How do dermal denticles differ from the scales of bony fishes?

Dermal denticles, also known as placoid scales, differ significantly from the scales of bony fishes in both structure and growth patterns:

• Composition: Dermal denticles are structurally similar to teeth, consisting of a central pulp cavity surrounded by dentine and covered by a hard, enamel-like substance called vitrodentine. In contrast, the scales of modern bony fishes (such as cycloid and ctenoid scales) are primarily composed of bone and lack these dentine and enameloid layers.1234
• Growth: The scales of bony fishes typically grow throughout the animal's life, often forming visible growth rings as they enlarge. Dermal denticles, however, have a limited size and do not grow once formed; as the cartilaginous fish increases in size, new denticles simply form between the existing ones.142
• Development: Dermal denticles originate from the skin and pierce through the ectoderm, after which no further enamel can be added. Bony fish scales are typically deposited within the dermis as thin, overlapping plates.1