Sharks may have up to 3,000 teeth at one time. These teeth are modified placoid scales that have the same structure as a tooth, having an outer layer of enamel, dentine and a central pulp cavity. The teeth are arranged in rows and when one tooth is damaged or lost, it is replaced by another. Their teeth are constantly replenished and have unlimited rows of teeth to replace others (they are polydonts). Most sharks have about 5 rows of teeth at any time.