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'Light micrograph of a section through a tooth. At the centre of the tooth is the dental pulp (white), which contains the blood vessels and nerves. The majority of the tooth consists of dentine (purple), which is mineralised connective tissue. The tooth is covered by a coat of white enamel (dark purple), the hardest tissue in the human body.'. COVER IMAGE ASTRID & HANNS-FRIEDER MICHLER/SCIENCE PHOTO LIBRARY