Teeth are accessory digestive organs located in sockets of the alveolar processes of the mandible and maxillae. The alveolar processes are covered by gums, sockets are lined by the periodontal ligament, that anchors the teeth to the socket and acts as a shock absorber during chewing.
A tooth has three regions-the crown, root, and neck. The crown is the visible portion above the level of the gums. Embedded in the socket are one to three roots. The neck is the constricted region between the crown and root.
- Dentin forms the majority of the tooth that gives the tooth its basic shape and rigidity. The dentin of the crown is covered by enamel, which consists of calcium phosphate and calcium carbonate. It serves to protect the tooth from the wear and tear of chewing.
- The dentin of the root is covered by cementum, which attaches the root to the periodontal ligament. The dentin of a tooth encloses a space, the pulp cavity lies within the crown and is filled with pulp, containing blood vessels, nerves, and lymphatic vessels.
- Narrow extensions of the pulp cavity, called root canals, run through the root of the tooth. Each root canal has an opening at its base, the apical foramen, through which blood vessels, lymphatic vessels, and nerves enter a tooth.
Humans have two sets of teeth-deciduous and permanent. The deciduous teeth which fall, also known as primary teeth or milk teeth begin to erupt at about 6 months of age and all 20 are present at the age of 2 years.
- The Incisors:– Are at the midline, adapted for cutting into food. They are central or lateral incisors based on their position.
- The Canines:– Have a pointed surface called a cusp are used to tear food. Incisors and Canines have only one root.
- First and second deciduous molars:– Which have four cusps. Maxillary molars have three roots, Mandibular molars have two roots. The molars crush and grind food for swallowing.
All of the deciduous teeth are lost between ages 6 and 12 years and are replaced by the permanent (secondary) teeth. The permanent dentition contains 32 teeth that erupt between age 6 and adulthood. The deciduous molars are replaced by the first and second premolars are used for crushing and grinding.
- The permanent molars, which erupt posterior to the premolars, do not replace any deciduous teeth and erupt as the jaw grows to accommodate them, the first permanent molars at age 6, the second permanent molars at age 12 and the third permanent molars (wisdom teeth) after age 17 or not at all.