Editing Human tooth (section)

==Development==
{{Main|Human tooth development}}

[[File:Molarsindevelopment11-24-05.jpg|thumb|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 [[embryo]]nic [[cell (biology)|cells]], [[cell growth|grow]], and erupt into the [[human mouth|mouth]]. Although many diverse [[species]] have teeth, their development is largely the same as in humans. For [[human]] teeth to have a healthy [[Human mouth|oral]] environment, [[tooth enamel|enamel]], [[dentin]], [[cementum]], and the [[periodontium]] must all develop during appropriate stages of [[fetal development]]. [[Deciduous teeth|Primary teeth]] start to form in [[human embryogenesis|the development of the embryo]] between the sixth and eighth weeks, and [[permanent teeth]] begin to form in the twentieth week.<ref>{{harvnb|Cate|1998|page=95}}</ref> 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.<ref name = "cate81"/>

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.<ref name="cate81">{{harvnb|Cate|1998|page=81}}</ref> 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 [[cell (biology)|cells]] that eventually forms a tooth. It is organized into three parts: the [[enamel organ]], the [[dental papilla]] and the [[dental follicle]].<ref name = "utmb"/> The ''enamel organ'' is composed of the [[outer enamel epithelium]], [[inner enamel epithelium]], [[stellate reticulum]] and [[stratum intermedium]].<ref name = "utmb"/> These cells give rise to [[ameloblast]]s, 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 [[odontoblast]]s, which are dentin-forming cells.<ref name="utmb">[https://web.archive.org/web/20070203050136/http://cellbio.utmb.edu/microanatomy/digestive/tooth.htm Lab Exercises: Tooth development]. University of Texas Medical Branch.</ref> Additionally, the junction between the dental papilla and inner enamel epithelium determines the crown shape of a tooth.<ref name="cate86and102">{{harvnb|Cate|1998|pages=86 and 102}}.</ref> The ''dental follicle'' gives rise to three important [[cell (biology)|cells]]: [[cementoblast]]s, [[osteoblast]]s, and [[fibroblast]]s. Cementoblasts form the cementum of a tooth. Osteoblasts give rise to the [[Alveolar process of maxilla|alveolar bone]] around the roots of teeth. Fibroblasts develop the [[periodontal ligament]]s which connect teeth to the alveolar bone through cementum.<ref name = "ross453">{{harvnb|Ross|2002|page=453}}</ref>

=== Eruption ===
{{Main|Tooth eruption}}

[[File:ToothLost-2917.jpg|thumb|right|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 that the periodontal ligaments play an important role in tooth eruption. Primary teeth erupt into the mouth from around six months until two years of age. These teeth are the only ones in the mouth until a person is about six years old. At that time, the first permanent tooth erupts. This stage, during which a person has a combination of primary and permanent teeth, is known as the mixed stage. The mixed stage lasts until the last primary tooth is lost and the remaining permanent teeth erupt into the mouth.

There have been many theories about the cause of tooth eruption. One theory proposes that the developing root of a tooth pushes it into the mouth. Another, known as the cushioned hammock theory, resulted from microscopic study of teeth, which was thought to show a [[ligament]] around the root. It was later discovered that the "ligament" was merely an [[Artifact (observational)|artifact]] created in the process of preparing the slide. Currently, the most widely held belief is that the periodontal ligaments provide the main impetus for the process.

The onset of primary tooth loss has been found to correlate strongly with somatic and psychological criteria of school readiness.<ref>Kranich, Ernst-Michael (1990) "Anthropologie", in F. Bohnsack and E-M Kranich (eds.), ''Erziehungswissenschaft und Waldorfpädagogik'', Reihe Pädagogik Beltz, Weinheim, p. 126, citing [[Frances Ilg]] and [[Louise Bates Ames]] (Gesell Institute), ''School Readiness'', p. 236 ff</ref><ref>{{cite journal|quote=...the loss of the first deciduous tooth can serve as a definite indicator of a male child's readiness for reading and schoolwork|title=Second Dentition and School Readiness|journal=New York State Dental Journal |volume=43 |pages=155–8|pmid=264640|year=1977|last1=Silvestro|first1=JR|issue=3}}</ref>{{Clarify|date=January 2012}}