Enamel development

• ENAMEL

Dr asmaa s

• PHYSICAL CHARACTERISTICS

• Hard, brittle, totally acellular , highly mineralized
• Secretory product of stratified squamous epithelium
• Calcified tissue
• Hydroxyapatite crystal arrange in prism or rods Density:-
• Decreases from the surface of enamel to the dentino-enamel junction.
• Thickness:-
• Thickness over the cusps of the molars where it measures 2.5 mm & incisal edges of incisors where it is
• 2.0 mm.

• ENAMEL

• Forms a protective covering (2 mm – knife edge).
• Forms a resistant covering (suitable for mastication).
• The hardest calcified tissue in human body.
• enamel is very brittle but the underlying dentin provides some resilience
• Acts as semipermeable membrane (selectively permeable).
• Color: yellowish white to grayish white depends on translucency.

•  Enamel gains mechanical strength by interweaving HAP crystals

•  Enamel rod – 5-12 million/tooth
•  Appatite crystal is hexagonal
•  Enamel initially starts with a high protein content, but these are removed and the voids backfilled with HAP as the tooth matures

• CHEMICAL PROPERTIES

•  96% inorganic - by weight
•  inorganic crystalline calcium phosphate – hydroxyapatite
•  various ions like strontium, magnesium, lead and fluoride are present at some point during enamel formation

• ENAMEL STRUCTURE

• 1)
• Enamel (rods(prisms)
• Rod sheaths
• Inter-rod substance.

• CROSS SECTION

•  Cross section of enamel rod shows the key hole pattern
•  Head represents the rod and key shows the inter rod region
•  Head is directed towards the occlusal aspect and tail towards the cervical region of the tooth

• CROSS SECTION OF ENAMEL

• ENAMEL

• CHARACTERISTICS - ENAMEL ROD/PRISM

• Number: 5 – 12 millions.
• Direction: Run in oblique direction and wavy course.
• Length: greater than the thickness.
• Diameter average: 4 µm.
• Appearance: Have a clear crystalline appearance.
• Cross-section: hexagonal, round, oval, or fish scales.

• Enamel Rod: Basic Structural Unit

• Cross section






•  Head of enamel rod is formed by one ameloblast and tail is formed by three ameloblasts
•  Thus each rod is formed by four ameloblasts

• 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.

• ROD SHEATH

•  the boundary between rod and interrod is delimited by a narrow space containing organic material – rod sheath
• 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.

• ALTERNATING ROD DIRECTIONALITY

• Hunter Schreger bands are alternating light and dark bands seen in a section of enamel when cut longitudinally and illuminated in a special way.
• The bands are produced by the orientation of groups of rods.
• If the light passes through rods cut in cross-section, the band appears light.
• If the light passes through rods cut in longitudinally, the band appears dark.

• STRIATIONS

•  E. rod is built-up of segments (dark lines).
•  Best seen in insufficient calcified E.
•  In a longitudinal section dark lines are seen that shows the daily deposition of enamel (rhythmic manner of E. matrix formation).
• These lines are known as cross striation
•  Segment length: about 4 µm.

• CROSS-STRIATIONS

• Cross striations

• DIRECTION OF RODS

•  Near the edge or cusp tip they are oblique
•  At the cusp tip they are almost vertical
•  Run from DEJ to surface of enamel
•  Usually at right angles to the Dentin surface.
•  Follow a wavy course in clockwise and anticlockwise deviation full thickness of enamel
•  At the cusps or incisal edges: gnarled enamel.
•  At pits and fissures: rods converge in their outward course.

• STRAIGHT ENAMEL RODS -LONGITUDINAL LABIOLINGUALSECTION

•  The enamel rods project in the direction of the arrow.
•  Can you see the striaof Retzius?

• Wavy course of enamel rod

• A more spiral course is noted at cusps & incisal areas Gnarled enamel

• GNARLED ENAMEL

•  Enamel rods are general not straight throughout their length.
•  In the cuspal region, the rods
• are very wavy.
•  This is referred to as gnarled enamel.
•  In this section, you can see the end of an odontoblasticprocess penetrating the enamel just past the DEJ.
•  This structure is called an
• enamel spindle.
•  Legend
•  Legend: A, Gnarled enamel; B, Enamel spindle

• DIRECTION OF ENAMEL RODS

• HUNTERSCHRAGER BANDS

•  Optical phenomenon seen in reflected light
•  Alternate light and dark bands
•  Seen in ground longitudinal section
•  Due to abrupt change in the direction of enamel rod
•  Originate from the DEJ.

• HUNTER-SCHREGER BANDS

• HUNTER-SCHREGER BANDS

• ENAMEL -TRANSVERSE GROUND SECTION

•  In a transverse section of tooth, the stria of Retzius appear as concentric bands parallel to the dentino-enamel junction (DEJ). In addition to the "hypo-mineralized" dark striaof Retzius, there also exist hypo-mineralized areas perpendicular to the DEJ. These are enamel lamellae(that traverse the entire thickness of enamel) and enamel tufts(that traverse the inner third of enamel adjacent to the DEJ

•  sectionLegend: A, Striaof Retzius; B, Enamel tuft; C, Enamel lamella; D, DEJ

• STRAE OF RETZIUS

•  Incremental lines of growth
•  Eccentric growth rings
•  DEJ to outer surface of enamel
•  Where they end as shallow furrows known as perikymata

• INCREMENTAL LINES OF RETZIUS:

• NEONATAL LINE

•  The E. of the deciduous teeth and the 1st permanent molar (It is incremental line that is the boundary between the enamel forms before and after the birth)
•  The neonatal line is usually the darkest and thickest striaof Retzius.
•  Etiology
•  Due to sudden change in the environment and nutrition.

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

• NEONATAL LINE

• 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

•  In this ground cross- section of tooth, you can see enamel lamellae and enamel tufts You can also see the neonatal line.
•  •What do all three of these structures have in common?
•  Answer: They are all hypocalcified.
•  Legend: A, Enamel lamella; B, Enamel tuft; C, Neonatal line

• ENAMEL LAMELLAE

• ENAMEL LAMELLAE

•  Enamel tufts are less mineralized areas of enamel in the inner third of enamel adjacent to the DEJ. They resemble tufts of grass.
•  •They are wavy due to the waviness of the adjacent rods.
•  •Structures rich in organic matter (i.e. less mineralized) that project to the surface of the enamel are enamel lamellae.
•  Legend: A, Enamel tufts; B, Enamel lamella

• ENAMEL TUFTS -TWO PLANES OF FOCUS

•  Enamel tufts consist of several unconnected "leaves" of hypo- calcified enamel.
•  •They display a wavy twisted appearance.
•  •Enamel spindles are the processes of odontoblastsprojecting into the enamel.
•  Legend: A, Enamel spindle; B, Enamel tuft

• ENAMEL TUFTS

• ENAMEL TUFTS

• 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

• ENAMEL SPINDLES

• Odontoblast processes usually end at the DEJ. However, sometimes the ends of the process become embedded in the enamel as it forms.
• These very small, usually straight structures that you can see adjacent to the DEJ are enamel spindles.
• They are only about one tenth the length of an enamel tuft. Legend: A, Enamel
• spindle;
• B:Odontoblastprocesses in dentin

•  Legend: A, Enamel spindle; B, Odontoblastprocess; C,

Enamel rod

• ODONTOBLASTIC PROCESSES AND ENAMEL SPINDLES

• THE RELATIONSHIP BETWEEN THE STRIAE OF RETZIUZ AND SURFACE PERIKYMATA

• PERIKYMATA (IMBRICATION LINES) ARE EXTERNAL MANIFESTATIONS OF RETZIUS STRIAE

• 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

• 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

• LIFE CYCLES OF THE AMELOBLASTS

•  According to their function, can be divided into six stages:
• Morphogenic stage.
• Organizing stage.
• Formative stage.
• Maturative stage.
• Protective stage.
• Desmolytic stage.

• Morphogenic stage.

•  React by differential growth
•  Produce shape of the crown
•  Terminal bar appears
•  Basal lamina separates the inner enamel epithelium and cells of the dental papilla
•  Pulpal layer adjacent to the basal lamina is a cell free zone
•  At cervical region – cell is relatively undifferentiated

• Organizing stage.

•  Inner enamel epithelium interact with the cells of dental papilla which differentiate into odontoblast
•  Cells become elongated
•  Proximal part contain nuclei
•  Distal end is nucleus free zone
•  Dentin formation begins
•  Cell free zone disappear

•  As dentine is formed nutrition supply of the inner enamel epithelium changes from dental papilla to the capillaries that surround the outer enamel epithelium
•  Reduction and gradual disappearance of the stellate reticulum

• Formative stage.

•  Formatve stage starts After the dentine formation
•  Enamel matrix formation starts
•  Development of blunt cell process on the ameloblast surface which penetrate the basal lamina and enter the predentin

• Maturative stage.

•  Maturation starts after most thickness of enamel matrix formation in occlusal and incisal area. In cervical area matrix formation is still in progress
•  Ameloblast reduce in length
•  Cells of stratum intermedium takes spindle shape

• Protective stage.

•  After enamel calcification cells on ameloblast can no longer be differentiated from stratum intermedium and outer enamel epithelium
•  These layer forms reduced enamel epithelium
•  Protect the enamel from connective tissue until the tooth erupts, if it contacts then anomalies develop enamel may be resorbed or cementum cover may form (afibrillar cementum)

• Desmolytic stage.

•  Reduced enamel epithelium induces atrophy of connective tissue separating it with oral epithelium thus fusion of the two epithelia can occur
•  Premature degeneration of the reduced enamel epithelium may prevent the eruption of he tooth

• AMELOGENESIS

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

• Mineralization.

• dpTP=distal portion of Tome’s process

• ppTP=proximal portion of Tome’s process

• Sg=secretory granules(E. protein)

• ORGANIC MATRIX FORMATION

• AMELOGENESIS

• Schematic representation of the various functional stages in the life cycle of ameloblasts as would occur in a human tooth.
• Morphogenetic stage;
• histodifferentiation stage;
• initial secretory stage (no Tomes’ process);
• secretory stage (Tomes’ process);
• ruffle-ended ameloblast of the
• maturative stage;
• 6. smooth-ended ameloblast of the maturative stage;
• 7.protective stage.

• AMELOGENESIS

• RUFFLED AND SMOOTH AMELOBLASTS