Why Do Fingers Prune?
After a long soak in the tub, fingers emerge looking like raisins. The real reason for this curious phenomenon lies under the skin.
Danielle is an Assistant Editor at The Scientist. She has a background in neuroscience and molecular psychiatry. She has previously written for BioTechniques News, The Scientist, and Drug Discovery News.
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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.
Scientists still do not understand how exactly water triggers vasoconstriction, although Wilder-Smith has his theories.1 “It's likely that there's a whole array of different electrolyte channels, and that these are selectively being stimulated,” he said.
Observational studies in patients with peripheral nerve damage further support the notion that wrinkling is an active process. The median nerve runs down the arm into the hand and regulates sweating and blood flow. Patients with nerve damage in their hands did not exhibit wrinkling in the affected fingers.2 In the 1970s, the mother of a child with nerve damage in the hand noticed the return of shriveling fingers following nerve repair. This observation inspired the hand surgeon Seamus O’Riain at University College to develop a simple test that uses wet fingertip wrinkling as a readout of nerve function, which is still used today.3
References
- Wilder-Smith E, Chow A. Muscle Nerve. 2003;27(3):307-311.
- Wilder-Smith E. Clin Auton Res. 2004;14(2):125-131.
- O’Riain S. Br Med J. 1973;3(5881):615-616.
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