Why enamel does not simply grow back

Mature tooth enamel is acellular, so it cannot restore lost structure the way living tissues do. In early decay, before a cavity forms, minerals from saliva and fluoride can help remineralize the enamel and reverse or arrest the lesion. But an established cavity or major crack does not heal away on its own. Dentin can form a limited protective tertiary layer in response to irritation, but this is not full tooth regeneration.