Why do JUST hands and feet shrivel up in water?

ok, so there are exceptions (extreme foot immersion, male genitalia, bum, etc.) but my question is...what makes certain body parts more susceptible to wrinkling. If you stayed in water long enough, would your whole body shrivel up? I understand the theory of osmosis, but I'd like to know what sets these places apart from the rest. Is it an oil, amount of fat, layers of skin, what is it?