Human tooth development
This article may be too technical for most readers to understand.(September 2010) |
Tooth development or odontogenesis is the complex process by which
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
Overview
The tooth germ is an aggregation of cells that eventually forms a tooth.[2] These cells are derived from the ectoderm of the first pharyngeal arch and the ectomesenchyme of the neural crest.[1][3][4] The tooth germ is organized into three parts: the enamel organ, the dental papilla and the dental sac or follicle.
The enamel organ is composed of the
The dental papilla contains cells that develop into odontoblasts, which are dentin-forming cells.[2] Additionally, the junction between the dental papilla and inner enamel epithelium determines the crown shape of a tooth.[1] Mesenchymal cells within the dental papilla are responsible for formation of tooth pulp.
The dental sac or follicle gives rise to three important entities:
NGF-R is present in the condensing ectomesenchymal cells of the dental papilla in the early cap stage tooth germ [10] and plays multiple roles during morphogenetic and cytodifferentiation events in the tooth.[11][12][13] There is a relationship between tooth agenesis and absence of the peripheral trigeminal nerve (see Hypodontia).
All stages (bud, cap, bell, crown), growth and morphogenesis of the teeth are regulated by a
Various phenotypic inputs modulate the size of the teeth.[18]
Parathyroid hormone is required for tooth eruption.[19]
Human tooth development timeline
The following tables present the development timeline of human teeth.[20] 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 I.U. | 16 wk I.U. | 17 wk I.U. | 15.5 wk I.U. | 19 wk I.U. | |||
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 I.U. | 16 wk I.U. | 17 wk I.U. | 15.5 wk I.U. | 18 wk I.U. | |||
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 |
Stages
Tooth development is commonly divided into the following stages: the initiation stage, the bud stage, the cap stage, the bell stage, 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.[1]
Initiation Stage
One of the earliest signs in the formation of a tooth that can be seen microscopically is the distinction between the vestibular lamina and the dental lamina. It occurs in the sixth to seventh week of the embryonic life. The dental lamina connects the developing tooth bud to the epithelial layer of the mouth for a significant time.[21] This is regarded as the initiation stage.[1]
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.[1] Typically, this occurs when the fetus is around 8 weeks old.[22] The tooth bud itself is the group of cells at the periphery of the dental lamina.
Along with the formation of the dental lamina, 10 round epithelial structures, each referred to as a bud, develop at the distal aspect of the dental lamina of each arch. These correspond to the 10 primary teeth of each dental arch, and they signify the bud stage of tooth development. Each bud is separated from the ectomesenchyme by a basement membrane. Ectomesenchymal cells congregate deep to the bud, forming a cluster of cells, which is the initiation of the condensation of the ectomesenchyme. The remaining ectomesenchymal cells are arranged in a more or less haphazardly uniform fashion.[citation needed]
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
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. The bell stage is divided into the early bell stage and the late bell stage.[1] Cells on the periphery of the enamel organ separate into four important layers. Cuboidal cells on the periphery of the dental organ are known as outer enamel epithelium (OEE).[2] The columnar cells of the enamel organ adjacent to the enamel papilla are known as inner enamel epithelium (IEE). The cells between the IEE and the stellate reticulum form a layer known as the stratum intermedium. The rim of the enamel organ where the outer and inner enamel epithelium join is called the cervical loop.[23]
In summary, the layers in order of innermost to outermost consist of dentin, enamel (formed by IEE, or 'ameloblasts', as they move outwards/upwards), inner enamel epithelium and stratum intermedium (stratified cells that support the synthetic activity of the inner enamel epithelium) What follows is part of the initial 'enamel organ', the center of which is made up of stellate reticulum cells that serve to protect the enamel organ. This is all encased by the OEE layer.[citation needed]
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.[1]
The crown of the tooth, which is influenced by the shape of the inner 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
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.[1]
Other structures that may appear in a developing tooth in this stage are enamel knots, enamel cords, and enamel niche.[1]
Advanced bell 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 IEE cells were dividing to increase the overall size of the tooth bud, but rapid dividing, called
The adjacent layer of cells in the dental papilla suddenly increases in size and differentiates into odontoblasts, which are the cells that form dentin.[24] Researchers believe that the odontoblasts would not form if it were not for the changes occurring in the IEE. As the changes to the IEE 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 termed predentin, they migrate toward the center 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.[1]
After dentin formation begins, the cells of the IEE secrete an organic matrix against the dentin. This matrix immediately mineralizes and becomes the initial layer of the tooth's enamel. Outside the dentin are the newly formed ameloblasts in response to the formation of dentin, 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.[citation needed]
Hard tissue formation
Enamel
Enamel formation is called amelogenesis and occurs in the crown stage (advanced bell stage) of tooth development. "Reciprocal induction" governs the relationship between the formation of dentin and enamel; dentin formation must always occur before enamel formation.[25] Generally, enamel formation occurs in two stages: the secretory and maturation stages.[26] Proteins and an organic matrix form a partially mineralized enamel in the secretory stage; the maturation stage completes enamel mineralization.[citation needed]
In the secretory stage, ameloblasts release enamel proteins that contribute to the enamel matrix, which is then partially mineralized by the enzyme alkaline phosphatase.[27] This mineralized phase occurs very early around the 3rd or 4th month of pregnancy. This marks the first appearance of enamel in the body. Ameloblasts make enamel at the location of where the cusps of the teeth are located. Enamel grows outwards, away from the center of the tooth.[citation needed]
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.[28] By the end of this stage, the enamel has completed its mineralization.
A residue may form on newly erupted teeth of both dentitions that may leave the teeth extrinsically stained. This green-gray residue, Nasmyth membrane, consists of the fused tissue of the reduced enamel epithelium and oral epithelium, as well as the dental cuticle placed by the ameloblasts on the newly formed outer enamel surface. Nasmyth membrane then easily picks up stain from food debris and is hard to remove except by selective polishing. The child's supervising adults may need reassurance that it is only an extrinsic stain on a child's newly erupted teeth.[29]
Patients with osteopetrosis display enamel abnormalities, suggesting that the a3 gene mutation found in V-ATPases also plays a role in the development of hypomineralized and hypoplastic enamel.[30]
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.[31]
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).[32] The odontoblasts begin to move toward the center of the tooth, forming an extension called the odontoblast process.[1] 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.[32]
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.[32]
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.
Cementum
Acellular cementum forms first. The cementoblasts differentiate from follicular cells, which can only reach the surface of the tooth's root once
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.[36] This type of cementum forms around the fiber 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.[36] Nonetheless, it is known that cellular cementum is usually not found in teeth with one root.[36] 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.[citation needed]
Formation of the periodontium
The periodontium, which is the supporting structure of a tooth, consists of the cementum, periodontal ligaments,
Periodontal ligament
Cells from the dental follicle give rise to the
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 ligament. This perpetual creation of periodontal ligament leads to the formation of groups of fibers in different orientations, such as horizontal and oblique fibers.[36]
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.[36] 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.[citation needed]
Like any other bone in the human body, alveolar bone is modified throughout life. Osteoblasts create bone and
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.[36]
Much about gingival formation is not fully understood, but it is known that
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.[1]
Nerve formation
Vascular formation
Blood vessels grow in the dental follicle and enter the dental papilla in the cap stage.[1] 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.[38] 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.[citation needed]
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.
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.[42]
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.[43] 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.[44] The primary teeth typically erupt in the following order: (1) central incisor, (2) lateral incisor, (3) first molar, (4) canine, and (5) second molar.[45] 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.[46] 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.
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 (
Primary teeth | ||||||||
---|---|---|---|---|---|---|---|---|
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 | ||||||||
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 |
Immediately after the eruption enamel is covered by a specific film: Nasmyth's membrane or 'enamel cuticle', structure of embryological origin is composed of keratin which gives rise to the enamel organ.[52][53]
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, and vitamins A, C, and D.[54] 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, although not a nutrient, is incorporated into the hydroxyapatite crystal of a developing tooth and bones. The dental theory is the low levels of fluoride incorporation and very mild fluorosis makes the tooth more resistant to demineralization and subsequent decay.[9]
Deficiencies of nutrients can have a wide range of effects on tooth development.[55] 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 ingestion has been noted to delay eruption of teeth for as much as a year or more from the accepted eruption dates since the initial 1940s fluoridation trials. Researchers theorize that the delay is a manifestation of fluoride's depressing impact on thyroid hormones. The delay in eruption has been suggested as the reason for the apparent difference in decay among the youngest children. Fluoride ingestion during tooth development can lead to a permanent condition known as fluorosis with varying levels of severity, the result of fluoride's interference with the normal osteoblast development.[56][57][58][59][60]
Undiagnosed and untreated celiac disease often causes dental enamel defects and can be the only manifestation of the disease, in absence of gastrointestinal symptoms or malabsorption signs.[61][62][63]
Bisphenol A (BPA) is a hormone-disrupting chemical that has been implicated in having negative effects on human health, including, but not limited to, fetal development. As shown in animal studies which mimic human enamel, the mother's consumption of products with BPA during pregnancy can lead to the child's tooth development being obstructed. Those children are shown to be prone to incisor and first molar hypomineralization, a weakened state of the enamel. Additionally, it is most important for mother's to avoid BPA during pregnancy, but also avoid BPA-use in the child's products up to five months of age.
Developmental disturbances
The absence of
Anodontia is a complete lack of tooth development. It is rare, most often occurring in a condition called hypohidrotic ectodermal dysplasia.
Hypodontia is a lack of some tooth development (not including third molars). It is one of the most common developmental abnormalities, affecting 3.5–8.0% of the population. Hypodontia is often associated with the absence of a dental lamina, which is vulnerable to environmental forces like infection and chemotherapy medications. It is also associated with many syndromes, such as Down syndrome and Crouzon syndrome.[64]
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.
Some systemic conditions may cause delayed tooth development, such as nutritional factors, endocrine disorders (
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).[72] Teeth affected by regional odontodysplasia nevAmelogenesis imperfecta is an autosomal dominant disease characterized by a defect in dental enamel formation. Teeth are often free of enamel, small, misshapen, and tinted brown. The cause of these deformities is due to a mutation in enamel in expression. Dental patients with this disease should be especially cautious and visit their dentist frequently.
Natal and neonatal teeth are an anomaly that involves teeth erupting in a newborn infant's mouth earlier than usual. The incidence ranges from 1:2,000 to 1:3,500 births. Natal teeth are more frequent, approximately three times more common than neonatal teeth. Some authors reported a higher prevalence in females than males. The most common location is the mandibular region of the central incisors.[73] Natal teeth and neonatal teeth are associated with genetics, developmental abnormalities and certain recognized syndromes. Additional names for this condition include precocious dentition, baby teeth, and milk teeth.
See also
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