Tooth development or odontogenesis is the process in which

teeth A tooth (: teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...

develop and grow into the

mouth A mouth also referred to as the oral is the body orifice through which many animals ingest food and animal communication#Auditory, vocalize. The body cavity immediately behind the mouth opening, known as the oral cavity (or in Latin), is also t ...

. Tooth development varies among species.

Tooth development in vertebrates

Fish

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

Hox gene Hox genes, a subset of homeobox, homeobox genes, are a gene cluster, group of related genes that Evolutionary developmental biology, specify regions of the body plan of an embryo along the craniocaudal axis, head-tail axis of animals. Hox protein ...

expression regulates mechanisms for

tooth A tooth (: teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tea ...

initiation. However,

shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...

s continuously produce new teeth throughout their lives via a drastically different mechanism. Shark teeth form from modified scales near the tongue and move outward on the jaw in rows until they are eventually dislodged. Their scales, called

dermal denticle A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scale (zoology), scales, which can also provide effective Underwater camouflage, camouflage through the us ...

s, and

are homologous organs.

Mammals

Generally, tooth development in non-human

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s is similar to

human tooth development Tooth development or odontogenesis is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth. For human teeth to have a healthy oral environment, all parts of the tooth must develop during appropriate sta ...

. The variations usually lie in the morphology,

number A number is a mathematical object used to count, measure, and label. The most basic examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can be represented in language with number words. More universally, individual numbers can ...

, development timeline, and types of teeth. However, some mammals' teeth do develop differently than humans'. In

mice A mouse (: mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...

, WNT signals are required for the initiation of

development.

Rodent Rodents (from Latin , 'to gnaw') are mammals of the Order (biology), order Rodentia ( ), which are characterized by a single pair of continuously growing incisors in each of the upper and Mandible, lower jaws. About 40% of all mammal specie ...

s' teeth continually grow, forcing them to wear down their teeth by gnawing on various materials. If rodents are prevented from gnawing, their teeth eventually puncture the roofs of their mouths. In addition, rodent incisors consist of two halves, known as the crown and root analogues. The labial half is made of enamel and resembles a crown, while the lingual half is made of dentin and resembles a root. The mineral distribution in

rodent Rodents (from Latin , 'to gnaw') are mammals of the Order (biology), order Rodentia ( ), which are characterized by a single pair of continuously growing incisors in each of the upper and Mandible, lower jaws. About 40% of all mammal specie ...

enamel is different from that of

monkey Monkey is a common name that may refer to most mammals of the infraorder Simiiformes, also known as simians. Traditionally, all animals in the group now known as simians are counted as monkeys except the apes. Thus monkeys, in that sense, co ...

dog The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the gray wolf. Also called the domestic dog, it was selectively bred from a population of wolves during the Late Pleistocene by hunter-gatherers. ...

pig The pig (''Sus domesticus''), also called swine (: swine) or hog, is an omnivorous, domesticated, even-toed, hoofed mammal. It is named the domestic pig when distinguishing it from other members of the genus '' Sus''. Some authorities cons ...

s, and

human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

s. In

horse teeth Horse teeth refers to the dentition of equine species, including horses and donkeys. Equines are both heterodontous and diphyodontous, which means that they have teeth in more than one shape (there are up to five shapes of tooth in a horse's m ...

, enamel and dentin layers are intertwined, which increases the strength and decreases the wear rate of the teeth. Contrary to popular belief, horse teeth do not "grow" indefinitely. Rather, existing tooth erupts from below the gumline. Horses start to "run out" of erupting tooth in their early 30s and in the rare case they live long enough, the roots of their teeth will fall out completely in the middle to latter part of their third decade. In

manatee Manatees (, family (biology), family Trichechidae, genus ''Trichechus'') are large, fully aquatic, mostly herbivory, herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing t ...

s, mandibular molars develop separately from the jaw and are encased in a bony shell separated by soft tissue. This also occurs in

elephant Elephants are the largest living land animals. Three living species are currently recognised: the African bush elephant ('' Loxodonta africana''), the African forest elephant (''L. cyclotis''), and the Asian elephant ('' Elephas maximus ...

s' successional teeth, which erupt to replace worn teeth. Elephants have six sets of molars in their life, all of which grown from the back of their mouth and are then pushed forward. All members of the

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

Tubulidentata Orycteropodidae is a family of afrotherian mammals. Although there are many fossil species, the only species surviving today is the aardvark, ''Orycteropus afer''. Orycteropodidae is recognized as the only family within the order Tubulidentata, ...

have no incisors or canines, their teeth have no enamel, and their molars grow continuously from the root.

Human tooth development

When it comes to tooth development in humans, there are many differences in the way humans and other primates' teeth develop slowly over the beginning of their life. Once the dental tissue in primates and humans is formed, there are little changes made throughout the remainder of the organism's lifetime, but there are still factors such as tooth wear and dental disease can alter the dental tissue of primates. The dental tissue in primates is a layered structure, and scientists are able to observe the layered structure chronologically to learn about the detailed development of their teeth. This layered structure is preserved throughout life for humans and primates.

Invertebrate "teeth"

True teeth are unique to

vertebrates Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

, although some invertebrates have

analogous structures Analogy is a comparison or correspondence between two things (or two groups of things) because of a third element that they are considered to share. In logic, it is an inference or an argument from one particular to another particular, as oppose ...

sometimes called "teeth". The

organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...

with the simplest

genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...

bearing such "

" is probably the

worm Worms are many different distantly related bilateria, bilateral animals that typically have a long cylindrical tube-like body, no limb (anatomy), limbs, and usually no eyes. Worms vary in size from microscopic to over in length for marine ...

genus Ancylostoma ( Ancylostoma duodenale,

Necator americanus ''Necator americanus'' is a species of hookworm (a type of helminth) commonly known as the New World hookworm. Like other hookworms, it is a member of the phylum Nematoda. It is an obligatory parasitic nematode that lives in the small intestin ...

). Molluscs have a structure called a

radula The radula (; : radulae or radulas) is an anatomical structure used by mollusks for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters ...

which bears a ribbon of

chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...

ous "teeth". However, these are histologically and developmentally different from vertebrate teeth, and are unlikely to be homologous. For example, vertebrate teeth develop from a

neural crest The neural crest is a ridge-like structure that is formed transiently between the epidermal ectoderm and neural plate during vertebrate development. Neural crest cells originate from this structure through the epithelial-mesenchymal transition, ...

mesenchyme Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood, or bone. The interactions between mesenchyme and epithelium help to form nearly ever ...

-derived dental papilla, and the neural crest is specific to vertebrates, as are tissues such as enamel.

Variation of tooth development across species

The study of tooth development across different species, invertebrates, and vertebrates indicates that tooth development varies greatly across different types of organisms. Organisms may vary from having few to no teeth at all to organisms that go through life with multiple layers of teeth. In some unique cases there are species with teeth that regenerate throughout their lifetime. Because of this, it is very important that scientists select specific species to study that may provide them with information as to how similar species' tooth development takes place. These organisms are chosen based on their specific early dental specification as well as tooth replacement ability. With an organism that has the ability to regenerate teeth, scientists have a unique opportunity to continuously study the specific odontogenic processes and mechanisms that lead to the development of teeth in different vertebrates.

Genetics

In reptilian tooth development, some scientist have chosen to determine the location and function of the SHH gene as well as the odontogenic band to help collect information on how reptiles develop teeth at a young age. The SHH gene gives instructions to produce a protein known as the Sonic Hedgehog protein, and this protein's function is to aid in certain required embryonic development processes. The SHH gene and proteins play major roles in not only the tooth development of organisms, but they also help in cell growth, specialization, and patterning of an organism's body.

References

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