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Human tooth

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"Teeth" redirects here. For the film, see Teeth (film).
The visible teeth of a smile.

Teeth (singular, tooth) are structures found in the jaws of many vertebrates. The primary function of teeth is to tear, scrape, 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.

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.

Some animals develop only one set of teeth (monophyodont) or develop many (polyphyodont). Sharks, for example, grow a new set of teeth every two weeks to replace worn teeth. Rodent incisors grow and wear away continually through the animal's gnawing, maintaining approximately constant length. Some rodent species such as the sibling vole[1] and the guinea pig[2] have also continuously growing molars.


Anatomy

A third molar.
Main article: Dental anatomy

Dental anatomy is a field of anatomy dedicated to the study of tooth structures. The development, appearance, and classification of teeth fall within its purvue, though the function of teeth as they contact one another is referred to as dental occlusion. Tooth formation begins prior to birth and their eventual morphology is dictated during this time. Dental anatomy is also a taxonomical science; it is concerned with the naming of teeth and the structures of which they are made. This information serves a practical purpose when rendering dental treatment.

The anatomic crown of a tooth is designated by the area above the cementoenamel junction (CEJ) and is consequently covered in enamel. The majority of the crown is composed of dentin, with the pulp chamber found in the center. The crown is only found within bone before eruption into the mouth. Afterwards, it is almost always visible. The anatomic root is found below the cementoenamel junction and is covered with cementum. As with the crown, dentin composes most of the root, which normally have pulp canals. The roots of teeth may be single in number or multiple. Canines and most premolars, except for maxillary (upper) first premolars, usually have one root. Maxillary first premolars and mandibular molars usually have two roots. Maxillary molars usually have three roots. Additional roots are referred to as supernumerary roots.

Usually, there are 20 primary ("baby") teeth and 32 permanent teeth. Among primary teeth, 10 usually are found in the maxilla and the other 10 in the mandible. The classes of teeth are incisors, canines, and molars. In the primary set of teeth, there are two types of incisors, centrals and laterals, and two types of molars, first and second molars. All of these are replaced with a permanent counterpart except for the molars, which are replaced by permanent premolars. Among permanent teeth, 16 are found in the maxilla while the other 16 in the mandible. The maxillary teeth are the maxillary central incisor, maxillary lateral incisor, maxillary canine, maxillary first premolar, maxillary second premolar, maxillary first molar, maxillary second molar, and maxillary third molar. The mandibular teeth are the mandibular central incisor, mandibular lateral incisor, mandibular canine, mandibular first premolar, mandibular second premolar, mandibular first molar, mandibular second molar, and mandibular third molar. The third molars are commonly called "wisdom teeth" and may never erupt into the mouth or form at all. Additional teeth besides those mentioned are referred to as supernumerary teeth. For example, fourth and fifth molars may form even though this is rare.[3]

Most of the teeth have identifiable features distinguishing them from the others. There are several different notation systems to refer to a specific tooth. The three most commons systems are the FDI World Dental Federation notation, Universal numbering system (dental), and Palmer notation method. The FDI system is used worldwide, and the universal is used widely in the USA.

Parts

Section of a human molar

Enamel

Main article: Tooth enamel

Enamel is the hardest and most highly mineralized substance of the body, and with dentin, cementum, and dental pulp is one of the four major tissues which make up the tooth.[4] It is the normally visible dental tissue of a tooth and must be supported by underlying dentin. Ninety-six percent of enamel consists of mineral, with water and organic material composing the rest.[5] The normal color of enamel varies from light yellow to grayish white. At the edges of teeth where there is no dentin underlying the enamel, the color sometimes has a slightly blue tone. Since enamel is semitranslucent, the color of dentin and any restorative dental material underneath the enamel strongly affects the appearance of a tooth. Enamel varies in thickness over the surface of the tooth and is often thickest at the cusp, up to 2.5 mm, and thinnest at its border, which is seen clinically as the cementoenamel junction (CEJ).[6]

Enamel's primary mineral is hydroxyapatite, which is a crystalline calcium phosphate.[7] The large amount of minerals in enamel accounts not only for its strength but also for its brittleness.[8] Dentin, which is less mineralized and less brittle, compensates for enamel and is necessary as a support.[7] Unlike dentin and bone, enamel does not contain collagen. Instead, it has two unique classes of proteins called amelogenins and enamelins. While the role of these proteins is not fully understood, it is believed that they aid in the development of enamel by serving as a framework support, among other functions.[9]

Dentin

Main article: Dentin

Dentin is the substance between enamel or cementum and the pulp chamber. It is secreted by the odontoblasts of the dental pulp. The formation of dentin is known as dentinogenesis. The porous, yellow-hued material is made up of 70% inorganic materials, 20% organic materials, and 10% water. Because it is softer than enamel, it decays more rapidly and is subject to severe cavities if not properly treated, but dentin still acts as a protective layer and supports the crown of the tooth.

Dentin is a mineralized connective tissue with an organic matrix of collagenous proteins. Dentin has microscopic channels, called dentinal tubules, which radiate outward through the dentin from the pulp cavity to the exterior cementum or enamel border. These canals have different configurations in different species and their diameter ranges between 0.8 and 2.2 micrometres. Although they may have tiny side-branches, they do not intersect with each other. Their length is dictated by the radius of the tooth. The three dimensional configuration of the dentinal tubules is under genetic control.

Cementum

Main article: Cementum

Cementum is a specialized bony substance covering the root of a tooth. It is approximately 45% inorganic material (mainly hydroxyapatite), 33% organic material (mainly collagen) and 22% water. Cementum is excreted by cementoblasts within the root of the tooth and is thickest at the root apex. Its coloration is yellowish and it is softer than either dentin or enamel. The principle role of cementum is to serve as a medium by which the periodontal ligaments can attach to the tooth for stability. At the cementoenamel junction, the cementum is acellular due to its lack of cellular components, and this type covers approximately ⅓ - ½ of the root. The more permeable form of cementum, cellular cementum, covers ⅓ - ½ of the root apex, where it binds to the dentin.

Pulp

Main article: Pulp (tooth)

The dental pulp is the part in the center of a tooth made up of living soft tissue and cells called odontoblasts. The odontoblasts are the sturcutral cells of the tooth, however there are also other cells in the pulp. Those include: Fibroblasts, Granulocites, Histiosites etc. It's commonly called 'the nerve', although it contains many other structures which are not nerves.

It is also the place where the blood vessels and nerve endings are located.

Development

Main article: Tooth development
Radiograph of lower right (from left to right) third, second, and first molars in different stages of development.

Tooth development is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth. Although many diverse species have teeth, non-human tooth development is largely the same as in humans. For human teeth to have a healthy oral environment, enamel, dentin, cementum, and the periodontium must all develop during appropriate stages of fetal development. Primary (baby) teeth start to form between the sixth and eighth weeks in utero, and permanent teeth begin to form in the twentieth week in utero.[10] If teeth do not start to develop at or near these times, they will not develop at all.

A significant amount of research has focused on determining the processes that initiate tooth development. It is widely accepted that there is a factor within the tissues of the first branchial arch that is necessary for the development of teeth.[11] The tooth bud (sometimes called the tooth germ) is an aggregation of cells that eventually forms a tooth and is organized into three parts: the enamel organ, the dental papilla and the dental follicle.[12]

The enamel organ is composed of the outer enamel epithelium, inner enamel epithelium, stellate reticulum and stratum intermedium.[12] These cells give rise to ameloblasts, which produce enamel and the reduced enamel epithelium. The growth of cervical loop cells into the deeper tissues forms Hertwig's Epithelial Root Sheath, which determines the root shape of the tooth. The dental papilla contains cells that develop into odontoblasts, which are dentin-forming cells.[12] Additionally, the junction between the dental papilla and inner enamel epithelium determines the crown shape of a tooth.[13] The dental follicle gives rise to three important entities: cementoblasts, osteoblasts, and fibroblasts. Cementoblasts form the cementum of a tooth. Osteoblasts give rise to the [[Alveolar process|alveolar bone around the roots of teeth. Fibroblasts develop the periodontal ligaments which connect teeth to the alveolar bone through cementum.[14]

Tooth development is commonly divided into the following stages: the bud stage, the cap, the bell, and finally maturation. The staging of tooth development is an attempt to categorize changes that take place along a continuum; frequently it is difficult to decide what stage should be assigned to a particular developing tooth.[11] This determination is further complicated by the varying appearance of different histologic sections of the same developing tooth, which can appear to be different stages.

Eruption

Main article: Tooth eruption

Tooth eruption in humans is a process in tooth development in which the teeth enter the mouth and become visible. It is currently believed that the periodontal ligaments play an important role in tooth eruption. Primary teeth erupt into the mouth from around 6 months until 2 years of age. These teeth are the only ones in the mouth until about a person is 6 years old. At that time, the first permanent tooth erupts and begins a time in which there is a combination of primary and permanent teeth. This stage, known as the mixed stage, lasts until the last primary tooth is lost. Then, the remaining permanent teeth erupt into the mouth.

There have been many theories about the cause of tooth eruption. One theory rested on the idea that the developing root of a tooth pushed it into the mouth.[15] Another belief, known as the cushioned hammock theory, resulted from microscopic study of teeth, which was thought to show a ligament around the root. It was later discovered that the "ligament" was merely an artifact created in the process of preparing the slide.[16] Currently, the most commonly held belief is that the periodontal ligaments provide the main impetus for the process.[17]

Periodontium

The periodontium is the supporting structure of a tooth, helping to attach the teeth to surrounding tissues. It consists of the cementum, periodontal ligaments, gingiva, and alveolar bone. Cementum is the only one of these that is a part of a tooth. Alveolar bone surrounds the roots of teeth to provide support and creates what is commonly called a "socket". Periodontal ligaments connect the alveolar bone to the cementum, and the gingiva is the surrounding tissue visible in the mouth.

Tooth decay

Plaque

Main article: Dental plaque

Plaque is a biofilm consisting of large amounts of various bacteria which forms on teeth. If not removed regularly, it can lead to dental cavities (caries) or periodontal problems (such as gingivitis). Given time, plaque can mineralize along the gingiva, forming tartar. The microorganisms that form the biofilm are almost entirely bacteria (mainly streptococcus and anaerobes), with the composition varying by location in the mouth. Streptococcus mutans is the most important bacteria associated with dental caries.

Certain bacteria in the mouth live off the remains of foods, especially sugars and starches. In the absence of oxygen they produce lactic acid, which dissolves the calcium and phosphorus in the enamel. This process, known as "demineralisation", leads to tooth destruction. 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. Nonetheless, in the presence of plaque, saliva is unable to penetrate through the plaque to neutralize the acid produced by the bacteria.

Caries (Cavities)

Tooth decay on a premolar.
Main article: Dental caries

Dental caries, also described as "tooth decay" or "dental cavities", is an infectious disease which damages the structures of teeth.[18] The disease can lead to pain, tooth loss, infection, and, in severe cases, death. There is a long history of dental caries, with evidence showing the disease was present in the Bronze, Iron, and Middle age but also prior to the neolithic period.[19] The largest increases in the prevalence of caries have been associated with diet changes.[19][20] Today, it remains one of the most common diseases throughout the world. In the United States, dental caries is the most common chronic childhood disease, being at least five times more common than asthma.[21] Countries that have experienced an overall decrease in cases of tooth decay continue to have a disparity in the distribution of the disease.[22] Among children in the United States and Europe, 60-80% of cases of dental caries occur in 20% of the population.[23]

Tooth decay is caused by certain types of acid-producing bacteria which cause the most damage in the presence of fermentable carbohydrates such as sucrose, fructose, and glucose.[24][25] The resulting acidic levels in the mouth affect teeth because a tooth's special mineral content causes it to be sensitive to low pH. Depending on the extent of tooth destruction, various treatments can be used to restore teeth to proper form, function, and aesthetics, but there is no known method to regenerate large amounts of tooth structure. Instead, dental health organizations advocate preventative and prophylactic measures, such as regular oral hygiene and dietary modifications, to avoid dental caries.[26]

Tooth care

Toothbrushes are commonly used to clean teeth.
Main article: Oral hygiene

Oral hygiene is the practice of keeping the mouth clean, and is means of prevention from dental caries, gingivitis, periodontal disease, bad breath, and other dental disorders. It consists of both professional and personal care. Regular cleanings, usually done by dentists and dental hygienists, are recommended to remove tartar (mineralized plaque) that may develop even with careful brushing and flossing. Professional cleaning includes tooth scaling, using various instruments or devices to loosen and remove deposits from the teeth.

Regular brushing is recommended by healthcare professionals twice a day (rather in the morning and in the evening, or after meal) in order to prevent formation of plaque and tartar.[26] A toothbrush is able to remove plaque on most surfaces of the teeth except for areas between teeth.

As a result, flossing is also considered a necessity to maintain oral hygiene. When used correctly, dental floss removes plaque from between teeth and at the gum line, where periodontal disease often begins and could develop caries. The purpose of cleaning teeth is to remove plaque, which consists mostly of bacteria.[27] Electric toothbrushes are not considered more effective than the manual variety.[28] The most important advantages of electric toothbrushes is the ability to aid people with dexterity difficulties, such as those associated with rheumatoid arthritis.

In addition, fluoride therapy is often recommended to protect against dental caries. It has been demonstrated that water fluoridation and fluoride supplements decrease the incidence of dental caries. Fluoride helps prevent dental decay by binding to the hydroxyapatite crystals in enamel.[29] The incorporated fluoride makes enamel more resistant to demineralization and, thus, resistant to decay.[30] Topical fluoride is also recommended to protect the surface of the teeth. This may include a fluoride toothpaste or mouthwash. Many dentists include application of topical fluoride solutions as part of routine visits.

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.

Some of the earliest artificial teeth were made by the Etruscans and their use was adopted in Ancient Rome for the wealthy citizens who often dined on food containing damaging sugars.

Abnormalities of the dentition

In animals

Section through the ivory tusk of a mammoth
  • Carnassials are used for slicing food in carnivores only.
  • Rodents' incisors grow continuously throughout their lives. This is referred to as Aradicular.
  • Reptiles' and sharks' teeth are replaced constantly. Before they wear out this is referred to as polyphedont. 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.
  • Walrus tusks are canine teeth that grow continuously throughout life.[31]
  • In Aardvarks, teeth lack enamel and have many pulp tubules, hence the name of the order Tubulidentata.

References

  1. ^ Tummers M and Thesleff I. Root or crown: a developmental choice orchestrated by the differential regulation of the epithelial stem cell niche in the tooth of two rodent species. Development (2003). 130(6):1049-57.
  2. ^ AM Hunt. A description of the molar teeth and investing tissues of normal guinea pigs. J Dent Res. (1959) 38(2):216-31.
  3. ^ Kokten G, Balcioglu H, Buyukertan M. Supernumerary Fourth and Fifth Molars: A Report of Two Cases. Journal of Contemporary Dental Practice, 2003 November; (4)4:067-076. Page accessed February 10, 2007.
  4. ^ Ross, Michael H., Gordon I. Kaye, and Wojciech Pawlina, "Histology: a Text and Atlas", 4th ed. (Baltimore: Lippincott Williams & Wilkins, 2002), p. 441.
  5. ^ Cate, A. R. Ten, "Oral Histology: Development, Structure, and Function", 5th ed. (Saint Louis: Mosby-Year Book, 1998), p. 1.
  6. ^ Cate, A. R. Ten, "Oral Histology: Development, Structure, and Function", 5th ed. (Saint Louis: Mosby-Year Book, 1998), p. 219.
  7. ^ a b Johnson, Clarke. "Biology of the Human Dentition," 1998. Page accessed on January 24, 2007.
  8. ^ Cate, A. R. Ten, "Oral Histology: Development, Structure, and Function", 5th ed. (Saint Louis: Mosby-Year Book, 1998), p. 219.
  9. ^ Cate, A. R. Ten, "Oral Histology: Development, Structure, and Function", 5th ed. (Saint Louis: Mosby-Year Book, 1998), p. 198.
  10. ^ A. R. Ten Cate, Oral Histology: Development, Structure, and Function, 5th ed. (Saint Louis: Mosby-Year Book, 1998), p. 95. ISBN 0-8151-2952-1.
  11. ^ a b A. R. Ten Cate, Oral Histology: Development, Structure, and Function, 5th ed. (Saint Louis: Mosby-Year Book, 1998), p. 81. ISBN 0-8151-2952-1.
  12. ^ a b c *University of Texas Medical Branch. "Lab Exercises: Tooth development." Page found here.
  13. ^ A. R. Ten Cate, Oral Histology: Development, Structure, and Function, 5th ed. (Saint Louis: Mosby-Year Book, 1998), pp. 86 and 102. ISBN 0-8151-2952-1.
  14. ^ *Ross, Michael H., Gordon I. Kaye, and Wojciech Pawlina. Histology: a text and atlas. 4th edition, p. 453. 2003. ISBN 0-683-30242-6.
  15. ^ Harris, Edward F. Craniofacial Growth and Development. In the section entitled "Tooth Eruption." 2002. pp. 1-3.
  16. ^ Harris, Edward F. Craniofacial Growth and Development. In the section entitled "Tooth Eruption." 2002. p. 3.
  17. ^ Harris, Edward F. Craniofacial Growth and Development. In the section entitled "Tooth Eruption." 2002. p. 5.
  18. ^ Dental Cavities, MedlinePlus Medical Encyclopedia, page accessed August 14, 2006.
  19. ^ a b Epidemiology of Dental Disease, hosted on the University of Illinois at Chicago website. Page accessed January 9, 2007.
  20. ^ Suddick, Richard P. and Norman O. Harris. "Historical Perspectives of Oral Biology: A Series". Critical Reviews in Oral Biology and Medicine, 1(2), pages 135-151, 1990.
  21. ^ Healthy People: 2010. Html version hosted on Healthy People.gov website. Page accessed August 13, 2006.
  22. ^ "Dental caries", from the Disease Control Priorities Project. Page accessed August 15, 2006.
  23. ^ Touger-Decker, Riva and Cor van Loveren. Sugars and dental caries, The American Journal of Clinical Nutrition, 78, 2003, pages 881S–892S.
  24. ^ Hardie, J.M. (1982). The microbiology of dental caries. Dental Update, 9, 199-208.
  25. ^ Holloway, P.J. (1983). The role of sugar in the etiology of dental caries. Journal of Dentistry, 11, 189-213.
  26. ^ a b Oral Health Topics: Cleaning your teeth and gums. Hosted on the American Dental Association website. Page accessed August 15, 2006.
  27. ^ Introduction to Dental Plaque. Hosted on the Leeds Dental Institute Website, page accessed August 14, 2006.
  28. ^ Thumbs down for electric toothbrush, hosted on the BBC News website, posted January 21, 2003. Page accessed January 23, 2007.
  29. ^ Cate, A.R. Ten. "Oral Histology: development, structure, and function." 5th edition, 1998, p. 223. ISBN 0-8151-2952-1.
  30. ^ Ross, Michael H., Gordon I. Kaye, and Wojciech Pawlina, 2003. "Histology: a text and atlas." 4th edition, p. 453. ISBN 0-683-30242-6.
  31. ^ The Permanent Canine Teeth, hosted on the University of Illinois at Chicago website. Page accessed February 5, 2007.

See also

Look up tooth in Wiktionary, the free dictionary.

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