Molar (tooth)

Molar (tooth)

Overview
Molars are the rearmost and most complicated kind of tooth
Tooth
Teeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...

 in most mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s. In many mammals they grind food; hence the Latin name mola, "millstone
Millstone
Millstones or mill stones are used in windmills and watermills, including tide mills, for grinding wheat or other grains.The type of stone most suitable for making millstones is a siliceous rock called burrstone , an open-textured, porous but tough, fine-grained sandstone, or a silicified,...

".

Adult humans have twelve molars, in four groups of three at the back of the mouth. The third, rearmost molar in each group is called a wisdom tooth
Third molar
A wisdom tooth, in humans, is any of the usually four third molars. Wisdom teeth usually appear between the ages of 17 and 25. Most adults have four wisdom teeth, but it is possible to have fewer , or more, in which case they are called supernumerary teeth...

. It is the last tooth to appear, breaking through the surface of the gum at about the age of twenty, although this varies from individual to individual.
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Encyclopedia
Molars are the rearmost and most complicated kind of tooth
Tooth
Teeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...

 in most mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s. In many mammals they grind food; hence the Latin name mola, "millstone
Millstone
Millstones or mill stones are used in windmills and watermills, including tide mills, for grinding wheat or other grains.The type of stone most suitable for making millstones is a siliceous rock called burrstone , an open-textured, porous but tough, fine-grained sandstone, or a silicified,...

".

Adult humans have twelve molars, in four groups of three at the back of the mouth. The third, rearmost molar in each group is called a wisdom tooth
Third molar
A wisdom tooth, in humans, is any of the usually four third molars. Wisdom teeth usually appear between the ages of 17 and 25. Most adults have four wisdom teeth, but it is possible to have fewer , or more, in which case they are called supernumerary teeth...

. It is the last tooth to appear, breaking through the surface of the gum at about the age of twenty, although this varies from individual to individual. Ethnicity can also have an impact on the age at which this occurs, with statistical variations between groups.

The human mouth contains upper (maxillary) and lower (mandibular) molars. They are: maxillary first molar
Maxillary first molar
The maxillary first molar is the tooth located laterally from both the maxillary second premolars of the mouth but mesial from both maxillary second molars....

, maxillary second molar
Maxillary second molar
The maxillary second molar is the tooth located distally from both the maxillary first molars of the mouth but mesial from both maxillary third molars. This is true only in permanent teeth. In deciduous teeth, the maxillary second molar is the last tooth in the mouth and does not have a third...

, maxillary third molar, mandibular first molar
Mandibular first molar
The mandibular first molar or six-year molar is the tooth located distally from both the mandibular second premolars of the mouth but mesial from both mandibular second molars...

, mandibular second molar
Mandibular second molar
The mandibular second molar is the tooth located distally from both the mandibular first molars of the mouth but mesial from both mandibular third molars. This is true only in permanent teeth...

, and mandibular third molar.

Molars among species

  • Tribosphenic: This kind is found in insectivore
    Insectivore
    An insectivore is a type of carnivore with a diet that consists chiefly of insects and similar small creatures. An alternate term is entomophage, which also refers to the human practice of eating insects....

    s and young platypus
    Platypus
    The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...

    es (adults have no teeth). Upper molars look like three-pointed mountain ranges; lowers look like two peaks and a third off to the side.
  • Quadrate: This kind is found in human
    Human
    Humans are the only living species in the Homo genus...

    s and various other species. Four cusps are arranged in a rectangle; there may be a fifth.
  • Bunodont: The cusps, instead of being sharp peaks, are rounded hills. The entire tooth is covered in enamel
    Tooth enamel
    Tooth enamel, along with dentin, cementum, and dental pulp is one of the four major tissues that make up the tooth in vertebrates. It is the hardest and most highly mineralized substance in the human body. Tooth enamel is also found in the dermal denticles of sharks...

    , and is most common among omnivores such as the pig
    Pig
    A pig is any of the animals in the genus Sus, within the Suidae family of even-toed ungulates. Pigs include the domestic pig, its ancestor the wild boar, and several other wild relatives...

    , the bear
    Bear
    Bears are mammals of the family Ursidae. Bears are classified as caniforms, or doglike carnivorans, with the pinnipeds being their closest living relatives. Although there are only eight living species of bear, they are widespread, appearing in a wide variety of habitats throughout the Northern...

     and humans.
  • Hypsodont: There is a lot of enamel and dentine above the gumline and the top of the pulp. This kind of molar is found in mammals that wear their teeth a lot, such as the horse
    Horse
    The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

    .
  • Zalambdodont: The tooth has two ridges that meet at an angle, forming the letter lambda.
  • Dilambdodont: Like zalambdodont, but there are two lambdas on one tooth.
  • Lophodont: The tooth has a few ridges perpendicular to the jaw.
  • Selenodont: The tooth has a crescent-shaped ridge or ridges.
  • Loxodont: The tooth has several parallel oblique ridges on its surface. The elephant
    Elephant
    Elephants are large land mammals in two extant genera of the family Elephantidae: Elephas and Loxodonta, with the third genus Mammuthus extinct...

     Loxodonta is named for this feature.

Tribosphenic molar


The molar design that is considered one of the most important characteristics of mammals is a three-cusped shape called a tribosphenic molar. This molar design has two important features: the trigonid, or shearing end, and the talonid, or crushing heel.
In modern mammals that have tribosphenic molars the trigonid is towards the front and the talonid towards the rear.

The tribosphenic design appears in all groups of mammals. Some paleontologists believe that it developed independently in monotreme
Monotreme
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...

s, rather than being inherited from an ancestor that they share with marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...

s and placentals; but this idea has critics and the debate is still going on.

Also some Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 mammals, such as Shuotherium
Shuotherium
Shuotherium is a fossil relative of the monotremes from the Jurassic. The original holotype is composed of a partial dentary and seven teeth . The holotypes for other species of this genus are solely represented by teeth...

and Pseudotribos
Pseudotribos
Pseudotribos is an extinct genus of mammal from the Middle Jurassic some 165 million years ago of China.-External links:*...

, have "reversed tribosphenic" molars in which the talonid is towards the front. This variant is regarded as an example of convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

.

Pathology

  • Buccal bifurcation cyst
    Buccal bifurcation cyst
    Buccal bifurcation cyst is an inflammatory odontogenic cyst, of the paradental cysts family, that typically appears in the buccal bifurcation region of the mandibular first molars in the second half of the first decade of life...

  • Dental caries
    Dental caries
    Dental caries, also known as tooth decay or a cavity, is an irreversible infection usually bacterial in origin that causes demineralization of the hard tissues and destruction of the organic matter of the tooth, usually by production of acid by hydrolysis of the food debris accumulated on the...

  • Dental trauma
    Dental trauma
    Dental trauma refers to trauma to the face, mouth, and especially the teeth lips and periodontium. The study of dental trauma is called dental traumatology.- Types :* Tooth fractures**Enamel infraction**Enamel fracture**Enamel-dentine fracture...

  • Dentigerous cyst
    Dentigerous Cyst
    A dentigerous cyst or Follicular Cyst is an odontogenic cyst - thought to be of developmental origin - associated with the crown of an unerupted tooth. The cyst cavity is lined by epithelial cells derived from the reduced enamel epithelium of the tooth forming organ...

    and other odontogenic cyst
    Odontogenic cyst
    Odontogenic cyst is a closed sac, having a distinct membrane derived from rests of odontogenic epithelium. It may contain air, fluids, or semi-solid material. Intra-bony cysts are most common in the jaws, because the mandible and maxilla are the only bones with epithelial components. That...

    s
  • Pericoronitis
    Pericoronitis
    Pericoronitis is a common problem in young adults with partial tooth impactions. It usually occurs within 17 to 24 years of age as it is when the third molars start erupting. It occurs when the tissue around the wisdom tooth has become inflamed because bacteria have invaded the area. Poor oral...

  • Periodontitis


External links


For pictures of various molars see The Diversity of Cheek Teeth.