Source: http://www.todayifoundout.com/index.php/...

It's not osmosis.

It’s the vasoconstriction that is the cause of wrinkling digits.  In the 1930s, Dr.’s T. Lewis and GW Pickering, noticed patients with damaged median nerves (one of the main nerves in your arms that can affect the movement and sensation in the hands) did not have their skin wrinkle in areas connected to that nerve.  Since this observation, there have been several studies that have confirmed vasoconstriction as the main cause of water-immersion wrinkling.