Enamel Structure and Properties Quiz

Enamel

• Enamel, the hardest tissue in the human body, is highly calcified.
• It covers the anatomical crown of the teeth.
• Enamel is produced by cells of ectodermal origin.

Enamel Composition

• Enamel is primarily composed of inorganic crystals (97% by weight).
• A small amount of water and organic matrix (3%) is also present.
• Amelogenins and enamelin are the organic components.
• Enamel crystals are primarily hydroxyapatite crystals.

Physical Properties

• Enamel thickness varies, with maximum thickness on cusps and premolars (2-2.5mm).
• Thickness thins to a knife edge near the cervical margin.
• Enamel is hard, but also brittle and susceptible to fracture if not supported by dentin.
• Deciduous teeth enamel is slightly less mineralized than permanent teeth enamel.

Hardness

• Enamel's hardness is due to its high mineral salt content and crystalline arrangement.
• Permanent teeth enamel is harder than deciduous teeth enamel.

Brittleness

• Enamel is brittle.
• It is liable to fracture if not supported by dentin.

Color

• Enamel color ranges from yellow to shades of gray or gray-blue.
• The color depends on the degree of translucency, which in turn depends on calcification.
• The yellow color of dentin is visible through thin translucent enamel.
• Deciduous teeth enamel is typically whiter than permanent teeth enamel.

Enamel Permeability

• Enamel is semi-permeable, allowing partial passage of saliva ions and dyes.
• The primary pathway for passage is from saliva to the upper part of the enamel.

Amelogenesis

• Amelogenesis is the process of enamel formation.
• Ameloblasts are the cells responsible for secreting organic and inorganic enamel components.
• Amelogenesis involves several stages:
  • Presecretory stage
  • Secretory stage
  • Maturation stage
  • Protective stage

1- Pre-secretory stage

• Cells are short columnar with large, centrally located oval nuclei.
• Synthetic organelles are few and scattered.
• Cells are separated from the dental papilla by a basement membrane and cell-free zone.

2- Secretory stage

• After the first layer of dentin is formed, the distal membrane of ameloblasts flattens and produces the first layer of enamel matrix.
• Hydroxyapatite crystals are parallel to each other and interdigitate with dentin crystals.
• Distal ends of ameloblasts extend blunt cytoplasmic processes (Tomes' processes).
• Tomes processes have numerous secretory vesicles containing enamel matrix with no organelles.
• A distal terminal web (distal junctional complex) forms a dark staining line encircling the secretory ameloblasts and separating them from the Tome's process.

3- Maturation stage

• Begins after the entire enamel thickness is formed.
• Ameloblasts shorten, decreasing the number of synthetic organelles.
• The function of ameloblasts converts enamel from 30% mineralization to 97%.

4- Protective stage

• Ameloblasts and enamel organ layers form the enamel epithelium after complete crown formation.
• Protects mature enamel from surrounding tissue.
• Protects enamel until tooth eruption.

Enamel Structure

• Number of enamel rods vary per tooth, reaching up to 12 million in the upper molars.
• Orientation is perpendicular to the dentin surface (generally horizontal in deciduous and oblique in permanent teeth at cervical regions)
• Enamel rod course is wavy near the surface, becoming straight.
• Enamel rod diameter increases from amelodentinal junction to the surface.
• In cross-section, enamel rods appear hexagonal, oval, or rounded.

Enamel Rods (Interprismatic)

• Enamel rods are cemented together by interrod substances.
• Crystal orientation differs from the enamel rods' orientation.

Rod Sheath

• Peripheral part of each enamel rod (contains more organic matrix than the other region).
• A darker stain and resistance to decalcification due to increased organic content.

Histological Structure of Enamel

• Each enamel rod is composed of segments separated by dark lines (striated appearance).
• Rhythmic apposition of the enamel matrix by ameloblasts gives striated appearance, representing the daily rate of matrix formation.

Incremental Lines

• Due to rhythmic deposition of enamel every four days (brownish bands in ground sections).
• In longitudinal view, striations are present at cusp and incisal edges, surrounding the dentin tips, and don't reach the enamel surface.
• Sometimes, they reach the outer enamel surface forming transverse depressions (perikymata).

Neonatal Line

• Separates enamel formed before birth from enamel formed after birth.
• Prenatal enamel quality is better due to constant fetal nutrition.
• Present in deciduous teeth and first permanent molar teeth.

Amelodentinal Junction

• Scalloped junction between enamel and dentin directed towards dentin.
• This arrangement increases surface area and adherence between enamel and dentin.

C. Gnarled Enamel

• Present on cusps and incisal edges.
• Due to twisting enamel rods over each other.
• Functional impact of rod orientation change: increased enamel strength.

Hunter-Schreger Bands

• An optical phenomenon viewed with reflected light.
• Appearing as alternating light and dark bands, mainly visible in the inner 2/3 of the enamel.
• Bands not apparent on the outer enamel surface.

Enamel Spindles

• Develop before enamel formation.
• Developing odontoblastic processes extend into the ameloblast layer, becoming trapped, to form enamel spindles.
• Found in great numbers at cusp regions.
• Dark spaces in dried ground sections.

Enamel Tufts

• Hypocalcified enamel rods projecting from the dentin-enamel junction.
• Appear branched, with higher enamel protein concentrations.
• Resemble grass tufts.
• Likely a developmental feature.

Enamel Lamellae

• Hypomineralized areas that extend from the outer enamel surface towards the dentin-enamel junction.
• Three types:
  • Type A: Hypomineralized enamel rods filled with proteins.
  • Type B: Cracks during development, filled with cellular debris.
  • Type C: Cracks formed after tooth eruption, filled with organic debris.

Rodless Enamel

• A thin layer of enamel surface with a thickness of 30μ.
• Adjacent to the DEJ.
• Consists of closely-packed crystals, parallel to each other and perpendicular to incremental lines of Retzius.
• More highly mineralized compared to the enamel below.

Perikymata

• Transverse, wave-like grooves on enamel surface, an outer manifestation of Retzius striae.

• Perikymata are continuous around the tooth and usually parallel to each other and to the cementoenamel junction.

• They are prominent in the cervical portion of the crown.

• With age, perikymata become less prominent, especially on facial and lingual crown surfaces.

Rod Ends

• Outer enamel surface of newly erupted teeth shows small indentations corresponding to Tomes' processes.

• Ameloblasts cease enamel production before complete retraction.

Pellicle and Plaque

• Erupting enamel is covered by pellicle (salivary protein precipitate).
• Pellicle reforms within hours after tooth cleaning.
• Microorganisms colonize pellicle to form dental plaque within a day or two.

Age Changes

• Attrition is the most apparent age-related change in enamel involving wear on occlusal and proximal surfaces due to mastication.
• Teeth darkening with age due to organic material accumulation or tooth attrition exposure of dentin's yellow color.

Alteration in Chemical Composition

• Fluoride incorporation strengthens enamel crystals, making them more resistant to acid dissolution.
• After tooth eruption, saliva exchange causes localized changes in enamel elements (e.g., fluorine).
• Decrease in enamel water content occurs.

Clinical Considerations

• Repairing damaged enamel requires operative procedures involving cavity preparation and restorative materials.
• Knowledge about enamel rod orientation is crucial in cavity preparation because enamel is brittle and relies on sound dentin support.

Fluoridation

• Fluoride incorporation into hydroxyapatite crystals renders them more resistant to acid dissolution.
• This explains fluoride's role in caries prevention.
• Demineralization process is initiated

Test your knowledge about enamel, the hardest tissue in the human body. This quiz covers its composition, physical properties, and differences between permanent and deciduous teeth enamel. Perfect for dental students and enthusiasts alike!