A tooth is divided into the crown, which is the part above the gum line, and the root, which is the part below the gum line.
The crown is covered with white enamel, which protects the tooth. Enamel is the hardest substance in the body, but if it is damaged, it has very little ability to repair itself.
Under the enamel is dentin, which is similar to bone but is harder. Dentin surrounds the central (pulp) chamber, which contains blood vessels, nerves, and connective tissue. Dentin is sensitive to touch and to temperature changes.
The blood vessels and nerves enter the pulp chamber through the root canals, which are also surrounded by dentin. In the root, dentin is covered by cementum, a thin bonelike substance. Cementum is surrounded by a membrane (periodontal ligament) that cushions the tooth and attaches the cementum layer, and thereby the whole tooth, firmly to the jaw bone.
People have two sets of natural teeth:
There are 20 primary teeth: one pair each of upper and lower central (front) incisors, lateral incisors, canines (cuspids), first molars, and second molars.
There are 32 permanent teeth: one pair each of upper and lower central incisors, lateral incisors, canines, first bicuspids, second bicuspids, first molars, second molars, and third molars (wisdom teeth). Wisdom teeth, however, vary—not everyone gets all four wisdom teeth, and some people do not get any wisdom teeth.
There is a broad range of normal times for teeth to push through the gum tissue (erupt) into the mouth. For primary teeth, the central incisors are the first teeth to erupt, occurring at about 6 months of age. These are followed by the lateral incisors, first primary molars, canines, and, finally, second primary molars. By about 2½ years of age, all the primary teeth can usually be seen in the child's mouth.
Each of these primary teeth will be pushed out by a permanent tooth, starting at about age 6. The permanent first (6-year) molars come into the mouth just behind the last primary molars and, therefore, do not replace any teeth. This lack of replacement is also true for the permanent second and third molars. The wisdom teeth are the last permanent teeth to come in, typically between the ages of 17 and 21.
An impacted tooth is one that cannot erupt because of a lack of space or because of the position of the teeth. Most teeth that become impacted are wisdom teeth.
In rare cases, a child is born with a tooth (a natal tooth), or a baby tooth erupts in the mouth within a month of birth (a neonatal tooth). These teeth are usually primary lower incisors, but they may be extra (supernumerary) teeth. These teeth are removed only if they interfere with nursing or if they become exceedingly loose.
In many children, the permanent lower incisors come in behind each other. Lack of space (due to crowding, rotated permanent teeth, or abnormalities in skeletal development) may be the problem, and early orthodontic therapy (braces) may be necessary. Thumb or finger sucking may also affect the position of teeth, sometimes requiring early orthodontic therapy.