How Come?: Why are the tips of our fingernails white?

Imagine having 10 soft, nail-less fingers, and 10 delicate, unprotected toes. Nails are nature's way of shielding our hands and feet from the slings and arrows of the world. Nails also act as a kind of back brace, helping our fingertips sense texture and other information through touch. Of course, nails are also handy for picking up a dime or scratching a mosquito bite.

Fingernails and toenails are made of a protein called keratin (from a Greek word meaning "horn," the kind on an animal's head). Keratin comes in many varieties and is a building block in everything from horses' hooves to the silky hairs on our head.

Our finger and toe shields, the nail plates, are made of up of layer upon layer of dead, "keratinized" cells. The ridged nail plates fit into the grooves of the nail bed underneath, with a semirigid keratin acting as a strong glue. The cells of the nail plate are translucent, and the nail bed underneath is full of crisscrossing blood vessels. So our nails look pink.

The nail tips, also known as the distal edges, are where this super-glued attachment ends. The translucent nail tips project beyond the blood-red nail bed, and so don't look pink. We see white when light reflects from the tips.

The dirt that tends to collect under the tips makes them more opaque, adding to the milky-gray look. But it's easy to prove that the tips are actually as translucent as the rest of the nail plate: Under a strong light, slide a piece of colored paper underneath a clean nail tip. You should easily see the edge of the paper.

The base of the nail can sport a whitish design, too. Look at your fingernails - especially your thumbs - and you should see a half-moon called the lunula (Latin for little moon). The lunula is actually the visible part of the matrix, giving us a peek at the nail-plate factory. Look closely at lunula, and you may notice that its curve mirrors that of the natural edge of your nails. That's no accident; the unique shape of the lunula determines the shape of your nails.

Why is the lunula white? Unlike the dead keratin cells in the nail plate, the keratin cells in the lunula have nuclei (centers). These nucleated cells reflect rather than transmit light, creating a grayish-white half-moon.

But the lunula and nail edge aren't the only whitish bits on our nails. Dermatologists say that small white spots turn up on everyone's nails sooner or later. The most common cause: bangs and dings to the fingernail base. Where the nail bed is injured, new nail cells are incompletely formed or keratinized, creating a white spot. Luckily, the spot rises as the nail is pushed up by new growth from below. Since a typical fingernail grows about 1/32 of an inch a week, it can take months for a white spot to reach the (white) tip - and meet its scissored-off fate.