Teeth (singular, tooth) are structures found in the jaws of many vertebrates. The primary function of teeth is to tear and chew food, and in some animals, particularly carnivores, for fighting and/or defence. The roots of the teeth are covered by gums. Adult teeth naturally darken with age as the pulp within the tooth shrinks and dentin is deposited in its place.
Teeth are among the most distinctive features of mammal species and fossils. Paleontologists use them to identify fossil species and their relationships. The shape of the teeth is related to the animal's diet. For example, plant matter is hard to digest, so herbivores have many molars for chewing. Carnivores need canines to kill and tear meat.
While humans develop two sets of teeth throughout life (diphyodont), some animals develop only one set (monophyodont) or develop many (polyphyodont). Sharks, for example, grow a new set of teeth every two weeks which is very useful since they munch a lot of food to keep alive in the sea like turtules and porpoises, and the wear and tear would mean they would die of starvation. Rodent teeth grow and wear away continually through the animal's gnawing, maintaining approximately constant length.
Humans are diphyodont, meaning that they develop two sets of teeth throughout life. The first set (the "baby," "milk," "primary" or "deciduous" set) normally starts to appear at about six months of age, although some babies are born with one or more visible teeth, known as neonatal teeth. Normal eruption of teeth starting at about six months is known as teething and can be quite painful for an infant. Human children have 20 deciduous teeth evenly distributed across the mouth's quadrants. Each quadrant of five teeth has a:
The second, permanent set of teeth consists of 32 teeth. Twenty-eight of them appear between the ages of about 6 and 12 years. Secondary teeth do not push deciduous teeth out of their sockets; instead, a group of cells (odontoclasts) forms in front of tip of second tooth and dissolves the base of first tooth. Finally, the first tooth is held in place only by tissues of gum. Deciduous molars are replaced by premolars. The third molars (the wisdom teeth) are the final teeth to erupt, usually around age 20. However, it is common for the wisdom teeth not to erupt at all; this is often the case in small jaws without room to support the extra teeth. It is possible, though rare, for a person to have fourth molars, and there have been instances where fifth molars have been present in the dentition. 
Permanent teeth are evenly distributed across the mouth's quadrants. Each quadrant of eight teeth has a:
- central incisor (upper jaw: maxillary central incisor; lower jaw: mandibular central incisor)
- lateral incisor (upper jaw: maxillary lateral incisor; lower jaw: mandibular lateral incisor)
- cuspid (canine) (upper jaw: maxillary canine; lower jaw: mandibular canine)
- first premolar (bicuspid) (upper jaw: maxillary first premolar; lower jaw: mandibular first premolar)
- second premolar (bicuspid) (upper jaw: maxillary second premolar; lower jaw: mandibular second premolar)
- first molar (upper jaw: maxillary first molar; lower jaw: mandibular first molar)
- second molar (upper jaw: maxillary second molar; lower jaw: mandibular second molar)
- third molar (wisdom teeth) (upper jaw: maxillary third molar; lower jaw: mandibular third molar)
The permanent set may last for life if cared for properly through a regular program of dental hygiene, including regular brushing and professional cleaning by a dentist or hygienist. Teeth that are susceptible to decay may be sealed for additional protection.
Teeth are attached to the underlying bone of the jaw via the periodontal ligament, though the teeth themselves are not made of bone. The white part of the tooth, which can be seen in the mouth, is the enamel. Immediately one to three mm below the enamel is a slightly softer, yellow tissue called dentin. Dentin is supported by the pulp (commonly called 'the nerve', although it contains many other structures which are not nerves), which lies in the center of the tooth. The teeth's composition is specialized to resist the harsh environment of the oral cavity and withstand the large forces imposed upon them by mastication, or chewing.
Dentists use several different notation systems to refer to a specific tooth. The three most commons systems are the Universal numbering system, the Palmer Notation Method, and the two-digit FDI World Dental Federation notation which is widely used internationally . 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 (toward the midline), distal (away from the midline), occlusal (the top surface), incisal (the cutting surface), "gingival" (toward the gumline), and "pulpal" (toward the center).
The usual numbers of roots are:
- 1 - incisors, canines, lower premolars, and second upper premolars
- 2 - first upper premolars
- 3 - lower and upper molars (often two fused together, but still distinct roots)
See also supernumerary roots.
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 and . 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 which cause 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) then the impact is limited and the teeth can repair themselves.
Caries is a term used to describe the process in which some bacteria produce cavitation of the teeth. This group of bacteria, which are a part of the normal oral flora, have a complex relationship with the host in which they use sugar that the host consumes to metabolize as energy and produce acid which subsequently dissolves the teeth. Caries refers to many, not one, species of bacteria including lactobacillus and viridians streptococci. Pits and fissures and interproximal smooth surfaces are the most commonly colonized areas of the teeth. Attempts to prevent dental caries involves 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 cavities. It toughens the teeth by replacing the hydroxyapatite and carbonated hydroxyapatite minerals of which the enamel is made with fluorapatite, which is harder to dissolve by acid. 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 76 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 is an infectious disease and is treated only by prevention. Once the decay process begins, caries can be eliminated only through the removal of the bacteria. Fluoride can be used to remineralize decalcified enamel, but new tooth structure cannot be regrown. When a patient gets a filling, the dentist mechanically removes the bacterial flora and dead tooth structure and replaces it with a restorative material (metal or glass). This was, traditionally, achieved using gold or a compound of metals called amalgam. Amalgam fillings have been the cause of some public concern because they contain mercury. Modern tooth-colored composite or ceramic are used in addition to traditional amalgam fillings.
As a last resort, teeth affected by caries may be extracted, preferably under local or general anaesthetic. The extracted teeth may be either replaced with a denture or if extracted much before permanent teeth erupts, the space may be maintained by specialized ortho appliances called Space Maintainers.
Regular brushing is recommended by healthcare professionals twice a day. . Studies show that the minimum required time interval is once every two days. A mirror may be used to see if the plaque is completely removed during brushing. The actual term 'brushing' is rather inaccurate. Though a toothbrush is used, it is not designed to deliver mechanical abrasive action. Rather, the surface of the brush is meant to be held at a forty five degree angle to remove plaque from the periodontal margin (the space in between the gum and tooth). Thus, the term 'cleaning teeth' is far more accurate.
In research, levels of plaque were recorded before and after brushing and found that plaque removal steadily improved as brushing times and pressure were increased. However, their results showed that when people brush for longer than two minutes, at a pressure higher than 150 grams (the weight of an orange), they are not removing any additional plaque, and may be causing permanent damage to the teeth and gums.
Brushing teeth immediately after eating acidic foods is not recommended, because acid softens the enamel, which can then be damaged by brushing. It is better to wait at least half an hour after eating acidic foods before brushing. Rinsing the mouth and eating non-acidic foods can also speed up this process.
Electric toothbrushes are no more effective than the manual variety, according to research. However, "rotation-oscillation" electric toothbrushes out-performed manual brushing, removing around 7% more plaque and leading to 17% less gum disease than manual brushes.  Any kind of electric toothbrush does tend to help people who are not as good at cleaning their teeth and as a result have had oral hygiene problems.
As noted above, eating certain cheeses such as cheddar or Parmesan soon after eating potentially harmful foods have been noted to be helpful in preventing tooth decay as well. This is possibly due to the alkalinity of cheese, which neutralizes acid produced by bacteria.
In the future, tooth decay may 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
- Amelogenesis imperfecta - a condition in which the tooth's 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
- Dental fluorosis - white spotted, yellow, brown, black and sometimes pitted teeth from over-ingesting fluoride
- Supernumerary roots - presence of a higher-than-normal number of roots on a tooth. Most common in maxillary bicuspids.
- Abnormalities with number of teeth
- Abnormalities with size of teeth
- Abnormalities in shape of teeth
- Tooth Gemination
- Tooth Fusion - the union of two adjacent tooth germs by dentin during formation)
- Talon cusp
- Cusp of Carabelli
- Dens Envaginus - cusp-like elevation of enamel
- Dens in Dente - also called dens invaginus
- Ectopic enamel
- Dilaceration - trauma to the tooth during formation causing damage to the root structure
- Supernumerary roots - presence of a higher-than-expected number of roots on a tooth
Development of teeth
There are four stages in the embryonic development of teeth, the bud Stage, the cap Stage, the bell Stage and the crown stage.
The bud stage begins at the 7th week of intrauterine life and is marked by the initial proliferation of the dental lamina into the underlying ectomesenchyme.
The cap stage is functionally characterized by increasing cellular proliferation. The epithelial bud swells and assumes a crescent shape. Together with the adjacent increase of ectomesenchymal cells, these two cell populations form the tooth germ, and the primitive structures that will eventually become the tooth can now be seen.
Facts about teeth in non-human animals
- Rodents' incisors grow continuously throughout 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
- The narwhals giant unicorn-like tusk is a tooth that contains millions of sensory pathways and may be used for sensing in feeding, navigation and mating. It is the most neurologically complex tooth known.
- Horse teeth can be used to estimate the animal's age, and some horses have a form of premolars called Wolf teeth.
People and characters famous for their teeth
- Allan Thompson
- Austin Powers
- Esther Rantzen
- Bugs Bunny
- Styles P
- Julia Roberts
- Steve Buscemi
- Harold Berman
- Jimmy Carter
- Tom Cruise
- Dr. Teeth
- Ken Dodd (Comedian who uses them frequently in his jokes. His teeth have been insured for this reason)
- George Washington (his teeth were false; though popularly thought to be wooden, they were actually ivory.)
- The Jackson Five (their teeth were famously tooth paste clean on publicity pictures)
- Elton John (gap-teeth)
- Jerry Lewis as his nerdy characters
- Ruud Lubbers, Dutch prime minister who had a gap between his teeth. At the start of the nineties however, he had it surgically fixed so from then on his teeth look normal.
- John Massis, Belgian artist who could pull trains with his strong teeth
- The Mad Hatter in Lewis Carroll's "Alice" stories
- Madonna (gap-teeth)
- Freddie Mercury, lead singer of Queen, well known for having a significant overbite.
- The shark in Jaws
- Thomas Pynchon, writer
- Jaws from the James Bond movies
- Professor Frink
- Shane McGowan
- Guy Verhofstadt, Belgian prime minister, whose teeth are often ridiculed in cartoons as resembling those of a rabbit
- The Wolf in the fairy tale of Little Red Riding Hood
- Gary Dell'Abate from the Howard Stern Show
- David Letterman (gap-teeth)
- Lauren Hutton (American supermodel with gap teeth)
- Terry-Thomas (English comedy actor with gap in his front teeth)
- Amy Sedaris for her character Jerri Blank in Strangers With Candy
- Kurt Nilsen (gap between teeth)
- Head and neck anatomy
- Fluoride therapy
- Tooth Fairy
- Dentist and Dentistry
- Dental tourism
- Dental Auxiliary