Claws are products of the skin, and are found at the tips of the digits of most four-legged animals. Claws consist of thick deposits of keratinized, flattened epithelial cells. Keratin is a fibrous protein that is also a major consistuent of hair, the outer layer of skin, reptile scales, and bird feathers.

A layer of cells called matrix cells, located at the base of the claw, produces new claw. The claw grows outward from the matrix, such that the newest claw cells are located at the base of the claw and the oldest cells at the tip.

Cross-section of a mammalian claw. The claw grows outward (blue arrows) from proliferating matrix cells at its base. Adapted from Kardong 1995.

The claw consists of two parts:

• The dorsal unguis (also called the nail plate). This is the hard, external part of the claw. In the unguis, the grain of the keratin is perpendicular to the direction of growth and is arranged in layers at an oblique angle.
• The ventral subunguis (also called the sole plate). This is the soft, flaky underside of the claw. The grain of the keratin is parallel to the direction of growth, and softer than that of the unguis. The subunguis is continued by a cusion-like pad.
  

Drawing from life of a side view (top) and bottom view (bottom) of a rat's claw.

The unguis curves around and surrounds the subunguis, except at the tip of the claw where the edges of the unguis do not meet. The unguis grows faster than the subunguis, producing the claw's familiar downward curve. The sides of the claw are thinner and wear faster than the top median line of the claw, thus producing a point.

The rat's claw has a sensitive pink "core" or "quick," called the nail bed, which you can see through the semi-transparent keratin. In humans, the nail bed is the flat pink part of the fingernail.

Claws are found in most four-legged animals. Primates have flattened versions of these claws, called nails. In fact, primate fingernails and toenails may actually be fetal versions of claws. Anatomically, our unguis is flat and our subunguis is very reduced -- just a thin band of softer material under the outer edge of the nail. Ungulates, like horses and cows, have an especially thick version of the claw called a hoof.

• Human nail development
• Legs, feet and cursorial locomotion
• Characteristics of mammals, including claw, nail and hoof structure
• Mammal characteristics
• Photo of a carnivore claw