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DIFFERENTIATE BETWEEN THE CERVICAL LOOP AND HERTWIG’S
EPITHELIAL ROOT SHEATH (HERS)
Introduction and Definitions
The Cervical loop and Hertwig’s Epithelial Root Sheath (HERS) are structural
features that are apparent during different stages of the developing tooth
The Cervical loop refers to the apical portion of the enamel organ which is a
zone of great mitotic activity formed by the meeting of the outer and inner
enamel epithelium meet as the enamel organ grows into the mesenchyme
The Hertwig Epithelial Root Sheath (HERS) is a bi-layer proliferation of
epithelial cells located at the cervical end of the tooth and is responsible for
directing the process of root development
Appearance during Odontogenesis
The Cervical loop is a highly proliferative region which first appears during the
cap stage and continues to grow through the mesenchyme as tooth
development crosses into the bell stage until enamel and coronal dentine are
formed.
The HERS appears after crown formation is complete and both coronal enamel
and dentine have been formed. The disappearance of certain signaling
molecules such as Egf, FGf-10 have been implicated in the transition from
crown to root development.
Structural differences
The cervical loop maintains the four cell layers of the enamel organ i.e., the
inner enamel epithelium, the outer enamel epithelium, stellate reticulum and
stratum intermedium. It is therefore a structure that has ectodermal
components enclosing mesenchymal components.
The HERS only has ectodermal cells of the outer and inner epithelium.
Functional Differences
The cervical loop is composed of stem cells that are involved in crown
formation but which degenerate just before root formation begins. Here, the
stellate reticulum and stratum intermedium cells disappear, and the inner and
outer enamel epithelium in the distal part of the cervical loop forms the HERS.
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HERS determines the number, size and shape of the root or roots of the tooth.
Through epithelial mesenchymal induction, its inner epithelium cells induce
dental papilla cells to lay down radicular dentine and it also maintains the
periodontal ligament space.
Fate of the Cervical loop and HERS
The cervical loop remains in place until the crown development is complete and
root formation begins. It then proliferates apically and loses its separating cells
as an epithelial bilayer now known as the HERS.
The HERS is only intact when root development is beginning. It then
disintegrates once dentine formation begins to allow growth of cementum onto
the root surface. The epithelial diaphragm is the part of the HERS that
determines the future position of the amelodentinal junction. Remnants of the
epithelial root sheath can remain in the periodontal ligament as the Epithelial
rest cells of Malassez which can cause dental cysts. Currently they are
postulated to have stem cell properties for periodontal repair and regeneration.
Molecular Signaling Differences
The cervical loop is the signaling center for enamel development
The HERS is the signaling center for the tooth root formation
Morphological Similarities.
They form structural boundaries: Both the cervical loop and the HERS are
boundaries of two dental mesenchymal tissues; the dental papilla and the
dental follicle.
Ecto mesenchymal interaction is present with the mesenchyme of the dental
papilla to induce differentiation of pre odontoblasts into odontoblasts for both
structures. In both structures, the inner enamel epithelium cells induces
proliferation of the underlying dental papilla cells into odontoblasts.
Although discussed as different entities the HERS can be viewed as distal
extension of the cervical loop that doesn’t have the stratum intermedium and
stellate reticulum.
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Both structures possess secretory function. The inner enamel epithelium of the
cervical loop secretes the enamel matrix for the cervical enamel while those of
the HERS secrete the hyaline layer of Hopewell smith.
Similar regulatory factors have been located in both outer enamel of the
cervical loop as well as in the HERS e.g. Notch 2 a regulatory factor for the
decision of cell fate.
Basement membrane: Both structures have a basement membrane on their cell
boundaries which possesses certain signaling factors for both processes of
crown and root formation.
References:
1.B. k Berkovitz, G R Holland, B j Moxham. Oral Anatomy, Histology and
Embryology 4 th edition.
2.Ten Cate’s Oral Histology: development, structure, and function 8 th edition.
3. Provenza DV, Sisca RF. The Dental Primordium, An Electron Microscopic
Study of the Cervical Loop. J Periodontol. 1973;44(9):551–8.
4. Guo Y, Guo W, Chen J, Tian Y, Chen G, Tian W, et al. Comparative study on
differentiation of cervical-loop cells and Hertwig’s epithelial root sheath cells
under the induction of dental follicle cells in rat. Sci Rep. 2018 Apr
25;8(1):6546.
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5. Sakano M, Otsu K, Fujiwara N, Fukumoto S, Yamada A, Harada H. Cell
dynamics in cervical loop epithelium during transition from crown to root:
implications for Hertwig’s epithelial root sheath formation. J Periodontal Res.
2013 Apr;48(2):262–7.