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Tooth development

2007 Schools Wikipedia Selection. Related subjects: Health and medicine

   Radiograph of lower right (from left to right) third, second, and first
   molars in different stages of development.
   Enlarge
   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.^ 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.^

Overview

   Histologic slide showing a tooth bud. A: enamel organ B: dental papilla
   C: dental follicle
   Enlarge
   Histologic slide showing a tooth bud.
   A: enamel organ
   B: dental papilla
   C: dental follicle

   The tooth bud (sometimes called the tooth germ) is an aggregation of
   cells that eventually forms a tooth.^ These cells are derived from the
   ectoderm of the first branchial arch and the ectomesenchyme of the
   neural crest.^ The tooth bud is organized into three parts: the enamel
   organ, the dental papilla and the dental follicle.

   The enamel organ is composed of the outer enamel epithelium, inner
   enamel epithelium, stellate reticulum and stratum intermedium.^ These
   cells give rise to ameloblasts, which produce enamel and the reduced
   enamel epithelium. The location where the outer enamel epithelium and
   inner enamel epithelium join is called the cervical loop.^ 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.^ Additionally, the junction between the
   dental papilla and inner enamel epithelium determines the crown shape
   of a tooth.^ Mesenchymal cells within the dental papilla are
   responsible for formation of tooth pulp.

   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 bone around
   the roots of teeth. Fibroblasts develop the periodontal ligaments which
   connect teeth to the alveolar bone through cementum.^

Human tooth development timeline

   The following tables present the development timeline of human teeth.^
   Times for the initial calcification of primary teeth are for weeks in
   utero. Abbreviations: wk = weeks; mo = months; yr = years.

                                Maxillary (upper) teeth
       Primary teeth     Central
                         incisor Lateral
                                 incisor
                                         Canine   First
                                                 molar   Second
                                                         molar
   Initial calcification 14 wk   16 wk   17 wk   15.5 wk 19 wk
   Crown completed       1.5 mo  2.5 mo  9 mo    6 mo    11 mo
   Root completed        1.5 yr  2 yr    3.25 yr 2.5 yr  3 yr
                               Mandibular (lower) teeth
   Initial calcification 14 wk   16 wk   17 wk   15.5 wk 18 wk
   Crown completed       2.5 mo  3 mo    9 mo    5.5 mo  10 mo
   Root completed        1.5 yr  1.5 yr  3.25 yr 2.5 yr  3 yr
   Maxillary (upper) teeth
   Permanent teeth Central
   incisor Lateral
   incisor
   Canine First
   premolar Second
   premolar First
   molar Second
   molar Third
   molar
   Initial calcification 3–4 mo 10–12 mo 4–5 mo 1.5–1.75 yr 2–2.25 yr at
   birth 2.5–3 yr 7–9 yr
   Crown completed 4–5 yr 4–5 yr 6–7 yr 5–6 yr 6–7 yr 2.5–3 yr 7–8 yr
   12–16 yr
   Root completed 10 yr 11 yr 13–15 yr 12–13 yr 12–14 yr 9–10 yr 14–16 yr
   18–25 yr
    Mandibular (lower) teeth
   Initial calcification 3–4 mo 3–4 mo 4–5 mo 1.5–2 yr 2.25–2.5 yr at
   birth 2.5–3 yr 8–10 yr
   Crown completed 4–5 yr 4–5 yr 6–7 yr 5–6 yr 6–7 yr 2.5–3 yr 7–8 yr
   12–16 yr
   Root completed 9 yr 10 yr 12–14 yr 12–13 yr 13–14 yr 9–10 yr 14–15 yr
   18–25 yr

The developing tooth bud

   One of the earliest steps in the formation of a tooth that can be seen
   microscopically is the distinction between the vestibular lamina and
   the dental lamina. The dental lamina connects the developing tooth bud
   to the epithelial layer of the mouth for a significant time.^

   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.^ 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.

Bud stage

   The bud stage is characterized by the appearance of a tooth bud without
   a clear arrangement of cells. The stage technically begins once
   epithelial cells proliferate into the ectomesenchyme of the jaw.^ The
   tooth bud itself is the group of cells at the end of the dental lamina.

Cap stage

   Histologic slide of tooth in cap stage.
   Enlarge
   Histologic slide of tooth in cap stage.

   The first signs of an arrangement of cells in the tooth bud occur in
   the cap stage. A small group of ectomesenchymal cells stops producing
   extracellular substances, which results in an aggregation of these
   cells called the dental papilla. At this point, the tooth bud grows
   around the ectomesenchymal aggregation, taking on the appearance of a
   cap, and becomes the enamel (or dental) organ. A condensation of
   ectomesenchymal cells called the dental follicle surrounds the enamel
   organ and limits the dental papilla. Eventually, the enamel organ will
   produce enamel, the dental papilla will produce dentin and pulp, and
   the dental follicle will produce all the supporting structures of a
   tooth.^
   Histologic slide of tooth in early bell stage. Note cell organization.
   Enlarge
   Histologic slide of tooth in early bell stage. Note cell organization.

Bell stage

   The bell stage is known for the histodifferentiation and
   morphodifferentiation that takes place. The dental organ is bell-shaped
   during this stage, and the majority of its cells are called stellate
   reticulum because of their star-shaped appearance.^ Cells on the
   periphery of the enamel organ separate into three important layers.
   Cuboidal cells on the periphery of the dental organ are known as outer
   enamel epithelium.^ The cells of the enamel organ adjacent to the
   dental papilla are known as inner enamel epithelium. The cells between
   the inner enamel epithelium and the stellate reticulum form a layer
   known as the stratum intermedium. The rim of the dental organ where the
   outer and inner enamel epithelium join is called the cervical loop.^

   Other events occur during the bell stage. The dental lamina
   disintegrates, leaving the developing teeth completely separated from
   the epithelium of the oral cavity; the two will not join again until
   the final eruption of the tooth into the mouth.^
   Histologic slide of tooth in late bell stage. Note disintegration of
   dental lamina at top.
   Enlarge
   Histologic slide of tooth in late bell stage. Note disintegration of
   dental lamina at top.

   The crown of the tooth, which is influenced by the shape of the
   internal enamel epithelium, also takes shape during this stage.
   Throughout the mouth, all teeth undergo this same process; it is still
   uncertain why teeth form various crown shapes—for instance, incisors
   versus canines. There are two dominant hypotheses. The "field model"
   proposes there are components for each type of tooth shape found in the
   ectomesenchyme during tooth development. The components for particular
   types of teeth, such as incisors, are localized in one area and
   dissipate rapidly in different parts of the mouth. Thus, for example,
   the "incisor field" has factors that develop teeth into incisor shape,
   and this field is concentrated in the central incisor area, but
   decreases rapidly in the canine area. The other dominant hypothesis,
   the "clone model", proposes that the epithelium programs a group of
   ectomesenchymal cells to generate teeth of particular shapes. This
   group of cells, called a clone, coaxes the dental lamina into tooth
   development, causing a tooth bud to form. Growth of the dental lamina
   continues in an area called the "progress zone". Once the progress zone
   travels a certain distance from the first tooth bud, a second tooth bud
   will start to develop. These two models are not necessarily mutually
   exclusive, nor does widely accepted dental science consider them to be
   so: it is postulated that both models influence tooth development at
   different times.^

   Other structures that may appear in a developing tooth in this stage
   are enamel knots, enamel cords, and enamel niche.^
   Histologic slide of developing hard tissues. Ameloblasts are forming
   enamel, while odontoblasts are forming dentin.
   Enlarge
   Histologic slide of developing hard tissues. Ameloblasts are forming
   enamel, while odontoblasts are forming dentin.

Crown stage

   Hard tissues, including enamel and dentin, develop during the next
   stage of tooth development. This stage is called the crown, or
   maturation, stage by some researchers. Important cellular changes occur
   at this time. In prior stages, all of the inner enamel epithelium cells
   were dividing to increase the overall size of the tooth bud, but rapid
   dividing, called mitosis, stops during the crown stage at the location
   where the cusps of the teeth form. The first mineralized hard tissues
   form at this location. At the same time, the inner enamel epithelial
   cells change in shape from cuboidal to columnar. The nuclei of these
   cells move closer to the stratum intermedium and away from the dental
   papilla.^
   Histologic slide of tooth. Note the tubular appearance of dentin. A:
   enamel B: dentin
   Enlarge
   Histologic slide of tooth. Note the tubular appearance of dentin.
   A: enamel
   B: dentin

   The adjacent layer of cells in the dental papilla suddenly increases in
   size and differentiates into odontoblasts, which are the cells that
   form dentin.^ Researchers believe that the odontoblasts would not form
   if it were not for the changes occurring in the inner enamel
   epithelium. As the changes to the inner enamel epithelium and the
   formation of odontoblasts continue from the tips of the cusps, the
   odontoblasts secrete a substance, an organic matrix, into their
   immediate surrounding. The organic matrix contains the material needed
   for dentin formation. As odontoblasts deposit organic matrix, they
   migrate toward the centre of the dental papilla. Thus, unlike enamel,
   dentin starts forming in the surface closest to the outside of the
   tooth and proceeds inward. Cytoplasmic extensions are left behind as
   the odontoblasts move inward. The unique, tubular microscopic
   appearance of dentin is a result of the formation of dentin around
   these extensions.^

   After dentin formation begins, the cells of the inner enamel epithelium
   secrete an organic matrix against the dentin. This matrix immediately
   mineralizes and becomes the tooth's enamel. Outside the dentin are
   ameloblasts, which are cells that continue the process of enamel
   formation; therefore, enamel formation moves outwards, adding new
   material to the outer surface of the developing tooth.

Hard tissue formation

   Sections of tooth undergoing development.
   Enlarge
   Sections of tooth undergoing development.

Enamel

   Enamel formation is called amelogenesis and occurs in the crown stage
   of tooth development. "Reciprocal induction" governs the relationship
   between the formation of dentin and enamel; dentin formation must
   always occur before enamel formation. Generally, enamel formation
   occurs in two stages: the secretory and maturation stages.^ Proteins
   and an organic matrix form a partially mineralized enamel in the
   secretory stage; the maturation stage completes enamel mineralization.

   In the secretory stage, ameloblasts release enamel proteins that
   contribute to the enamel matrix, which is then partially mineralized by
   the enzyme alkaline phosphatase.^ The appearance of this mineralized
   tissue, which occurs usually around the third or fourth month of
   pregnancy, marks the first appearance of enamel in the body.
   Ameloblasts deposit enamel at the location of what become cusps of
   teeth alongside dentin. Enamel formation then continues outward, away
   from the centre of the tooth.

   In the maturation stage, the ameloblasts transport some of the
   substances used in enamel formation out of the enamel. Thus, the
   function of ameloblasts changes from enamel production, as occurs in
   the secretory stage, to transportation of substances. Most of the
   materials transported by ameloblasts in this stage are proteins used to
   complete mineralization. The important proteins involved are
   amelogenins, ameloblastins, enamelins, and tuftelins.^ By the end of
   this stage, the enamel has completed its mineralization.

Dentin

   Dentin formation, known as dentinogenesis, is the first identifiable
   feature in the crown stage of tooth development. The formation of
   dentin must always occur before the formation of enamel. The different
   stages of dentin formation result in different types of dentin: mantle
   dentin, primary dentin, secondary dentin, and tertiary dentin.

   Odontoblasts, the dentin-forming cells, differentiate from cells of the
   dental papilla. They begin secreting an organic matrix around the area
   directly adjacent to the inner enamel epithelium, closest to the area
   of the future cusp of a tooth. The organic matrix contains collagen
   fibers with large diameters (0.1-0.2 μm in diameter).^ The odontoblasts
   begin to move toward the centre of the tooth, forming an extension
   called the odontoblast process.^ Thus, dentin formation proceeds toward
   the inside of the tooth. The odontoblast process causes the secretion
   of hydroxyapatite crystals and mineralization of the matrix. This area
   of mineralization is known as mantle dentin and is a layer usually
   about 150 μm thick.^

   Whereas mantle dentin forms from the preexisting ground substance of
   the dental papilla, primary dentin forms through a different process.
   Odontoblasts increase in size, eliminating the availability of any
   extracellular resources to contribute to an organic matrix for
   mineralization. Additionally, the larger odontoblasts cause collagen to
   be secreted in smaller amounts, which results in more tightly arranged,
   heterogeneous nucleation that is used for mineralization. Other
   materials (such as lipids, phosphoproteins, and phospholipids) are also
   secreted.^

   Secondary dentin is formed after root formation is finished and occurs
   at a much slower rate. It is not formed at a uniform rate along the
   tooth, but instead forms faster along sections closer to the crown of a
   tooth.^ This development continues throughout life and accounts for the
   smaller areas of pulp found in older individuals.^ Tertiary dentin,
   also known as reparative dentin, forms in reaction to stimuli, such as
   attrition or dental caries.^
   Cross-section of tooth at root. Note clear, acellular appearance of
   cementum.A: dentinB: cementum
   Enlarge
   Cross-section of tooth at root. Note clear, acellular appearance of
   cementum.
   A: dentin
   B: cementum

Cementum

   Cementum formation is called cementogenesis and occurs late in the
   development of teeth. Cementoblasts are the cells responsible for
   cementogenesis. Two types of cementum form: cellular and acellular.^

   Acellular cementum forms first. The cementoblasts differentiate from
   follicular cells, which can only reach the surface of the tooth's root
   once Hertwig's Epithelial Root Sheath (HERS) has begun to deteriorate.
   The cementoblasts secrete fine collagen fibrils along the root surface
   at right angles before migrating away from the tooth. As the
   cementoblasts move, more collagen is deposited to lengthen and thicken
   the bundles of fibers. Noncollagenous proteins, such as bone
   sialoprotein and osteocalcin, are also secreted.^ Acellular cementum
   contains a secreted matrix of proteins and fibers. As mineralization
   takes place, the cementoblasts move away from the cementum, and the
   fibers left along the surface eventually join the forming periodontal
   ligmaments.

   Cellular cementum develops after most of the tooth formation is
   complete and after the tooth occludes (in contact) with a tooth in the
   opposite arch.^ This type of cementum forms around the fibre bundles of
   the periodontal ligaments. The cementoblasts forming cellular cementum
   become trapped in the cementum they produce.

   The origin of the formative cementoblasts is believed to be different
   for cellular cementum and acellular cementum. One of the major current
   hypotheses is that cells producing cellular cementum migrate from the
   adjacent area of bone, while cells producing acellular cementum arise
   from the dental follicle.^ Nonetheless, it is known that cellular
   cementum is usually not found in teeth with one root.^ In premolars and
   molars, cellular cementum is found only in the part of the root closest
   to the apex and in interradicular areas between multiple roots.
   Histologic slide of tooth erupting into the mouth. A: tooth B: gingiva
   C: bone D: periodontal ligaments
   Enlarge
   Histologic slide of tooth erupting into the mouth.
   A: tooth
   B: gingiva
   C: bone
   D: periodontal ligaments

Formation of the periodontium

   The periodontium, which is the supporting structure of a tooth,
   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.

Periodontal ligaments

   Cells from the dental follicle give rise to the periodontal ligaments
   (PDL). Specific events leading to the formation of the periodontal
   ligaments vary between deciduous (baby) and permanent teeth and among
   various species of animals.^ Nonetheless, formation of the periodontal
   ligaments begins with ligament fibroblasts from the dental follicle.
   These fibroblasts secrete collagen, which interacts with fibers on the
   surfaces of adjacent bone and cementum.^ This interaction leads to an
   attachment that develops as the tooth erupts into the mouth. The
   occlusion, which is the arrangement of teeth and how teeth in opposite
   arches come in contact with one another, continually affects the
   formation of periodontal ligaments. This perpetual creation of
   periodontal ligaments leads to the formation of groups of fibers in
   different orientations, such as horizontal and oblique fibers.^

Alveolar bone

   As root and cementum formation begin, bone is created in the adjacent
   area. Throughout the body, cells that form bone are called osteoblasts.
   In the case of alveolar bone, these osteoblast cells form from the
   dental follicle.^ Similar to the formation of primary cementum,
   collagen fibers are created on the surface nearest the tooth, and they
   remain there until attaching to periodontal ligaments.

   Like any other bone in the human body, alveolar bone is modified
   throughout life. Osteoblasts create bone and osteoclasts destroy it,
   especially if force is placed on a tooth.^ As is the case when movement
   of teeth is attempted through orthodontics, an area of bone under
   compressive force from a tooth moving toward it has a high osteoclast
   level, resulting in bone resorption. An area of bone receiving tension
   from periodontal ligaments attached to a tooth moving away from it has
   a high number of osteoblasts, resulting in bone formation.

Gingiva

   The connection between the gingiva and the tooth is called the
   dentogingival junction. This junction has three epithelial types:
   gingival, sulcular, and junctional epithelium. These three types form
   from a mass of epithelial cells known as the epithelial cuff between
   the tooth and the mouth.^

   Much about gingival formation is not fully understood, but it is known
   that hemidesmosomes form between the gingival epithelium and the tooth
   and are responsible for the primary epithelial attachment.^
   Hemidesmosomes provide anchorage between cells through small
   filament-like structures provided by the remnants of ameloblasts. Once
   this occurs, junctional epithelium forms from reduced enamel
   epithelium, one of the products of the enamel organ, and divides
   rapidly. This results in the perpetually increasing size of the
   junctional epithelial layer and the isolation of the remenants of
   ameloblasts from any source of nutrition. As the ameloblasts
   degenerate, a gingival sulcus is created.

Nerve and vascular formation

   Frequently, nerves and blood vessels run parallel to each other in the
   body, and the formation of both usually takes place simultaneously and
   in a similar fashion. However, this is not the case for nerves and
   blood vessels around the tooth, because of different rates of
   development.^

Nerve formation

   Nerve fibers start to near the tooth during the cap stage of tooth
   development and grow toward the dental follicle. Once there, the nerves
   develop around the tooth bud and enter the dental papilla when dentin
   formation has begun. Nerves never proliferate into the enamel organ.^

Vascular formation

   Blood vessels grow in the dental follicle and enter the dental papilla
   in the cap stage.^ Groups of blood vessels form at the entrance of the
   dental papilla. The number of blood vessels reaches a maximum at the
   beginning of the crown stage, and the dental papilla eventually forms
   in the pulp of a tooth. Throughout life, the amount of pulpal tissue in
   a tooth decreases, which means that the blood supply to the tooth
   decreases with age.^ The enamel organ is devoid of blood vessels
   because of its epithelial origin, and the mineralized tissues of enamel
   and dentin do not need nutrients from the blood.

Tooth eruption

   Tooth eruption occurs when the teeth enter the mouth and become
   visible. Although researchers agree that tooth eruption is a complex
   process, there is little agreement on the identity of the mechanism
   that controls eruption.^ Some commonly held theories that have been
   disproven over time include: (1) the tooth is pushed upward into the
   mouth by the growth of the tooth's root, (2) the tooth is pushed upward
   by the growth of the bone around the tooth, (3) the tooth is pushed
   upward by vascular pressure, and (4) the tooth is pushed upward by the
   cushioned hammock.^ The cushioned hammock theory, first proposed by
   Harry Sicher, was taught widely from the 1930s to the 1950s. This
   theory postulated that a ligament below a tooth, which Sicher observed
   on under a microscope on a histologic slide, was responsible for
   eruption. Later, the "ligament" Sicher observed was determined to be
   merely an artifact created in the process of preparing the slide.^

   The most widely held current theory is that while several forces might
   be involved in eruption, the periodontal ligaments provide the main
   impetus for the process. Theorists hypothesize that the periodontal
   ligaments promote eruption through the shrinking and cross-linking of
   their collagen fibers and the contraction of their fibroblasts.^

   Although tooth eruption occurs at different times for different people,
   a general eruption timeline exists. Typically, humans have 20 primary
   (baby) teeth and 32 permanent teeth.^ Tooth eruption has three stages.
   The first, known as deciduous dentition stage, occurs when only primary
   teeth are visible. Once the first permanent tooth erupts into the
   mouth, the teeth are in the mixed (or transitional) dentition. After
   the last primary tooth falls out of the mouth—a process known as
   exfoliation—the teeth are in the permanent dentition.

   Primary dentition starts on the arrival of the mandibular central
   incisors, usually at eight months, and lasts until the first permanent
   molars appear in the mouth, usually at six years.^ The primary teeth
   typically erupt in the following order: (1) central incisor, (2)
   lateral incisor, (3) first molar, (4) canine, and (5) second molar.^ As
   a general rule, four teeth erupt for every six months of life,
   mandibular teeth erupt before maxillary teeth, and teeth erupt sooner
   in females than males.^ During primary dentition, the tooth buds of
   permanent teeth develop below the primary teeth, close to the palate or
   tongue.

   Mixed dentition starts when the first permanent molar appears in the
   mouth, usually at six years, and lasts until the last primary tooth is
   lost, usually at eleven or twelve years.^ Permanent teeth in the
   maxilla erupt in a different order from permanent teeth on the
   mandible. Maxillary teeth erupt in the following order: (1) first molar
   (2) central incisor, (3) lateral incisor, (4) first premolar, (5)
   second premolar, (6) canine, (7) second molar, and (8) third molar.
   Mandibular teeth erupt in the following order: (1) first molar (2)
   central incisor, (3) lateral incisor, (4) canine, (5) first premolar,
   (6) second premolar, (7) second molar, and (8) third molar. Since there
   are no premolars in the primary dentition, the primary molars are
   replaced by permanent premolars.^ If any primary teeth are lost before
   permanent teeth are ready to replace them, some posterior teeth may
   drift forward and cause space to be lost in the mouth.^ This may cause
   crowding and/or misplacement once the permanent teeth erupt, which is
   usually referred to as malocclusion. Orthodontics may be required in
   such circumstances for an individual to achieve a straight set of
   teeth.

   The permanent dentition begins when the last primary tooth is lost,
   usually at 11 to 12 years, and lasts for the rest of a person's life or
   until all of the teeth are lost (edentulism). During this stage, third
   molars (also called " wisdom teeth") are frequently extracted because
   of decay, pain or impactions. The main reasons for tooth loss are decay
   or periodontal disease.^

   CAPTION: Eruptions times for primary and permanent teeth ^

   Primary teeth
   Central
   incisor Lateral
   incisor
   Canine First
   premolar Second
   premolar First
   molar Second
   molar Third
   molar
   Maxillary teeth 10 mo 11 mo 19 mo 16 mo 29 mo
   Mandibular teeth 8 mo 13 mo 20 mo 16 mo 27 mo
   Permanent teeth
   Central
   incisor Lateral
   incisor
   Canine First
   premolar Second
   premolar First
   molar Second
   molar Third
   molar
   Maxillary teeth 7–8 yr 8–9 yr 11–12 yr 10–11 yr 10–12 yr 6–7 yr
   12–13 yr 17–21 yr
   Mandibular teeth 6–7 yr 7–8 yr 9–10 yr 10–12 yr 11–12 yr 6–7 yr 11–13
   yr 17–21 yr

Nutrition and tooth development

   As in other aspects of human growth and development, nutrition has an
   effect on the developing tooth. Essential nutrients for a healthy tooth
   include calcium, phosphorus, fluoride, and vitamins A, C, and D.^
   Calcium and phosphorus are needed to properly form the hydroxyapatite
   crystals, and their levels in the blood are maintained by Vitamin D.
   Vitamin A is necessary for the formation of keratin, as Vitamin C is
   for collagen. Fluoride is incorporated into the hydroxyapatite crystal
   of a developing tooth and makes it more resistant to demineralization
   and subsequent decay.^

   Deficiencies of these nutrients can have a wide range of effects on
   tooth development.^ In situations where calcium, phosphorus, and
   vitamin D are deficient, the hard structures of a tooth may be less
   mineralized. A lack of vitamin A can cause a reduction in the amount of
   enamel formation. Fluoride deficency causes increased demineralization
   when the tooth is exposed to an acidic environment, and also delays
   remineralization. Furthermore, an excess of fluoride while a tooth is
   in development can lead to a condition known as fluorosis.

Abnormalities

   There are a number of tooth abnormalities relating to development.

   Anodontia is a complete lack of tooth development, and hypodontia is a
   lack of some tooth development. Anodontia is rare, most often occurring
   in a condition called Hypohidrotic ectodermal dysplasia, while
   hypodontia is one of the most common developmental abnormalities,
   affecting 3.5–8.0% of the population (not including third molars). The
   absence of third molars is very common, occurring in 20–23% of the
   population, followed in prevalence by the second premolar and lateral
   incisor. Hypodontia is often associated with the absence of a dental
   lamina, which is vulnerable to environmental forces, such as infection
   and chemotherapy medications, and is also associated with many
   syndromes, such as Down syndrome and Crouzon syndrome.^

   Hyperdontia is the development of extraneous teeth. It occurs in 1–3%
   of Caucasians and is more frequent in Asians.^ About 86% of these cases
   involve a single extra tooth in the mouth, most commonly found in the
   maxilla, where the incisors are located.^ Hyperdontia is believed to be
   associated with an excess of dental lamina.

   Dilaceration is an abnormal bend found on a tooth, and is nearly always
   associated with trauma that moves the developing tooth bud. As a tooth
   is forming, a force can move the tooth from its original position,
   leaving the rest of the tooth to form at an abnormal angle. Cysts or
   tumors adjacent to a tooth bud are forces known to cause dilaceration,
   as are primary (baby) teeth pushed upward by trauma into the gingiva
   where it moves the tooth bud of the permanent tooth.^

   Regional odontodysplasia is rare, but is most likely to occur in the
   maxilla and anterior teeth. The cause is unknown; a number of causes
   have been postulated, including a disturbance in the neural crest
   cells, infection, radiation therapy, and a decrease in vascular supply
   (the most widely held hypothesis).^ Teeth affected by regional
   odontodysplasia never erupt into the mouth, have small crowns, are
   yellow-brown, and have irregular shapes. The appearance of these teeth
   in radiographs is translucent and "wispy," resulting in the nickname
   "ghost teeth".^

Tooth development in animals

   Generally, tooth development in non-human mammals is similar to human
   tooth development. The variations lie in the morphology, number,
   development timeline, and types of teeth, not usually in the actual
   development of the teeth.

   Enamel formation in non-human mammals is almost identical to that in
   humans. The ameloblasts and enamel organ, including the dental papilla,
   function similarly.^ Nonetheless, while ameloblasts die in humans and
   most other animals—making further enamel formation impossible— rodents
   continually produce enamel, 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 covered with enamel and resembles a
   crown, while the lingual half is covered with dentin and resembles a
   root. Both root and crown develop simultaneously in the rodent incisor
   and continue to grow for the life of the rodent.

   The mineral distribution in rodent enamel is different from that of
   monkeys, dogs, pigs, and humans.^ In horse teeth, the enamel and dentin
   layers are intertwined, which increases the strength and decreases the
   wear rate of the teeth.^

   Supporting structures that create a "socket" are found exclusively in
   Mammalia and Crocodylia.^ In manatees, mandibular molars develop
   separately from the jaw, and are encased in a bony shell separated by
   soft tissue. This also occurs in elephants' successional teeth, which
   erupt to replace lost teeth.

   Unlike most animals, sharks continuously produce new teeth throughout
   life ^via a drastically different mechanism. Because shark teeth have
   no roots, sharks easily lose teeth when they feed (zoologists estimate
   that a single shark can lose up to 2,400 teeth in one year ^) -- they
   must therefore be continually replaced. Shark teeth form from modified
   scales near the tongue and move outward on the jaw in rows until they
   fully develop, are used, and are eventually dislodged.^
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