Danielle Gerhard in The Scientist:
Why fingers shrivel up in water is an age-old question that every child asks at bath time, but the answer may come as a surprise. “The whole body doesn’t wrinkle, and that says a lot,” said Einar Wilder-Smith, a neurologist at Luzerner Kantonsspital. Unlike the surrounding skin, the outermost layer of the fingertip is highly innervated with and tethered to vasculature, setting the stage for pruney fingers.
While osmosis, particularly into dead skin cells, seems like a compelling explanation for this curious phenomenon, Wilder-Smith clarified, “In fact, it’s the opposite.”
In the early 2000s, while working at the National University Hospital in Singapore, Wilder-Smith suspected that the surrounding vasculature drives the wrinkling. He found that after a long soak, blood vessels nestled just below the skin constrict, resulting in negative pressure and downward tugging of the outermost layer of skin. The uneven puckering pattern likely results from varied skin tautness, or tethering, throughout the fingertip.
More here.