt and osmosis first, but we don't gain or lose water that easily. Unlike many amphibians, we can live out of water because we are covered with a layer of dead, keratinized skin cells that serve as a moisture barrier. It is this stratum corneum that absorbs water and expands. It is thickest on our hands and feet because they get the most wear. Our skin is attached to the tissues below, so it wrinkles to accommodate the greater surface area. It's just our skin doing it's job.


I did one honest experiment in the hot tub by sitting in the water with one hand outside. I figured that if osmosis were a factor, my dry hand would still wrinkle because of diffusion through the rest of my skin. That didn't happen. Anyway, I'm sure osmosis would work in the other direction. We're pretty salty inside, so we would lose salt and accumulate water, at least when soaking in fresh water. But water retention or edema looks and feels quite different from wrinkling. Skin is not water permeable.

Salt water may be different, but I'm sure I used to get wrinkled back in my surfing days when we used to stay out in the ocean for hours at a time. On the other hand, doesn't soaking in salt water toughen or tighten the skin?

This stumper also answers another question that's long puzzled me. When sun screen or skin lotion soaks in, where does it go? It must be absorbed by the same keratinized layer. It doesn't really get into us at all.

Keratin is a remarkable fibrous protein that also makes up hair, feathers, horns, and hoofs, as well as egg white. (That's another stumper: how can liquid egg white be made of the same material?) The outermost stratum corneum layer of the epidermis is also known as the cornified or "horny" layer. I suffer from painful corns and callosus on my feet, so I know exactly what this means. A corn feels like a sharp horn growing into you!

Here are some links for more information.

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