Oral Biology

Type of cells that teeth develop from

• Epithelium (from oral ectoderm) - Determines shape of tooth (permissive influence)

• Ectomesenchyme (from neural crest cells) - Determines fate of tooth

Morphology vs Differentiation

• Morphology - Changes in shape of developing tooth

• Differentiation - Different cell type/function (odontoblast vs ameloblast)

When does odontogenesis begin?

6 weeks in utero

Dental Lamina Stage

• Initial induction

• Primary Epithelial Band - Horseshoe thickening of oral ectoderm in upper and lower jaw

• Dental lamina - Sheet growing on epithelial band where tooth buds can develop

• Vestibular lamina (vestibule) - Space separating lips/cheek from teeth/gingiva (found buccal to teeth)

Bud Stage

• Proliferation

• Bud forms on dental lamina (epithelial cells)

• 10 buds on each jaw

• Ectomesenchyme condense around bud

• Congenital absence from interruption in this stage

Cap Stage

• Proliferation, Differentiation, Morphogenesis

• Shape of tooth becomes evident from unequal proliferation of bud cells (morpho-differentiation)

• Successional lamina forms on lingual side of teeth

• Enamel knot - Cluster of non-dividing epithelial cells

  • All teeth have primary knots

  • Molars have additional secondary knots

• Tooth germ becomes enamel organ, dental papilla, and dental sac

Enamel Organ

• From Oral Epithelium

• Forms ameloblasts and eventually enamel for crown shape

• Induces dentin formation

Dental Papilla

• From ectomesenchyme

• Forms odontoblasts which is responsible for dentin and pulp shape

• Determines tooth made

Dental Sac

• From ectomesenchyme

• Forms cementoblast and cementum

• Also forms periodontium, PDL, and alveolar bone

Early Bell Stage

• Proliferation, Differentiation, Morphogenesis

• DEJ location is determined

• Enamel organ differentiates to 4 layers

• Cervical Loop is junction of IEE and OEE. Dental papilla cells proliferate to outer dental papilla (future odontoblasts) and inner dental papilla cells

Outer Enamel Epithelium

• Outer protective barrier

• Capillary Plexus

• Aids with transport of material inwards for oxygen and nutrients

Stellate Reticulum

• Secretes GAGs which draw water into enamel organ and increase volume

• Provides support so crown shape is maintained until hard tissue is formed

Stratum Intermedium

• Enables IEE cells to become ameloblasts

• Forms functional unit with IEE

• Contains alkaline phosphatase for mineralization

Inner Enamel Epithelium

• Become ameloblasts

• Initiate dentin formation (reciprocal induction)

• Forms enamel knots in cap stage

• Makes cusps, fissures, and ridges of final crown pattern

Late Bell Stage

• Proliferation and Induction

• Hard tissue is formed

• Ameloblast (forming enamel) and Odontoblast (forming dentin) using reciprocal induction

Reciprocal Induction

• IEE becomes preameloblast

• Preameloblast helps dental papilla cells differentiate to preodontoblast

• Preodontoblast becomes odontoblast and secretes predentin

• Odontoblast induces preameloblast to become ameloblasts

• Ameloblasts secrete enamel until they reach 30% calcification

Additional Reciprocal Induction Notes

• First enamel and dentin formed at DEJ

• Dentin formed before enamel

• Last enamel formed at outermost layer of tooth

• Last dentin formed at dentino-pulpal junction

Hertwig’s Epithelial Root Sheath

• Sheath formed when IEE and OEE join

• Cervical loop grows apically

• HERS does not form root but induces root odontoblast differentiation to synthesize dentin

• Eventually detach and migrate so dental sac mesenchymal cells take their place and become cementoblasts to lay cementum around root

• Each medial ingrowth of HERS causes additional tooth root

Epithelial Rests of Malassez

• HERS remnant that detaches from root surface

• Can become tumor-forming cells

• Those that do not disappear will calcify and become “cementicles”

• Lateral periodontal cysts occur

  • Usually in adult males and not associated with pain

  • Does not need treatment unless pulpal tissue is necrotic

HERS disturbance anomalies

• Exposed Root Dentin - HERS detaches too late for dental sac mesenchymal cells to be placed on dentin and cementum will not appear on that portion of the tooth as a result

• Enamel Pearls - HERS cells do not detach, causing odontoblasts to form enamel pearls

• Accessory Root Canals - Lack of HERS prevents dental papilla cell differentiation, causing lack of dentin and cementum formation

Amelogenesis

• Presecretory Stage - IEE → Pre-ameloblast → Ameloblast

• Secretory Stage - Ameloblast → Enamel

  • ~30% mineralized

  • Initial secretory ameloblasts secrete prism-less enamel

  • Secretory ameloblasts secrete enamel prisms (Tome’s Process)

• Maturation Stage

  • >96% mineralized

  • Ruffle-ended ameloblasts - Have tight junctions in distal end to prevent enamel from leaking

  • Smooth-ended ameloblasts - Have leaky junctions in distal end to remove water and organic proteins

• Protective - Ameloblasts become cuboidal covering mature enamel

Reduced Enamel Epithelium

• Remnant of enamel organ

• All 4 enamel organ layers and papillary layer contribute to REE

Mature Dentin

• Light yellow color

• Resilient and hard

• 70% inorganic (HA), 20% organic (Collagen I), 10% water

• Derived from dental papilla

• Apatite crystals with collagen fibers (not parallel to dentin tubules) in matrix

Odontoblasts

• Only cells in dentin

• Reside within dentin tubules

• Projection as odontoblastic process (Tome’s fiber)

Dentinogenesis Process

• Differentiation of odontoblasts (IEE induces dental papilla cells to become odontoblast)

• Predentin formation

• Mantle mineralization

• Predentin formation

• Circumpulpal mineralization

Mantle Dentin

• Outer, first-formed layer

• Thick von Korff’s fibers (Type III collagen)

Circumpulpal Dentin

• Outlines pulp

• Has part of primary, and all of secondary and tertiary dentin

Primary, Secondary, and Tertiary Dentin

• Primary Dentin - Formed prior to and during tooth eruption. Forms fast

• Secondary Dentin - After root formation and continuous through life, causing pulp recession. Has directional change and deposition is not even

• Tertiary Dentin - Responds to stimuli and has osteodentin cells

  • Reactive Dentin - Pre-existing odontoblasts

  • Reparative Dentin - Newly differentiated odontoblasts

Dentinal Tubules

• Contain odontoblastic process and interstitial fluid

• 1 in 10 have nerves

• S-shaped in crown and straight in roots

• Tapered contour

• Canaliculi

Intratubular vs Intertubular Dentin

• Intratubular Dentin

  • Lines wall of tubule

  • 40% more mineralized than intertubular dentin

  • Dentin Sialoprotein, No Collagen fibers

  • Not in mantle or interglobular dentin

• Intertubular Dentin

  • Type I collagen

  • Between dentinal tubules

Interglobular Dentin

• Irregular and hypomineralized

• Between circumpulpal and mantle dentin

• Caused by Vitamin D deficiency or high fluoride during development

Incremental growth lines of von Ebner

• 5 day cyclic growth lines

• Exaggerated change in fiber orientation

Contour Lines of Owen

• Wider, thicker incremental lines

• Deficiencies during mineralization

Neonatal Line

• Thickest line

• Made during birth

• On all primary teeth and cusp of first permanent molar

Sclerotic Dentin

• Occludes dentin tubules with material

• Appears transparent

• Commonly in apical 1/3 of root and crown

• Increases with age

• Caused by mineral deposition and spread of peritubular dentin

Dead Tract Dentin

• Empty dentinal tubules caused by sclerotic dentin

• Odontoblastic process retracts/dies

• Appears black when transmitted light

• Sealed off by tertiary dentin

Granular Layer of Tomes

• In ground section only

• Granular appearing layer in root dentin between dentin and cementum

• Caused by arrangement of collagenous and noncollagenous proteins

Age effects on Dentin

• Secondary Dentin - Decreases volume of pulp with age and may cause pulp stones

• Tertiary Dentin - Decreases volume of pulp

• Sclerotic Dentin and Dead Tract dentin - May appear

General Enamel Details

• Epithelial origin

• Made by ameloblasts

• Has no cells and can’t self-repair

• 96% mineralized (HA crystal arrangement)

• Permeable

• Can be demineralized/remineralized

• Has long and thin crystals (larger than dentin crystals)

5 Enamel Physical Properties

• Color

  • Birefringence - light double refracts (splits to two rays)

  • Not translucent in young enamel

• Thickness

  • Thicker at crown and thins until it reaches gingival level

• Hardness

  • Mature enamel has higher enamel to protein ratio than earlier stages

  • Hard and brittle

• Solubility

  • Organic acid decalcifies enamel → increases soluble calcium → increases caries susceptibility

• Permeability

  • Permeable to dyes and radioactive isotopes

  • Makes demineralization and remineralization possible

  • Dehydrated enamel looks chalky and lighter in color

Calcium HA Substitutions

• Carbonate replace phosphates

• Magnesium replaces calcium

• Fluoride replaces hydroxyl ion (more resistance from acid dissolution)

Organic matrix composition in enamel

• Proteins (amelogenins and non-amelogenins) and lipids (cell membrane remnants)

• Distributed between HA crystals

Rods and Interrods

• Rods - Long axis

• Interrod - Angled from rod

Enamel Patterns

• Pattern 1 - DEJ and Surface (formed slowly)

• Pattern 3 - Keyhole patterns

  • Head - Occlusal (parallel to long axis)

  • Tail - Cervical (65-70 degree angulation to long axis)

  • 4 Ameloblasts needed to make a single enamel rod

Enamel Rod Orientation and Support

• Perpendicular to DEJ and surface

• Slightly S-shaped

• Enamel rods unsupported by dentin fracture

  • Full-length and shortened enamel rods that are supported will remain

Rod Sheath

• Boundary between rod and interrod containing organic matter

• Has amelogenin and ameloblastin

• Caries penetrate through sheath

Etching Patterns

• Type 1 - Remove rods

• Type 2 - Remove interrods

• Type 3 - Random removal pattern

Cross Striations

• Diurnal (every 24 hours)

• Perpendicular to long axis of enamel prisms

Striae of Retzius

• Weekly

• Striae of Retzius

• Increased organic content

Perikymata

• Imbrication lines of Pickerill

• Found on surface of tooth

Hunter-Schreger Band

• Optical phenomenon making light and dark bands

Gnarled Enamel

• Complex twisting of rods near DEJ

• Harder and can break burs

• Resists masticatory load at cusps/incisal edges

Non-prismatic enamel

• Outer layer of enamel

• First formed and is highly mineralized due to lack of organic material and rods

• Caused by absence of Tomes process of ameloblasts in first and last stages of enamel deposition

Enamel Spindles

• Elongated odontoblastic process extending into enamel

Enamel Tuft

• Tuft of grass in DEJ

• Contains organic enamel proteins

• Developmental origin

Enamel Lamellae

• Crack across enamel

• Contains organic material

• Developmental and post-eruptive origin

Attrition, Abrasion, Erosion

• All cause enamel loss

• Attrition - Tooth to tooth contact

• Abrasion - External object

• Erosion - Acidic agents (including bulimic erosion)

Enamel Coatings

• Developmental coatings

  • REE/Nasmyth’s membrane (primary enamel cuticle) - Worn away after mastication and eating

  • Afibrillar Cementum (Secondary enamel cuticle)

• Acquired coatings

  • Pellicle

  • Plaque

  • Calculus

Orders of DEJ

• 1st order - Scallops

• 2nd order - Microscallops

• 3rd order - Nanostructual interaction between collagen of dentin and enamel

Age effects on enamel

• Attrition

• Decreased permeability

• Stains and dark color

• Less water content

• More brittle

• Increase fluoride use and reduced complex carbs → Decrease in caries

Enamel Disturbances during Tooth Development

• Febrile disease

• Tetracycline defects (bands of color)

• Fluoride-induced defects (hypermineralized)

• Ectodermal dysplasia (can cause loss of teeth or modified tooth shape)

• Amelogenesis Imperfecta (teeth are colored, sensitive, and at risk of decay)

• Enamel pearl (Enamel sphere in crown or root furcation of teeth)

• Enamel in furcation (enamel surface extends to root furcation)