Enamel is the hardest and most highly mineralized tissue in the body, consisting of 96% inorganic material (hydroxyapatite) and 4% organic material. It is formed through the process of amelogenesis, which involves three stages - the presecretory, secretory, and maturation stages. Ameloblasts are the cells responsible for enamel formation and organization into rods and interrod enamel. Enamel acquires its structural properties through mineral deposition and maturation over several years. Its unique composition and structure provide protection and function for teeth.

1. Enamel: Composition, Formation
and Structure Dr. Yumna
Lecturer
Dental Hygiene and Technology

2. Introduction
• Enamel is a protective covering for teeth
• Cells responsible for formation of enamel are
ameloblasts
• Enamel acquires a complex structural organization
• It possess high degree of mineralization

3. Composition

4. Physical Characteristics
• Enamel is the most highly mineralized extracellular
matrix
• It consists of approximately 96% mineral and 4%
organic material
• Inorganic content of enamel is a crystalline calcium
phosphate (hydroxyapatite)

5. Physical Characteristics
• Various ions strontium, magnessium, lead and
fluoride may also be incorporated into the crystals of
enamel
• High mineral content renders enamel extremely hard
• When crystals start dissolution, dental caries starts

6. Physical Characteristics
• Hardness of enamel is comparable to that of steel
• Enamel is brittle
• Dentine is resilient and it maintains integrity of
enamel
• Enamel is translucent

7. Physical Characteristics
• Enamel varies in color from light yellow to gray white

8. Structure of
Enamel

9. Structure
• Conventional demineralized sections of enamel only
have an empty space which was previously occupied
by mature enamel because the mineral has been
dissolved and traced organic material has been
washed away

10. Structure
• Fundamental organizational
units of enamel are the
– Rods (prisms)
– Interrod enamel
(interprismatic
substance)

12. Structure
• Enamel rod was first described as hexagonal and
prism like in cross section
• Enamel is built from closely packed and long, ribbon
like carbonatoapatite crystals
• Fully mature enamel crystals are no longer hexagonal

14. Amelogenesis

15. Amelogenesis
• It is a two step process
• Enamel first mineralizes to only 30%, later on crystals
grow wider
• Organic matter and water are lost and mineral is
added to reach full thickness of enamel
• At this stage greater than 96% mineral content is
found

16. Amelogenesis
• Ameloblasts secrete matrix proteins and are
responsible for creating and maintaining an
extracellular environment favorable to mineral
deposition
• Amelogenesis has been described in as many as six
phases

17. Amelogenesis
• Generally it is subdivided into three main functional
stages referred to as the
– Presecretory stage
– Secretory stage
– Maturation stage

18. Presecretory Stage
• Differentiating ameloblasts acquire their phenotype
• Change their polarity
• Develop extensive protein apparatus
• Secrete organic matrix of enamel

19. Secretory Stage
• Also called formative stage
• Ameloblasts elaborate and organize the entire
enamel thickness
• Results in the formation of highly ordered tissue

20. Maturation Stage
• Ameloblasts modulate
• Ameloblasts transport specific ions required for
concurrent accretion of enamel
• Ameloblasts are considered cells that carry out
multiple activities throughout their life cycle

21. Maturation Stage
• When differentiation of the ameloblasts occurs and
dentin starts forming, these cells are distanced from
the blood vessels
• Vascular supply is achieved by blood vessels
invaginating outer enamel epithelium
• There is loss of stellate reticulum

22. Light Microscopy of Amelogenesis
• At the late bell stage, most of light microscopic
features of amelogenesis can be seen
• In the region of cervical loop, low columnar cells in
inner enamel epithelium are identifiable

24. Light Microscopy of Amelogenesis
• An important event for the production of enamel is
the development of cytoplasmic extensions on
ameloblasts called Tome’s process
• Tome’s process give the junction between the
enamel and the ameloblast a saw-toothed
appearance

25. Light Microscopy of Amelogenesis
• As the inner enamel epithelium
is traced coronally in a crown
stage tooth germ, it cells
become tall and columnar

27. Light Microscopy of Amelogenesis
• Blood vessels invaginate deeply into
the cell, without disrupting the
basal lamina associated with the
outer aspect of enamel organ to
form a convoluted structure called
papillary layer

28. Light Microscopy of Amelogenesis
• Finally when enamel is fully mature, the ameloblast
layer and the adjacent papillary layer regress and
together constitute the reduced enamel epithelium
• At this stage ameloblasts stop modulating, reduce
their size and assume a cuboidal appearance

30. Light Microscopy of Amelogenesis
• As the tooth passes
through oral epithelium,
incisally reduced enamel
epithelium is destroyed
but remain cervically to
form the junctional
epithelium

32. Presecretory
Stage

33. Presecretory Stage
Morphogenetic Phase:
• During bell stage shape of the crown is determined
• Cells of inner enamel epithelium still can undergo
mitotic division
• They are cuboidal or low columnar cells

34. Presecretory Stage
Differentiation Phase:
• As the cells of inner enamel epithelium differentiate
into ameloblasts they elongate
• Ameloblasts become polarized
• These cells can no longer divide

35. Presecretory Stage
• Junctional complexes are present between
ameloblasts
• They play an important role in amelogenesis by
tightly holding together ameloblasts and pass on
required nutrients to them

36. Secretory
Stage

37. Secretory Stage
• Ameloblasts reflects their intense synthetic and
secretory activity
• The cell granules migrate to the distal extremity of
the cell that is into Tome’s process

38. Secretory Stage
• When enamel formation begins, Tomes’ process
comprises only a proximal portion
• The content of secretory granules is
• Secretion from the proximal part of the process and
from adjoining ameloblasts, results in the formation
of enamel pits

39. Maturation
Stage

40. Maturation Stage
• Amelogenesis is a slow developmental process
• It can take as long as 5 years to complete
• Maturation stage ameloblasts are referred to as post
secretory cells

41. Maturation Stage
• In this stage messenger RNA and protein signal for
– Amelogenin and
– Ameloblastin
• The two enamel proteins found in maturation
ameloblasts

42. Maturation Stage
Transitional Phase
• Ameloblasts undergo programmed cell death
(apoptosis)
• Approximately 25% cells die during transitional phase
• Other 25
% cells die as enamel maturation proceeds

43. Maturation Stage
• In embryogenesis, cells die at specific times during
development to permit orderly morphogenesis
• Two major ways by which cell death can occur are
– Necrosis (pathological)
– Apoptosis (physiological)

44. Maturation Stage
• Specialized proteinases (caspases) also inactivate
cellular survival pathways and activate factors that
promote death

45. Maturation Stage
Maturation Proper:
• Next principal activity of ameloblasts is the bulk
removal of water and organic material from the
enamel to allow introduction of additional inorganic
material
• The most dramatic activity is the modulation of cells

46. Maturation Stage
• They maintain the environment that allows accretion
of mineral content
• Maintain pH for functioning of the matrix degrading
enzymes

47. Enamel
Proteins

48. Enamel Proteins
• The organic matrix of enamel is made of non-
collagenous proteins only and made up of enamel
proteins and enzymes
• 90% of proteins are heterogenous group of low
molecular weight proteins known as amelogenins
• Remaining 10% are enamelins and ameloblastins

49. Enamel Proteins
• Amelogenins are hydrophobic proteins
• They have molecular weight between 5 and 45kDa
• Amelogenins regulate growth in thickness and width
of crystals
• Ameloblastins are concentrated near the cell surface
at sites where they are secreted

51. Structural and
Organizational
Features of
Enamel

52. Rod Interrelationships
• Rods tend to maintain in group
• Rods run in a perpendicular direction to the surface
of the dentin with a slight inclination towards the
cusp

54. Striae of Retzius
• They are identified using ground sections of calcified
teeth
• They are seen as a series of dark lines extending from
dentino-enamel junction towards tooth surface
• In cross sections they appear as concentric rings

55. Striae of Retzius
• The basis for their production is
still not clear
• A proposal suggests that they
reflect appositional or
incremental growth of the
enamel layer

57. Neonatal Line
• The neonatal line when present, is an enlarged stria
of Retzius that reflects changes occurring at birth
• Accentuated incremental lines also are produced by
systemic disturbances (e.g. fever) that affect
amelogenesis

59. Cross Striations
• Human enamel is known to form at a rate of 4µm per day
• Ground sections of enamel reveal periodic bands or cross
striations make up rods of enamel
• Alternating expansion and contraction of bands is
sometimes visible
• This pattern reflect diurnal rhythmicity in rod formation

61. Hunter Schreger Bands
• The bands of Hunter and Schreger are an optical
phenomenon produced by changes in direction
between adjacent groups of rods
• They are seen more clearly in longitudinal ground
sections
• Found in inner two thirds of enamel
• Appear as dark and light alternating zones

63. Gnarled Enamel
• Over the cusps of teeth
the rods appear twisted
around each other in a
seemingly complex
arrangement known as
gnarled enamel

65. Enamel Tufts and Lamellae
• They are like geological faults
• No known clinical significance
• Seen in transverse sections of enamel
•

66. Enamel Tufts
• Enamel tufts project from
dentino-enamel junction into
the enamel
• Contain greater concentration
of enamel proteins then rest of
enamel
• They occur developmentally

68. Enamel Lamellae
• They extend for varying depths from the surface of
enamel
• They contain linear, longitudinally oriented defects
filled with organic material
• Cracks in enamel can be sometimes mistaken for
lamellae but generally donot contain organic matter

69. Enamel Spindles
• Before enamel forms, some developing odontoblasts
processes extend into the ameloblast layer
• When enamel formation begins, become trapped to
form enamel spindles
• The shape and nature of DEJ prevent shearing of
enamel during function

71. Enamel Surface
• The striae of Retzius often extend from the
dentinoenamel junction to the outer surface of
enamel where they end in shallow furrows known as
perikymata
• Perikymeta runs in circumferentially horizontal lines
across the face of the crown

73. Enamel Surface
• Structure of enamel varies with age.
• In unerupted teeth enamel surface consists of a
structureless surface layer (finalenamel) that is lost
abruptly by abrasion, attrition and erosion in erupted
teeth

74. Enamel Surface
• As the tooth erupts it is covered with pellicle,
consisting of debris from enamel organ
• Salivary pellicle always reapperas on surface of teeth
shortly after mechanical polishing of teeth
• Dental plaque forms readily on teeth

76. Age Changes of Enamel
• With age enamel becomes
progressively worn in
regions of masticatory
attrition
• Wear facets increase in old
ages

77. Age Changes of Enamel
• Characteristics of aging include
– Discoloration
• Teeth darkens with age
– Reduced permeability
– Modifications in the surface layer

78. Age Changes of Enamel
• Darkening of teeth can be caused by addition of
organic matter
• It may also be caused by darkening of dentin layer
• Enamel becomes less permeable with age
• Young enamel acts as semipermeable membrane

79. Defects of
Amelogenesis

80. Defects of Amelogenesis
• Many conditions produce defects in enamel structure
• Defects occur because amelobalsts are cells sensitive
to changes in environment

81. Defects of Amelogenesis
• Defects in enamel can be caused by
– Febrile disease
– Nutrient deficiency
– Protein malformation in enamel
– Disarrangement of ameloblasts
– Maternal issues
– Fluoridation

82. Clinical
Implications

83. Fluoridation
• If fluoride is incorporated into the hydroxyapatite
crystals, the crystal becomes more resistant to acid
dissolution
• fluoride play role in caries prevention
• Presence of fluoride enhances chemical reactions
that lead to the precipitation of calcium phosphate

86. Acid Etching
• Use of fissure sealants, bonding of restorative
materials to enamel and cementing of orthodontic
brackets involve acid etching
• In first step etching removes plaque and other debris
along with a thin layer of enamel

87. Acid Etching
• Second step: it increases
porosity of the exposed
surface of enamel which
helps in proper bonding for
restoratives and adhesive
materials