Tooth, plural teeth, are hard structures found in the jaws of many of the higher animals. They have various structures to allow them to fulfill their many different purposes. The primary function of teeth is to tear and chew food and in some animals, particularly carnivores, as a weapon. The roots of the teeth are covered by gums. The color of teeth are supposed to be white, but it heavily depends on the person, and how well they care for their teeth. Teeth are covered by a protective stucture, called the "enamel, " that helps to prevent cavities on the teeth.
The form teeth take and their mode of development in a species is called the species' dentition. Dentists sometimes refer to the inner surface of teeth as the lingual surface (meaning towards the tongue), and the outer surface as the labial surface (meaning towards the lips) or "buccal" (meaning towards the cheek). Other terms are "Mesial", or toward the midline, "Distal", which is away from the midline, "Occlusal", the top surface, or incisal, the cutting surfaces.
Teeth are among the most distinctive features of different mammal species, and one that fossilizes well. Paleontologists use them to identify fossil species and, often, their relationships. The shape of the teeth is related to the animal's diet, as well as its evolutionary descent. For example, herbivore diets are harder to digest thus herbivores have more molars for chewing. Carnivores need canines to kill and tear and since meat is easier to digest, can swallow without the need for molars to chew the food well.
Humans grow two sets of teeth, though some animals grow many more: sharks grow a new set of teeth every two weeks. Some other animals grow just one set. Rodent teeth grow and wear away continually through the animal's gnawing, maintaining constant length.
In humans, the first (a.k.a. milk, primary or deciduous) set of teeth appears at about six months of age. This stage is known as teething and can be quite painful for an infant. Human children have 20 milk teeth evenly distributed across the quadrants. Each quadrant of 5 teeth consists of:
- central incisor
- lateral incisor
- first molar
- second molar
The second, permanent set is formed between the ages of six and twelve years. The new set replaces the 20 teeth of the old set. A new tooth forms underneath the old one, pushing it out of the jaw. Apart from this another 8-12 teeth grow. This set can last for life if cared for properly through a regular program of dental hygiene, including brushing with water or toothpaste as well as periodic professional cleaning by a dentist or hygienist. If a person's teeth are susceptible to decay, for example if the molars include deep pits and fissures, then complete prevention of decay may require treatment with dental sealants.
Adult humans have 32 teeth evenly distributed across the quadrants. Each quadrant of 8 teeth consists:
- central incisor
- lateral incisor
- first bicuspid
- second bicuspid
- first molar
- second molar
- third molar
The last molar of each quadrant (i.e. the third molar and commonly referred to as wisdom teeth) may or may not erupt.
The teeth are numbered, in the most common American system, 1-16 and 17-32, with 1 being the upper right wisdom tooth (whether or not it is present) and 32 being the lower right wisdom tooth.
Plaque is a soft white layer which forms on teeth, containing large amounts of bacteria of various types, particularly Streptococcus mutans. Left unchecked for a few days plaque will harden, especially near the gums, forming tartar.
Certain bacteria in the mouth live off the remains of foods, especially sugars. In the absence of oxygen they produce lactic acid, which dissolves the calcium and phosphorus in the enamel in a process known as demineralisation. Enamel demineralisation takes place below the critical pH of about 5.5
Saliva gradually neutralises the acids causing the pH of the tooth surface to rise above the critical pH. This causes 'remineralisation', the return of the dissolved minerals to the enamel. If there is sufficient time between the intake of foods (two to three hours) and the damage is limited the teeth can repair themselves.
Dental caries (cavitation) occurs when over a period of time the process of demineralisation is greater than remineralisation. Attempts to prevent dental caries involve reducing the factors that cause demineralisation, and increasing the factors leading to remineralisation. Unchecked demineralisation leads to cavities, which may penetrate the underlying dentine to the tooth's nerve-rich pulp and lead to toothache.
In moderation, fluoride is known to protect the teeth against caries. It toughens the teeth by replacing the hydroxyapatite and carbonated hydroxyapatite minerals of which the enamel is made with fluorapatite, which is harder. It also reduces the production of acids by bacteria in the mouth by reducing their ability to metabolize sugars. The addition of fluoride (sodium monofluorophosphate) to toothpaste is now very common, and may explain the decline in dental caries in the Western world in the past 30 years.
Some believe that a diet rich in fluorine salts, particularly in childhood, can lead to a stronger enamel which is less susceptible to decay. Fluoridation of drinking water remains a controversial issue. However, in many parts of the world, the natural water supply may be sufficiently rich in fluorides to supply the needs of children without additional sources being required.
Caries may be treated by filling cavities with a long-lasting material. This was, traditionally, achieved using gold or a compound of metals called amalgam, which contains mercury. For cosmetic reasons, and because it is thought mercury may seep from fillings into the circulation over time, a ceramic or other white filler may be preferred to amalgam. As a last resort, teeth affected by caries may be extracted, preferably under local or general anaesthetic.
Most foods endanger teeth to some extent. By far the best protection is brushing after meals and snacks. However, some foods are worse than others.
Some foods may protect against caries. Milk and especially cheese appear to be able to raise pH values in the mouth and so reduce tooth exposure to acid. Milk and cheese are both rich in calcium and phosphate and may also encourage remineralisation. Plus, they may increase saliva production which increases the pH level in the mouth. Foods high in fibre may also help to increase the flow of saliva. Unsweetened (sugar free) chewing gum stimulates saliva production, and helps to clean the surface of the tooth (even sugary gum may be helpful, since the sugar dissolves out very quickly).
Sugars are commonly associated with dental caries. Other carbohydrates, especially cooked starches, eg crisps, may also damage teeth, although to a much lesser degree. This is because starch is not an ideal food for the bacteria. It has to be converted (by enzymes in saliva) first.
Sucrose (table sugar) is most commonly associated with caries, although glucose and maltose seem equally cariogenic (likely to cause caries). The amount of sugar consumed at any one time is less important than how often sugar containing foods and drinks are consumed. The more frequently sugars are consumed, the greater the time during which the tooth is exposed to low pH levels, at which demineralisation occurs. It is important therefore to try to encourage infrequent consumption of food and drinks containing sugar so that teeth have a chance to repair themselves. Obviously, limiting sugar-containing foods and drinks to meal times is one way to reduce the incidence of caries.
Fresh fruit (and fruit juices) contains not only sugars, but some (oranges, lemons, limes, apples) also contain acids which lower the pH.
Another factor which affects the risk of developing caries is the stickiness of foods. Some foods or sweets may stick to the teeth and so reduce the pH in the mouth for an extended time, particularly if they are sugary. It is important that teeth are cleaned at least twice a day, preferably with a toothbrush and fluoride toothpaste, to remove any food sticking to the teeth. Regular brushing and the use of dental floss also removes the dental plaque coating the tooth surface.
Regular brushing is recommended by healthcare professionals, though not too hard  . Two minutes maximum, without pressing the brush too hard against the teeth and gums.
Brushing teeth immediately after eating sugar is not recommended, because sugar softens the enamel, which can then be damaged by brushing. Better to wait half an hour after eating sugary foods before brushing.
Electric toothbrushes are no more effective than the manual variety, according to research  .
In future, tooth decay could be banished by treatment with a genetically modified bacterium, according to research at the University of Florida. 
Dentures and false teeth
In societies that have high sugar diets, tooth decay can damage teeth badly enough that they need to be removed. This leads to the creation of replacement teeth such as dentures and other tooth replacements.
Abnormalities of the Dentition
- Ameleogenesis Imperfecta - A condition in which the tooths primary surface, the enamel, does not form properly or at all.
- Dentinogenesis Imperfecta - A similar condition to above, but affects the underlying layer of the tooth
- Deossification - Loss of bone tissue
- Dens in Dente (also dens invaginus)
- Dens Envaginus (Opposite of above)
- Supernumerary Roots - More than the normal number of roots for a tooth. Most common in maxillary bicuspids.
- Dilaceration - Trauma to the tooth during formation causing damage to the root structure
- Fusion - The union of two adjacent tooth germs by dentin during formation
- Abnormalities with number of teeth
- Hyperdontia (More than the average number of teeth)
- Hypodontia (Missing teeth)
- Abnormalities with size of teeth
Facts about teeth in non-human animals
- Rodents' teeth grow all their lives.
- Reptiles' and sharks' teeth are replaced constantly, before they wear out. A crocodile replaces its teeth over forty times in a lifetime.
- Elephants' tusks are specialized incisors for digging food up and fighting.
- Turtles and tortoises are toothless.
- An overview of dental anatomy