8 - Morphology + Physiology of enamel

Question 1

Morphology - Enamel rods

Answer 1

• Extend from DEJ to enamel outer surface ⇢ each rod formed by 4 ameloblasts
• Crystal orientation of rods ⇢ longitudinal (in tail = diagonal/perpendicular)
• Rod shape ⇢ keyhole shaped ⇢ size ± same as red blood cell
• Rods are interlocked ⇢ additional support ⇢ prevents tooth fracture
• Rod’s surface = rod sheath ⇢ contains more organic matter than core
• Rod’s direction ⇢ perpendicular to DEJ + curves towards cusp tip

Question 2

Striae of Retzius

Answer 2

• Dark lines extending from DEJ to tooth’s surface ⇢ reflects incremental growth of enamel layer
• Longitudinal sections ⇢ seen as lines
• Cross sections ⇢ seen as concentric rings
• Neonatal line = enlarged stria of retzius ⇢ reflects physiological changes occurring at birth

Question 3

Cross striations of rods

Answer 3

• Enamel is formed at rate of ±4mm daily
• every 4mm ⇢ a periodic band appears across the rod = cross striation

Question 4

Hunter-Schreger bands

Answer 4

• Dark + light bands ⇢ appear by reflecting light on enamel surface
• Rods in cross section are cut differently due to wavy course

Question 5

Gnarled enamel

Answer 5

• Found at cusps near DEJ
• Enamel rods form intertwining bundles

Question 6

Enamel tufts + spindles

Answer 6

Enamel tufts

• Occur during development ⇢ due to change in direction of groups of rods in scalloped DEJ

Enamel spindles

• Before enamel forms ⇢ some odontoblast processes extend in ameloblast layer
• When enamel formation begins ⇢ they become trapped to form spindles

Question 7

Enamel lamellae

Answer 7

• Faults in enamel development + extend from enamel surface
• Consists of ⇢ linear, longitudinal orientated defects filled w/organic material

Question 8

Enamel surface

Answer 8

Characterised by:

• Perikymata ⇢ shallow furrows resulting from extensions of retzius striae
• Final enamel ⇢ outer layer of enamel without rods
• Pellicle ⇢ covers tooth when erupts + consists of debris from enamel organ ⇢ Lost rapidly + salivary pellicle is formed

Question 9

Physiology of enamel

Answer 9

Enamel ⇢ hardest + highly mineralised substance in the body

• Secreted by ameloblasts
• Outermost layer of tooth’s crown
• Avascular + acellular tissue ⇢ CANNOT renew itself if damaged

Question 10

Synthesis of organic matrix in enamel

Answer 10

• Matrix is synthesised in RER ⇢ migrates to golgi apparatus
• Matrix is condensed + packaged in membrane- bound vesicles
• Vesicles migrate to apical end of cell + release content along DEJ

Early enamel matrix

• 65% H2O
• Proteoglycans, lipids, citrate, electrolytes
• Organic matrix = enamelins + amelogenins

Mature enamel matrix

• 2-3% H2O
• 95-97% inorganic matrix ⇢ mainly hydroxyapatite + ions
• >1% organic matrix ⇢ Ameloblastin, enamelins, amelogenins + other proteins

Question 11

Events in enamel deposition

Answer 11

• Growth + thickening of crystallites
• Degradation + removal of ECM components
• Mineralisation ⇢ starts w/formation of crystals arranged in prisms + their ⇡ size
• Maturation of crystals ⇢ formation of organic matrix
• Initial enamel deposition = 25% ⇢ 75% a result of crystal growth

Mineralisation - 2 phases

1. Deposition of enamel matrix + size increase
2. From DEJ ⇢ first cusp tip, crown sides, then cervical region mineralised

Question 12

Information about enamel

Answer 12

• Inner layer ⇢ more permeable than outer layer
• Space between crystals ⇢ ⇣w/age ⇢ ⇣permeability
• Enamel has ⇣Thermal conductivity + ⇣Expansion = keeps pulp safe from any temp range