Enamel Formation and Structure

Enamel Formation

• Enamel is a crucial part of tooth structure, along with cementum, PDL (periodontal ligament), and alveolar bone.
• Enamel interacts with dentin and gingiva, contributing to the overall tooth structure.

Enamel Structure

• Enamel is the hardest and most highly calcified tissue in the human body
• Composed of a crystalline structure, it is insensitive due to the absence of nerves.
• Acid-soluble and demineralizes at a pH of 5.5 or lower
• Enamel cannot be renewed and darkens with age as the enamel is lost.
• Fluoride and saliva assist in remineralization

Chemical Characteristics

• 96% of enamel is inorganic material, mainly hydroxyapatite crystals that are hexagonal in shape.
• These crystals are arranged to form enamel rods or enamel prisms.
• The core of the crystals is rich in magnesium and carbonate which accounts for acid solubility in peripheral portions
• Pores exist between crystals, especially at the boundaries of rods and these pores are filled with water.
• 4% of enamel consists of organic material, notably proteins exclusively found in enamel.
• Amelogenins make up 90% of the organic material and are low molecular weight proteins that are hydrophobic.
• Nonamelogenins represent 10% of the organic material, consisting of high molecular weight proteins which do not contribute to enamel structuring.

Amelogenesis

• This process has six phases but is generally subdivided into presecretory, secretory, and maturation stages.
• The six phases include Morphogenetic, Organizing (differentiation), Secretory (formative), Maturative, Protective, and Desmolytic.

Presecretory Stage

• It includes morphogenetic, and differentiation phases.

Morphogenetic Phases

• Before ameloblasts fully differentiate, they interact with adjacent mesenchymal cells to define the ADJ (amelodentinal junction) and crown shape.
• Cells present as short columnar with large, oval nuclei that fill the cell body
• The Golgi apparatus and centrioles are located at the basal end, facing the stratum intermedium while the mitochondria are evenly dispersed in the cytoplasm.

Differentiation Phases

• Cells of the inner enamel epithelium differentiate into ameloblasts, they elongate, and their nuclei shift toward the stratum intermedium.
• Dentin formation by odontoblasts begins by the end of the differentiation phase.
• The basement membrane supporting preameloblasts disintegrates after dentin deposition and during ameloblast differentiation.

Secretory Stage

• Ameloblasts develop blunt processes that penetrate the basal lamina and protrude into the predentin which fragments the basement membrane.
• The cell organelles of ameloblasts increase in number, initiating enamel secretion.
• Hydroxyapatite crystals deposit more or less parallel to each other, interdigitating with dentin crystals in an initial aprismatic enamel layer that appears structureless
• Ameloblasts move away from the dentin surface, forming conical projections called Tomes processes and it all works in an outward direction.

Tomes Process

• Organizes enamel crystallites into rod and interrod enamel

Secretory Stage - Tomes Process Granules

• Tomes process secretes granules perpendicular to its membrane resulting in enamel prismatic and inter-prismatic substance appearance.
• Enamel protein secretion becomes confined to two sites
• The first involves secretion from proximal TP sites, forming interrod enamel.
• The second site involves secretion from the distal sloping portion of TP which later fills the pit with matrix, forming enamel rods.
• The difference between rod and interrod enamel is in the orientation of crystals
• The crystals of the rod meet those of interrod at sharp angles.

Maturative Stage

• Occurs after most of the enamel matrix thickness has formed in the occlusal or incisal area
• Enamel matrix continues forming in cervical crown parts
• Ameloblasts undergo significant height reduction, decreasing in volume and organelle content.
• Water and organic materials are selectively removed from the enamel with additional inorganic material introduced.

Enamel Maturation

• Initial or partial mineralization occurs immediately during matrix formation, accounting for 25-30% of total mineralization.
• A second stage completes mineralization to ~96% inorganic substance.
• Progresses from cusp tip or incisal edge cervically.
• Crystal maturation starts at the dentinal end and progresses towards the outer surface.
• Maturation initially runs parallel to the DEJ (dentinoenamel junction) and later to the outer enamel surface.
• Maturation happens via growth of primary crystals that subsequently fuse together.
• The fibrils between crystals become thinner as maturation progresses.

Protective Stage

• After complete formation and mineralization, ameloblasts form 3-4 layers of stratified epithelium.
• These layers cover the enamel and are referred to as reduced enamel epithelium that protects the enamel by separating it from connective tissue until tooth eruption.

Desmolytic Stage

• Reduced enamel epithelium secretes desmolytic enzymes that cause degeneration of connective tissue, separating the tooth from the oral epithelium.

Histological Structure

• Enamel is formed of enamel crystallites, enamel rods, rod sheaths (packaging of the rods), and interrod substance.

Enamel Crystallites

• They are the smallest units of enamel.
• They are hexagonal in form.
• They are approximately 160 nm in length and 40-70 nm in width.

Rod and Interrod Enamel

• Tomes' processes control enamel crystallites into rod enamel (prisms of ~100 crystallites each) and interrod enamel.
• Enamel crystallites that elongate near the tip of Tomes' process form rod enamel.
• Those near intercellular junctions form the interrod enamel
• The rod and interrod enamel differs in crystallite orientation

Enamel Rod Orientation

• Under electron microscopy a common keyhole paddle-shape is seen in cross section.
• The head is toward the occlusal or incisal surface, and the tail is cervically.

Enamel Rod Numbers

• Rod numbers range from five million in the lower lateral incisor to approximately 12 million in the upper first permanent molar.
• The number of rods equals the count of ameloblasts.
• On the tooth surface there are around 20,000-30,000 enamel rods/mm².
• The density of the rods at the DEJ is approximately 10% more than at the enamel surface.

Enamel Rod Diameter

• The Enamel Rod diameter is about 5µm and in keyhole types is about 9µm in height.
• The diameter increases from the dentino-enamel junction to the outer surface at a ratio of approximately 1:2
• Enamel rods are perpendicular to the dentin surface
• In deciduous teeth, enamel rods are vertical at the cusp tip or incisal edge then oblique toward the occlusal surface, and finally horizontal at the cervical area (so end cervically abruptly).

Rod Direction

• The rods are at a right angle to the dentin surface.
• The rods are vertical in the cuspal region and more oblique as they proceed cervically.
• In deciduous teeth, enamel rods are horizontal cervically and in permanent teeth, they are directed more gingivally.

Enamel Rod Course

• Enamel rods start straight at the dentino-enamel junction (DEJ) for about 30µm
• They then follow a wavy course until near the outer surface of enamel where they become straight again.

Rod Sheath

• The peripheral part of the enamel rod forms an incomplete envelope around the prisms and is less calcified than the rod itself.

Inter-Rod Substance

• The enamel rods aren’t in direct contact, but instead cemented by inter-rod substance
• This appears to be minimal or absent in certain areas

Gnarled Enamel

• At the incisal edge or cusp tip, the enamel rod has a twisted course.

Rodless Enamel

• Rodless enamel is located at the enamel surface to a thickness of approximately 30µm, and may exist as a thin layer adjacent to the DEJ
• Results from the absence of Tomes processes on the ameloblasts and responsible for development of enamel rods
• It consists of closely packed crystals that run parallel to one another and perpendicular to incremental lines of Retzius (more highly mineralized than bulk enamel beneath it).

Gnarled Enamel - More Facts

• A complex arrangement of twisted rods appears over the cusps and incisal edges
• Rod direction changes between the layers, increasing structural strength and is making the enamel less prone to fracture and wear.

Dentinoenamel Junction

• The junction is seen as a scalloped border with the convexities directed toward the dentin, increasing surface area and adherence

Dentino-enamel-Junction Function

• This irregular surface assures the union between enamel and dentin

Enamel Spindles

• They are odontoblastic processes extending into the ameloblast layer before the onset of enamel formation.

Enamel Tufts

• They arise from the DEJ (dentinoenamel junction) up to 1/5 to 1/3 of the way from it and resemble tufts of grass
• These are always evident in transverse ground sections.
• Tufts are hypocalcified prisms and inter-prismatic substances, and the shape of the tufts is a result of the wavy course of the enamel rod occurring in several layers

Enamel Lamellae

• Thin, leaf-like faults between enamel rod groups, extend on the enamel surface toward the DEJ, and may extend into the dentin
• Primarily organic material and susceptible to bacterial entry or caries
• False enamel lamellae are cracks in enamel and frequently extend into the dentin.
• True enamel lamellae are hypocalcified rods due to disturbances in the ameloblasts

Enamel Lamellae Types

• Type A (True Enamel Lamella): Hypomineralized enamel rods formed developmentally and filled with enamel protein
• Type B (False Enamel Lamella): Cracks formed during development at planes of tension and filled with cellular debris.
• Type C: Cracks produced after eruption and filled with organic debris from the oral cavity.
• To find if a lamella is true or not, the sample must be decalcified, in which case the true lamella will not disappear, but the crack will.
• Enamel lamellae may act as a route of caries spread.

Enamel Rod Formation

• Crystals don't grow continuously during the secretory stage, but in increments
• The enamel rod forms rhythmically, every segment being approximately 4 µm in length, which manifests structurally as prism cross-striations

Retzius Incremental Lines

• Appear as brownish bands in ground sections of enamel.
• Illustrate the successive apposition of enamel layers during crown formation.
• In longitudinal sections, they surround the tip of the dentin, or appear as concentric circles in transverse sections.

Striations

• Each rod is built of segments separated by darklines giving them striated appearance
• More noticeable in enamel that's insufficiently calcified.
• Striations are ~5 μm apart
• This distance corresponds to approximately one day of enamel deposition.
• Cause: segmented enamel owing to enamel matrix deposition

Striae of Retzius - General Facts

• Prominent cross-striations are approximately every 4 days/1 week, and are known as striae of Retzius or incremental lines.
• In longitudinal ground section they appear as dark bands to reflect rhythmic enamel formation,
• They arise from the DEJ(dentinoenamel junction), go upward and outward around the tip, and join the DEJ again, therefore not the outer enamel surface.

Perikymata

• These are the surfaces in which Striae of Retzius are incomplete and result in a series of alternating grooves called the imbrication lines of Pickerill.
• The elevations between the grooves are called perikymata
• They run parallel to each other and cementoenamel
• Both disappear from the enamel surface from abrasion and wear
• Transverse, wavelike elevation, continuous, regular and generally found on the cervical region

Enamel Cuticle

• A primary enamel cuticle covers the entire tooth and erupted teeth being removed after mastication
• Secreted by ameloblasts after formation

Cracks

• Fissure-like, found in every enamel surface extending at varying lengths in all enamel surfaces
• They are actually the edges of the enamel lamellae, outer edges and disappear with decalcification.

Permeability, Color and Attrition

• Enamel becomes gradually less permeable with age
• The outer surface of old enamel alters, allowing organic parts of enamel and crystals to be more prone to ions increasing pore size
• Teeth darken with age due to addition of organic material
• From the environment due to attrition, also leads to staining and leads to thin enamel exposing yellow Dentin
• Most frequent age change of enamel is attrition
• This relates to the diet, teeth hardness, certain oral habits, and occlusion

Clinical Considerations

• Enamel cannot repair itself because ameloblasts are lost after the enamel has been deposited, however topical re-mineralization can occur
• Diseased or fractured enamel can only be repaired with operative procedures
• The hard enamel is permeable to bacteria causing decay
• Enamel may be lost due to long term conditions involving low pH, carbs and microorganisms that promote decay.
• Demineralized surfaces may promote remineralization via fluoride, phorosous and/or calcium
• Topical application of fluoride via toothpaste or fissure sealant applications help fight decay

Dental Restorations

• Most entail enamel removal to access underlying decay in Dentin
• In both, it is important to keep in mind the orientation of enamel rods for proper dentin support
• It may also be removed before decay present, like removing and replacing enamel during dental sealant.
• Removal can happen during crowns, veneers, and prostheses to increase aesthetic effect