Most likely you've noticed your fingertips when you come out of the pool or bathtub after a long soak: They are wrinkled, much more so than when you went in. Yet, soon after you dry off, they return to normal. This is part of a complex process to keep your skin moist and flexible and is related to the reason most people need to apply moisturizing oil or lotion.
The skin is made up of several layers. The top layer is the stratum corneum--which in Latin simply means horny layer. This layer is particularly thick at your fingertips and toes. This is the layer that flakes off and may appear as dandruff and appears in your bed linens, in your clothes and elsewhere.
To counteract the flaking, your epidermis, the main outer layer of skin, produces sebum. Sebum is a protein that repels water. However, when you are in the tub, scrubbing away the dead cells on your statum corneum, you also soak away some of the sebum. Thus your skin can't repel the water as before and so it swells up, especially in your toes and fingers. You don't notice this swelling because you are surrounded by fluids.
When you get out of the tub, the excess water quickly evaporates, however, it takes your skin longer to shrink back to its original appearance. At this point, your skin is particularly dry because it doesn't have as much sebum, so it's a good idea to use moisturizer for added protection for your skin.
Interestingly enough, wrinkling may not happen if you are in saltwater. According to Grace Fields at Argonne National Laboratory, if the water is saltier than your skin, it will draw water out of your skin. If the water is less salty, your skin will absorb the water around it. This is because water travels from the areas with the least salt concentration to those with the highest salt concentration. The water in your body is saltier than fresh water, hence the wrinkles.
When there is nerve damage in the fingers or toes, the skin does not wrinkle in water. When that nerve damage has been repaired, the skin wrinkling reoccurs, at least in part. Many hospitals and other medical facilities check for nerve damage by submerging the body part in water and checking for wrinkling, according to Gregory Borah, MD, in a letter to "Discover" magazine.
Similarly, diabetes, leprosy and other disorders affecting the nerves may not develop skin wrinkles because of their condition while those with cystic fibrosis have been shown to develop such wrinkling quicker.