Editing Dentin (section)

==Types==
Dentin is classified into three types: primary, secondary, and tertiary.<ref name="pmid156038322">{{cite journal |vauthors=Kinney JH, Nalla RK, Pople JA, Breunig TM, Ritchie RO |date=June 2005 |title=Age-related transparent root dentin: mineral concentration, crystallite size, and mechanical properties |journal=Biomaterials |volume=26 |issue=16 |pages=3363–76 |doi=10.1016/j.biomaterials.2004.09.004 |pmid=15603832}}</ref><ref name="Towle_20192">{{Cite journal |vauthors=Towle I |date=March 2019 |title=Tertiary dentine frequencies in extant great apes and fossil hominins. |journal=Open Quaternary |volume=5 |issue=2 |page=2 |doi=10.5334/oq.48 |doi-access=free}}</ref> Secondary dentin is a layer of dentin formed after the tooth's root has fully formed. Tertiary dentin develops as a result of a stimulus, such as a carious attack or wear.<ref>{{cite journal |last1=Mondéjar-Fernández |first1=Jorge |last2=Janvier |first2=Philippe |date=2021 |title=Finned Vertebrates |url=https://www.taylorfrancis.com/chapters/edit/10.1201/9781351189590-15/finned-vertebrates-jorge-mond%C3%A9jar-fern%C3%A1ndez-philippe-janvier |journal=Vertebrate Skeletal Histology and Paleohistology |publisher=CRC Press |pages=294–324 |doi=10.1201/9781351189590-15 |isbn=978-1-351-18959-0 |s2cid=241700775|url-access=subscription }}</ref>

===Primary dentin===

'''Primary dentin''', the most prominent dentin in the tooth, lies between the enamel and the pulp chamber (near dentinoenamel junction). The outer layer closest to enamel is known as '''mantle dentin'''.  This layer is unique to the rest of primary dentin.  Mantle dentin is formed by newly differentiated odontoblasts and forms a layer consistently 15-20 micrometers (μm) wide. Unlike primary dentin, mantle dentin lacks phosphorylation, has loosely packed collagen fibrils and is less mineralized. Below it lies the circumpulpal dentin, more mineralized dentin which makes up most of the dentin layer and is secreted after the mantle dentin by the odontoblasts. Circumpulpal dentin is formed before the root formation is completed.

Newly secreted dentin is unmineralized and is called predentin. It is easily identified in hematoxylin and eosin stained sections since it stains less intensely than dentin. It is usually 10-47μm and lines the innermost region of the dentin. It is unmineralized and consists of collagen, glycoproteins, and proteoglycans. It is similar to osteoid in bone and is thickest when dentinogenesis is occurring.<ref name=":0">Ten Cate's Oral Histology, Nanci, Elsevier, 2013, page 194</ref>

=== Secondary dentin ===

Secondary dentin is formed after root formation is complete, normally after the tooth has erupted and is functional. It grows much more slowly than primary dentin but maintains its incremental aspect of growth. It has a similar structure to primary dentin, although its deposition is not always even around the pulp chamber. It appears greater in amounts on the roof and floor of the coronal pulp chamber, where it protects the pulp from exposure in older teeth. The secondary dentin formed is not in response to any external stimuli, and it appears very much similar to the primary dentin.  It is the growth of this dentin that causes a decrease in the size of the pulp chamber with age. This is clinically known as pulp recession; cavity preparation in young patients, therefore, carries a greater risk of exposing the pulp. If this occurs, the pulp can be treated by different therapies such as direct pulp capping. Previously it was thought that pulp capping was most successful if followed by a stainless steel crown, however this procedure is most of the times unnecessary in children. It requires the unnecessary removal of enamel which is key to the life of the tooth. Adhesive dentistry allows for conservative restoration techniques that minimize the loss of tooth structure and should be used. In order to maintain space in the primary dentition, attempts are made not to extract a pulpal exposure.

===Tertiary dentin (including reparative dentin and reactionary dentin) – pathologic===

[[Tertiary dentin]] is dentin formed as a reaction to external stimulation such as cavities and wear.<ref name="Towle_2019">{{Cite journal |vauthors=Towle I |date=March 2019 |title=Tertiary dentine frequencies in extant great apes and fossil hominins. |journal=Open Quaternary |volume=5 |issue=2 |page=2 |doi=10.5334/oq.48 |doi-access=free}}</ref> It is of two types, either reactionary, where dentin is formed from a pre-existing odontoblast, or reparative, where newly differentiated odontoblast-like cells are formed due to the death of the original odontoblasts, from a pulpal [[progenitor cell]]. Tertiary dentin is only formed by an odontoblast directly affected by a stimulus; therefore, the architecture and structure depend on the intensity and duration of the stimulus, e.g., if the stimulus is a carious lesion, there is extensive destruction of dentin and damage to the pulp, due to the differentiation of bacterial metabolites and toxins. Thus, tertiary dentin is deposited rapidly, with a sparse and irregular tubular pattern and some cellular inclusions; in this case, it is referred to as "osteodentin". Osteodentin is seen in Vit.A deficiency during development. However, if the stimulus is less active, it is laid down less rapidly with a more regular tubular pattern and hardly any cellular inclusions.<ref name="pmid15603832">{{cite journal | vauthors = Kinney JH, Nalla RK, Pople JA, Breunig TM, Ritchie RO | title = Age-related transparent root dentin: mineral concentration, crystallite size, and mechanical properties | journal = Biomaterials | volume = 26 | issue = 16 | pages = 3363–76 | date = June 2005 | pmid = 15603832 | doi = 10.1016/j.biomaterials.2004.09.004 }}</ref> The speed at which tertiary dentin forms also varies substantially among primate species.<ref name="Towle_2019" />