Enamel is the hardest and most highly mineralized tissue in the human body. It forms a protective covering on the crown of teeth. Enamel is composed primarily of hydroxyapatite crystals arranged in prisms or rods. The basic structural unit of enamel is the enamel rod or prism, which are formed by ameloblasts during enamel development. Enamel rods extend from the dentinoenamel junction to the outer enamel surface in a wavy pattern.

1. ENAMEL
‫الرحيم‬ ‫الرحمن‬ ‫هللا‬ ‫بسم‬
Upload By : Ahmed Ali Abbas
Babylon University College of Dentistry
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2. 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.

3. ENAMEL
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. enamel is very brittle but the underlying dentin
provides some resilience
5. Acts as semipermeable membrane (selectively
permeable).
6. Color: yellowish white to grayish white
depends on translucency.

4.  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

5. 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

6. ENAMEL STRUCTURE
1) Enamel
rods(prisms)
2) Rod sheaths
3) Inter-rod substance.

7. 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

8. CROSS SECTION OF ENAMEL

9. ENAMEL

10. 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.

11. • Enamel Rod: Basic Structural
Unit
Cross section

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

13. 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.

14. 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.

15. 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.

16. 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.

17. CROSS-STRIATIONS
Cross striations

18. 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.

19. STRAIGHT ENAMEL RODS -LONGITUDINAL
LABIOLINGUALSECTION
 The enamel rods
project in the direction
of the arrow.
 Can you see the striaof
Retzius?

20. Wavy course of enamel rod
• A more spiral course is noted at cusps & incisal areas Gnarled enamel

21. 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

22. DIRECTION OF ENAMEL RODS

23. 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.

24. HUNTER-SCHREGER BANDS

25. HUNTER-SCHREGER BANDS

26. 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

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

28. 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

29. INCREMENTAL LINES OF RETZIUS:

30. 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.

31. NEONATAL LINE

32. 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.

33. 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

34. ENAMEL LAMELLAE

35. ENAMEL LAMELLAE

36.  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

37. 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

38. ENAMEL TUFTS

39. ENAMEL TUFTS

40. 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.

41. DENTINO-ENAMEL JUNCTION

42. 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

43.  Legend: A, Enamel
spindle; B,
Odontoblastprocess; C,
Enamel rod

44. ODONTOBLASTIC PROCESSES AND
ENAMEL SPINDLES

45. THE RELATIONSHIP BETWEEN THE STRIAE OF
RETZIUZ AND SURFACE PERIKYMATA

46. PERIKYMATA (IMBRICATION LINES)
ARE EXTERNAL MANIFESTATIONS
OF RETZIUS STRIAE

47. C. ROD ENDS
 Are concave and vary in depth and
shape.
 Are shallow in the cervical regions.
 Deep near the incisal or occlusal
edges.

48. ROD ENDS

49. 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.

50. CRACKS

51. 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.

52. 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

53. 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

54.  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

55. 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

56. 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

57. 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)

58. 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

59. AMELOGENESIS
1. Organic matrix formation (follows
incremental pattern – brown striae of
Retzius).
2. Mineralization.

60. dpTP=distal portion of Tome’s process
ppTP=proximal portion of Tome’s process
Sg=secretory granules(E. protein)
ORGANIC MATRIX FORMATION

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

62. AMELOGENESIS

63. RUFFLED AND SMOOTH AMELOBLASTS