Histology of Enamel

What percentage of enamel is inorganic and what is it made of?

96%; calcium & phosphorus in the form of hydroxyapatite

What percentage of enamel is organic and what does it form?

4%; forms fibers left by ameloblasts

What does the enamel matrix provide during enamel formation?

Pathway/framework for calcification; initial shape/support

What happens to enamel matrix after calcification?

Its role is finished and it is no longer visible under the microscope

What protective role does enamel serve for the tooth?

Provides an insulating barrier against physical, thermal, and chemical forces

How does enamel thickness vary across a tooth?

Thinner at cervical region, thicker at masticatory surfaces

In a perfect state, what is the color of enamel?

Colorless; reflects underlying tooth structure/stains

Why may enamel appear darker in color clinically?

Staining occurs in the interrod enamel (interprismatic region)

Why is enamel more radiopaque than dentin or pulp?

More mineralized

Which enamel component is most calcified?

Enamel rod (prism)

What shape are enamel rods?

Hexagonal, long cylindrical units

How are enamel rods oriented relative to the DEJ?

Perpendicular

How do enamel rods fan out, and what clinical importance does this have?

outward; decay follows their path

Which enamel structure encloses enamel rods and is least calcified?

Enamel rod sheath

Which enamel structure is found between rods, acts as cement, and is the 2nd most calcified?

Interrod substance

What are Hunter-Schreger bands caused by?

Twisting/curling of rods, reflecting alternating light/dark bands

What direction do Hunter-Schreger bands run?

Perpendicular to DEJ toward enamel surface

What are Striae of Retzius?

Brownish incremental lines every 4 days during enamel development

How do Striae of Retzius appear in transverse sections?

Thin concentric rings

Why are Striae of Retzius clinically significant?

Less mineralized → easier path for decay

What is the neonatal line and what does it represent?

A prominent Retzius line marking ameloblast trauma during birth

What are enamel tufts?

Hypocalcified enamel rods extending brush-like from DEJ outward; seen histologically

What are enamel spindles, and where are they found?

• Come from odontoblast

• cytoplasmic extension of dentinal fibers that cross into enamel near DEJ

• seen histologically

What clinical effect may enamel spindles contribute to in young patients?

Slight hypersensitivity

What are enamel lamellae, and what causes them?

Hair-like cracks extending inward; caused by enamel development defects or trauma (e.g. temperature changes)

What are perikymata

Surface depressions where Striae of Retzius reach enamel

Where are perikymata most visible?

labial maxillary incisors as horizontal lines

What is gnarled enamel, and what is its function?

Interdigitated enamel rods at cusp tips; resist occlusal stress

Residue covering newly erupted teeth (enamel cuticle)

Nasmyth’s membrane

Calcified ameloblast product at cervical enamel

primary enamel cuticle

List characteristics of the primary enamel cuticle

protects enamel; stains yellow/green in children; removable by polishing

What is the secondary enamel cuticle?

Non-calcified product of reduced enamel epithelium (REE)

What is the function of the secondary enamel cuticle

attaches gingiva to tooth; secretes epithelial attachment

Describe the pattern of pit & fissure caries in enamel

Begins small in pits, then fans out triangularly along enamel rods

Why does decay spread rapidly once through enamel?

Dentin is less mineralized

What is the enamel rod orientation in smooth surface caries?

Rods diverge

What are the two stages of enamel development?

Apposition and maturation

What happens during the apposition stage?

Ameloblasts secrete enamel matrix in successive layers

What happens during the maturation stage?

Mineralization of enamel matrix in two waves

Where does the first wave of enamel mineralization occur and move to?

Incisal/occlusal crown near DEJ → moves to nonmasticatory surfaces

What happens to ameloblasts during maturation?

Reduce in height/volume, fuse into reduced enamel epithelium, then are lost at eruption

What does the loss of ameloblasts mean for enamel growth?

Prevents any further enamel apposition

What is posteruptive maturation, and what contributes to it?

Continued mineralization after eruption; fluoride and calcium aid deposition

What causes enamel to appear darker in color?

Staining in interrod enamel (interprismatic region)

What happens to ameloblasts after enamel maturation?

Reduce in size, fuse, and are lost forever during eruption (no further appositional growth possible)

Where does the second wave of mineralization occur?

Cervically, overlapping the first wave

How does enamel appear on radiographs compared to dentin and pulp?

More radiopaque (lighter)

Apposition of enamel matrix by ameloblasts

amelogenesis

The study of microscopic structure and function of tissues

histology

Hair-like cracks/defects in enamel extending inward

enamel lamella

What can enamel lamella lead to

decay

What is enamel lamellae caused by

development defects or trauma (rapid temp changes)

Hypocalcified enamel rods extending from DEJ outward; seen histologically, not clinically

Enamel tufts

What is the hardest structure in the human body?

Enamel

Enamel is approximately how much harder than bone?

2x as hard

Enamel can withstand about how much crushing pressure per square inch?

100,000 lbs/sq. in.