عرض مصدر Cementum

{{Infobox anatomy
| Name        = Cementum
| Latin       = caementum
| GraySubject =
| GrayPage    =
| Image       = Periodontium.svg
| Caption     = The '''cementum''' is the surface layer of the tooth root. Rather than being a passive entity like paint on a wall, cementum is a dynamic entity within the [[periodontium]].  It is attached to the [[alveolar bone]] ''(C)'' by the fibers of the [[periodontal ligament]] and to the soft tissue of the [[gingiva]] by the [[gingival fibers]] ''(H)''.
| Image2      =
| Caption2    =
| System      =
| MeshName    =
| MeshNumber  =
| DorlandsPre =
| DorlandsSuf =
| Code        =
}}
'''Cementum'''<ref>{{cite web
| url   = http://www.dentalfind.com/glossary/cementum.html
| title = Cementum
| date  = 2007-01-01}}</ref> is a specialized calcified substance covering the root of a [[tooth]]. The cementum is the part of the [[periodontium]] that attaches the teeth to the [[alveolar bone]] by anchoring the [[periodontal ligament]].<ref name=":0">Illustrated Dental Embryology, Histology, and Anatomy, Bath-Balogh and Fehrenbach, Elsevier, 2011, page 170.</ref>

==Development==
Cementum is excreted by cells called [[cementoblasts]] within the root of the tooth and is thickest at the root apex. These cementoblasts develop from undifferentiated [[mesenchymal cell]]s in the connective tissue of the [[dental follicle]] or sac.<ref name=":2"/>

Unlike [[ameloblast]]s and [[odontoblast]]s, which leave no cellular bodies in their secreted products, during the later steps within the stage of [[wiktionary:apposition|apposition]], many of the cementoblasts become entrapped by the cementum they produce, becoming cementocytes. Thus again, cementum is more similar to alveolar bone, with its osteoblasts becoming entrapped [[osteocyte]]s.<ref name=":0"/>

Cementum is capable of repairing itself to a limited degree and is not resorbed under normal conditions.<ref name=":4">Ten Cate's Oral Histology, Nanci, Elsevier, 2013, pages 205- 207</ref>

==Features==
Cementum is slightly softer than [[dentin]] and consists of about 45% to 50% inorganic material ([[hydroxylapatite]]) by weight and 50% to 55% [[organic matter]] and water by weight.<ref>American Academy of Periodontology 2010 In-Service Exam, question A-38</ref> The organic portion is composed primarily of [[collagen]] and [[proteoglycans]].<ref>{{cite book|last1=Kumar|first1=G.|title=Orban's Oral Histology & Embryology|date=15 Jul 2011|publisher=Elsevier India|isbn=9788131228197|page=152|edition=13th|url=https://books.google.com/books?id=061EAwAAQBAJ&pg=PA152&lpg=PA152&dq=what+is+protein+polysaccharides+in+cementum&source=bl&ots=DnZFSRcP3s&sig=b_ahEFEiN4183HtUGrmLunAtVWg&hl=en&sa=X&ei=SKh8VJecPIKRoQTm9YD4Cg&ved=0CD8Q6AEwBQ#v=onepage&q=what%20is%20protein%20polysaccharides%20in%20cementum&f=false|accessdate=1 December 2014}}</ref> Cementum is avascular, receiving its nutrition through its own imbedded cells from the surrounding vascular [[periodontal ligament]].<ref name=":0"/>

The cementum is light yellow and slightly lighter in color than [[dentin]]. It has the highest fluoride content of all mineralized tissue. Cementum also is permeable to a variety of materials. It is formed continuously throughout life because a new layer of cementum is deposited to keep the attachment intact as the superficial layer of cementum ages. Cementum on the root ends surrounds the apical foramen and may extend slightly onto the inner wall of the pulp canal.

==Structure==
[[File:Human tooth diagram-en.svg|thumb|Cementum situated around a human molar]]
The cells of cementum are the entrapped cementoblasts, the cementocytes. Each cementocyte lies in its [[Wikt:lacuna|lacuna]], similar to the pattern noted in bone. These lacunae also have canaliculi or canals. Unlike those in bone, however, these canals in cementum do not contain nerves, nor do they radiate outward. Instead, the canals are oriented toward the periodontal ligament and contain cementocytic processes that exist to diffuse nutrients from the ligament because it is vascularized.

After the apposition of cementum in layers, the cementoblasts that do not become entrapped in cementum line up along the cemental surface along the length of the outer covering of the periodontal ligament. These cementoblasts can form subsequent layers of cementum if the tooth is injured.

[[Sharpey's fibers|Sharpey fibers]] are part of the principal collagenous fibers of the [[periodontal ligament]] embedded in the cementum and [[alveolar bone]] to attach the tooth to the alveolus.<ref name=":0"/>

===Cementoenamel junction===
{{main|Cementoenamel junction}}
The cementum joins the enamel to form the [[cementoenamel junction]] (CEJ), which is referred to as the ''cervical line''.

Three possible types of transitional interfaces may be present at the CEJ. The traditional view was that certain interfaces dominated in certain oral cavities. The CEJ may exhibit all of these interfaces in an individual’s oral cavity, and there is even considerable variation when one tooth is traced circumferentially.<ref name=":0" />

===Dentinocemental junction===
When the cementoid reaches the full thickness needed, the cementoid surrounding the cementocytes becomes mineralized, or matured, and is then considered cementum. Because of the apposition of cementum over the dentin, the dentinocemental junction (DCJ) is formed. This interface is not as defined, either clinically or histologically, as that of the dentinoenamel junction (DEJ), given that cementum and dentin are of common embryological background, unlike that of enamel and dentin.<ref name=":0"/>

The dentinocemental junction (DCJ) is a relatively smooth area in the permanent tooth, and attachment of cementum to the dentin is firm but not understood completely.<ref name=":2">Dieter D. Bosshardt, Knut A. Selvig, Dental cementum: the dynamic tissue covering of the root, ''Periodontol 2000''. 1997 Feb;13:41-75. at http://www.fodonto.uncu.edu.ar/upload/41-75.pdf</ref>

===Types===
Two kinds of cementum are formed: acellular and cellular, and fibers can be intrinsic or extrinsic, resulting in four possible permutations; the first cementum to be formed during tooth development is acellular extrinsic fiber cementum.<ref name=":4" /><ref>AAP 2010 In-Service Examination, question A-9</ref> The acellular layer of cementum is living tissue that does not incorporate cells into its structure and usually predominates on the coronal half of the root; cellular cementum occurs more frequently on the apical half.<ref name=":2"/>

==Pathology==
* Some [[root resorption]] of the apical portion of the root may occur, however, if orthodontic pressures are excessive and movement is too fast. Some experts also agree on a third type of cementum, ''afibrillar cementum'', which sometimes extends onto the enamel of the tooth. 
* The excessive buildup of cementum on the roots of a tooth is a pathological condition known as [[hypercementosis]]. Cementum thickness can increase on the root end to compensate for attritional wear of the occlusal/incisal surface and passive eruption of the tooth.<ref name="dental.pitt.edu">Developmental Anomalies, Max A. Listgarten, University of Pennsylvania and Temple University at http://www.dental.pitt.edu/informatics/periohistology/en/gu0509.htm</ref>   
* When cementum is exposed through gingival recession, it quickly undergoes abrasion by mechanical friction because of its low mineral content and thinness. The exposure of the deeper dentin can lead to problems such as extrinsic staining and dentinal hypersensitivity.<ref name=":0"/> 
* The incidence of cemental caries increases in older adults as gingival decline occurs from either trauma or periodontal illness. It is a chronic condition that forms a large, shallow lesion and slowly invades first the root’s cementum and then dentin to cause a chronic infection of the pulp. Because dental pain is a late finding, many lesions are not detected early, resulting in restorative obstacles and increased tooth loss.<ref name=":0" />  
* [[Cementicle]]s are small, spherical or ovoid [[Calcification|calcified]] masses embedded within or attached to the cementum layer on the root surface of a tooth, or lying free within the [[periodontal ligament]].<ref name="Chiego2014">{{cite book|author=Chiego Jr. DJ|title=Essentials of Oral Histology and Embryology: A Clinical Approach|url=https://books.google.com/books?id=LMrsAwAAQBAJ&pg=PA133|date=14 April 2014|publisher=Elsevier Health Sciences|isbn=978-0-323-29100-2|page=133}}</ref><ref name="Bath-Balogh2010">{{cite book|author1=Bath-Balogh M|author2=Fehrenbach MJ|title=Illustrated Dental Embryology, Histology, and Anatomy|url=https://books.google.com/books?id=uPMLBAAAQBAJ&pg=PA174|date=10 December 2010|publisher=Elsevier Health Sciences|isbn=978-1-4377-2934-4|page=174}}</ref>
* Cemental spurs can be found at or near the CEJ. These are symmetrical spheres of cementum attached to the cemental root surface, similar to enamel pearls. Cemental spurs result from irregular deposition of cementum on the root. They can present some clinical problems in differentiation from calculus and may be noted on radiographs; yet, because they are hard dental tissue, they are not easily removed, and thus may also interfere with periodontal treatment.<ref name=":0"/>

===DNA studies===
A 2010 archeological study has found that cementum has five times the amount of [[mitochondrial DNA]] compared to [[dentine|dentin]], which is commonly sampled.<ref>{{cite journal|last=Adler|first=C.J.|author2=Haak, W.|author3=Donlon, D.|author4=Cooper, A.|title=Survival and recovery of DNA from ancient teeth and bones|journal=Journal of Archaeological Science|year=2010|volume=38|issue=5|pages=956–964|doi=10.1016/j.jas.2010.11.010}}</ref> Teeth are increasingly utilized as a source of nuclear DNA to aid identification of human remains. DNA extraction and the results of genetic analysis from the tissue are extremely variable and to some extent unpredictable. However, the quantity of DNA available in dentin is affected by age and dental disease, whereas that in cementum is not.<ref>Denice Higginsa, John Kaidonisa, Jeremy Austinb, Grant Townsenda, Helen Jamesa & Toby Hughes, Dentine and cementum as sources of nuclear DNA for use in human identification, ''Australian Journal of Forensic Sciences''. Volume 43, Issue 4, 2011 at http://www.tandfonline.com/doi/abs/10.1080/00450618.2011.583278#preview</ref>

== See also ==
*[[Tooth development]]
* [[Dentin]]
* [[Periodontal ligament]]
* [[Periodontium]]
* [[Cementoma]]
* [[Hypercementosis]]

== References ==
{{reflist|30em}}

== External links ==
# Cementum, Max A. Listgarten, University of Pennsylvania and Temple University at http://www.dental.pitt.edu/informatics/periohistology/en/gu0501.htm
# Harvesting cementum from root surface: A new paradigm in the study of cementum and the cemento-dentinal junction, George Cherian, Journal of Academy of Advanced Dental Research, Vol 2; Issue 2: May 2011 at http://www.joaor.org/userfiles/Vol-2-Issue-2-May-Aug-2011/03George.pdf
# Survival and recovery of DNA from ancient teeth and bones at http://www.icb.ufmg.br/labs/lbem/pdf/AdlerJ_Archae_Sci_(2011)_38(5)_956-964.pdf
{{Periodontology}}
{{Cranial fibrous joints}}
{{Mouth anatomy}}

[[Category:Teeth]]
[[Category:Periodontology]]
[[Category:Human mouth anatomy]]