Michael Alexander, age 9, of Charlotte, N.C., for his question:
WHAT ARE TEETH MADE OF?
In the normal human adult's mouth are 32 permanent teeth positioned symmetrically in horizontal arches of 16 teeth each in the lower and upper jaw. The teeth are divided into four quadrants of eight teeth each. Each quadrant has four specific types of teeth which are shaped and sized differently.
Teeth are important for chewing food. During his lifetime, a person will receive two sets of teeth: first a set of baby teeth and later a set of permanent, adult teeth.
Teeth are made of hard, calcified tissues embedded in bony sockets in the upper and lower jaws.
Each tooth has a crown which is the visible and functional part, and a root, which is embedded in the bony socket and anchors the tooth in the jawbone. A layer of tissue called the periodontal ligament covers the root. It is a highly specialized structure which holds the tooth in place and lessens the pressure of the occlusion or biting during chewing
A tooth is composed of four tissues: enamel, cementum, dentin and dental pulp or nerve. The bulk of a tooth is formed by dentin, which is a hard, glistening substance consisting of 67 percent inorganic salts.
The part of the tooth covered by enamel is called the crown and it extends above the gum line. Enamel is composed almost entirely of inorganic salts and it forms the very hardest tissue in the entire human body.
The cementum, a tissue almost identical with bone, covers the surface of the roots.
In most cases, incisors, cuspids and bicuspids have one root, while molars have two or three. The tooth core is filled with the pulp containing not only the nerves but also arteries, veins and lymphatics that enter the tooth through an opening at the root tip.
The periodontal ligament is attached to the cementum on one side and to the bone on the alveolus or jawbone on the other. A slight construction between the crown and root is called the tooth neck, or cervix.
The color of the crown depends largely on the color and translucency of the enamel. Normal enamel is light yellow and is translucent enough to permit the darker yellow dentin to show through.
The cervical parts of the crown are normally darker than the incisal or occlusal parts. When the enamel has worn and the dentin becomes exposed, the tooth darkens and becomes stained from the color contained in food and beverages.
In the bony socket where the tooth enters the jaw, the bone extends upward like a collar above the tooth. This collar is called the alveolar process.