Hertwig's epithelial root sheath
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(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 initates 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).
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.
# 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.
*Cate, A.R. Ten. Oral Histology: development, structure, and function. 5th ed. 1998. ISBN 0815129521.
*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
