# Hard tissue

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{{Short description|Mineralized body tissue}}
'''Hard tissue''', refers to "normal" calcified [tissue](/source/Tissue_(biology)), is the tissue which is  [mineralized](/source/Mineralized_tissues) and has a firm [intercellular matrix](/source/intercellular_matrix).<ref>{{cite web|url=http://medical-dictionary.thefreedictionary.com/hard+tissue|publisher=Farlex and Partners|title=Medical Dictionary|accessdate=25 October 2015}}</ref> The hard tissues of humans are [bone](/source/bone), [tooth enamel](/source/tooth_enamel), [dentin](/source/dentin), and [cementum](/source/cementum).<ref>{{cite book|author1=Berkovitz BKB|author2=Holland GR|author3=Moxham BJ|title=Oral Anatomy, Histology and Embryology|url=https://books.google.com/books?id=M4hQAQAAIAAJ|year=2009|publisher=Mosby/Elsevier|isbn=978-0-7234-3551-8|page=7}}</ref> The term is in contrast to [soft tissue](/source/soft_tissue).

==Bone==
{{main|Bone}}
Bone is a [rigid](/source/Stiffness) [organ](/source/organ_(anatomy)) that constitutes part of the [vertebral](/source/vertebrate) [skeleton](/source/skeleton). Bones support and protect the various organs of the body, produce [red](/source/red_blood_cell) and [white blood cell](/source/white_blood_cell)s, store [mineral](/source/mineral)s and also enable mobility. [Bone tissue](/source/Bone_tissue) is a type of dense [connective tissue](/source/connective_tissue). Bones come in a variety of shapes and sizes and have a complex internal and external structure. They are lightweight yet strong and hard, and serve multiple [functions](/source/Function_(biology)). [Mineralized](/source/Mineralized_tissue) [osseous tissue](/source/osseous_tissue) or bone tissue, is of two types – [cortical](/source/cortical_bone) and [cancellous](/source/cancellous_bone) and gives it rigidity and a [coral](/source/coral)-like three-dimensional internal structure. Other types of tissue found in bones include [marrow](/source/bone_marrow), [endosteum](/source/endosteum), [periosteum](/source/periosteum), [nerve](/source/nerve)s, [blood vessel](/source/blood_vessel)s and [cartilage](/source/cartilage).

Bone is an active tissue composed of different cells. [Osteoblast](/source/Osteoblast)s are involved in the creation and mineralisation of bone; [osteocyte](/source/osteocyte)s and [osteoclast](/source/osteoclast)s are involved in the reabsorption of bone tissue. The mineralised matrix of bone tissue has an organic component mainly of [collagen](/source/collagen) and an inorganic component of [bone mineral](/source/bone_mineral) made up of various salts.

==Enamel==
{{main|Tooth enamel}}
Enamel is the hardest substance in the human body and contains the highest percentage of minerals,<ref name="ross485">Ross ''et al.'', p. 485</ref> 96%, with water and organic material composing the rest.<ref name=":4">Ten Cate's Oral Histology, Nancy, Elsevier, pages 70-94</ref> The primary mineral is [hydroxyapatite](/source/hydroxyapatite), which is a [crystal](/source/crystal)line [calcium phosphate](/source/calcium_phosphate).<ref name="staines" /> Enamel is formed on the tooth while the tooth is developing within the gum, before it [erupts](/source/Tooth_eruption) into the mouth. Once fully formed, it does not contain blood vessels or nerves. [Remineralisation of teeth](/source/Remineralisation_of_teeth) can repair damage to the tooth to a certain degree but damage beyond that cannot be repaired by the body.  The maintenance and repair of human tooth enamel is one of the primary concerns of [dentistry](/source/dentistry).

In humans, enamel varies in thickness over the surface of the tooth, often thickest at the [cusp](/source/Cusp_(dentistry)), up to 2.5&nbsp;mm, and thinnest at its border with the [cementum](/source/cementum) at the [cementoenamel junction](/source/cementoenamel_junction).<ref name="cate219">Ten Cate's Oral Histology, Nanci, Elsevier, 2013, page 122</ref>

The normal color of enamel varies from light yellow to grayish (bluish) 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](/source/translucent), the color of dentin and any material underneath the enamel strongly affects the [appearance](/source/Human_physical_appearance) of a tooth. The enamel on primary teeth has a more opaque crystalline form and thus appears whiter than on permanent teeth.

<!-- article quote "Enamel is a composite material, consisting of about 87 vol% mineral, nominally hydroxyapatite, Ca5(PO4)3OH, 2% organic component, and 11% water" -->The large amount of mineral in enamel accounts not only for its strength but also for its brittleness.<ref name=":3">Ten Cate's Oral Histology, Nanci, Elsevier, pages 70-94</ref> Tooth enamel ranks 5 on [Mohs hardness scale](/source/Mohs_scale_of_mineral_hardness) and has a [Young's modulus](/source/Young's_modulus) of 83 GPa.<ref name="staines">{{cite journal|journal=Journal of Materials Science|year=1981|title=Spherical indentation of tooth enamel|author=M. Staines, W. H. Robinson and J. A. A. Hood| doi = 10.1007/bf01113595 | volume=16 |issue =9 | pages= 2551–2556 |s2cid=137704231}}</ref> Dentin, less mineralized and less brittle, 3–4 in hardness, compensates for enamel and is necessary as a support.<ref name="johnson">Johnson</ref> On radiographs, the differences in the mineralization of different parts of the tooth and surrounding periodontium can be noted; enamel appears lighter than dentin or pulp since it is denser than both and more [radiopaque](/source/Radiodensity).<ref name=":0">Illustrated Dental Embryology, Histology, and Anatomy, Fehrenbach and Popowics, Elsevier, 2026, page 164</ref>

Enamel does not contain collagen, as found in other hard tissues such as dentin and bone, but it does contain two unique classes of [protein](/source/protein)s: [amelogenin](/source/amelogenin)s and [enamelin](/source/enamelin)s. While the role of these proteins is not fully understood, it is believed that they aid in the development of enamel by serving as a framework for minerals to form on, among other functions.<ref name=":3"/> Once it is mature, enamel is almost totally without the softer organic matter. Enamel is avascular and has no nerve supply within it and is not renewed, however, it is not a static tissue as it can undergo mineralization changes.<ref>Fehrenbach, Popowics p. 164</ref>

==Dentin==
{{main|Dentin}}
By weight, 70% of dentin consists of the mineral [hydroxyapatite](/source/hydroxyapatite), 20% is organic material, and 10% is water.<ref name=":0" /> Yellow in appearance, it greatly affects the color of a tooth due to the [translucency](/source/translucency) of enamel. Dentin, which is less mineralized and less brittle than enamel, is necessary for the support of enamel.<ref>Johnson, Clarke. "[http://www.uic.edu/classes/orla/orla312/BHDTwo.html Biology of the Human Dentition] {{webarchive|url=https://web.archive.org/web/20151030052831/http://www.uic.edu/classes/orla/orla312/BHDTwo.html |date=2015-10-30 }}." Page accessed July 18, 2007.</ref> Dentin rates approximately 3 on the [Mohs scale](/source/Mohs_scale) of mineral hardness.<ref>Marshall GW Jr, Marshall SJ, Kinney JH, Balooch M.J. The dentin substrate: structure and properties related to bonding ''J'' ''Dent.'' 1997 Nov;25(6):441-58.</ref>

==Cementum==
{{main|Cementum}}
Cementum is slightly softer than [dentin](/source/dentin) and consists of about 45% to 50% inorganic material (hydroxyapatite) by weight and 50% to 55% [organic matter](/source/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](/source/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&q=what+is+protein+polysaccharides+in+cementum&pg=PA152|accessdate=1 December 2014}}</ref> Cementum is avascular, receiving its nutrition through its own imbedded cells from the surrounding vascular [periodontal ligament](/source/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. <ref>Fehrenbach, Popowics p. 191</ref>

==References==
{{reflist}}

Category:Tissues (biology)

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Adapted from the Wikipedia article [Hard tissue](https://en.wikipedia.org/wiki/Hard_tissue) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Hard_tissue?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.