NOTE: All research used for this article is linked in the bottom of the post.
The Short Answer:
Main Point: Skin on you hands wrinkles because the blood vessels in the hand constrict after long exposure to water. The constriction of the blood vessel pushes the skin inward, causing the wrinkles we all know and love.
BUT there is A LOT more to it. There is still a lot we do not know about why our skin wrinkles. Read along if you want to learn all of what scientists do and don’t know about skin wrinkling.
The History of Studying Hand Wrinkling
Skin wrinkling in water starts 5-30 minutes after immersion. The first study on skin wrinkling due to water immersion was done in 1936. The study showed that after patients’ nerves were removed in their hands, their fingers no longer wrinkled in water. The same phenomenon was observed patients whose fingers were amputated then replanted. Skin wrinkling has since become the standard test to assess nerve function in fingers. There is even a standard test doctors commonly use which relates the amount of wrinkles after water immersion to nerve function.
Does All the Skin on the Body Wrinkle?
Only glabrous skin with sweat glands wrinkles due to contact with water. Glabrous skin is hair free skin and can be found on the palms of our hands, the soles of our feet, the penis and clitoris. However, only glabrous skin with sweat glands wrinkles. Sexual organs do not wrinkle in water despite being made of glabrous skin.
Glabrous skin is the only skin to wrinkle in part due to its unique properties. Glabrous skin has a much thicker dermis and epidermis (the top layers of our skin). When blood vessels constrict, they push in the epidermis towards the dermis at to varying degrees. Blood vessels are found in the dermis. The strength of the connection between the two layers determines how far the skin is pulled in. The change in connection strength causes the wrinkling we see in our hands.
What Exactly Causes this Skin to Wrinkle?
On the surface level, it is a phenomenon known as vasoconstriction. That is just a fancy way of saying that the blood vessels constrict or shrink. Vasoconstriction and vasodilation (blood vessels increasing in size) are controlled by sympathetic nerves. The deeper we dig into it though, the less we know. The neurons in the hand instruct the blood vessels to constrict when in water. There is also likely a connection between sweat glands and constriction, since skin wrinkling is only found on skin with sweat glands.
Some researchers hypothesize that the water enters the sweat glands, changing the electrolyte composition. Due to the imbalance of electrolytes, the neurons fire off, constricting the blood vessels. Blood vessels constrict best in warm ~40°C water, higher pH and lower tonicity. Normally blood vessels dilate in warm temperatures. Blood vessel dilation helps the body release heat. This anomaly helps support the hypothesis related to sweat glands, since nothing else seems to be different in the two environments.
The optimal conditions for skin wrinkling are also the optimal conditions for water absorption in the skin. When conditions are in favor of diffusion of fluids into the skin, more skin wrinkling is observed.
Diabetics have decreased amount of skin wrinkling, possible due to damaged nerves. Manual workers also have less wrinkling, possibly due to thicker stratum corneum. On the other hand, people with cystic fibrosis exhibit more skin wrinkling.
It is worth noting, the aforementioned hypothesis has not been fully proven. Scientists still do not know the full mechanism behind skin wrinkling.
How Does Skin Wrinkling Help Us?
Doctors use skin wrinkling as a test for nerve function in fingers. Fingers with no skin wrinkling are deemed to have minimal nerve function. Replanted fingers often do not wrinkle at all due to minimal nerve function. The wrinkling has 4 levels of wrinkling, which help doctors determine nerve function. Recent research has suggested using blood flow as a more accurate and in depth test to determine nerve function in the fingers.
Do Replanted Fingers Ever Regain Nerve Function?
Yes. Some replanted fingers eventually start to regain nerve function, but it takes a long time. Replanted fingers need at least 4 months before they start to see any levels of wrinkling. Even after 14-16 months, fingers still had less than 50% wrinkling compared to normal fingers. The wrinkling also changes based on the distance from the replant site. The parts closer to the replant site begin wrinkle quicker than further parts.
Does Skin Wrinkling Have Any Evolutionary Benefits?
Skin wrinkling has no known evolutionary benefits. One study hypothesized that skin wrinkling might help with gripping. The researchers tested a variety of different cases and skin wrinkling showed no statistical benefit in any of the tests. Like many phenomenon in the human, there is still much we do not know about skin wrinkling.
Works Cited:
Physiology of the Skin (Gerard McKnight, Jasmine Shah, Rachel Hargest)
DOI: https://doi.org/10.1016/j.mpsur.2021.11.005
Paradoxical response to water immersion in replanted fingers (Ching-Hua Hsieh, Kuo-Feng Huang, Po-Chou LiLiang)
DOI: 10.1007/s10286-006-0340-9
Skin Wrinkling on Immersion of Hands (Jackson Braham)
DOI: 10.1007/s10286-004-0172-4
Water immersion wrinkling Physiology and use as an indicator of sympathetic function (Einar P. V.Wilder-Smith)
DOI: 10.1007/s10286-004-0172-4
Water-Induced Finger Wrinkles Do Not Affect Touch Acuity or Dexterity in Handling Wet Objects (Julia Haseleu, Damir Omerbašić, Henning Frenzel)
DOI: 10.1371/journal.pone.0084949
WATER-IMMERSION WRINKLING IS DUE TO VASOCONSTRICTION (Einar P V Wilder-Smith, Adeline Chow)
DOI: 10.1002/mus.10323.
FINGERTIP SKIN WRINKLING – THE EFFECT OF VARYING TONICITY (N Tsai, S Kirkham)
DOI: 10.1016/j.jhsb.2004.12.010