dental anatomy: enamel

ENAMELDr. Mohsen S. MohamedBDS, Misr International Universtiy, Cairo, Egypt.Certification, Universitätsklinikum Carl Gustav Carus.Owner and Author of OziDent.com

Dental Anatomy

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Physical Characteristics

1. Forms a protective covering (2 mm – knife edge).

2. Forms a resistant covering (suitable for mastication).

3. The hardest calcified tissue in human body.

4. Brittle.5. The specific gravity is 2.8.6. Acts as semipermeable membrane.7. Color: yellowish white to grayish white.

Tooth Layers

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longitudinal cross section of the Tooth showing :

Enamel, Dentine, Pulp and Cementum

Chemical Properties

• Inorganic materials (apatite crystals) 96%

By weight

• Organic substances and water 4%

• In volume the organic matter and water are nearly equal to the inorganic contents.

Structure

I. Prisms or rods.II. Rod sheath.III. Inter-prismatic substance.IV. Striations.V. Direction of rods.VI. Hunter-Schreger bands.VII. Incremental lines.VIII. Surface structures.IX. Enamel lamellae.X. Enamel tufts.XI. Dentino-enamel junction.XII. Odontoblastic processes and enamel spindles.

Enamel Rods or Prisms

Characteristics

Number: 5 – 12 millions.

Direction: Run in oblique direction and wavy course.

Length: greater than the thickness of E.

Diameter average: 4 µm.

Appearance: Have a clear crystalline appearance.

Cross-section: hexagonal, round, oval, or fish scales.

Enamel Rods

Submicroscopic Structure Of Enamel Rods

Keyhole or paddle-shaped. Separated by interrod substance. About 5 µm in breadth and 9 µm in length. The bodies are near the occlusal or incisal

surface. The tails point cervically. The crystals; parallel to the long axis of

the prism heads. Deviate about 65° from the tails.

Keyhole shaped E. rodsHexagonal ameloblasts

Note crystal orientation

Enamel Rod’s Shape

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Crystals in rod and inter-rod enamel are similar in structure but diverge in orientation

Enamel Crystal

Crystals length: 0.05 – 1 µm.

Thickness: about 300 A°.

Average width: about 900 A°.

Cross sections: somewhat irregular.

Enamel Crystal

Longitudinal Section Transverse Section

A thin peripheral layer.

Darker than the rod.

Relatively acid-resistant.

Less calcified and contains more organic matter than the rod itself.

Electron Microscope : often incomplete.

The Rod Sheath

•Cementing E. rods together.

•More calcified than the rod sheath.

•Less calcified than the rod itself.

•Appears to be minimum in human teeth.

Inter-prismatic Substance

•E. rod is built-up of segments (dark lines).

•Best seen in insufficient calcified E.

•Represent rhythmic manner of E. matrix formation.

•Segment length: about 4 µm.

Striations

Cross-striations

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•Usually at right angles to the D. surface.

•Follow a wavy course in clockwise and anticlockwise deviation.

•At the cusps or incisal edges: gnarled enamel.

•At pits and fissures: rods converge in their outward course.

Direction of Rods

Direction of Enamel Rods

•Alternating dark and light strips.

•Have varying width.

•Seen in large ground section (oblique reflected light).

•Originate from the DEJ.

Hunter-Schreger Bands

Hunter-Schreger Bands

Hunter-Schreger Bands

Hunter-Schreger Bands

This is Due to:

1. Change in the direction of E. rods.

2. Variation in calcification of the E.

3. Alternate zones having different permeability and organic material.

4. Optical phenomenon.

A. Incremental Lines of RetziusB. Neonatal Line

Incremental Lines

Incremental Lines of Retzius: Brownish bands in ground sections.

Reflect variation in structure and mineralization.

Broadening of these lines occur in metabolic disturbances.

Etiology

1. Periodic bending of E. rods.

2. Variation in organic structure.

3. Physiologic calcification rhythm.

Incremental Lines of Retzius:

Neonatal Line

The E. of the deciduous teeth and the 1st permanent molar develop partly before birth and partly after birth, the boundary between both is marked by neonatal line or ring.

Etioloyg

Due to sudden change in the environment and nutrition.

The antenatal E. is better calcified than the postnatal E.

Neonatal Line

SURFACE STRUCTURES

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Surface Structures

a. Structureless layer (E. skin)

b. Perikymata

c. Rod ends

d. Cracks

e. Enamel cuticle

a. Structureless layer

About 30 µm thick.

In 70% permanent teeth and all deciduous teeth.

Found least often over the cusp tips.

Found commonly in the cervical areas.

No E. prisms.

All the apatite crystals area parallel to one another

and perpendicular to the striae of Retzius.

More mineralized than the bulk of E. beneath it.

b. Perikymata

Transverse wave like grooves.

Thought to be the external manifestation of the striae of Retzius.

Lie parallel to each other and to CEJ.

Number:

About 30 perik./mm at the CEJ.

About 10 perik./mm near the incisal edge.

Their course is regular, but in the cervical region, it may be quite irregular.

Powdered graphite demonstrates them.

It is absent in the occlusal part of deciduous teeth but present in postnatal cervical part (due to undisturbed and even development of E. before birth)

The relationship between the striae of Retziuz and surface perikymata

Striae of Retziuz Perikymata

c. Rod ends

Are concave and vary in depth and shape.

Are shallow in the cervical regions.

Deep near the incisal or occlusal edges.

Rod ends

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d. Cracks

Narrow fissure like structure.

Seen on almost all surfaces.

They are the outer edges of lamellae.

Extend for varying distance along the surface.

At right angles to CEJ.

Long cracks are thicker than the short one.

May reach the occlusal or incisal edge.

Cracks

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e. Enamel cuticle

1. Primary E. cuticle (Nasmyth’s membrane).

2. Secondary E. cutile (afibrilar cementum).

3. Pellicle (a precipitate of salivary proteins.

Primary enamel cuticle

Covers the entire crown of newly erupted tooth.

Thickness: 0.2 µm. Removed by mastication (remains intact

in protective areas). Secreted by postamloblasts. EM: similar to basal lamina.

Secondary enamel cuticle

Covered the cervical area of the enamel. Thickness: up to 10 µm. Continuous with the cementum. Probably of mesodermal origin or may be

elaborated by the attachment epithelium. Secreted after E.O. retracted from the

cervical region during tooth development.

Pellicle

Re-form within hours after mechanical cleaning .

May be colonized by microorganisms to form a bacterial plaque.

Plaque may be calcified forming calculus.

ENAMEL LAMELLAE Created For www.Ozident.com

Enamel Lamellae

Are thin, leaf like structures,

Develop in planes of tension.

Extends from E. surface towards the DEJ.

Confused with cracks caused by grinding (decalcification).

Extend in longitudinal and radial direction.

Represent site of weakness in the tooth and three types; A, B, and C.

Enamel Lamellae

Type A Type B Type C

Consistency Poorly calcified rod seg.

Degenerated cells Organic matter from saliva

Tooth Unerupted Unerupted Erupted

Location Restricted to the E. Reach into the D. Reach into the D.

Occurrence Less common Less common More common

Enamel Lamellae

Enamel Lamellae

ENAMEL TUFTS Created For www.Ozident.com

Enamel Tufts

Arise from DEJ. Reach to 1/5 – 1/3 the thickness of E. In ground section: resemble tufts of grass. Do not spring from a single small area. The inner end arises at the dentin. Consist of hypocalcified E. rods and

interprismatic substance. The extend in the direction of the long axis of

the crown (best seen in horizontal sections).

Enamel Tufts

Enamel Tufts

DENTINO-ENAMEL JUNCTION

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Dentino-Enamel Junction

Scalloped junction – the convexities towards D.

At this junction, the pitted D. surface fit rounded projections of the enamel.

The outline of the junction is performed by the arrangement of the ameloblasts and the B. M.

Dentino-Enamel Junction

ODONTOBLASTIC PROCESSES AND ENAMEL SPINDLES

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Odontoblastic Processes and Enamel Spindles

The odontoblasts processes may cross DEJ (before the hard substance is formed) to the E. and ends as E. spindles.

They are filled with organic matter. The processes and spindles are at right angle

to the surface of the dentin. The direction of spindles and rods is divergent. Spindles appear dark in ground sections under

transmitted light.

Odontoblastic Processes and Enamel Spindles

LIFE CYCLES OF THE AMELOBLASTS

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Life Cycles of the Ameloblasts

According to their function, can be divided into six stages:

1. Morphogenic stage.

2. Organizing stage.

3. Formative stage.

4. Maturative stage.

5. Protective stage.

6. Desmolytic stage.

1

2

3

4 5

6

Amelogenesis

1. Organic matrix formation (follows incremental pattern – brown striae of Retzius).

2. Mineralization.

Organic Matrix Formation

a. Amelodentinal membrane.

b. Development of Tome’s processes.

c. Distal terminal bars.

d. Ameloblasts covering maturing enamel.

dpTP=distal portion of Tome’s process

ppTP=proximal portion of Tome’s process

Sg=secretory granules(E. protein)

Organic Matrix Formation

Ameloblasts are perpendicular to the rods

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(arrow=cell membrane, p=Tome’s process, s=incomplete septum)

Depression in enamel surface which were occupied by Tome’s processes

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Mineralization

a. Partial mineralization (25-30%).

b. Maturation (gradual completion of mineralization).

Crystal Mineralization

Recently formed crystals Mature crystals

Abnormalities

Interference during E. matrix formation may cause Enamel hypoplasia.

Interference during Enamel maturation may cause Enamel hypocalcification.

Each condition may be caused by systemic, local, or hereditary factors.

Abnormalities

Enamel Hypocalcification Enamel Hypoplasia

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