The Hertwig epithelial root sheath (HERS) or epithelial root sheath is a proliferation of

epithelial Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellula ...

cells located at the

cervical loop The cervical loop is the location on an enamel organ in a developing tooth where the outer enamel epithelium and the inner enamel epithelium join. The cervical loop is a histologic term indicating a specific epithelial structure at the apical s ...

of the

enamel organ The enamel organ, also known as the dental organ, is a cellular aggregation seen in a developing tooth and it lies above the dental papilla. The enamel organ which is differentiated from the primitive oral epithelium lining the stomodeum.The ename ...

in a developing tooth. Hertwig epithelial root sheath initiates the formation of

dentin Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) 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 ena ...

in the root of a

tooth A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...

by causing the differentiation of

odontoblast 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 t ...

s from the

dental papilla 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 ...

. 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 In dentistry, the epithelial cell rests of Malassez (ERM) or epithelial rests of Malassez (''pax epithelialis pediodontii'') are part of the periodontal ligament cells around a tooth. They are discrete clusters of residual cells from Hertwig's ep ...

(ERM). These rests can become cystic, presenting future periodontal infections.

Structure

Hertwig epithelial root sheath is derived from the inner and outer enamel epithelium of the enamel organ.Illustrated Dental Embryology, Histology, and Anatomy, Bath-Balogh and Fehrenbach, Elsevier, 2011, p. 66

Function

The sheath is also responsible for multiple or accessory roots (medial growth) and lateral or accessory canals in the root (break in epithelium).Ten Cate's Oral Histology, Nanci, Elsevier, 2013, p. 174 It is controversial, but HERS may be involved in

cementogenesis 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 ( ...

and the secreting of cementum, or that HERS-derived products might be related to enamel-related molecules, and that these proteins might initiate acellular

cementum 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.Illustrated Dental Embryology, Histology, a ...

formation.

Other animals

While in mammals the HERS is rather a transient structure, in amphibians it is more or less a permanent one. Here the root epithelium does not

fenestrate Fenestration may refer to: * Fenestration (architecture), the design, construction, or presence of openings in a building * Used in relation to fenestra in anatomy, medicine and biology * Fenestration, holes in the rudder of a ship to reduce the w ...

like in mammalians. Within vertebrates 3 distinct stages of HERS development can be observed. # In teleosts and chondrichthyans no HERS or root is really formed, and tooth development is restricted to crown development. An inflexible joint is formed between the tooth and the bone at the apical end of the tooth where the epithelium remains open. # In amphibians and non-crocodilian reptiles a continuous root sheath or HERS is formed without fragmentation of the epithelium. Once again a rather rigid connection between bone and tooth is formed at the apical end of the tooth where no epithelium is present. # In crocodilians and mammals the HERS is a transient structure and fragments to form the epithelial cell rests of Malassez. Through the gaps in the root epithelium elements of the periodontal ligament can migrate and form a flexible connection between bone and root.

History

Hertwig epithelial root sheath was not discovered in any mammalian species. Instead this epithelial structure was discovered by

Oskar Hertwig Oscar Hertwig (21 April 1849 in Friedberg – 25 October 1922 in Berlin) was a German embryologist and zoologist known for his research in developmental biology and evolution. Hertwig is credited as the first man to observe sexual reproduction ...

in 1874 in an amphibian (see notes below for further discussion in other animals).

References

*Hertwig, O. (1847) Über das zahnsystem der amphibien und seine bedeutung für die genese des skelets der mundhöhle. Arch. Mikrosk. Anat. EntwMech. 11 (suppl): 55–56 {{Authority control Tooth development

Structure

Function

Other animals

History

See also

References