Hard tissue

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Hard tissue, refers to "normal" calcified tissue, is the tissue which is mineralized and has a firm intercellular matrix. [1] The hard tissues of humans are bone, tooth enamel, dentin, and cementum. [2] The term is in contrast to soft tissue.

Contents

Bone

Bone is a rigid organ that constitutes part of the vertebral skeleton. Bones support and protect the various organs of the body, produce red and white blood cells, store minerals and also enable mobility. Bone tissue is a type of dense 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. Mineralized osseous tissue or bone tissue, is of two types – cortical and cancellous and gives it rigidity and a coral-like three-dimensional internal structure. Other types of tissue found in bones include marrow, endosteum, periosteum, nerves, blood vessels and cartilage.

Bone is an active tissue composed of different cells. Osteoblasts are involved in the creation and mineralisation of bone; osteocytes and osteoclasts 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.

Enamel

Enamel is the hardest substance in the human body and contains the highest percentage of minerals, [3] 96%, with water and organic material composing the rest. [4] The primary mineral is hydroxyapatite, which is a crystalline calcium phosphate. [5] Enamel is formed on the tooth while the tooth is developing within the gum, before it erupts into the mouth. Once fully formed, it does not contain blood vessels or nerves. 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.

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. [6]

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, 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. [7] Tooth enamel ranks 5 on Mohs hardness scale and has a Young's modulus of 83 GPa. [5] Dentin, less mineralized and less brittle, 3–4 in hardness, compensates for enamel and is necessary as a support. [8] On radiographs, the differences in the mineralization of different portions 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. [9]

Enamel does not contain collagen, as found in other hard tissues such as dentin and bone, but it does contain two unique classes of proteins: amelogenins and enamelins. 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. [7] 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. [10]

Dentin

By weight, 70% of dentin consists of the mineral hydroxyapatite, 20% is organic material, and 10% is water. [9] Yellow in appearance, it greatly affects the color of a tooth due to the translucency of enamel. Dentin, which is less mineralized and less brittle than enamel, is necessary for the support of enamel. [11] Dentin rates approximately 3 on the Mohs scale of mineral hardness. [12]

Cementum

Cementum is slightly softer than dentin and consists of about 45% to 50% inorganic material (hydroxyapatite) by weight and 50% to 55% organic matter and water by weight. [13] The organic portion is composed primarily of collagen and proteoglycans. [14] Cementum is avascular, receiving its nutrition through its own imbedded cells from the surrounding vascular periodontal ligament. [9]

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.

Related Research Articles

<span class="mw-page-title-main">Human tooth</span> Calcified whitish structure in humans mouths used to break down food

Human teeth function to mechanically break down items of food by cutting and crushing them in preparation for swallowing and digesting. As such, they are considered part of the human digestive system. Humans have four types of teeth: incisors, canines, premolars, and molars, which each have a specific function. The incisors cut the food, the canines tear the food and the molars and premolars crush the food. The roots of teeth are embedded in the maxilla or the mandible and are covered by gums. Teeth are made of multiple tissues of varying density and hardness.

<span class="mw-page-title-main">Cementum</span> Specialized calcified substance covering the root of a tooth

Cementum 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.

<span class="mw-page-title-main">Tooth enamel</span> Major tissue that makes up part of the tooth in humans and many animals

Tooth enamel is one of the four major tissues that make up the tooth in humans and many animals, including some species of fish. It makes up the normally visible part of the tooth, covering the crown. The other major tissues are dentin, cementum, and dental pulp. It is a very hard, white to off-white, highly mineralised substance that acts as a barrier to protect the tooth but can become susceptible to degradation, especially by acids from food and drink. In rare circumstances enamel fails to form, leaving the underlying dentin exposed on the surface.

<span class="mw-page-title-main">Dentin</span> Calcified tissue of the body; one of the four major components of teeth

Dentin or dentine is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by enamel on the crown and cementum on the root and surrounds the entire pulp. By volume, 45% of dentin consists of the mineral hydroxyapatite, 33% is organic material, and 22% is water. Yellow in appearance, it greatly affects the color of a tooth due to the translucency of enamel. Dentin, which is less mineralized and less brittle than enamel, is necessary for the support of enamel. Dentin rates approximately 3 on the Mohs scale of mineral hardness. There are two main characteristics which distinguish dentin from enamel: firstly, dentin forms throughout life; secondly, dentin is sensitive and can become hypersensitive to changes in temperature due to the sensory function of odontoblasts, especially when enamel recedes and dentin channels become exposed.

<span class="mw-page-title-main">Ameloblast</span>

Ameloblasts are cells present only during tooth development that deposit tooth enamel, which is the hard outermost layer of the tooth forming the surface of the crown.

<span class="mw-page-title-main">Hydroxyapatite</span> Naturally occurring mineral form of calcium apatite

Hydroxyapatite is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), often written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. It is the hydroxyl endmember of the complex apatite group. The OH− ion can be replaced by fluoride or chloride, producing fluorapatite or chlorapatite. It crystallizes in the hexagonal crystal system. Pure hydroxyapatite powder is white. Naturally occurring apatites can, however, also have brown, yellow, or green colorations, comparable to the discolorations of dental fluorosis.

<span class="mw-page-title-main">Human tooth development</span> Process by which teeth form

Tooth development or odontogenesis is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth. For human teeth to have a healthy oral environment, all parts of the tooth must develop during appropriate stages of fetal development. Primary (baby) teeth start to form between the sixth and eighth week of prenatal development, and permanent teeth begin to form in the twentieth week. If teeth do not start to develop at or near these times, they will not develop at all, resulting in hypodontia or anodontia.

Amelogenesis is the formation of enamel on teeth and begins when the crown is forming during the advanced bell stage of tooth development after dentinogenesis forms a first layer of dentin. Dentin must be present for enamel to be formed. Ameloblasts must also be present for dentinogenesis to continue.

<span class="mw-page-title-main">Dental papilla</span>

In embryology and prenatal development, the dental papilla is a condensation of ectomesenchymal cells called odontoblasts, seen in histologic sections of a developing tooth. It lies below a cellular aggregation known as the enamel organ. The dental papilla appears after 8–10 weeks intra uteral life. The dental papilla gives rise to the dentin and pulp of a tooth.

<span class="mw-page-title-main">Odontoblast</span> Type of cell that produces dentin in teeth

In vertebrates, an odontoblast is a cell of neural crest origin that is part of the outer surface of the dental pulp, and whose biological function is dentinogenesis, which is the formation of dentin, the substance beneath the tooth enamel on the crown and the cementum on the root.

Dentinogenesis is the formation of dentin, a substance that forms the majority of teeth. Dentinogenesis is performed by odontoblasts, which are a special type of biological cell on the outer wall of dental pulps, and it begins at the late bell stage of a tooth development. The different stages of dentin formation after differentiation of the cell result in different types of dentin: mantle dentin, primary dentin, secondary dentin, and tertiary dentin.

<span class="mw-page-title-main">Dental follicle</span>

The dental follicle, also known as dental sac, is made up of mesenchymal cells and fibres surrounding the enamel organ and dental papilla of a developing tooth. It is a vascular fibrous sac containing the developing tooth and its odontogenic organ. The dental follicle (DF) differentiates into the periodontal ligament. In addition, it may be the precursor of other cells of the periodontium, including osteoblasts, cementoblasts and fibroblasts. They develop into the alveolar bone, the cementum with Sharpey's fibers and the periodontal ligament fibers respectively. Similar to dental papilla, the dental follicle provides nutrition to the enamel organ and dental papilla and also have an extremely rich blood supply.

A cementoblast is a biological cell that forms from the follicular cells around the root of a tooth, and whose biological function is cementogenesis, which is the formation of cementum. The mechanism of differentiation of the cementoblasts is controversial but circumstantial evidence suggests that an epithelium or epithelial component may cause dental sac cells to differentiate into cementoblasts, characterised by an increase in length. Other theories involve Hertwig epithelial root sheath (HERS) being involved.

<span class="mw-page-title-main">Epithelial root sheath</span>

The Hertwig epithelial root sheath (HERS) or epithelial root sheath is a proliferation of epithelial cells located at the cervical loop of the enamel organ in a developing tooth. Hertwig epithelial root sheath initiates the formation of dentin in the root of a tooth by causing the differentiation of odontoblasts from the dental papilla. The root sheath eventually disintegrates with the periodontal ligament, but residual pieces that do not completely disappear are seen as epithelial cell rests of Malassez (ERM). These rests can become cystic, presenting future periodontal infections.

Cementogenesis is the formation of cementum, one of the three mineralized substances of a tooth. Cementum covers the roots of teeth and serves to anchor gingival and periodontal fibers of the periodontal ligament by the fibers to the alveolar bone.

<span class="mw-page-title-main">Dentinogenesis imperfecta</span> Medical condition

Dentinogenesis imperfecta (DI) is a genetic disorder of tooth development. It is inherited in an autosomal dominant pattern, as a result of mutations on chromosome 4q21, in the dentine sialophosphoprotein gene (DSPP). It is one of the most frequently occurring autosomal dominant features in humans. Dentinogenesis imperfecta affects an estimated 1 in 6,000-8,000 people.

Dr. Anne George is a Professor of Oral Biology at the University of Illinois at Chicago College of Dentistry and holds the Allan G. Brodie Endowed Professorship, and she also is an adjunct professor in the Department of Cell and Anatomy and the Department of Engineering at the University of Illinois Medical School. She has been a faculty member at Northwestern University in Chicago, where she was given a Teaching Excellence Award in 1999.

The junctional epithelium (JE) is that epithelium which lies at, and in health also defines, the base of the gingival sulcus. The probing depth of the gingival sulcus is measured by a calibrated periodontal probe. In a healthy-case scenario, the probe is gently inserted, slides by the sulcular epithelium (SE), and is stopped by the epithelial attachment (EA). However, the probing depth of the gingival sulcus may be considerably different from the true histological gingival sulcus depth.

In dentistry, enamel matrix derivative (EMD) is an extract of porcine fetal tooth material used to biomimetically stimulate the soft and hard tissues surrounding teeth to regrow following tissue destruction.

<span class="mw-page-title-main">Tooth discoloration</span> Medical condition

Tooth discoloration is abnormal tooth color, hue or translucency. External discoloration is accumulation of stains on the tooth surface. Internal discoloration is due to absorption of pigment particles into tooth structure. Sometimes there are several different co-existent factors responsible for discoloration.

References

  1. ↑ "Medical Dictionary". Farlex and Partners. Retrieved 25 October 2015.
  2. ↑ Berkovitz BKB; Holland GR; Moxham BJ (2009). Oral Anatomy, Histology and Embryology. Mosby/Elsevier. p. 7. ISBN   978-0-7234-3551-8.
  3. ↑ Ross et al., p. 485
  4. ↑ Ten Cate's Oral Histology, Nancy, Elsevier, pages 70-94
  5. 1 2 M. Staines, W. H. Robinson and J. A. A. Hood (1981). "Spherical indentation of tooth enamel". Journal of Materials Science. 16 (9): 2551–2556. doi:10.1007/bf01113595. S2CID   137704231.
  6. ↑ Ten Cate's Oral Histology, Nanci, Elsevier, 2013, page 122
  7. 1 2 Ten Cate's Oral Histology, Nanci, Elsevier, pages 70-94
  8. ↑ Johnson
  9. 1 2 3 Illustrated Dental Embryology, Histology, and Anatomy, Bath-BaloghFehrenbach, Elsevier, 2011, page 180
  10. ↑ Bath-Balogh, Fehrenbach, p. 179
  11. ↑ Johnson, Clarke. "Biology of the Human Dentition Archived 2015-10-30 at the Wayback Machine ." Page accessed July 18, 2007.
  12. ↑ Marshall GW Jr, Marshall SJ, Kinney JH, Balooch M.J. The dentin substrate: structure and properties related to bonding JDent. 1997 Nov;25(6):441-58.
  13. ↑ American Academy of Periodontology 2010 In-Service Exam, question A-38
  14. ↑ Kumar, G. (15 Jul 2011). Orban's Oral Histology & Embryology (13th ed.). Elsevier India. p. 152. ISBN   9788131228197 . Retrieved 1 December 2014.
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