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Hard tissue, refers to "normal" calcified tissue, is the tissue which is mineralized and has a firm intercellular matrix. The hard tissues of humans are bone, tooth enamel, dentin, and cementum. The term is in contrast to soft tissue.
Bone is an active tissue composed of different cells. are involved in the creation and mineralisation of bone; and are involved in the reabsorption of bone tissue. The mineralised matrix of bone tissue has an organic component mainly of collagen and an inorganic component of bone mineral made up of various salts.
In humans, enamel varies in thickness over the surface of the tooth, often thickest at the cusp, up to 2.5 mm, and thinnest at its border with the cementum at the cementoenamel junction.Ten Cate's Oral Histology, Nanci, Elsevier, 2013, page 122
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 translucent, the color of dentin and any material underneath the enamel strongly affects the appearance of a tooth. The enamel on primary teeth has a more opaque crystalline form and thus appears whiter than on permanent teeth.
The large amount of mineral in enamel accounts not only for its strength but also for its brittleness.Ten Cate's Oral Histology, Nanci, Elsevier, pages 70-94 Tooth enamel ranks 5 on Mohs hardness scale and has a Young's modulus of 83 GPa. Dentin, less mineralized and less brittle, 3–4 in hardness, compensates for enamel and is necessary as a support.Johnson 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 Radiodensity.Illustrated Dental Embryology, Histology, and Anatomy, Fehrenbach and Popowics, Elsevier, 2026, page 164
Enamel does not contain collagen, as found in other hard tissues such as dentin and bone, but it does contain two unique classes of : and . 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. 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.Fehrenbach, Popowics p. 164
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. Fehrenbach, Popowics p. 191
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