Enamel & Dentin

Hardest substance in the body

Enamel (hydroxyapatite crystals, larger than in bone/dentin)

When is enamel formed?

Only before eruption

Enamel matrix proteins

Amelogenin (most abundant), enamelin, tuftelin, ameloblastin — NO collagen

Is enamel cellular?

No — acellular, no cell processes

Why do older teeth look more yellow?

Enamel becomes more translucent, so dentin color shows

Can enamel repair or turn over?

No — ameloblasts are gone

Decalcified section: immature vs mature enamel

Immature: protein-rich, stains pink. Mature: ~96% mineral → looks empty/white

Ground section: what looks dark?

Hypomineralized (protein-rich) areas → air spaces: tufts, lamellae, spindles, tubules, interglobular dentin, Tomes' granular layer

Tomes' process

Conical apical end of the differentiated ameloblast that makes enamel crystals

Tomes' fibers

Odontoblast processes in dentin tubules

Tomes' granular layer

Hypomineralized root dentin under cementum

Honeycomb surface of enamel matrix

Each depression is where a Tomes' process sat

What keeps enamel crystals thin during deposition?

Amelogenin (nanospheres) surrounding them

Enamel crystal growth: deposition vs maturation

Deposition: LENGTH. Maturation: WIDTH (1.5 → 25 nm)

What stops crystal length growth?

Unknown — possibly capping proteins like enamelin

Enamel rod (prism)

Bundle of crystals running from DEJ to near the surface (~5 µm wide)

Rod shapes

Round (rarer) and keyhole (head toward crown, tail toward root)

What makes rods visible?

Different crystal orientations in rod vs interrod — no physical boundary between them

Why do crystals in rod and interrod differ in direction?

Crystals leave the Tomes' process perpendicular to its membrane, and head and tail membranes face different ways

Why are rods mechanically important?

Prevent flat fracture planes — enamel chips rather than cleaves

Aprismatic enamel

Enamel without rods, formed when there's no Tomes' process

First (DEJ) aprismatic layer

Formed before Tomes' process develops; clinically insignificant

Surface aprismatic layer

Formed after Tomes' process is lost (maturation); 20–130 µm; clinically significant — must be etched for bonding

Why etch enamel before bonding?

Remove aprismatic surface layer → expose rod ends (tags) → rough surface bonds better

3 groups of mature enamel features

DEJ-associated (spindles, tufts, lamellae); appositional growth (cross striations, striae of Retzius, perikymata); rod orientation (Hunter-Schreger bands)

Enamel spindles

Odontoblast processes trapped in enamel across the DEJ; most at cusp tip; dark in ground section

Enamel tufts

Grass/tornado-like hypomineralized rods from the DEJ, rich in tuftelin; evenly spaced

Enamel lamellae

Sheet-like hypomineralized planes from DEJ toward surface; look like cracks; contain enamel proteins

Lamella vs true crack

Lamella contains enamel proteins; a true crack contains salivary proteins

When do tufts and lamellae become visible?

Only after neighboring enamel matures (they retain protein)

Cross striations

DAILY enamel growth lines, 2.5–6 µm apart

Striae of Retzius

WEEKLY enamel growth lines; always run from the DEJ toward the cusp tip

Cross striations between striae of Retzius

~7

Perikymata

Surface ridges/grooves where striae of Retzius meet the crown surface (imbrication lines of Pickerill)

Neonatal line

Prominent stria of Retzius formed at birth; separates prenatal and postnatal enamel

Hunter-Schreger bands

Alternating light (parazones) and dark (diazones) bands from alternating rod orientations; extend ~halfway from DEJ

Parazones vs diazones

Parazones light; diazones dark

Maturation direction

Starts at the cusp tip, ends at the neck

Enzyme that clears amelogenin in maturation

Enamel serine protease

Events of enamel maturation

Tomes' process lost; protease degrades amelogenin; protein removed; crystals grow in width and squeeze out protein → ~96% mineral

Final protein content of enamel

1–2% (amelogenin ~90% of it)

Enamel vs dentin: inorganic content

Enamel 96%; dentin 70%

Enamel vs dentin: formation

Enamel before eruption only; dentin throughout life

Enamel vs dentin: nerves and cells

Enamel none; dentin has odontoblast processes and nerves

Enamel vs dentin: proteins

Enamel: enamel proteins, no collagen. Dentin: collagens

Enamel vs dentin: crystal size

Enamel large; dentin small

Enamel vs dentin: mineralization

Enamel two-step (months–years delay); dentin one-step (24 hr: predentin → dentin)

Dentinal tubules

Hold odontoblast processes (and nerves); empty → black lines in ground sections

3 types of dentin

Mantle, primary (circumpulpal), secondary

Mantle dentin

First dentin under the DEJ (crown only); 20–50 µm; collagen PERPENDICULAR to DEJ; less mineralized

Primary (circumpulpal) dentin

Most of the dentin, formed until root completion; collagen PARALLEL to DEJ; S-shaped tubules

Secondary dentin

Formed after root completion, slowly throughout life; irregular tubules; narrows pulp

S-shaped tubule direction

From DEJ curve toward the cusp tip, then back toward the pulp

Incremental lines of von Ebner

DAILY dentin lines, 4–8 µm (equivalent to cross striations)

Lines of Owen

WEEKLY dentin lines (equivalent to striae of Retzius)

Protein that starts dentin mineralization

Dentin phosphoprotein (DPP) — highly phosphorylated, binds calcium

Calcospherite

Calcium-rich mineralization sphere that grows and fuses with neighbors to mineralize dentin

Predentin mineralization time

~24 hours

Interglobular dentin

Hypomineralized CROWN dentin where calcospherites fail to fuse; dark in ground section

Tomes' granular layer (cause)

Hypomineralized ROOT dentin under cementum; highly branched tubules ('palm trees')

Hypomineralized dentin: crown vs root

Crown = interglobular dentin; root = Tomes' granular layer

Translucent dentin

Increases linearly with age, starting at root apex toward crown; used in forensics; similar to sclerotic dentin

Why can enamel not be repaired?

Ameloblasts are gone after eruption

Why does mature enamel look empty in a decalcified section?

~96% mineral, only ~4% protein left

Aprismatic enamel: where and why?

At DEJ (before Tomes' process) and at the surface (after it is lost); surface layer must be etched for bonding

Collagen orientation in mantle vs primary dentin?

Mantle perpendicular to DEJ; primary parallel

Von Ebner vs Owen lines?

Daily 4–8 µm; weekly (equal to striae of Retzius)

Enamel mineral

Hydroxyapatite — same mineral as bone and dentin but much larger crystals (formula NOT required)

'Apatite' means…

A phosphorylated mineral

Translucency of enamel with age

Increases (dentin color shows through → older teeth look more yellow)

Is there an 'amelocyte'?

No — enamel has no cells or cell processes inside

Mature enamel % mineral vs protein (decalcified section)

~96% mineral, ~4% protein — not enough to hold structure, so it looks EMPTY/white

Immature enamel matrix in a decalcified (H&E) section

Protein-rich, so it stains (pink)

Ground section: how is it made and what is lost?

Ground thin enough for light to pass; all organic material (protein) is lost, leaving air spaces

Mature enamel in a ground section looks…

Clear

Preferred word for dark structures in ground sections

'Hypomineralized' (rather than 'excess protein')

First mineralized tissue formed

Dentin (predentin, then dentin a day later)

When does the ameloblast fully differentiate?

Only after it sees dentin

What does Tomes' process make?

The enamel crystals

What does each depression in the honeycomb/sawtooth enamel matrix surface represent?

Where a Tomes' process sat

'This guy Tomes got around' — the three Tomes' eponyms

Tomes' process (ameloblast, enamel); Tomes' fibers (odontoblast processes, dentin); Tomes' granular layer (hypomineralized root dentin)

What does Tomes' process secrete?

Ca²⁺ and PO₄³⁻ (and amelogenin) → nucleate a crystal

Why must crystal growth be controlled?

It would otherwise keep growing spontaneously and too fast (like an ice crystal)

Amelogenin nanospheres

Amelogenin forms nanospheres (small, then large) that surround growing crystals so they can't grow in width (nanosphere terms NOT required)

Enamelin (green ovals) on the slide

Next to growing crystal ends; 'regulate growth in length???' — mechanism uncertain, don't memorize a definite one

ENAMEL MATRIX =

Crystals + a lot of protein (+ water) — why demineralized immature enamel still stains

Rods vs prisms

Interchangeable terms

Which rod shape is more common?

Keyhole ('tennis racket'/fish shape); round is rarer

Keyhole rod orientation

Round head toward the crown, tail toward the root

Where do rods run?

From the DEJ to (almost) the outer crown surface

Enamel rod size (slide 11)

5 µm × 2.5 mm (width × length)

Can rod length really be measured?

Not without serial sections

Rod sheath (sheathlin)

Small spaces between rods holding a little water and protein; drawn artifactually enlarged on slide 11

Mature enamel composition (rods)

~95–96% mineral; rest protein and water

What lies above the ameloblast?

Stratum intermedium

Crystals leave the Tomes' process at what angle?

Roughly PERPENDICULAR to the plasma membrane

Why do rod and interrod crystals point different ways?

Head and tail membrane regions of Tomes' process face different directions

Is the line between rod and interrod a physical structure?

No — only the juxtaposition of two crystal orientations; nothing between adjacent rods

'It is only the juxtaposition of different crystal orientations (in rod and interrod regions) that permits ___'

Rods to be visualized

If enamel crystals were uniform like a diamond…

No rods; flat fracture planes; enamel would facet/cleave easily (hardness same)