human tooth human tooth human teeth function to mechanically break down items of food by cutting and crushing them in preparation for swallowing and digesting. as such, they are considered part of the human digestive system. humans have four types of teeth: incisors, canines, premolars, and molars, which each have a specific function. the incisors cut the food, the canines tear the food and the molars and premolars crush the food. the roots of teeth are embedded in the maxilla (upper jaw) or the mandible (lower jaw) and are covered by gums. teeth are made of multiple tissues of varying density and hardness. teethimage showing incisors and canine teeth, situated in gums above and below.diagram of a human molar showing its major constituentsdetailsidentifierslatindentesta98a05.1.03.001ta22818fma75150anatomical terminology humans, like most other mammals, are diphyodont, meaning that they develop two sets of teeth. the first set, deciduous teeth, also called "primary teeth", "baby teeth", or "milk teeth", normally eventually contains 20 teeth. primary teeth typically start to appear ("erupt") around six months of age and this may be distracting and/or painful for the infant. however, some babies are born with one or more visible teeth, known as neonatal teeth or "natal teeth". anatomy main article: dental anatomy there are four main types of teeth in humans, shown labelled here. dental anatomy is a field of anatomy dedicated to the study of tooth structure. the development, appearance, and classification of teeth fall within its field of study, though dental occlusion, or contact between teeth, does not. dental anatomy is also a taxonomic science as it is concerned with the naming of teeth and their structures. this information serves a practical purpose for dentists, enabling them to easily identify and describe teeth and structures during treatment. the anatomic crown of a tooth is the area covered in enamel above the cementoenamel junction (cej) or "neck" of the tooth. most of the crown is composed of dentin ("dentine" in british english) with the pulp chamber inside. the crown is within bone before eruption. after eruption, it is almost always visible. the anatomic root is found below the cej and is covered with cementum. as with the crown, dentin composes most of the root, which normally has pulp canals. canines and most premolars, except for maxillary first premolars, usually have one root. maxillary first premolars and mandibular molars usually have two roots. maxillary molars usually have three roots. additional roots are referred to as supernumerary roots. humans usually have 20 primary (deciduous, "baby" or "milk") teeth and 32 permanent (adult) teeth. teeth are classified as incisors, canines, premolars (also called bicuspids), and molars. incisors are primarily used for cutting, canines are for tearing, and molars serve for grinding. most teeth have identifiable features that distinguish them from others. there are several different notation systems to refer to a specific tooth. the three most common systems are the fdi world dental federation notation (iso 3950), the universal numbering system, and the palmer notation. the fdi system is used worldwide, the universal only in the united states, while the older palmer notation still has some adherents only in the united kingdom. primary teeth among deciduous (primary) teeth, ten are found in the maxilla (upper jaw) and ten in the mandible (lower jaw), for a total of 20. the dental formula for primary teeth in humans is 2.1.0.22.1.0.2. in the primary set of teeth, in addition to the canines, there are two types of incisors—centrals and laterals—and two types of molars—first and second. all primary teeth are normally later replaced with their permanent counterparts. the universal numbering system for adult human teeth. the view is from a dental practitioner's perspective, meaning tooth 1 is the upper right rear (third) molar. permanent teeth among permanent teeth, 16 are found in the maxilla and 16 in the mandible, for a total of 32. the dental formula is 2.1.2.32.1.2.3. permanent human teeth are numbered in a boustrophedonic sequence. the maxillary teeth are the maxillary central incisors (teeth 8 and 9 in the diagram), maxillary lateral incisors (7 and 10), maxillary canines (6 and 11), maxillary first premolars (5 and 12), maxillary second premolars (4 and 13), maxillary first molars (3 and 14), maxillary second molars (2 and 15), and maxillary third molars (1 and 16). the mandibular teeth are the mandibular central incisors (24 and 25), mandibular lateral incisors (23 and 26), mandibular canines (22 and 27), mandibular first premolars (21 and 28), mandibular second premolars (20 and 29), mandibular first molars (19 and 30), mandibular second molars (18 and 31), and mandibular third molars (17 and 32). third molars are commonly called "wisdom teeth" usually emerge at ages 17 to 25. these molars may never erupt into the mouth or form at all. when they do form, they often must be removed. if any additional teeth form—for example, fourth and fifth molars, which are rare—they are referred to as supernumerary teeth (hyperdontia). development of fewer than the usual number of teeth is called hypodontia. there are small differences between the teeth of males and females, with male teeth along with the male jaw tending to be larger on average than female teeth and jaw. there are also differences in the internal dental tissue proportions, with male teeth consisting of proportionately more dentine while female teeth have proportionately more enamel. parts enamel main article: tooth enamel enamel is the hardest and most highly mineralized substance of the body. it has its origin from oral ectoderm. it is one of the four major tissues which make up the tooth, along with dentin, cementum, and dental pulp. it is normally visible and must be supported by underlying dentin. 96% of enamel consists of mineral, with water and organic material comprising the rest. the normal color of enamel varies from light yellow to grayish white. at the edges of teeth where there is no dentin underlying the enamel, the color sometimes has a slightly blue tone. since enamel is semitranslucent, the color of dentin and any restorative dental material underneath the enamel strongly affects the appearance of a tooth. enamel varies in thickness over the surface of the tooth and is often thickest at the cusp, up to 2.5mm, and thinnest at its border, which is seen clinically as the cej. the wear rate of enamel, called attrition, is 8 micrometers a year from normal factors. enamel's primary mineral is hydroxyapatite, which is a crystalline calcium phosphate. the large amount of minerals in enamel accounts not only for its strength but also for its brittleness. dentin, which is less mineralized and less brittle, compensates for enamel and is necessary as a support. unlike dentin and bone, enamel does not contain collagen. proteins of note in the development of enamel are ameloblastins, amelogenins, enamelins and tuftelins. it is believed that they aid in the development of enamel by serving as framework support, among other functions. in rare circumstances enamel can fail to form, leaving the underlying dentine exposed on the surface. dentin main article: dentin dentin is the substance between enamel or cementum and the pulp chamber. it is secreted by the odontoblasts of the dental pulp. the formation of dentin is known as dentinogenesis. the porous, yellow-hued material is made up of 70% inorganic materials, 20% organic materials, and 10% water by weight. because it is softer than enamel, it decays more rapidly and is subject to severe cavities if not properly treated, but dentin still acts as a protective layer and supports the crown of the tooth. dentin is a mineralized connective tissue with an organic matrix of collagenous proteins. dentin has microscopic channels, called dentinal tubules, which radiate outward through the dentin from the pulp cavity to the exterior cementum or enamel border. the diameter of these tubules range from 2.5 μm near the pulp, to 1.2 μm in the midportion, and 900 nm near the dentino-enamel junction. although they may have tiny side-branches, the tubules do not intersect with each other. their length is dictated by the radius of the tooth. the three dimensional configuration of the dentinal tubules is genetically determined. there are three types of dentin, primary, secondary and tertiary. secondary dentin is a layer of dentin produced after root formation and continues to form with age. tertiary dentin is created in response to stimulus, such as cavities and tooth wear. cementum main article: cementum cementum is a specialized bone like substance covering the root of a tooth. it is approximately 45% inorganic material (mainly hydroxyapatite), 33% organic material (mainly collagen) and 22% water. cementum is excreted by cementoblasts within the root of the tooth and is thickest at the root apex. its coloration is yellowish and it is softer than dentin and enamel. the principal role of cementum is to serve as a medium by which the periodontal ligaments can attach to the tooth for stability. at the cement to enamel junction, the cementum is acellular due to its lack of cellular components, and this acellular type covers at least ⅔ of the root. the more permeable form of cementum, cellular cementum, covers about ⅓ of the root apex. dental pulp main article: pulp (tooth) the dental pulp is the central part of the tooth filled with soft connective tissue. this tissue contains blood vessels and nerves that enter the tooth from a hole at the apex of the root. along the border between the dentin and the pulp are odontoblasts, which initiate the formation of dentin. other cells in the pulp include fibroblasts, preodontoblasts, macrophages and t lymphocytes. the pulp is commonly called "the nerve" of the tooth. development main article: tooth development radiograph of lower right third, second, and first molars in different stages of development tooth development is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth. although many diverse species have teeth, their development is largely the same as in humans. for human teeth to have a healthy oral environment, enamel, dentin, cementum, and the periodontium must all develop during appropriate stages of fetal development. primary teeth start to form in the development of the embryo between the sixth and eighth weeks, and permanent teeth begin to form in the twentieth week. if teeth do not start to develop at or near these times, they will not develop at all. a significant amount of research has focused on determining the processes that initiate tooth development. it is widely accepted that there is a factor within the tissues of the first pharyngeal arch that is necessary for the development of teeth. tooth development is commonly divided into the following stages: the bud stage, the cap, the bell, and finally maturation. the staging of tooth development is an attempt to categorize changes that take place along a continuum; frequently it is difficult to decide what stage should be assigned to a particular developing tooth. this determination is further complicated by the varying appearance of different histologic sections of the same developing tooth, which can appear to be different stages. the tooth bud (sometimes called the tooth germ) is an aggregation of cells that eventually forms a tooth. it is organized into three parts: the enamel organ, the dental papilla and the dental follicle. the enamel organ is composed of the outer enamel epithelium, inner enamel epithelium, stellate reticulum and stratum intermedium. these cells give rise to ameloblasts, which produce enamel and the reduced enamel epithelium. the growth of cervical loop cells into the deeper tissues forms hertwig's epithelial root sheath, which determines a tooth's root shape. the dental papilla contains cells that develop into odontoblasts, which are dentin-forming cells. additionally, the junction between the dental papilla and inner enamel epithelium determines the crown shape of a tooth. the dental follicle gives rise to three important cells: cementoblasts, osteoblasts, and fibroblasts. cementoblasts form the cementum of a tooth. osteoblasts give rise to the alveolar bone around the roots of teeth. fibroblasts develop the periodontal ligaments which connect teeth to the alveolar bone through cementum. eruption main article: tooth eruption bottom teeth of a seven-year-old, showing primary teeth (left), a lost primary tooth (middle), and a permanent tooth (right) tooth eruption in humans is a process in tooth development in which the teeth enter the mouth and become visible. current research indicates