1. The Enamel By
Dr. Suhair Majeed

2. The Enamel
Enamel, the hard protective substance that covers the crown of the tooth, is the hardest biologic tissue in the body.
It is able to resist fractures during the stress of mastication.
Enamel provides shape and contour to the crowns of teeth and covers the part of the tooth that is exposed to the oral environment.

3. Cont.
The cells that are responsible for formation of enamel, the ameloblasts, are lost as the tooth erupts into the oral cavity, and hence enamel cannot renew itself.

4. AMELOGENESIS (ENAMEL FORMATION)
The formation of any hard tissue involves the laying down of an organic matrix and mineralization of this matrix. Enamel is different from other hard tissues, which are all derived from connective tissue. Enamel is ectodermal in origin; it has a unique organic matrix and a different pattern of mineralization .

5. Cont.
Amelogenesis (enamel formation) is a complex process. Two processes are involved in the formation of enamel: A. Organic matrix formation, B. Mineralization and maturation of enamel matrix.

6. A- Organic Matrix Formation
The ameloblasts start their secretory activity after a thin layer of dentin has been laid down, so a thin continuous layer of enamel is formed along the dentin. This is called the dentinoenamel membrane.
The freshly secreted E. matrix contain 30% minerals as hydroxyapatite crystals,and 70% water ,and E. proteins.

7. Cont.
E.proteins include : - 90% amelogenin protein ,and - 10% nonamelogenins protein ( enamelin and ameloblastin). These E. proteins which are secreted by ameloblasts are responsible for creating and maintaining an extracellular environment favorable to mineral deposition. Amelogenins play important role in maintaining the spaces between the crystals.

8. Cont.
Ameloblastin and enamelin help in crystal growth and nucleation. When the first layer of E. is laid down , ameloblasts will begin to retreat from DEJ towards E. surface and begins to secrete the next layer of Enamel. Enamel matrix appear as a deep staining layer in H& E staining sections. Ameloblasts secrete the E. in rods or prisms , the intially secreted E. matrix is described as rodless or prismless E.

9. B- Mineralization and Maturation of the Organic Enamel Matrix
Mineralization started when the full thickness of E. matrix has been deposited. This process involved additional minerals with the removal of organic material and water to reach 96% mineral content. This minerals makes the initial E. crystals that formed in first stage to grow wider and thicker due to the deposition of large amount of hydroxy apitite crystals.

10. Cont.
the source of minerals during maturation are from : 1- ameloblast itself ,and other cells of E. organ like stratum intermedium. 2- Capillaries which are approach to the outer E. epith. and is very close to the enamel.

11. Life cycle of Ameloblasts
Ameloblasts show different morphological features and organelle content during different stages of enamel formation.As , ameloblasts are assigned to stages that start befor enamel apposition and continue through enamel formation as well as after enamel is completely formed. These stages collectively are referred to as life cycle of ameloblasts , which include the following stages :

12. Cont. a-Morphogenic stage :
1- presecretory stage :
a-Morphogenic stage :
during this stage ,inner E. epith. Begins it’s differentiation first into pre-ameloblasts which induce adjacent dental papilla cells to differentiate into odontoblasts which form dentin . At this stage inner E. epith. consists of cuboidal cells with centrally located nuclei.

14. B- differentiating stage :
the inner E. epith. Cells grow into columnar cells with more organelles mostly protein synthesizing organelles. At this stage, the polarity of the cells is reversed , because Golgi complex shifts from a proximal to distal position and the nuclei of cells shift to proximal side towards stratum intermedium.

16. 2- Secretory stage :
When the first layer of dentin is formed ,it induces the adjacent preameloblasts to complete their differentiation into ameloblasts which secrete enamel. Secretory ameloblasts are polarized tall columnar cells with Tome’s processes ( conical shaped processes )at their distal ends .
Tome’s processes interdigitate with the surface of the forming enamel giving it a picket fence appearance.

18. Cont.
Tome’s processes determine the orientation of newly formed (nucleated) enamel hydroxyapatite crystals .
Organelle content of secretoty ameloblasts is mainly protein synthesizing organelles i.e. Golgi complex and granular endoplasmic reticulum. Numerous mitochondria and secretory granules are also present.
Junctional complexes,tight junctions and desmosomes are present at the distal and proximal ends of ameloblasts.

19. Tome’s processes

20. Cont.
also, desmosomes and gap junctions are present along their lateral surfaces. As the secretory stage is ending Tome’s processes are lost and accordingly the last formed layers of enamel are rodless or prismless.

21. 3- Transitional stage :
when enamel reaches it’s full thickness ,ameloblasts enter a brief transitional stage. Their height is decreased and protein synthesizing organelles are completely reduced. Many lysosomes and autophagic vacuoles are also present. The overall number of ameloblasts is reduced by programmed cell death (apoptosis) and it is estimated that by the end of this stage of ameloblast population is reduced by as much as 50%.

22. 4- Maturation stage :
During maturation massive influx of calcium and phosphates occurs and at the same time there is selective loss of proteins , mainly amelogenin and water.
ameloblasts modulate between two phenotypes depending on the morphology of their distal ends. Ameloblasts either have numerous microvilli forming a ruffled border or their distal ends are even straight, thus forming two morphologically different types,named Ruffled – ended ameloblasts

24. Cont.
(80% of maturation ameloblasts ) and smooth –ended ameloblasts (20%) respectively The two morphological types of ameloblasts are grouped into alternating bands during this stage,thus maturation ameloblasts modulate i.e. change their morphology from one type to the other and back.

25. 5- protective stage :
at this stage ,ameloblasts lose their differentiation and become short cuboidal cells which together with the remenants of the other layers of the E. organ from a multilayered structure, named reduced enamel epithelium.
This structure remains on the surface of fully formed enamel untill the tooth erupts. It seperates the enamel from dental sac and thus protect it from being in contact with connective tissue cells in dental sac.

26. Cont.
if this contact accidentally happens, either enamel is resorbed resulting in pitting or dental sac cells in the contact area differentiate into cementoblasts and lay down cementum on the enamel surface. Both produce adverse effects on enamel appearance .the reduced enamel epithelium and the oral epithelium jointly form the dentogingival junction of the erupting tooth.

27. Functions of inner E.epithelial cells in tooth development and amelogenesis
1- establish the crown morphological pattern during bell stage. 2-induce adjacent surface cells of the dental papilla to differentiate into coronal odontoblasts. 3- differentiate into ameloblasts that secrete enamel. 4- differentiate into maturation ameloblasts which exhibit cyclic modulation between ruffle- ended and smooth ended ameloblasts.

28. Cont.
5- form with the outer layers of E. organ ,the reduced enamel epithelium which protects the enamel surface untill the tooth erupts. 6-contribute to the formation of the dentogingival junction of erupted teeth.

29. Enamel structures physical properties of E.
* Enamel is translucent, and varies in color from light yellow to gray-white. * It varies in thickness, from a maximum of approximately 2.5 mm over working surfaces to a feather edge at the cervical line. This variation influences the color of enamel because the underlying yellow dentin is seen through the thinner regions.

30. Cont.
* Enamel is selectively permeable,it’s partially permeable to some fluids, bacteria and other products of oral cavity, it’s due to presence of cracks and microscopic spaces on enamel surface which allows pentration of fluids. The permeability of enamel decreases and hardness increases with age. *very brittle and low tensile strength , thus enamel requires base of dentin to withstand forces

31. Cont.
(If this supportive layer of dentin is destroyed by caries or improper cavity preparation, unsupported enamel fractures easily).
*unlike other calcified structures in the body , E. is unique as it’s totally acellular.
*cracks are often seen in the enamel of teeth.

32. chemical composition of Enamel :
Fully formed enamel consists of approximately 96% mineral and 4% organic material and water .
The inorganic content of enamel is a crystalline calcium phosphate (hydroxyapatite) substituted with carbonate ions. Various ions—strontium,magnesium, lead, and fluoride—if present during enamel formation, may be incorporated into the crystals. The susceptibility of these crystals to dissolution by acid provides the chemical basis for dental caries.

33. Cont.
The organic matrix of enamel is made from non-collagenous proteins and enzymes. 90% of enamel proteins are amelogenins and 10% are non-amelogenins.
The different types of non-amelogenins associated with formation of enamel are ameloblastin ,enamelin and tuftelin. The primary function of organic material is to direct the growth of enamel crystals.

34. Structure of enamel : 1. Enamel rods (prisms) 2. Rod sheaths
Enamel is composed of the following :
1. Enamel rods (prisms)
2. Rod sheaths
3. Inter rod substance (cement)

35. 1- Enamel Rods :
enamel is composed of millions of enamel rods or prisms. An enamel rod is a long, thin structure extending from the dentinoenamel junction to the surface of enamel. The enamel rod follows a tortuous course; thus the length of an enamel rod may be greater than the thickness of enamel. Each rod is formed by four ameloblasts.

36. Cont.
rods in the transverse section are shaped with a rounded head or body section and a tail section. The heads or bodies of the rods are closer to the occlusal or incisal surface, while the tail points in the cervical direction . E. rods connected to one another in such a way that appear like key holes. In a cross section of enamel, many rods resemble fish-scales. the rods are directed at right angles to the dentinoenamel junction and the tooth surface .

37. Enamel rod

38. Cont.
- each rod is made up of millions of crystallites,
- crystals that surround each rod is called inter rod enamel,rod and inter rod enamel is formed from Tome’s process of ameloblasts.
- crystals are long hexagonal needle – like in shape , with average 1600 A length by A
width.

39. fish-scale ( ground section )

40. Transverse section of enamel rods (keyhole shaped rods )

41. 2- Rod sheath :
- the boundary between rod and inter rod enamel is marked by a narrow space filled with organic materials known as rod sheath .
- It is less calcified and contains more organic substance like enamel protein.

42. 3- Inter-rod Substance (Cement)
the rods are cemented together by inter-rod substance, which has slightly higher refractive index than the rods. The crystals are arranged in a different direction in the inter-rod region.

43. Other Structures Hunter-Schreger Bands :
are an optical phenomena and are seen in reflected light, they can be seen in longitudinal sections as dark and light bands. dark bands represent the cross sectional enamel rods ,and light bands represent longitudinally sectioned inter rod enamel.

44. Cont.
these bands occur due to change in the direction of rods that is, one group of rods extends toward the surface with a mesial drift while the adjacent group might show a distal drift .

45. Hunter - Schreger band

46. Gnarled enamel :
The enamel rods at the cuspal and incisal region appear , twisted and inter-twisted, and are more irregular from DEJ to the tooth surface . They are more at the cuspal region than incisal region. Gnarled enamel aids in resisting the high masticatory loads without fracture that the cusps have to bear.

47. Gnarled enamel

49. Incrimental lines of enamel :
include : 1- striae of Retzuis 2- cross striations 3- neonatal line

50. 1-incremental Lines of Retzius or Striae of Retzius
represent incremental growth, in ground ,cross sections they appear like concentric growth rings . In longitudinal sections they appear to be dark line extending from DEJ to the tooth surface. Striae of Retzius often extend from DEJ to the outer surface of enamel,where they end in a shallow furrows known as perikymata (or imbrication lines ). these lines represent a hypomineralized or rhythmic formation of the enamel .

51. Incremental lines of Retzius (G/S). A. In cuspal region B
Incremental lines of Retzius (G/S). A. In cuspal region B. In cervical region

53. 2- cross striations
They are periodic bands that appear along the full length of enamel rod.Because of this enamel rod appears like a ladder with cross striations being the rungs of the ladder . They appear at regular intervals ,that is in agreement with the rate of enamel deposition .

55. 3- neonatal line :
Is a Striae of Retzuis that formed at birth ,because it reflects the great physiologic changes occur at birth .so these lines demarcating the boundary between enamel formed before, and after birth.

56. Neonatal lines (NL)&LINES OF RETZUIS(LR)

57. Enamel structures near DEJ :
Include :
1- enamel spindles
2- enamel tufts

58. 1- Enamel spindles :
They are originated from odontoblastic process that cross the DEJ .befor enamel forms , some developing odontoblasts process extend into the ameloblast layer , and when enamel formation begins become trapped to form enamel spindles and formed a tubule. Tubules are found single or in groups and are shorter than tufts .fingure like spindles appear quite different than broader and longer tufts.

60. ES- enamel spindles GE= Gnarled enamel.

61. 2- enamel tufts
they also originate from DEJ , run a short distance in the enamel or to half of it’s thickness. They represent protein (enamelin) rich areas in enamel matrix that fail to mature.
they are formed during the formative stages of enamel ,they are considered to be “faults” by some researchers while others consider them to be necessary to anchor dentine to enamel .

62. a- enamel tufts b-Two enamel lamellae white arrows / lines of Retzius

63. ET- enamel tuft DT-dentinal tubules

64. Surface structures of enamel:
Include : 1-perikymata 2- enamel cuticle 3- enamel pellicle 4- E. Lamllae

65. 1- perikymata
are wave-like, transverse grooves. They are shallow furrows and most probably the external manifestation of incremental lines of Retzius. They are continuous around a tooth and usually lie parallel to each other and to the cementoenamel junction. their course is usually fairly regular , but in the cervical region it may be quite irregular.

66. Perikymata(imbrication lines)

67. 2- E. cuticle
Nasmyth’s Membrane is a nonmineralized, membrane usually found between the epithelium of dentogingival junction and the enamel surface. This delicate membrane covers the entire enamel of newly erupted tooth and is worn away by mastication and cleaning ,it’s secreted by ameloblasts when E. formation is completed.

68. 3- E. pellicle
After tooth is cleaned, salivary proteins having strong affinity to adsorbed to the enamel surface and form a very thin layer called salivary pellicle. In the beginning it is bacteria-free . Erupted enamel is usually covered by pellicle. This pellicle is removed by mastication or strong brushing and is again formed by itself within hours. After 24 to 48 hours of pellicle formation, colonies of microorganisms develop over it. Then it is called bacterial .

70. 4- E. Lamllae :
are very thin, leaf-like structures,. They extend from the enamel surface towards the dentinoenamel junction, rarely extending into dentin. The enamel lamellae contain mostly organic material ,with little mineral content. Lamellae may develop in planes of tension .

71. Cont.
they can be differentiated into three types: Type A lamellae composed of poorly calcified rod segment. Type B lamellae composed of degenerated cells. Type C lamellae arising in erupted teeth where the cracks are filled with organic matter and debris from saliva. Type A is restricted to enamel and type B and C may reach the dentin.

72. surface layer of enamel(SLE) Enamel rods (ER) and enamel lamellae (EL)

73. cracks
are very narrow, fissure-like structures that are present on almost all surfaces. They are actually the outer edges of enamel lamellae. They originate from dentinoenamel junction and run at right angles to it.

75. Age changes in enamel :
1- with age enamel becomes worn out because of masticatory attrition . 2- age also causes a decraese in permeability of enamel . 3- other characteristics of aging of enamel are discoloration and a change in the surface layer.

76. Defects of enamel formation :
disturbances in either secretion or maturation of enamel matrix ,can lead to defects in enamel structure . E. hypoplasia : due to decrease in the amount of matrix synthesized by ameloblasts . E.hypomineralization : is caused by a lack of sufficient mineral incorporation. generally three conditions affect enamel during it’s formative stages :

77. Cont.
1- defects caused by febrile disease 2- defects caused by tetracycline 3- defects caused by excess fluoride , dental fluorosis ( mottled enamel).

78. Thank you