(1) The HERS, (2) ERM, (3) Dental follicle, (4) cementoblasts, (5) periodontal ligament, (6) alveolar cells, (7) bone, (8) odontoblasts.

The Hertwig's epithelial root sheath (frequently abbreviated as "HERS") is a proliferation of epithelial cells located at the cervical loop of the enamel organ in a developing tooth. Hertwig's 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, but residual pieces that do not completely disappear are seen as epithelial cell rests of Malassez (ERM).

[edit] Evolution of HERS

Hertwig's epithelial root sheath was not discovered in any mammalian species. Instead this epithelial structure was discovered by Oskar Hertwig in 1874 in an amphibian. While in mammalian the HERS is rather a transient structure in amphibians it is more or less a permanent one. Here the root epithelium does not fenestrate like in mammalians. Within vertebrates 3 distinct stages of HERS development can be observed.

Hertwigs sheath is epithelium that is derived from the stellate reticulum and stratum intermedium of the enamel organ. The sheath is also responsible for muliple roots (medial growth) and lateral canals (break in epithelium)

1. 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.
2. 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.
3. 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.

[edit] References

• Ten Cate, A.R. Oral Histology: development, structure, and function. 5th ed. 1998. ISBN 0-8151-2952-1.
• 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
• Luan, X., Ito, Y. and Diekwish, T.G.H. (2006) Evolution and development of Hertwig's Epithelial Root Sheath. Dev. Dyn. 235: 1167-1180.
• Thomas, H.F. (1995) Root formation. Int. J. Dev. Biol. 39: 231-237

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