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Tooth

A tooth is a hard, calcified structure embedded in the jaws (maxilla and mandible) of vertebrates, consisting of a crown above the gum line and one or more roots below, primarily functioning to masticate food through cutting, tearing, and grinding actions.^[1] Composed mainly of enamel (the hardest substance in the human body, covering the crown), dentin (a resilient layer beneath the enamel), cementum (covering the root), and pulp (containing nerves and blood vessels), teeth are anchored by the periodontal ligament to the alveolar bone.^[2]^[1] In humans, teeth develop in two generations: primary (deciduous) dentition with 20 teeth erupting in infancy and childhood, and permanent dentition with 32 teeth typically emerging from ages 6 to 21.^[1] The four main types of human teeth—incisors (for incising), canines (for tearing), premolars (for crushing and tearing), and molars (for grinding)—are heterodont, meaning their shapes and positions are specialized for efficient food processing.^[1] Beyond mastication, teeth contribute to speech phonation, facial aesthetics, and overall oral health, with the World Health Organization defining adequate function as at least 20 teeth including 9-10 occluding pairs.^[1]^[3] Tooth development begins in utero through odontogenesis, involving epithelial-mesenchymal interactions that form the enamel organ and dental papilla, leading to eruption and root formation postnatally.^[3] Maintenance of tooth integrity relies on protective mechanisms like saliva's remineralization and the pulp's defensive responses to injury, though common issues such as decay and periodontal disease can compromise function if untreated.^[3]

General Aspects

Etymology

The English noun "tooth" originates from Middle English toth, which evolved from Old English tōþ (also spelled toþ), denoting a hard, bony structure in the mouth used for biting and chewing.^[4]^[5] This Old English form is inherited from Proto-Germanic *tanþs or *tanþuz, a reconstructed root shared across Germanic languages, including Old Saxon tand, Old Norse tann, and Old High German zant.^[4]^[6] The Proto-Germanic term derives from the Proto-Indo-European root *h₁dent-, signifying "tooth" or "to bite," which also underlies words in other Indo-European branches, such as Latin dens (as in "dental") and Ancient Greek odous (as in "orthodontia").^[4]^[5] The plural form "teeth," from Old English tēþ, follows an irregular Germanic pattern of vowel mutation (i-mutation), preserving the ancient stem while adapting to phonetic shifts over time.^[7]^[6]

Evolutionary Origin

The evolutionary origin of teeth traces back to the early vertebrates, specifically jawless forms known as agnathans, where tooth-like structures called odontodes first appeared as part of the dermal exoskeleton. Odontodes are mineralized projections composed primarily of dentine with overlying enameloid caps, serving both protective and sensory functions in the skin and oropharyngeal regions. Fossil evidence from Middle Ordovician deposits (approximately 470 million years ago) reveals these structures in early vertebrates like Eriptychius, featuring open pulp cavities and large dentinal tubules indicative of innervation, suggesting an initial sensory role before specialization for feeding.^[8] This "odontode explosion" represents a rapid diversification of such structures across the ectoderm and endoderm, driven by conserved gene regulatory networks involving epithelial and mesenchymal interactions.^[9] Debate persists on the precise pathway of tooth evolution, with two primary hypotheses: the "outside-in" model, positing that external dermal odontodes migrated into the oral cavity, and the "inside-out" model, proposing origins from internal endodermal pharyngeal denticles. A revised "outside-in" hypothesis, supported by developmental studies in extant fishes like zebrafish, indicates that teeth predated jaws, arising when odontode-competent ectoderm invaded the oropharyngeal cavity through the mouth and gill slits, interacting with neural crest-derived mesenchyme to initiate tooth formation.^[10] Fossil records bolster this view, including Silurian thelodonts (circa 425 million years ago) with pharyngeal odontode whorls resembling proto-teeth, and placoderms showing ordered denticles on gnathal bones that transitioned into true teeth. Developmental evidence further confirms ectodermal necessity, as endoderm alone fails to induce teeth in experimental models.^[11] In jawed vertebrates (gnathostomes), this odontode heritage facilitated the evolution of complex dentitions integrated with emerging jaws around the Devonian period (419–359 million years ago), marking a pivotal adaptation for enhanced feeding efficiency. The transition involved modular regulation of odontode size, position, and replacement, evolving from non-growing, scattered structures to aligned, replaceable teeth. While teeth were lost or modified in some lineages, such as cyclostomes (modern jawless vertebrates), the core odontode-based dentition persists across gnathostomes, underscoring its foundational role in vertebrate diversification.^[10]^[8]

Anatomy

Gross Anatomy

The human tooth is a hard, calcified structure embedded in the alveolar processes of the maxilla and mandible, consisting of a visible crown and an embedded root, connected to the jawbone by supporting tissues.^[1] The crown protrudes into the oral cavity for mastication, while the root anchors the tooth, typically extending deeper into the bone. The boundary between the crown and root is marked by the cementoenamel junction, a slight constriction also known as the cervical line.^[12] Teeth are arranged in two arches: the maxillary (upper) and mandibular (lower), with the permanent dentition comprising 32 teeth and the deciduous dentition 20 teeth.^[1] The crown forms the exposed portion of the tooth, varying in shape according to function, and is covered by enamel, the hardest substance in the human body, composed primarily of hydroxyapatite crystals. Enamel provides a protective barrier against wear and decay but lacks regenerative capacity as it is acellular.^[12] Beneath the enamel lies dentin, a dense, tubular tissue that constitutes the bulk of the tooth's core, offering structural support and elasticity. The pulp, a soft connective tissue containing blood vessels, nerves, and lymphatics, occupies the central pulp chamber in the crown and extends into the root canal, supplying vitality to the tooth.^[1] The root, usually one per tooth but multiple in molars, is covered by cementum, a thin mineralized layer similar to bone that facilitates attachment to the periodontal ligament. This ligament, a fibrous connective tissue, suspends the tooth in the alveolar socket, allowing slight mobility while absorbing shock during chewing. The alveolar bone, part of the jaw's socket, encases the root and provides bony support, with the periodontal ligament mediating the connection. At the root apex, the apical foramen permits passage of neurovascular structures from the pulp to the surrounding tissues.^[1] Human teeth are classified into four types based on morphology and function: incisors for cutting, canines for tearing, premolars for crushing and grinding, and molars for grinding. In permanent dentition, there are 8 incisors, 4 canines, 8 premolars, and 12 molars (including 4 wisdom teeth), symmetrically distributed with 16 per arch. Deciduous teeth lack premolars, featuring 8 incisors, 4 canines, and 8 molars. Each tooth type exhibits distinct crown shapes—incisors with chisel-like edges, canines with pointed cusps, premolars with two cusps, and molars with multiple cusps—and root configurations, such as single roots in incisors and canines versus two or three in molars.^[1]

Microscopic Anatomy

The microscopic anatomy of the tooth encompasses the cellular and tissue-level organization of its hard and soft components, which provide structural integrity, protection, and sensory functions. The outermost layer, enamel, is a highly mineralized, acellular tissue covering the crown, composed primarily of hydroxyapatite crystals arranged in prisms that extend from the dentino-enamel junction to the surface. These prisms, each formed by a single ameloblast during development, measure approximately 4–6 µm in diameter and are surrounded by interprismatic enamel, contributing to the tissue's hardness and resistance to wear. Enamel constitutes about 96% mineral by weight, with the remainder being trace organic proteins like amelogenin and water, and its crystallites grow to 50–100 nm wide and up to 1 mm long along the c-axis.^[13]^[12] Beneath the enamel lies dentin, the primary supportive tissue forming the bulk of the tooth, characterized by a tubular microstructure with dentinal tubules (2–4 µm diameter, 18,000–21,000 per mm²) that radiate outward from the pulp. These tubules house odontoblast processes and fluid, transmitting sensations and facilitating reparative responses, while the surrounding intertubular dentin consists of mineralized collagen fibrils arranged in a felt-like pattern perpendicular to the tubules. Dentin is approximately 70% mineral (hydroxyapatite), 20% organic matrix (90% type I collagen and non-collagenous proteins like dentin sialophosphoprotein), and 10% water by weight, with peritubular dentin—lacking collagen and rich in proteoglycans—forming hypermineralized cuffs around tubules. Odontoblasts, tall columnar cells lining the pulp periphery, secrete the predentin matrix, which mineralizes via matrix vesicles and extracellular matrix interactions, producing distinct layers such as mantle dentin (outer, 15–30 µm thick) and circumpulpal dentin.^[14]^[12] The central pulp cavity houses the dental pulp, a soft, gelatinous connective tissue that nourishes the tooth and mediates sensory input. Microscopically, it features four zones: an outermost odontoblastic layer of cuboidal to columnar cells, a cell-free zone (Weil's zone) with nerve endings, a cell-rich zone with fibroblasts and immune cells, and a central pulp core rich in blood vessels and nerves. Composed of loose connective tissue with fibroblasts as the predominant cells, the pulp includes unmyelinated nerve fibers from the trigeminal ganglion and vascular networks entering via the apical foramen, enabling defensive responses to stimuli. The coronal pulp is larger and more branched than the radicular portion, with its vascular supply supporting ongoing dentin formation throughout life.^[12]^[15] Covering the root surface, cementum is a bone-like, avascular mineralized tissue that anchors the tooth to the alveolar bone, increasing in thickness apically with age (up to 1500 µm in molars). It consists of type I collagen fibers and non-collagenous proteins such as bone sialoprotein, with cementocytes embedded in lacunae and exhibiting regenerative potential that diminishes over time. Two main types exist: acellular extrinsic fiber cementum (AEFC), a thin (50–200 µm) coronal layer with densely packed Sharpey's fibers from the periodontal ligament but no cementocytes; and cellular mixed stratified cementum (CMSC), a thicker apical layer containing both intrinsic (cementoblast-derived) and extrinsic fibers, along with cementocytes for adaptive remodeling. Cementicles, small calcified masses, may form in older cementum.^[16]^[12] The periodontal ligament (PDL), interfacing the cementum with alveolar bone, is a specialized connective tissue comprising principal collagen fibers (types I and III) organized into bundles—such as alveolar crest, horizontal, oblique, and apical groups—that insert as Sharpey's fibers into both surfaces for tooth support and force distribution. It contains fibroblasts, cementoblasts, osteoblasts, and immune cells within a ground substance of glycosaminoglycans and water, enabling rapid turnover with collagen half-life of approximately 3–23 days and proprioceptive functions via mechanoreceptors. Blood vessels and nerves course through the ligament, with wider fiber spacing near bone and denser packing near cementum.^[17]^[12]

Development

Embryogenesis

Tooth embryogenesis involves a series of reciprocal interactions between the oral ectoderm and neural crest-derived mesenchyme, leading to the formation of tooth primordia through distinct morphological stages. This process begins in humans around the 6th week of gestation, when the oral epithelium thickens to form the dental lamina, a band of epithelial cells that serves as the origin for all teeth. The dental lamina arises from the ectoderm of the stomodeum and interacts with the underlying mesenchyme to initiate odontogenesis, with the mesenchyme providing inductive signals essential for tooth development.^[18] The initiation stage, occurring at approximately 5-6 weeks of gestation, features the formation of dental placodes as localized thickenings of the oral epithelium within the dental lamina. These placodes mark the sites of future teeth and induce the adjacent mesenchyme to adopt an odontogenic fate, characterized by the expression of key transcription factors such as Pax9 and Msx1 in the mesenchyme. Signaling pathways, including Wnt (e.g., Wnt10a) and BMP (e.g., Bmp4), play critical roles in regulating placode formation and mesenchymal condensation, ensuring proper positioning and number of teeth. In mammals, this stage corresponds to embryonic day 11.5 (E11.5) in mice, highlighting conserved mechanisms across species.^[19]^[20] Progressing to the bud stage around the 8th week of gestation in humans (E12.5 in mice), the epithelial placodes invaginate into the condensed mesenchyme, forming bulbous enamel buds surrounded by a mesenchymal follicle. This stage involves proliferation and migration of epithelial cells, driven by fibroblast growth factor (FGF) signaling such as Fgf8, which promotes bud outgrowth, while Sonic hedgehog (Shh) expression in the epithelium regulates the transition by inhibiting proliferation and inducing bending. Mutations in genes like Eda (involved in ectodysplasin signaling) can disrupt this stage, leading to conditions such as hypohidrotic ectodermal dysplasia with reduced tooth number. The bud stage establishes the basic architecture, with five buds per jaw quadrant corresponding to primary teeth.^[21]^[19]^[20] By the cap stage, typically in the 9th-10th week of gestation (E14.5 in mice), the enamel bud develops into a cap-shaped enamel organ, with the epithelium folding to enclose the dental papilla (future pulp) and dental follicle (future periodontal ligament). A primary enamel knot—a cluster of non-proliferating epithelial cells—emerges at the cusp tip, acting as a signaling center that secretes BMP, FGF, Wnt, and Shh to pattern the cusps and inhibit epithelial proliferation, thereby controlling tooth shape. Transcription factors like Lef1 and p63 in the epithelium, along with Dlx2 in the mesenchyme, further refine morphogenesis during this phase. This stage highlights the shift from proliferation to differentiation, setting the foundation for multicusped teeth in mammals.^[18]^[20] The bell stage, beginning around the 11th week and continuing through the 14th week of gestation (E15.5-E17.5 in mice), involves further invagination of the enamel organ to form a bell-like structure, where epithelial cells differentiate into ameloblasts and mesenchymal cells into odontoblasts. Secondary enamel knots form to dictate additional cusp patterns, regulated by balanced antagonism between activators (e.g., Bmp4, ActivinβA) and inhibitors (e.g., Fgf4, Spry2) that create reaction-diffusion dynamics for cusp spacing. Ectodysplasin (EDA) signaling via Edar maintains knot integrity, while the overall process ensures cytodifferentiation without significant growth. By the end of this stage, the crown shape is largely determined, transitioning embryogenesis toward matrix secretion and mineralization.^[18]^[19]^[20] Following the bell stage, the apposition stage involves the secretion of extracellular matrix: odontoblasts deposit dentin, followed by ameloblasts laying down enamel matrix on the crown portion. This occurs primarily from the 14th week of gestation through postnatal periods for primary teeth. The maturation stage then follows, where enamel hardens through mineralization and protein removal, while the enamel organ reduces. Root development begins late in gestation or postnatally, guided by Hertwig's epithelial root sheath, which induces odontoblasts along the root to form dentin; cementum is later deposited by cementoblasts from the dental follicle. For primary teeth, roots complete around age 3; for permanent, by late teens. These stages complete tooth formation, with primary crown mineralization finishing perinatally and permanent around ages 2-6 years.^[21]

Eruption Process

Tooth eruption is the axial movement of a tooth from its intraosseous crypt within the alveolar bone to its functional position in the oral cavity, encompassing both intraosseous and supraosseous phases that require precise coordination of bone remodeling and soft tissue adaptation.^[22] This process begins after crown formation is complete and root development initiates, typically driven by interactions among the dental follicle, enamel organ, and surrounding bone tissues.^[23] In humans, primary teeth erupt between 6 and 30 months of age, while permanent teeth follow from around 6 years to the late teens, with the sequence generally adhering to mandibular central incisors and first molars first, followed by maxillary counterparts, lateral incisors, premolars, canines, second molars, and third molars.^[24] The eruption process unfolds in distinct stages, starting with pre-eruptive positioning where the tooth germ undergoes minor rotational and vertical movements within the alveolar crypt to align for eruption, facilitated by the gubernacular ligament or cord that guides the path.^[22] This is followed by the intraosseous eruptive stage, during which the tooth moves through the bone toward the oral mucosa at a rate of approximately 1-10 μm per day (0.03-0.3 mm per month) in humans, primarily through asymmetric bone resorption coronal to the tooth and deposition apical to it.^[25] Mucosal penetration then occurs as the reduced enamel epithelium fuses with the oral mucosa to form an eruption pathway, allowing the crown to emerge without significant soft tissue trauma.^[22] The supraosseous phase includes a pre-occlusal spurt where the tooth rapidly advances into the oral cavity to reach near-occlusion at a rate of approximately 1-2 mm per month, followed by post-occlusal eruption that continues throughout life to compensate for occlusal wear and alveolar bone growth, at a slower rate of about 0.015 mm per month in adults.^[23]^[26] At the cellular and molecular level, the dental follicle plays a central orchestrating role by regulating osteoclastogenesis and osteogenesis through secreted factors.^[24] It releases colony-stimulating factor-1 (CSF-1) and monocyte chemoattractant protein-1 (MCP-1) to recruit monocytes, which differentiate into osteoclasts under the influence of receptor activator of nuclear factor kappa-B ligand (RANKL), enabling coronal bone resorption.^[22] Parathyroid hormone-related protein (PTHrP), produced by the enamel organ and dental follicle, binds to PTH1 receptors on osteoblasts and osteoclast precursors to further promote bone remodeling and prevent ankylosis; disruptions in PTHrP signaling, as seen in mutations, lead to eruption failures.^[23] Bone morphogenetic protein-2 (BMP-2) from the follicle supports osteoblast activity for basal bone formation, maintaining the tooth's directional movement.^[24] Vascular endothelial growth factor (VEGF) may contribute to tissue remodeling during the supraosseous phase, while root elongation provides additional force via hydrostatic pressure in the periodontal ligament, though it is not the primary driver.^[22] Current understanding favors the dental follicle theory as the dominant mechanism, integrating bone remodeling with genetic and hormonal regulation, though elements of vascular pressure and periodontal traction may assist in later stages.^[23] The process is genetically programmed but influenced by systemic factors like thyroid hormones and local conditions such as space availability in the dental arch.^[24] Continuous eruption post-occlusion ensures functional adaptation, with teeth moving occlusally at a rate that matches alveolar bone growth in children and compensates for attrition in adults.^[22]

Functions

Mastication and Digestion

Mastication, or chewing, is the initial mechanical process of digestion in which the teeth break down food into smaller particles, increasing its surface area for subsequent enzymatic action. This process involves the coordinated action of the jaws, teeth, tongue, and salivary glands, transforming solid food into a soft, lubricated bolus suitable for swallowing. The primary function of human teeth in mastication is to cut, tear, crush, and grind ingested material, facilitating efficient nutrient extraction downstream in the gastrointestinal tract.^[3] Different types of teeth specialize in specific aspects of mastication to handle diverse food textures. Incisors, located at the front of the mouth, are chisel-shaped and primarily cut food with a biting force of approximately 43.3 kg, enabling initial shearing of items like vegetables or fruits. Canines, positioned next to the incisors, feature pointed cusps for tearing fibrous or tough foods, such as meat, while also stabilizing the jaw during lateral movements. Premolars, with broader surfaces and two cusps each, crush and grind semi-solid foods, exerting forces up to 99.11 kg. Molars, the largest teeth at the back, possess multiple cusps for thorough grinding and pulverizing, capable of forces reaching 120.66 kg on the first molars, which is essential for processing grains, nuts, and other hard substances. These specialized functions ensure comprehensive breakdown, with the occlusal surfaces of opposing teeth working in tandem during rhythmic chewing cycles.^[3]^[27] The masticatory process is powered by the temporomandibular joint (TMJ) and supported by the periodontal ligament, which absorbs shock and allows vertical and lateral jaw movements. During chewing, the tongue positions food between the teeth, while saliva lubricates particles and initiates chemical digestion through enzymes like amylase, which begins starch breakdown. Mastication reduces food particle size to form a swallowable bolus, typically with particles smaller than a few millimeters, increasing surface area for enzymatic action; further gastric grinding ensures particles under 2 mm pass to the small intestine for enhanced nutrient absorption and digestibility. Inadequate mastication, such as from tooth loss, can impair bolus formation, leading to reduced nutrient absorption and altered gut signaling.^[28]^[29]^[3] Overall, teeth's role in mastication not only mechanically prepares food for swallowing but also optimizes the entire digestive process by promoting efficient enzymatic interactions and preventing digestive overload in later stages. This integration underscores the evolutionary adaptation of dentition for dietary versatility in humans.^[30]^[31]

Sensory and Other Functions

Teeth are equipped with extensive sensory innervation, primarily from the maxillary and mandibular divisions of the trigeminal nerve (cranial nerve V), which provides afferent input for detecting touch, pressure, temperature, and pain within the oral cavity.^[32] The dental pulp, the soft connective tissue at the core of the tooth, contains numerous sensory nerve endings that respond to thermal stimuli, osmotic changes, and mechanical irritation, serving as a protective mechanism to alert against potential damage such as cracks or decay.^[15] This pulp innervation is crucial for transmitting nociceptive signals, enabling rapid withdrawal from harmful stimuli during eating or injury.^[33] The periodontal ligament surrounding the tooth root further enhances sensory capabilities through mechanoreceptors that monitor occlusal forces, tooth mobility, and position, providing proprioceptive feedback essential for coordinated jaw movements. These sensory elements collectively contribute to tactile discrimination of food texture and consistency, aiding in the modulation of bite force to prevent overload while facilitating precise oral manipulations.^[3] Beyond protection, this somatosensory system supports homeostasis by influencing vascular responses and dentin formation in response to stimuli.^[34] In addition to sensory roles, teeth fulfill non-masticatory functions critical to communication and physiology. The anterior teeth, particularly incisors and canines, are vital for speech articulation, shaping airflow and tongue positioning to produce sibilant sounds (e.g., /s/ and /z/) and labiodental fricatives (e.g., /f/ and /v/), with their alignment directly impacting intelligibility.^[35] Malocclusions or tooth loss can impair phonation kinetics, leading to lisps or muffled speech.^[3] Teeth also contribute to facial aesthetics and structural support by maintaining the vertical dimension of the face, propping up the lips and cheeks to preserve soft tissue contours and prevent premature aging appearances associated with edentulism.^[3] Furthermore, they aid in airway patency by supporting the tongue's position against the palate, facilitating unobstructed breathing and reducing risks of obstruction in relaxed states.^[3] These roles underscore the teeth's integration into broader orofacial harmony.

Diversity Across Species

Invertebrates

Invertebrates lack true teeth homologous to those of vertebrates, instead possessing diverse analogous structures adapted for feeding, such as rasping, grinding, or piercing mechanisms. These structures vary widely across phyla and are often composed of chitin, calcium carbonate, or other biomaterials, reflecting evolutionary adaptations to specific diets and environments.^[36] In mollusks, the radula serves as the primary tooth-like feeding apparatus, consisting of a chitinous ribbon embedded with thousands of microscopic teeth arranged in transverse rows. This structure, unique to most molluscan classes except bivalves, functions like a file or conveyor belt, rasping food particles from substrates such as algae, detritus, or prey. The teeth, termed denticles, exhibit phylum-specific morphologies; for instance, in gastropods, central, lateral, and marginal teeth vary in size and shape to suit herbivorous or carnivorous habits, with some species like chitons featuring hardened, mineralized radular teeth for scraping rock surfaces.^[37]^[36]^[38] Arthropods, particularly insects and crustaceans, utilize mandibles as paired, sclerotized jaws often equipped with tooth-like projections for biting, cutting, or grinding food. These mandibles, arising from the head capsule, feature incisor processes or molar surfaces that enhance mechanical efficiency; in ants, for example, mandibular teeth incorporate zinc gradients, increasing hardness up to threefold during maturation to enable precise cutting akin to a scalpel. In peracarid crustaceans, an asymmetrical tooth-like lacinia mobilis on the mandibles aids in shredding plant or animal matter.^[39]^[40]^[41] Annelids, such as polychaetes and leeches, possess jaw-like structures or pharyngeal teeth for capturing and processing prey. In eunicidan polychaetes, chitinous jaws with embedded teeth form a robust apparatus for predation, while leeches feature triradial jaws armed with sharp, pointed teeth that facilitate blood-feeding by incising host tissue. The pharynx in species like Nereis virens everts to expose these teeth, enabling the seizure of small invertebrates.^[42]^[43]^[44] Among echinoderms, only sea urchins (Echinoidea) have prominent tooth-like structures within the Aristotle's lantern, a five-fold symmetric masticatory apparatus comprising five calcareous teeth that continuously grow and erode as they grind algae from substrates. Each tooth, formed from magnesium calcite, exhibits a wedge-shaped cross-section for efficient scraping, with microstructural adaptations like polycrystalline plates enhancing wear resistance.^[45]^[46]^[47]

Fish

Fish teeth represent an early evolutionary stage in vertebrate dentition, originating from odontodes—dermal denticles that first appeared in ancient jawless fishes as pharyngeal structures before migrating to the oral jaws in jawed vertebrates.^[48] In modern fish, teeth are typically homodont, meaning all teeth in an individual's mouth are structurally similar, though they vary in size and shape to suit feeding needs; most species exhibit polyphyodonty, with continuous tooth replacement throughout life via a dental lamina that generates new teeth lingually to replace worn or lost ones.^[48] Unlike mammalian heterodonty, fish teeth prioritize grasping, piercing, or crushing over complex mastication, reflecting adaptations to aquatic diets ranging from plankton to hard-shelled prey.^[49] Fish dentition shows remarkable diversity across major clades, broadly divided into cartilaginous fishes (Chondrichthyes) and bony fishes (Osteichthyes). In Chondrichthyes, such as sharks and rays, teeth are embedded in the skin of the jaws rather than bony sockets, forming multiple rows with the functional set at the front and replacements moving forward; these are often triangular and serrated for cutting flesh, as seen in great white sharks (Carcharodon carcharias) where cusps facilitate slicing through prey.^[50] Rays like the spotted eagle ray (Aetobatus narinari) have flattened, molar-like teeth for crushing mollusks and crustaceans.^[49] In Osteichthyes, teeth attach directly to the bony jaws, vomer, and palatine bones, with greater variation; for instance, gars (Lepisosteus spp.) feature long, villiform teeth in multiple rows for impaling fish, while piranhas (Serrasalmus spp.) possess sharp, triangular teeth with lateral cusps for shearing.^[50] Common tooth morphologies in bony fishes include canines—elongated, pointed structures for piercing and holding prey, as in moray eels (Muraenidae) where pharyngeal teeth swing forward to secure slippery victims; incisors—chisel-like for cropping vegetation or cutting, exemplified by parrotfish (Scaridae) beaks formed from fused teeth that scrape algae from coral; and molars—broad and rounded for grinding, such as the pharyngeal tooth plates in lungfish (Dipnoi) that crush shelled invertebrates.^[49] Villiform teeth, small and needle-like, form rasping patches on jaws or tongues for directing food, common in piscivores like the black dragonfish (Idiacanthus antrostomus).^[49] Among piscivorous teleosts, dentition clusters into morphotypes: edentulous (toothless jaws for suction feeding), villiform (numerous fine teeth for raking, e.g., groupers Epinephelus spp.), and macrodont (few large teeth for puncturing, e.g., coral trout Plectropomus leopardus with back-fanged arrangements enhancing bite force by up to 42%).^[51] This diversity correlates with ecological niches, where tooth shape and arrangement optimize prey capture efficiency in aquatic environments; for example, herbivorous grass carp (Ctenopharyngodon idella) use robust pharyngeal teeth to mill plant material, while mollusk-crushing species like the river redhorse (Moxostoma carinatum) employ sturdy, plate-like dentition.^[50] Jawless fishes like lampreys (Petromyzontiformes) lack true teeth but have rasping oral structures with horn-like points for blood-feeding, highlighting the basal condition before odontode specialization.^[50] Overall, fish teeth underscore adaptive radiation, with over 30,000 species exhibiting forms that balance replacement rates (often weekly in sharks) against dietary demands.^[48]

Amphibians

Amphibians exhibit considerable diversity in dental morphology, reflecting adaptations to their aquatic and terrestrial lifestyles, with teeth primarily serving to grasp and hold prey rather than for chewing or grinding. Unlike mammalian teeth, amphibian dentition is characterized by pedicellate teeth, where each tooth consists of a calcified crown and pedicel separated by a flexible, non-calcified zone that allows for replacement and absorption. This structure is evident across the three major amphibian clades: Anura (frogs and toads), Urodela (salamanders and newts), and Gymnophiona (caecilians).^[52] In anurans, teeth are generally simple and conical, located on the maxillary and premaxillary bones of the upper jaw, as well as on the vomer and palatine bones of the palate; the lower jaw (dentary) is typically edentulous. Many anuran species have independently lost teeth multiple times over evolutionary history, with over 20 instances of tooth loss documented across frog lineages, resulting in some groups being completely toothless. For example, most toads (Bufonidae) lack teeth entirely, while the Australian tree frog (Litoria caerulea) retains small vomerine teeth for securing insects. Only one known anuran species, the South American Gastrotheca guentheri, possesses true lower jaw teeth, highlighting the rarity of mandibular dentition in this group. These teeth function to prevent prey escape during swallowing, aided by the tongue's adhesive properties.^[53] Urodeles display more complex dental arrangements, with teeth present on both upper and lower jaws, often in multiple rows. Marginal teeth line the premaxilla, maxilla, and dentary, while palatal teeth occur on the vomer and palatine; some species, like the hellbender (Cryptobranchus alleganiensis), also have teeth on the parasphenoid. Tooth morphology varies: larval teeth are conical for grasping soft prey, while adults may have recurved or bifid cusps adapted for holding slippery aquatic invertebrates or small vertebrates. Replacement occurs continuously through resorption of old teeth at the non-calcified zone, enabling rapid turnover suited to predatory lifestyles. In species like the axolotl (Ambystoma mexicanum), teeth aid in capturing live food by providing a firm grip during ingestion.^[54] Caecilians, the limbless, burrowing amphibians, possess the most specialized dentition among the group, featuring small, sharp, recurved teeth arranged in two rows on both jaws, with additional palatal teeth on the vomer. These teeth are often bicuspid or tricuspid, facilitating the capture of earthworms and other soil-dwelling prey in their subterranean habitats. The robust jaw structure, supported by fused bones, enhances biting force for piercing tough integuments. Tooth replacement is polyphyodont, with multiple generations forming simultaneously, allowing for efficient renewal in response to wear from abrasive environments. For instance, in the ringed caecilian (Siphonops annulatus), these adaptations support a diet of annelids seized through chemosensory detection.^[55]

Reptiles

Reptilian teeth are typically simple, conical structures adapted primarily for seizing prey, with variations in implantation and replacement reflecting the group's evolutionary adaptations to diverse diets and lifestyles. Unlike mammals, most reptiles exhibit homodont dentition, where teeth are uniform in shape and function across the jaw, lacking specialized incisors, canines, or molars. This uniformity supports their predominantly carnivorous or omnivorous habits, though some herbivorous species like iguanas have evolved broader, leaf-shearing teeth. In terms of implantation, reptile teeth display three main types: acrodont, pleurodont, and thecodont. Acrodont teeth, found in agamids (such as bearded dragons) and some chameleons, ankylose directly to the crest of the jawbone without sockets, resulting in non-replaceable teeth that wear down over time and are suited to insectivorous diets. Pleurodont teeth, common in iguanas, snakes, and most lizards, attach to the medial or labial surface of the jawbone, allowing for some mobility and periodic replacement; these are often curved and recurved for gripping slippery prey. Thecodont implantation, characteristic of crocodilians and some extinct reptiles, embeds teeth in deep sockets (alveoli) within the jaw, enabling powerful biting forces and continuous replacement throughout life, with up to 80 teeth in adults that regenerate every few months. Tooth replacement in reptiles is polyphyodont, occurring continuously rather than in waves as in mammals, with successional teeth developing lingual to functional ones. In squamates (lizards and snakes), replacement cycles vary by species; for instance, in the green anole (Anolis carolinensis), teeth are replaced every 1-2 months to maintain sharpness for capturing insects. Crocodilians exhibit a more complex system involving dental lamina that produces multiple generations of teeth, supporting their role as apex predators with bite forces exceeding 3,700 pounds per square inch in species like the saltwater crocodile (Crocodylus porosus). Herbivorous reptiles, such as the green iguana (Iguana iguana), have slower replacement rates and teeth adapted for grinding plant material, with pleurodont batteries that form shearing edges. Diversity in reptilian dentition also extends to specialized forms, such as the venom-conducting fangs in viperid snakes, which are enlarged, hollow pleurodont teeth that fold against the palate when not in use, injecting hemotoxic venom to subdue prey. In turtles and tortoises, which lack teeth entirely, beak-like structures (rhamphotheca) derived from scales replace dental function for cropping vegetation or crushing mollusks, representing a derived loss of dentition in the group. Fossil evidence from archosaurs, including dinosaurs, shows further variations like serrated carnivorous teeth in theropods or battery-like arrangements in hadrosaurs for herbivory, highlighting the evolutionary plasticity of reptilian dental systems.

Birds

Modern birds (Neornithes) are universally edentulous, lacking true teeth and instead possessing keratinous beaks (rhamphothecae) that cover the bony jaws, a condition that evolved as a replacement for dentition in food acquisition and processing.^[56] This tooth loss occurred gradually during the Mesozoic era, with complete edentulism emerging in the lineage leading to crown-group birds around 90 million years ago, later than previously estimated.^[57] The transition involved developmental arrests in odontogenesis, facilitated by the rhamphotheca, which inhibited tooth formation, and was accompanied by enhancements in the muscular gizzard for mechanical food breakdown.^[56] Fossil evidence from Mesozoic avialans reveals diverse dentition patterns, with no overarching macroevolutionary trend toward tooth loss across the group; instead, reductions happened independently in specific jaw regions (premaxilla, maxilla, dentary) due to localized selective pressures.^[57] For instance, early avialans like Jeholornis retained teeth only in the anterior dentary, while enantiornithines such as Falcatakely exhibited sequential loss starting in the maxilla before the premaxilla.^[57] Later groups, including the ornithurine Ichthyornis and hesperornithiform Hesperornis, possessed conical, unserrated teeth adapted for piscivory, with Hesperornis showing more robust, ornamented teeth differing markedly from the slender ones in Ichthyornis.^[58] Tooth replacement in these birds followed an alternating pattern in the premaxilla, similar to that in non-avian dinosaurs and crocodilians, indicating conservation of the underlying cycling mechanism during the reptile-to-bird transition.^[59] The evolutionary drivers of edentulism in birds are linked to life history traits, such as faster incubation periods in open-nest brooding theropods, where prolonged tooth development in embryos constrained hatching times to reptilian scales (3–6 months); selection for rapid development favored genetic inactivation of tooth-forming pathways.^[60] Key genetic changes include mutations disrupting Bmp4 signaling, which halts tooth bud initiation, and heterochrony—early cessation of tooth replacement—as evidenced in fossils like Limusaurus.^[60] Modern bird embryos retain vestigial tooth primordia, underscoring the latent genetic potential for dentition.^[60] Although true teeth are absent in extant birds, some lineages evolved tooth-like analogs known as pseudoteeth, which are bony or keratinous projections rather than enamel-dentin structures. In parrots (Psittaciformes), pseudoteeth form via epithelial-mesenchymal evaginations during embryonic development, resembling the ontogeny of scales and feathers but lacking dental tissues or replacement cycles.^[61] Similarly, extinct Cenozoic odontopterygiforms like Pelagornis bore sequential bony pseudoteeth along the jaws, increasing in size posteriorly, which likely aided in grasping slippery prey and arose from dynamic morphogenetic fields rather than true odontogenic processes.^[62] These structures highlight convergent adaptations for predatory or foraging functions in the absence of genuine dentition.

Human Teeth

Human teeth exhibit diphyodont dentition, consisting of two successive sets: primary (deciduous or milk) teeth and permanent teeth.^[21] The primary dentition comprises 20 teeth, erupting between approximately 6 months and 3 years of age, while the permanent dentition includes 32 teeth, replacing the primary set and erupting from around 6 years to late adolescence or early adulthood.^[63] This heterodont arrangement features four distinct tooth classes—incisors, canines, premolars, and molars—adapted for cutting, tearing, and grinding functions, reflecting adaptations to an omnivorous diet.^[1] The structure of a human tooth includes an outer crown covered by enamel, the hardest substance in the body, composed primarily of hydroxyapatite crystals for protection against wear.^[1] Beneath the enamel lies dentin, a bonelike tissue forming the bulk of the tooth, which is less mineralized and contains microscopic tubules housing odontoblastic processes.^[14] The central pulp cavity houses the dental pulp, a soft connective tissue with nerves, blood vessels, and lymphatics, providing nourishment and sensory innervation.^[64] Roots, embedded in the alveolar bone, are covered by cementum, a mineralized layer that anchors the periodontal ligament, facilitating tooth support.^[1] Incisors are chisel-shaped anterior teeth, with eight in the permanent dentition (four maxillary and four mandibular), designed for incising food; central incisors are larger and more centrally positioned.^[1] Canines, four in total, are pointed for tearing and are the longest teeth, prominent in both arches to aid in food manipulation.^[1] Premolars (bicuspids), numbering eight, feature two cusps for crushing and grinding, absent in primary dentition, and exhibit broader occlusal surfaces than canines.^[65] Molars, twelve in the permanent set (including four wisdom teeth or third molars), have multiple cusps for thorough mastication; first and second molars erupt earlier, while third molars typically emerge between 17 and 25 years.^[1] Tooth development begins in the sixth week of embryonic life through interactions between oral ectoderm and ectomesenchyme, forming dental lamina that initiates tooth buds.^[21] Stages include initiation, proliferation, histodifferentiation, morphodifferentiation, bell stage (where enamel organ and dental papilla form), and apposition (matrix deposition by ameloblasts and odontoblasts).^[21] Primary teeth calcification starts in utero around the 14th week, with eruption timelines as follows: central incisors at 6-10 months, lateral incisors at 8-21 months, canines and first molars at 15-21 months, and second molars at 20-24 months.^[63] Permanent tooth eruption follows primary exfoliation, governed by root formation, alveolar bone remodeling, and gubernacular cords guiding emergence.^[66] The sequence typically begins with mandibular first molars and central incisors at 6-7 years, followed by maxillary incisors at 7-8 years, premolars at 10-12 years, canines at 9-12 years, and second molars at 11-13 years, with third molars last.^[66] Variations occur due to genetics, nutrition, and gender, with females often erupting earlier than males.^[67] Human dentition is characterized by a reduction in tooth size and number compared to many mammals, with aligned arches supporting efficient occlusion.^[68]

Other Mammals

Mammalian dentition exhibits remarkable diversity, characterized by heterodonty with four primary tooth types: incisors for cutting and gnawing, canines for piercing and tearing, premolars for initial processing, and molars for grinding or crushing. This variation primarily reflects dietary adaptations, with carnivores featuring specialized shearing structures, herbivores emphasizing grinding surfaces, and omnivores balancing both functions. In carnivores such as felids (cats) and canids (dogs), carnassial teeth—typically the upper fourth premolar and lower first molar—form bladed edges for slicing meat, while reduced molars minimize grinding capability.^[69] Herbivores like equids (horses) and bovids (cows) possess hypsodont (high-crowned) molars that resist wear from abrasive plant material, often with complex lophs (ridges) for efficient trituration.^[69] Omnivores, including suids (pigs) and ursids (bears), display bunodont molars with low, rounded cusps suitable for crushing both vegetation and animal matter, allowing dietary flexibility.^[69] A subset of mammals features ever-growing (hypselodont) teeth, which lack roots and continuously erupt to compensate for heavy wear, evolving convergently in response to abrasive diets or specific ecological pressures. Rodents, such as mice and rats, exemplify this with chisel-like incisors that grow throughout life, featuring enamel on one side for self-sharpening during gnawing of wood or seeds; this adaptation is regulated by dental stem cell niches involving BMP and FGF signaling pathways.^[70] Lagomorphs (rabbits and hares) similarly have hypselodont incisors and molars for processing fibrous vegetation. In proboscideans like elephants (Loxodonta and Elephas), elongated incisors form tusks used for foraging, defense, and manipulation, though lacking enamel and thus prone to asymmetric wear.^[70] Marine mammals such as walruses (Odobenus rosmarus) exhibit ever-growing canines adapted for prying open bivalves and social displays.^[70] Specialized dentitions further highlight mammalian adaptability. In manatees and dugongs (sirenians), molars continuously replace each other in a conveyor-belt fashion to handle seagrass abrasion, with no incisors or canines present. Bats (Chiroptera) show extreme variation, from toothless species relying on echolocation for insect capture to frugivorous forms with simplified molars for soft fruit. Tooth complexity, measured by cusp number, has increased evolutionarily in herbivores to enhance plant processing, while carnivores often retain simpler, unicuspid forms for prey capture, correlating with dietary niches and speciation rates.^[71] Overall, these traits underscore how dental morphology drives mammalian ecological diversification, with phylogenetic constraints like the ancestral tribosphenic molar influencing modern forms.^[69]

Fossilization and Preservation

Fossilization Processes

Teeth, composed primarily of durable mineralized tissues such as enamel and dentin, exhibit exceptional preservation potential in the fossil record due to their resistance to biological and chemical degradation. Enamel, the hardest substance in the vertebrate body, consists of hydroxyapatite crystals that provide structural integrity, allowing teeth to withstand post-mortem processes far better than softer skeletal elements. This durability stems from the low organic content and high mineralization, which minimizes bacterial breakdown and physical abrasion.^[72]^[73] The initial stage of tooth fossilization involves taphonomic burial, where detached teeth—commonly shed by polyphyodont species such as sharks—are rapidly covered by sediments in aquatic or terrestrial environments. This burial shields the remains from scavengers, weathering, and exposure, preventing fragmentation and oxidation. In marine settings, such as those yielding abundant shark tooth fossils, teeth settle into fine-grained sediments like mud or sand, initiating preservation. Rapid burial is crucial, as prolonged exposure leads to bioerosion or dissolution, reducing the likelihood of fossilization.^[74]^[75] Following burial, permineralization is the dominant fossilization process for teeth, in which mineral-rich groundwater percolates through the sediment, depositing ions like silica, calcite, or phosphate into the microscopic pores of enamel and dentin. This impregnates the tooth structure without replacing the original bioapatite, increasing density and hardness while preserving fine details such as cusps and serrations. For instance, fossil shark teeth from Miocene deposits often show this silicification, where silica fills voids, enhancing durability against further diagenetic alteration. The process can occur over thousands to millions of years, depending on groundwater chemistry and sediment permeability.^[76]^[75] Diagenesis, encompassing post-burial chemical transformations, further modifies tooth composition during fossilization. In enamel, common changes include uptake of trace elements like uranium and fluoride, which substitute into the crystal lattice, and minor recrystallization that maintains overall structure but alters isotopic signatures. Dentin, being more porous and organic-rich, undergoes greater diagenetic alteration, such as collagen degradation and secondary mineralization, compared to enamel. These processes are influenced by environmental factors like pH and temperature, with acidic conditions accelerating dissolution. Despite these changes, enamel's resistance ensures that teeth often retain morphological and geochemical fidelity for paleobiological analyses.^[77]^[73]^[72] In rare cases, replacement occurs if original minerals dissolve and are substituted by others, such as pyrite or hematite, though this is less common in teeth than in bones due to enamel's stability. Carbonization, involving organic residue compression, is minimal in teeth given their inorganic dominance. Overall, these processes explain the abundance of isolated tooth fossils in the record, providing key insights into ancient diets and phylogenies.^[76]^[77]

Taphonomy of Teeth

Taphonomy encompasses the processes that affect organic remains from death to fossilization, with teeth exhibiting exceptional preservation potential due to their highly mineralized structure. Enamel, comprising approximately 96% inorganic material, renders teeth resistant to chemical dissolution and microbial degradation, far surpassing the durability of bone, which contains more organic components prone to rapid breakdown.^[78] This durability ensures that teeth often represent the majority of vertebrate microfossils in sedimentary deposits, providing critical insights into ancient faunas where skeletal elements are scarce.^[79] Following death, teeth undergo minimal initial decay compared to soft tissues, but they can be subject to scavenging, abrasion, and disarticulation, leading to isolated preservation. Transport by water currents or wind may relocate teeth, resulting in concentrations in depositional environments like floodplains or riverbeds, where low-energy conditions limit further mechanical damage.^[74] Rapid burial in fine sediments protects teeth from weathering and exposure, facilitating permineralization, where minerals such as silica or calcite infiltrate pores, reinforcing the structure without significant alteration.^[80] In contrast, prolonged exposure can cause surface etching or cracking, though enamel's hardness mitigates these effects more effectively than in bones. Diagenetic alterations during burial vary by environment; for instance, teeth from petroleum seeps exhibit superior microstructural preservation, with hydrocarbons sealing dentinal tubules and enamel prisms against diagenetic fluids, despite external discoloration.^[81] Conversely, karstic sinkholes promote dentin degradation through acidic groundwater, leading to mineral replacement like calcite infilling and reduced enamel crystallinity.^[81] In Late Cretaceous floodplain deposits, such as those in the Bostobynskaya Formation of Kazakhstan, dinosaur teeth show low abrasion and high abundance, reflecting minimal transport and polyphyodont shedding behaviors that increase their taphonomic input.^[82] These processes underscore teeth's role as robust archives, though post-mortem modifications like surface textural changes can bias analyses of original morphology if not accounted for.^[83]

References

[1]
Anatomy, Head and Neck, Teeth - StatPearls - NCBI Bookshelf
Jul 24, 2023 · Structure and Function · Teeth are calcified structures found in the oral cavity embedded to the upper jaw (maxilla) and lower jaw (mandible).
[2]
Tooth anatomy: MedlinePlus Medical Encyclopedia
Jul 8, 2023 · The root contains blood vessels and nerves, which supply blood and feeling to the whole tooth. This area is known as the "pulp" of the tooth.
[3]
Physiology, Tooth - StatPearls - NCBI Bookshelf
The primary function of teeth is mastication, which involves the cutting, mixing, and grinding of food to allow the tongue and oropharynx to shape it into a ...
[4]
Tooth - Etymology, Origin & Meaning
Tooth originates from Middle English, Old English, and Proto-Germanic roots meaning "tooth." It refers to a human or animal tooth and related actions like ...
[5]
TOOTH Definition & Meaning - Merriam-Webster
Etymology. Middle English, from Old English tōth; akin to Old High ... Play Blossom: Solve today's spelling word game by finding as many words as you can.Sweet tooth · Milk tooth · Baby tooth · Permanent tooth
[6]
tooth, n. meanings, etymology and more | Oxford English Dictionary
The earliest known use of the noun tooth is in the Old English period (pre-1150). tooth is a word inherited from Germanic. See etymology. Nearby entries. toot ...
[7]
Teeth - Etymology, Origin & Meaning
Middle English toth "human or animal tooth," from Old English toð from Proto-Germanic *tanthu- (source also of Old Saxon, Danish, Swedish, Dutch tand, ...
[8]
The origin of vertebrate teeth and evolution of sensory exoskeletons
May 21, 2025 · Although teeth evolved from structures in the dermal exoskeleton of jawless vertebrates known as odontodes, their origin and function remains ...
[9]
The Odontode Explosion: The origin of tooth-like structures in ... - NIH
Some evidence suggests that the first jawed vertebrates (albeit derived, placoderms [16–18]) possessed teeth on their jaws (gnathal bones), ordered denticles on ...
[10]
Evolutionary and developmental origins of the vertebrate dentition
We suggest that teeth may have arisen before the origin of jaws, as a result of competent, odontode-forming ectoderm invading the oropharyngeal cavity.
[11]
Evolutionary origins of the vertebrate dentition - PubMed
The new fossil evidence suggests that teeth may have evolved from these more specialised oropharyngeal denticles in agnathan vertebrates. Publication types.
[12]
Histology, Tooth - StatPearls - NCBI Bookshelf - NIH
Jun 26, 2023 · The tooth can be divided into two main parts: the crown and the root. An indentation (cervical line) encircles the tooth marking a distinction between the ...
[13]
Dental Enamel Formation and Implications for Oral Health and ...
The formed enamel has a characteristic prismatic appearance composed of rods, each formed by a single ameloblast and extending from the dentino-enamel junction ...
[14]
Dentin: Structure, Composition and Mineralization - PubMed Central
A thick dentin layer forms the bulk of dental mineralized dental tissues. Dentin is capped by a crown made of highly mineralized and protective enamel.
[15]
Anatomy, Head and Neck, Pulp (Tooth) - StatPearls - NCBI Bookshelf
Put simply, the main four functions of the pulp are formation and nutrition of the dentin, as well as the innervation and defense of the tooth. Dentin formation ...
[16]
Histology of human cementum: Its structure, function, and development
Cementum, or root cementum, is a mineralized tissue covering the entire root surface. According to Denton [1], cementum was first demonstrated microscopically ...
[17]
Histology, Periodontium - StatPearls - NCBI Bookshelf - NIH
The periodontium is a connective tissue consisting of four components: cementum, the periodontal ligament (PDL), alveolar bone, and gingival tissue.
[18]
Early development of the human dentition revisited - PubMed Central
The deciduous second lower molar has already reached the cap stage at prenatal week 8, when its equivalent in the upper jaw is not yet detectable in the same ...Introduction · Figure 1 · The Early Prenatal...
[19]
Concise Review: Cellular and Molecular Mechanisms Regulation of Tooth Initiation
### Summary of Cellular and Molecular Mechanisms of Tooth Initiation
[20]
Understanding Early Tooth Development in Four Dimensions - NIH
During late odontogenic stages, dental epithelial and mesenchymal cells differentiate into ameloblasts and odontoblasts that lay down enamel and dentin, ...
[21]
Embryology, Teeth - StatPearls - NCBI Bookshelf
The different layers of tooth i.e., enamel, dentin, pulp, and cementum, play different roles—enamel functions to protect the dentin.
[22]
How do teeth erupt? | British Dental Journal - Nature
Aug 9, 2024 · This process involves significant axial movement of the tooth, initiated at the right time and requiring both intra-osseous and intra-oral movement.
[23]
Recent advances in understanding theories of eruption (evidence ...
The eruptive phase begins with the onset of root formation and terminates by tooth appearance in the oral cavity, just before function (pre-functional phase) [4] ...
[24]
The tooth eruption and its abnormalities - A narrative review
[2] The mechanism behind the tooth eruption is an interaction between osteoblasts, osteoclasts and the dental follicle, with the presence of genetically ...
[25]
Mastication Development: Aspects in Early Childhood
These teeth serve different purposes: incisors are for cutting and canines are for cutting and tearing, while molars are mainly for chewing and shearing.
[26]
Physiology, Digestion - StatPearls - NCBI Bookshelf
Sep 12, 2022 · Mechanical digestion in the oral cavity consists of grinding food into smaller pieces by the teeth, a process called mastication.
[27]
Prehension, Mastication, Swallowing
During mastication, the tongue and, to a lesser extent, the lips and cheeks acts to keep food between the grinding surfaces of the teeth. This can be ...
[28]
Chewing and its influence on swallowing, gastrointestinal and ...
Jul 14, 2022 · Efficient chewing affects gut signaling and ultimately digestive and absorptive processes (Li et al. 2011).
[29]
The cost of chewing: The energetics and evolutionary significance of ...
Aug 17, 2022 · An essential outcome of mastication is the comminution of a food into small particles, lubricating them with saliva, so promoting the formation ...The Cost Of Chewing: The... · Materials And Methods · Selection Of Chewing...
[30]
Trigeminal Nerve: What It Is, Anatomy, Function & Conditions
Your trigeminal nerves help your face recognize pain and touch sensations, as well as heat and cold. The nerves also help you chew.Overview · Function · Conditions And Disorders<|separator|>
[31]
The anatomy, neurophysiology, and cellular mechanisms of ...
Mar 25, 2024 · Somatosensory innervation of the oral cavity enables the detection of a range of environmental stimuli including minute and noxious mechanical forces.
[32]
The Role of Sensory Nerves in Dental Pulp Homeostasis
Jan 17, 2024 · These findings indicate that sensory nerves play an essential role in pulp homeostasis maintenance and dental pulp cell fate decisions.
[33]
A Contemporary Review of Clinical Factors Involved in Speech ...
Jul 18, 2023 · The rehabilitation of the maxillary anterior teeth, in particular, plays a vital role in optimal speech intelligibility and articulation; their ...
[34]
Radula - an overview | ScienceDirect Topics
Gastropods are primarily herbivores and detritivores, and feed with a file-like, toothed structure called the radula, which is attached to the cartilaginous ...
[35]
Not just scratching the surface: distinct radular motion patterns in ...
Oct 21, 2020 · Radulae and their teeth can be quite distinct in their morphology and had been of high research interest, but only a few studies have examined ...
[36]
Elemental composition and material properties of radular teeth in the ...
Aug 14, 2023 · The molluscan feeding structure is the radula, a chitinous membrane with teeth, which are highly adapted to the food and the substrate to which ...
[37]
Mouthparts – ENT 425 – General Entomology - NC State University
They include the labrum, a flattened sclerite that serves as an upper lip, a pair of mandibles with toothed inner surfaces for grinding or crushing food, a pair ...
[38]
The lacinia mobilis and Similar Structures – a Valuable Character in ...
The lacinia mobilis of Peracarida can be characterized precisely by asymmetry on the left and right mandibles, as a strong tooth-like structure.
[39]
Tooth hardness increases with zinc-content in mandibles of young ...
We show that the hardness of the mandibular teeth increases nearly three-fold as the adults age and that hardness correlates with Zn content.
[40]
Functional Morphology of Eunicidan (Polychaeta) Jaws
Feb 26, 2018 · As the only hard structures in these soft-bodied animals, eunicidan teeth and occasional intact jaw apparatuses are im- portant fossil evidence ...
[41]
Unraveling the structure, chemical composition, and conserved ...
May 11, 2024 · Leech teeth are sharp, pointed, and along three jaws. They have chemical similarities to mouse teeth and conserved signaling molecules like ...
[42]
Nereis virens - Annelida - Lander University
The inner walls of the pharynx bear teeth and jaws. Eversion of the pharynx during feeding brings the jaws and teeth to the outside where they can be used to ...
[43]
Sea urchins have teeth? A review of their microstructure ...
The jaw structure or Aristotle's lantern has five-fold symmetry, with five jaw sections or pyramids each containing a single tooth (Figure 1).
[44]
Echinodermata: Morphology
... teeth arranged in a structure known as "Aristotle's lantern." Echinoderms (except holothurians) generally lack respiratory systems, and many have only ...
[45]
[PDF] Design strategies of sea urchin teeth - MIT
Oct 30, 2009 · The five teeth are part of the masticatory apparatus (Aristotle's lantern), which itself has five- fold radial symmetry.
[46]
Teeth – Morphology of the Vertebrate Skeleton
For the majority of vertebrate taxa, teeth are homodont, meaning that they are structurally similar (though they may vary in size). Many fish and non-mammalian ...
[47]
Tooth Types & Patches – Discover Fishes
Mar 27, 2018 · Common fish teeth include canines (for piercing), molars (for crushing), and incisors (for cutting). Villiform teeth are for stabbing and ...Missing: diversity | Show results with:diversity
[48]
Amazing Diversity in Fish Dentition | OSU Bio Museum
May 13, 2016 · Fish dentition varies widely, from lampreys with sharp teeth and radulae, to sharks with triangular teeth, lungfish with tooth plates, and ...
[49]
Functional implications of dentition-based morphotypes in ... - NIH
Sep 11, 2019 · The three morphotypes are edentulate, villiform, and macrodont. Edentulate/villiform have larger gape sizes, while macrodont have smaller gape ...
[50]
15.11 Amphibians – Concepts of Zoology – Hawaiʻi Edition
Amphibian teeth are pedicellate, which means that the root and crown are calcified, separated by a zone of noncalcified tissue. Amphibians have image-forming ...
[51]
Overview of Amphibians - Exotic and Laboratory Animals
Frogs have teeth only in the upper jaw; toads have none. Teeth are not generally used for chewing but simply for prehending food. These animals swallow ...
[52]
Amphibians - Biology 2e
Amphibians also have multiple small teeth at the edge of the jaws. In salamanders and caecilians, teeth are present in both jaws, sometimes in multiple rows. In ...
[53]
AMPHIBIANS - Veterian Key
Oct 1, 2016 · Unique Anatomy of Newts and Salamanders. MUSCULOSKELETAL ... Caecilians possess small sharp teeth that are positioned in two layers.
[54]
From snout to beak: the loss of teeth in birds - ScienceDirect.com
The evolution of diverse extreme beak shapes was completed during the first half of the Cenozoic, following tooth loss: in pelicans, stork-like birds, duck-like ...Missing: scientific review
[55]
Macroevolutionary dynamics of dentition in Mesozoic birds reveal no ...
Mar 19, 2021 · We also found no evidence for a general trend toward edentulism throughout avialan evolutionary history. Loss of teeth in one region of the ...
[56]
Synchrotron imaging of dentition provides insights into the biology of ...
Sep 23, 2016 · Our results allow comparison with other archosaurs and also mammals, with implications regarding dental character evolution across amniotes.
[57]
Dental replacement in Mesozoic birds: evidence from newly ... - Nature
Sep 30, 2021 · Our results imply that the control mechanism for tooth cycling is conserved during the transition between non-avian reptiles and birds. These ...
[58]
The origin of the bird's beak: new insights from dinosaur incubation ...
May 23, 2018 · Combining evidence from fossils and experimental developmental biology revealed the genetic regulation of edentulism in modern birds and ...
[59]
Evolution and development of parrot pseudoteeth
Dec 15, 2021 · Parrot pseudoteeth arose by epithelial and mesenchymal evagination, and their early development was similar to the ontogeny of scales and feathers.
[60]
Bony pseudoteeth of extinct pelagic birds (Aves ... - Nature
Aug 28, 2018 · The extinct neornithine Odontopterygiformes possessed bone excrescences (pseudoteeth) which resembled teeth, distributed sequentially by size along jaws.
[61]
Development of baby teeth: MedlinePlus Medical Encyclopedia Image
Oct 20, 2024 · Development of baby teeth. Overview. Both baby teeth (deciduous or milk teeth) and permanent teeth have fairly well-defined times of eruption.
[62]
Dental Anatomy
Mammals have teeth of different sizes and shapes, a condition known as heterodonty, allowing different teeth to be specialized for different tasks.Missing: functions | Show results with:functions
[63]
Anatomy, Permanent Dentition - StatPearls - NCBI Bookshelf - NIH
May 22, 2023 · Humans have two sets of teeth during their lifetime: the initial deciduous (primary) teeth and the successive permanent (secondary) teeth.
[64]
Anatomy, Head and Neck, Tooth Eruption - StatPearls - NCBI
Sep 20, 2023 · Tooth eruption is when a developing tooth moves from its initial nonfunctional position within the alveolar bone to its final functional location within the ...
[65]
Changes in the Sequence of Eruption of Permanent Teeth
The sequence of eruption observed was: males (maxilla) 1-6-2-4-3-5-7 and (mandible) 1-6-2-3-4-5-7; females (maxilla) 6-1-2-4-3-5-7 and (mandible) 1-6-2-3-4-5-7.
[66]
Dental Anatomy of Humans
Dental Anatomy of Humans ; Deciduous, Permanent ; Incisors, 6 - 10 months, 7 - 8 years ; Canine, 16 - 20 months, 11 years ; Premolars, 11 - 13 years.
[67]
https://pmc.ncbi.nlm.nih.gov/articles/PMC7586486/
[68]
https://vivo.colostate.edu/hbooks/pathphys/digestion/pregastric/humanpage.html
[69]
https://doi.org/10.3389/fdmed.2023.1158482
[70]
Teeth of Past and Present Elephants: Microstructure ... - AGU Journals
Apr 7, 2021 · Therefore, fossilized teeth are an attractive research model in paleontology. They provide information about the animal (i.e., age, diet ...
[71]
Diagenetic trends of dental tissues - ScienceDirect.com
Enamel is more durable than dentine during fossilisation, and consequently is less altered in terms of its composition.
[72]
Taphonomy—Death & Decay - Fossils and Paleontology (U.S. ...
Dec 11, 2024 · Taphonomy is the study of what happens to the remains of an organism between the time that it dies and when it becomes fossilized.
[73]
Fossil Shark Teeth - Florida Museum of Natural History
Sep 4, 2018 · Teeth fossilize through a process called permineralization. As water seeps through sediments over the teeth, it transports the minerals that are ...
[74]
Permineralization and Replacement (U.S. National Park Service)
Aug 16, 2024 · A permineralized fossil is denser and heavier than the original organic material because its pores have been filled by silica or other minerals.Permineralization · Petrification · Types of Permineralization... · Silicification
[75]
From bone to fossil: A review of the diagenesis of bioapatite
Sep 1, 2016 · This review will evaluate our current understanding of bioapatite diagenesis and fossilization, focusing on the biogeochemical transformations that occur ...
[76]
Are shark teeth fossils true fossils? - Florida Museum of Natural History
Oct 16, 2019 · Because the thin outer layer of enamel on the crown of the tooth starts out as nearly 100% mineral, it is less altered than the root portion of ...
[77]
Paleontologists reveal what fossilized teeth can tell us
Jul 30, 2021 · “Teeth reflect how mammals interacted with their environment,” says paleontologist Prof. Dr. Thomas Martin, who has published a book, Mammalian ...
[78]
[PDF] preservation of fossils: an introduction to taphonomy
Taphonomy is the general name for the study of post-mortem effects. ... In other words, the original bones, shell or teeth are preserved in an essentially ...
[79]
https://www.uni-bonn.de/en/news/paleontologists-reveal-what-fossilized-teeth-can-tell-us
[80]
https://cdn.serc.carleton.edu/files/NAGTWorkshops/paleo/activities/suny_oneonta_taphonomy_lab.pdf
[81]
Post-mortem enamel surface texture alteration during taphonomic ...
Jan 14, 2022 · In fossil specimens especially, DMTA can be biased by post-mortem alteration caused by mechanical or chemical alteration of the enamel surface.