Dermal denticles (placoid scales) are tough "scales" that cover the skin of elasmobranchs (sharks and rays). Even though denticles are similar to scales, they are really modified teeth and are covered with a hard enamel. These structures are packed tightly together and grow with their tips facing backwards. This gives the skin a rough feel. The function of these denticles is for protection against predators, although in some sharks, they may also have a hydrodynamic function.
Like our teeth, dermal denticles have an inner core of pulp (made up of connective tissues, blood vessels, and nerves), which is covered by a layer of dentine (hard calcareous material). This is covered enamel-like vitrodentine, which provides a hard outer casing.
Scales in bony fish grow as the fish gets larger, but dermal denticles stop growing after they reach a certain size, and then more denticles are added as the fish grows.