Topic summary
Placoid scales
Extracted from the Wikipedia article Fish scale.
Placoid scales
Placoid (pointed, tooth-shaped) scales are found in the cartilaginous fishes: sharks, rays. They are also called dermal denticles. Placoid scales are structurally homologous with vertebrateteeth ("denticle" translates to "small tooth"), having a central pulp cavity supplied with blood vessels, surrounded by a conical layer of dentine, all of which sits on top of a rectangular basal plate that rests on the dermis. The outermost layer is composed of vitrodentine (also called enameloid), a largely inorganic enamel-like substance. Placoid scales cannot grow in size, but rather more scales are added as the fish increases in size.