1. Tissue of the teeth
Dr Jamal Naim
PhD in Orthodontics
Enamel

2. Amelogenesis Enamel formation Enamel maturation
Maturation means also mineralization and does not wait the complete formation of Enamel.

3. Amelogenesis Enamel formation: Dentino-enamel membrane:
Induction after dentin formation (first layer is called mantel dentin)
Enamel proteins formed in RER and transported to the distal end with secrete granules and released extracellulary
This is the aprismatic Enamel (dentino-enamel membrane)

4. Initial and final enamel formation

5. Amelogenesis Enamel formation: Tomes process:
After formation of the DEM, the ameloblasts will move away from dentin and develop the TP.
TP will secrete the granules perpendicular to the membrane of the TP
This direction is responsible for the appearance of the enamel prismatic and inter-prismatic substance.

6. Amelogenesis Tomes process Rod enamel Tomes process Interrod enamel
Rod sheath

7. Amelogenesis Enamel maturation:
Initial or partial mineralization (immediate) during matrix formation, 25-30% of the total mineralization.
In a second stage the mineralization will be completed, about 96% inorganic substance.

8. Amelogenesis Enamel maturation:
From cusp tip or incisal edge and progress cervically
The maturation of the crystals begins at its dentinal end and progress to the outer surface.
At first parallel to D.E.J and later to the outer surface of enamel
Maturation occurs by the growth of the primary crystals till they fuse together
The fibrils between the crystals will become thinner.

9. Maturation occurs by the growth of the primary crystals till they fuse together

10. Amelogenesis
After eruption, the maturation continues by deposition of ions from saliva to reach 96 % of its weight inorganic substance

11. Properties & histological structure

12. COLOUR
THICKNESS
Physical
properties
of Enamel
PERMEABILITY
HARDNESS
BRITTLNESS

13. COLOUR Yellowish white to grayish white; it depends on: Translucency
Degree of calcification
Homogeneity
So:
Yellowish enamel is more translucent, better calcified and homogenous.
Grayish enamel is opaque, less calcified and less homogenous

14. COLOUR

15. THICKNESS
It is thick at the incisal edge and cusp tip of molars and premolars (2-2.5 mm) and ends cervically as knife edge.

16. Hardness and Brittleness
It is the hardest calcified tissue in human body because of its high calcification and crystal orientation.
It is greater at the outer surface and decrease at the DEJ.
It is greater at the cusp tip or incisal edge and decrease at the cervical line.
Although of its hardness, enamel is brittle especially when looses the underlying elastic healthy dentin.

17. Permeability
Enamel acts as a semi-permeable membrane for certain ions from:
the saliva to the outer layer of enamel
the pulp to the inner layer of enamel across dentin.

18. Histological Structure
Ground section
Decalcified section
the organic substance is burnt and the inorganic substance remain
the inorganic substance is dissolved and the organic substance remain

19. Histological Structure
Enamel is formed of:
Enamel crystallites
Enamel Rod
Rod Sheath (packaging of rods) and
Interrod Substance

20. Histological Structure
Enamel crystallites
crystallites are the smallest units of enamel
They are hexagonal in form
about 160 nm in length and nm in width

21. Histological Structure
Rod and interrod enamel:
The Tomes’ process organizes enamel crystallites into rod enamel (prism = about 100 crystallites) and interrod enamel.
Enamel crystallites that elongate near the tip of the Tomes’ process form the rod enamel.
Crystallites that lengthen near the intercellular junctions form the interrod enamel.
The rod enamel and interrod enamel differ in the orientation of their crystallites

22. Histological Structure
Tomes process
Rod enamel
Tomes process
Interrod enamel
Rod sheath

23. Histological Structure
The border between rod and interrod enamel is distinct because part of the ameloblast membrane is “nonsecretory,” which creates gaps in the mineralization front.
The apatite crystals are oriented parallel to the long axes of the rod in its body and deviate about 65 as they fan out into the margin and the tail.

24. Histological Structure
The rod and interrod enamel differ in the orientation of their crystallites

25. The enamel rods are arranged in rows with alternating orientation.
The interrod enamel

26. Histological Structure
By electron microscopy a common key-hole or paddle-shape rod is seen in cross section.
interrod enamel
rod enamel

27. Histological Structure
The head of the rod is toward the occlusal or incisal surface where the tail is cervically.
coronal
cervical

28. Histological Structure
The number of the enamel rods varies from 5 millions in lower lateral incisor to 12 millions in the upper first permanent molar.
The number of the rods equals the number of the ameloblasts.
At the tooth surface there are about enamel rods in 1 mm2
The density of the rods is at the DEJ about 10% higher than at the enamel surface.

29. Histological Structure
The diameter of the enamel rod is about 5 µm. In key-hole type the height is about 9 µm.
It increases from the dentino-enamel junction to the outer enamel surface by a ratio of 1:2.

30. Histological Structure
The enamel rod is perpendicular to the dentin surface;
In deciduous teeth ; the enamel rod is vertical at the cusp tip or incisal edge then become oblique toward the occlusal surface at the middle part and become horizontal at the cervical area (so enamel ends cervically abruptly).

31. Histological Structure
In permanent teeth; the direction of the enamel rods are similar to that of the deciduous teeth at the occlusal 2/3 but at the cervical region are directed root wise (so the enamel ends cervically as a knife edge).
permanent teeth
deciduous teeth

32. Histological Structure
Course: the enamel rod starts straight at dentino-enamel junction (D.E.J.) for about 30 µ then has a wavy course till near the outer surface of enamel where it become straight once more.
Wavy course of enamel rods

33. Histological Structure
Wavy course of enamel rods

34. Enamel Histology
At the incisal edge or cusp tip the enamel rod has a twisted course and is called gnarled enamel
Twisted course of enamel rods
Gnarled enamel D

35. Histological Structure
Dentino-enamel-Junction
In ground section D.E.J. appears as scalloped line.
The presence of this irregular surface assures the union between enamel and dentine.

36. Histological Structure
Dentin
Enamel
Dentino-enamel-Junction

37. Electron micrographs of the Dentino-enamel-Junction
The surface of Dentin after removing of Enamel

38. Histological Structure
Enamel spindle:
It is an odontoblastic process which extends in between the cells of inner dental epithelium before the formation of enamel.

39. Histological Structure
Enamel Tufts:
They arise from D.E.J. to about 1/5 to 1/3 of the enamel thickness as tufts of grass.
It always appears in transverse ground section.
They are hypo-calcified prisms and inter-prismatic substance.
It takes this shape because of the wavy course of the enamel rod for several layers leading to this tuft form.

40. Histological Structure
Enamel Tufts

41. Histological Structure
If a tuft runs till the outer enamel layers (more than 1/3 of enamel thickness), then we speak about Enamel lamella (true lamella, Type A lamella).
They are also hypo-calcified prisms and inter-prismatic substance.
True lamella should be distinguished from other types of lamellae (cracks, type B and type C lamella)
Enamel lamella

42. Histological Structure
Enamel lamella
type A,
True lamella
Enamel lamella
type B
Enamel lamella
type C
hypo-calcified prisms and inter-prismatic substance
Enamel cracks post-eruptive, can reach dentine
Enamel cracks pre-eruptive,
can reach dentine
Filled with epithelial cell or connective tissue
Filled with organic contents of saliva
It is limited to enamel

43. Histological Structure
To differentiate between true lamella and crack we do careful decalcification:
the true lamella will remain where cracks will disappear.
Enamel lamella could act as caries spread way.

44. Histological Structure
During the secretory stage, enamel crystals do not grow continuously, but rather extend in increments.
The enamel rod is formed in a rhythmic manner, every segment of 4 µm in length and formed in a day. It is manifested structurally as prism cross-striations

45. Histological Structure
cross striations
4 µ/day

46. Histological Structure
More prominent cross-striations occur in a regular period of about every 4 days/1 week and are known as striae of Retzius or incremental lines.
In longitudinal ground section they appear as dark bands reflecting the rhythmic enamel formation.
At the incisal edge and cusp tip they arise from the D.E.J. then go upward and outward surrounding the tip of dentine and come to D.E.J. again so, they do not reach the outer surface of enamel.

47. Histological Structure
striae of Retzius
perikymata

48. Histological Structure
striae of Retzius

49. Histological Structure
striae of Retzius

50. Histological Structure
striae of Retzius

51. Histological Structure
Neonatal line:
It is an enlarged stria of Retzius which present in all deciduous teeth and 1st permanent molar.
This line separates between enamel formed before birth and enamel formed after birth.
This line is due to the sudden change of nutrition and environment due to birth.
The quality of enamel formed before birth is better than that formed after birth, because of the more protected conditions and constant nutrition of the fetus.

52. Histological Structure
Prenatal enamel
Neonatal line
Postnatal enamel

53. Histological Structure
Hunter - shreger bands :
It is an optical phenomenon caused by changes of rod direction (the wavy course).
They are seen clearly by longitudinal ground section viewed by reflected light at cervical 2/3.
The phenomenon appears as dark and light alternating bands, starting from the D.E.J. and ends shortly from the outer surface of enamel where the enamel rods run straight to the surface.

54. Histological Structure
Hunter - shreger bands

55. Histological Structure
The dark bands (Diazones) absorb the light where the light bands (Parazones) reflect the light.
If the light passes from the opposite side, the light and dark bands will be reversed.

56. Histological Structure
The external manifestation of the incremental lines of Retzius represented as transverse wave like grooves on the surface of the enamel are known as perikymata.

57. Histological Structure
Structure less enamel:
This layer is highly mineralized than the rest of enamel and its thickness is 30 microns.
This surface layer of enamel is aprismatic.

58. Histological Structure
A fibrilar cementum:
A type of cementum formed on the cervical part of the enamel for a short distance.
It is formed due to the degeneration of the reduced dental epithelium covering the cervical area of the enamel before eruption.

59. Histological Structure
Enamel
Dentin
A fibrilar cementum
cementum