A guy who absentmindedly scratches his chin late in the day feels the stubble that has grown in since his morning shave. But if his girlfriend were to pass her hand along that same chin, there's a good chance that she would be better able to feel the individual hairs.
A few months ago, Ellen Lumpkin of Baylor College of Medicine in Houston provided strong evidence that we require Merkel cells in order to interpret Braille, compare rougher and finer grains of sand paper or manipulate strands of dental floss. She created mice with altered genes that grew and developed normally but lacked Merkel cells in certain parts of their bodies. The modified rodents could still feel vibrations, temperature changes and pain, but their nerve cells stopped responding to light touches.
Merkel cells are difficult to spot in a human finger. So instead of looking for them directly, Goldreich looked closely at the sweat glands on his subjects' fingertips, which are usually found above Merkel cells. He noticed that in larger fingers the sweat glands -- and presumably the Merkel cells beneath -- were spaced farther apart.