Enameloid

Enameloid is a mineralized dental tissue found in a broad array of jawed vertebrates, particularly fishes. It forms an outer layer on teeth and denticles and is similar in function to enamel, providing hardness and wear resistance, but it differs in developmental origin and microstructure.

Unlike true enamel, which is produced by ameloblasts in the enamel organ, enameloid is formed by odontoblasts, the same cells that lay down dentin. The tissue is usually rich in hydroxyapatite crystals and may incorporate an organic matrix. In many fishes, enameloid covers a dentin core and may be organized into layered crystal structures or prism-like patterns that resemble enamel.

The distribution of enameloid is broad among fishes, including cartilaginous and bony fishes, and it has also been reported in some extinct vertebrates. The relationship between enameloid and enamel is a subject of ongoing study and debate: some view enameloid as an enamel analogue produced by a different cell lineage, while others consider it a primitive enamel-like tissue that has been retained or modified in various lineages.

Enameloid figures prominently in discussions of vertebrate dental evolution and paleoecology. Its presence can influence interpretations of feeding behavior and the evolutionary transitions from denticles to enamel-coated teeth, contributing to broader understandings of tooth tissue diversity across vertebrates.