Location and Borders
Enamel externally covers the crown of the tooth (corona dentis), the part of the tooth projecting from the dental alveolus. The bulk of the tooth, dentin (dentinum), lies beneath the enamel.
The neck of the tooth (cervix dentis) is located at the border between the crown and the root. In this contact zone (area contingens), enamel joins the root cementum. There are four variants of this junction:
- Enamel and cementum meet end-to-end.
- Enamel overlaps cementum.
- Cementum overlaps enamel.
- Enamel does not reach the edge of the cementum (leaving an exposed area of dentin between them).
A thin and quite durable film, the enamel cuticle, covers the surface of intact teeth.
Clinical Note: In a newly erupted tooth, the clinical crown is smaller than the anatomic crown. With age, as areas of the tooth not covered by enamel become exposed, the clinical crown becomes larger than the anatomic crown.
Chemical Composition and Origin
While dentin and cementum are hard tissues of connective tissue origin, tooth enamel has an epithelial origin.
The high hardness of enamel is due to its high mineral salt content:
- Inorganic substances: 96–99% (mineral salts).
- Organic substances: 1–4% (proteins and water).
Histological Structure
The primary structural unit of the tissue is the enamel prism. Enamel contains several million such prisms.
- Prism characteristics: Diameter is 4–6 µm, with a predominantly arcade-shaped cross-section. Structurally, the prism consists of calcified ultra-thin fibrils.
- Arrangement: Prisms originate at the dentinoenamel junction and terminate on the crown surface. Their length varies and most often exceeds the thickness of the enamel layer itself.
- Bundles: Enamel prisms are grouped into bundles of 10–20. They follow an S-shaped course.
Histological ground sections of enamel reveal characteristic optical phenomena:
- Hunter-Schreger bands — alternating light and dark bands. Optical heterogeneity occurs because some enamel prism bundles are cut transversely while others are cut longitudinally.
- Striae of Retzius — lines running in an oblique direction. Their formation is related to the cyclic nature of enamel mineralization during its development.
Clinical Significance: Pathogenesis of Caries
Streptococcus mutans and Streptococcus sanguis dominate on tooth enamel due to their high ability to adhere to hard dental tissues.
The leading role in the development of dental caries is attributed to the pathogenic effect of microbial dental plaque:
- Pellicle formation: An acquired pellicle forms on the tooth surface — a film of salivary mucopolysaccharides and glycoproteins. The biofilm fixes to the tooth surface via an electrostatic mechanism, which also facilitates bacterial adhesion.
- Colony growth: Without regular removal of dental deposits, plaque accumulates, and microbial colonies actively proliferate.
- Fermentation: Acidogenic micro-organisms participate in the fermentation of dietary sugars; this is promoted by a cariogenic diet high in carbohydrates.
- pH drop: Fermentation produces organic acids, dropping the local pH in the plaque to a critical level (4.5–5.0).
- Demineralization: At the critical pH, enamel demineralization is triggered, forming a demineralized lesion.