Enamel is the hardest substance in the human body. It is composed primarily of hydroxyapatite crystals arranged in rods (prisms) that run from the dentin-enamel junction to the outer enamel surface. The basic structural unit of enamel is the enamel rod, which is hexagonal or oval in cross-section. Between the rods is the interrod enamel. Enamel gains its strength through the interweaving of tightly packed hydroxyapatite crystals in parallel alignment within the rods.

1. Enamel
04/09/2013

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. 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. The specific gravity is 2.8.
6. Khn-343
7. Acts as semipermeable membrane (selectively
permeable).
8. 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. Hydroxyappatite

6. Chemical properties
• 96% inorganic - by weight
• 4% organic - by weight
• inorganic crystalline calcium phosphate –
hydroxyapatite
• various ions like strontium, magnesium, lead
and fluoride are present at some point during
enamel formation
• In volume the organic matter and water
are nearly equal to the inorganic contents.

7. 1) Enamel rods(prisms)
2) Rod shealths
3) Cementing inter-rod substance.
STRUCTURE OF ENAMEL

8. Enamel rods
• Basic unit of enamel

9. 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

10. Cross section of enamel

11. Enamel
• built from closely packed ribbon-
like hydroxyapatite crystals – 60
to 70 nm in width and 25 to 30
nm in thickness
• the rod is shaped somewhat like a
cylinder and is made up of
crystals that organize with their
long axes parallel to the
longitudinal axis of the entire rod
– this organization is tighter
around the center of each rod
• the interrod region surrounds
each rod – its crystals are
oriented along different axes
from the rod
• Rods are oriented at right angles
to the dentin surface
• In cervical and central part of a
permanent teeth they are
horizontal

12. 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.

13. • Enamel Rod: Basic Structural Unit
Cross section

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

15. Enamel Rods

16. 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.

17. Keyhole shaped E. rods
Hexagonal ameloblasts
Note crystal orientation
Enamel Rod’s Shape

18. Enamel Rod
• Longitudinal section

19. Crystals in rod and inter-rod enamel are similar in structure but diverge
in orientation

20. 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.

21. Enamel rods sectioned in cross-
section
• In this electron micrograph enamel rods
are cut perpendicular to their long axis.
The ligherareas are the rod coresin which
hydroxyapatitecrystals are tightly packed
in alignment with each other. The darker
areas surrounding the rod cores are the
rod sheathsin which the crystals are
loosely packed at various angles. There
are two main parts to a rod: the rod
headand rod tail. The head has the
central core (light area), and is
sometimes referred to as the "rod". The
tail is made of the rod sheath (less
mineralized enamel). During
development, one ameloblast(in position
1 in the inset diagram) makes the rod
core for the rod at position 1, while three
other ameloblasts(in positions 2, 3 and 4)
produce the rod tail of rod 1. The tail is
located between 2 and 3 and above 4.
• Legend:A, Rod core; B, Rod sheath; C, Rod
tail; D, Rod head

22. Box diagram of human enamel
• This diagram represents a 25 X 25
X 25 μm of enamel.
• •It demonstrates the
arrangements of
hydroxyapatitecrystals in the
enamel rods in three planes of
section.
• •One rod is highlighted in blue to
demonstrate the typical human
rod shape.
• •In the rod core,
hydroxyapatitecrystals are aligned
with the long axis of the rod.
• •In the tail the crystals are
aligned diagonally or
perpendicularly to the long axis of
the rod.

23. Alternating rod directionality
• Hunter Schregerbands 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.
• Legend: A, Rods cut
longitudinally; B, Rods cut cross-
sectionally

24. Enamel Crystal
• Crystals length: 0.05 – 1 µm.
• Thickness: about 300 A°.
• Average width: about 900 A°.
• Cross sections: somewhat irregular.

25. Enamel Crystal
Longitudinal Section Transverse Section

26. Interprismatic substance
• Cementing E. rods together.
• More calcified than the rod sheath.
• Less calcified than the rod itself.
• Appears to be minimum in human teeth.

27. 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.

28. Cross-striations
Cross striations

29. 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.

30. Straight enamel rods -longitudinal
labiolingualsection
• The enamel rods project
in the direction of the
arrow.
• Can you see the striaof
Retzius?

31. Enamel cut
• In enamel cut in perfect cross-
section the shape of the
enamel rod exhibits a
"keyhole"-shaped pattern.
However, in a normal cross-
section of enamel, as seen
here, most rods are cut
obliquely.
• This is because they do not
travel in a straight line.
• The micrograph on the left is
produced by differential
interference microscopy while
the micrograph on the right is
from transmitted light
microscopy.

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

35. Gnarled enamel
• Optical appearance of enamel cut in oblique
plane
• Bundles of rods appear interwine more
irregularly
• Makes enamel stronger
• Seen in the incisal or cuspal region
• The enamel at the cusp of the tooth generally
exhibits a wavy pattern. This enamel is called
gnarled enamel. This is NOT hypo-mineralized.

36. 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

37. Direction of Enamel Rods

38. 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.

39. Hunter-Schreger Bands

40. Hunter-Schreger Bands

41. Hunter-Schreger Bands
This is Due to:
1. Change in the direction of E. rods.
2. Variation in calcification of the E.
3. Alternate zones having different
permeability and organic material.

42. Enamel -transverse ground section
• In a transverse section of tooth, the striaof
Retziusappear 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

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

44. 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

45. Incremental Lines of Retzius:
• Brownish bands in ground sections.
• Reflect variation in structure and mineralization.
• Broadening of these lines occur in metabolic disturbances.
• Etiology
1. Periodic bending of E. rods.
2. Variation in organic structure.
3. Physiologic calcification rhythm.

46. Microscopic picture of Enamel
Brown striae ofRetzius

47. Incremental Lines of Retzius:

48. 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.

49. Neonatal Line

50. 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.

51. Enamel lamellae
• Filled with organic material and water
• Type A B C can be seen
• A- poorly calcified rod segment
• B- Degenerated epithelia cells
• C- organic matter

52. Enamel Lamellae
Type A Type B Type C
Consistency Poorly calcified rod
seg.
Degenerated cells Organic matter
from saliva
Tooth Unerupted Unerupted Erupted
Location Restricted to the E. Reach into the D. Reach into the D.
Occurrence Less common Less common More common

53. 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

54. Enamel Lamellae

55. Enamel Lamellae

56. Enamel Tufts
• Arise from DEJ.
• Reach to 1/5 – 1/3 the thickness of E.
• In ground section: resemble tufts of grass.
• Do not spring from a single small area.
• The inner end arises at the dentin.
• Consist of hypocalcified E. rods and interprismatic
substance.
• The extend in the direction of the long axis of the
crown (best seen in horizontal sections).

57. Enamel tufts
• Runs short distance in the enamel
• Forms in the formative stage in the enamel

58. • 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

59. 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

60. Enamel Tufts

61. Enamel Tufts

62. Decalcified tooth
• In a decalcified section
of tooth, only the
organic material is left
behind.
• •In this micrograph you
can see an enamel
lamella and enamel
tufts.
• Legend: A, Enamel
lamella; B, Enamel tuft

63. 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.

64. DEJ
• Convexity of the enamel fits into the concavity
of the dentin
• Spindle tufts and lamellae are present at DEJ

65. Dentino-Enamel Junction

66. Odontoblastic Processes and Enamel
Spindles
• The odontoblasts processes may cross DEJ (before the hard
substance is formed) to the E. and ends as E. spindles.
• Odontoblasts process trapped in the enamel
• More in the cuspal region
• They are filled with organic matter.
• The processes and spindles are at right angle to the surface
of the dentin.
• The direction of spindles and rods is divergent.
• Spindles appear dark in ground sections under transmitted
light.

67. Enamel Spindles
• Odontoblastprocesses 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

68. • In this high magnification
of the DEJ you can clearly
see the bifurcation of the
ends of some of the
odontoblastprocesses as
well as enamel spindles.
• Legend: A, Enamel
spindle; B,
Odontoblastprocess; C,
Enamel rod

69. Odontoblastic Processes and Enamel
Spindles

70. Surface Structures
a. Structureless layer (E. skin)
b. Perikymata
c. Rod ends
d. Cracks
e. Enamel cuticle

71. a. Structureless layer
• About 30 µm thick.
• In 70% permanent teeth and all deciduous teeth.
• Found least often over the cusp tips.
• Found commonly in the cervical areas.
• No E. prisms.
• All the apatite crystals area parallel to one another and
perpendicular to the striae of Retzius.
• More mineralized than the bulk of E. beneath it.

72. b. Perikymata
• Transverse wave like grooves.
• Thought to be the external manifestation of the striae of Retzius.
• Lie parallel to each other and to CEJ.
• Number:
– About 30 perik./mm at the CEJ.
– About 10 perik./mm near the incisal edge.
• Their course is regular, but in the cervical region, it may be quite
irregular.
• Powdered graphite demonstrates them.
• It is absent in the occlusal part of deciduous teeth but present in
postnatal cervical part (due to undisturbed and even development
of E. before birth)

73. The relationship between the striae of
Retziuz and surface perikymata
Striae of Retziuz Perikymata

74. Perikymata (imbrication lines)
Are external manifestations
of Retzius striae

75. c. Rod ends
• Are concave and vary in depth and shape.
• Are shallow in the cervical regions.
• Deep near the incisal or occlusal edges.

76. Rod ends

77. 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.

78. Cracks

79. e. Enamel cuticle
1. Primary E. cuticle (Nasmyth’s
membrane).
2. Secondary E. cutile (afibrilar
cementum).
3. Pellicle (a precipitate of salivary
proteins.

80. Primary enamel cuticle
• Covers the entire crown of newly erupted
tooth.
• Thickness: 0.2 µm.
• Removed by mastication (remains intact in
protective areas).
• Secreted by postamloblasts.
• EM: similar to basal lamina.

81. Secondary enamel cuticle
• Covered the cervical area of the enamel.
• Thickness: up to 10 µm.
• Continuous with the cementum.
• Probably of mesodermal origin or may be elaborated
by the attachment epithelium.
• Secreted after E.O. retracted from the cervical region
during tooth development.

82. Pellicle
• Precipitate of salivary protein
• Covers the crown
• Re-form within hours after mechanical
cleaning .
• May be colonized by microorganisms to form a
bacterial plaque.
• Plaque may be calcified forming calculus.

83. 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.

84. 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

85. 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

86. • 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

87. 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

88. 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

89. 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)

90. 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

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

92. Organic Matrix Formation
a. Amelodentinal membrane.
b. Development of Tome’s processes.
c. Distal terminal bars.
d. Ameloblasts covering maturing enamel.

93. Enamel matrix
• Secretory activity of ameloblast starts after
the small dentin layer formation
• Ameloblast lose their projections separating
them from predentin
• Islands of enamel matrix deposit along the
dentin
• This layer is known as dentino enamel
membrane

94. Tomes process
• Surface of ameloblast facing the enamel is not
smooth
• The projection of ameloblast into the enamel
matrix is tomes process.
• There is an interdigitation of cells of
ameloblast and enamel rod because long axis
of rods and ameloblast are not parallel

95. • Picket fence arrangements
• Atleast two ameloblasts are involved in the
synthesis of each enamel rod

96. Distal terminal bars
• Appear at the distal end of the ameloblast
• Separate the tomes process from the cell
proper
• They are localized condensation of
cytoplasmic substance

97. Ameloblasts covering maturing enamel
• These are shorter than the ameloblast
covering incompletely formed enamel
• During enamel maturation 90% initially
secreted protein is lost
• Organic content and water is lost in
mineralization

98. dpTP=distal portion of Tome’s process
ppTP=proximal portion of Tome’s process
Sg=secretory granules(E. protein)
Organic Matrix Formation

99. Ameloblasts are perpendicular to the rods
(arrow=cell membrane, p=Tome’s process, s=incomplete septum)

100. Depression in enamel surface which were occupied by
Tome’s processes

101. Mineralization
a. Partial mineralization (25-30%).
b. Maturation (gradual completion of
mineralization).

102. Maturation
• Maturation seems to begin at the dentinal end
of the rod
• Each rod mature from the depth to the
surface
• The sequence of maturing rod is from cusps or
incisal edge toward the cervical line
• Incisal and occlusal region reach maturity
ahead of the cervical region

103. • Original ribbon shaped crystal increase in
thickness more rapidly than in width

104. Crystal Mineralization
Recently formed crystals Mature crystals

105. Abnormalities
• Interference during E. matrix formation may
cause Enamel hypoplasia.
• Interference during Enamel maturation may
cause Enamel hypocalcification.
• Each condition may be caused by systemic,
local, or hereditary factors.

106. Abnormalities
Enamel Hypocalcification Enamel Hypoplasia

107. Amelogenesis
• two-step process
• first step produces a partially mineralized
enamel – approximately 30% mineralized
• second step involves an influx of additional
mineral content – coincident with the removal
of organic material and water – results in 96%
mineralization
• the influx of mineral results in growth of the
crystals in width and thickness

108. Amelogenesis- life cycle of ameloblast
• ameloblasts – derived from the
inner dental epithelium
• secrete matrix proteins that are
responsible for creating and
maintaining and extracellular
environment favorable to mineral
deposition
• possess a unique life cycle – each
stage reflects its primary activity
during enamel stages
• can be divided into three
main stages
– presecretory, secretory,
maturation stages

109. Amelogenesis
Figure 7-14 Representative
micrographs of amelogenesis in the
cat. A, Tooth formation shows an
occlusal-to-cervical developmental
gradient so that on some crowns
finding most of the stages of the
ameloblast life cycle is possible. The
panels on the right (B corresponds with
B1 and C with B2) are enlargements of
the boxed areas: B, Secretory stage,
initial enamel formation; C, secretory
stage, inner enamel formation. D and E
are from the incisal tip of the tooth
(see Fig. 7-15). D, Midmaturation stage,
smooth-ended ameloblasts; and E, late
maturation stage, ruffle-ended
ameloblasts. Am, Ameloblasts; D,
dentin; E, enamel; N, nucleus; Od,
odontoblasts; PL, papillary layer; RB,
ruffled border; SB, smooth border; SI,
stratum intermedium.

110. Presecretory stage
• morphogenetic phase
– during the cell stage – DEJ and shape of the crown
is determined
– cells of the inner dental epithelium are cuboidal or
low columnar with large centrally located nuclei
and poorly developed Golgi
– separated from the dental papilla by a basement
membrane
• Organizing and differentiation phase
– as the cells of the IDE differentiate into ameloblasts
they elongate and their nuclei shift toward the
stratum intermedium
– the basement membrane fragments by the
cytoplasmic projections of the ameloblasts – during
the formation of predentin
• this allows contact between the pre-
ameloblasts and pre-odontoblasts
– the Golgi complex increases in volume and
migrates distally to occupy a major portion of the
cytoplasm
– the amount of RER increases significantly and most
of the mitochondria clusters in the proximal region
of the cell
• -therefore the cell becomes polarized with
most of the organelles distal to the nucleus

111. – at the distal end of the cell – extensions form called Tome’s processes
- against which enamel forms
– research now shows the these differentiating ABs secrete enamel
proteins at the early stage – even before the basement membrane
disintegrates
• pre-ameloblasts also express dentin sialoprotein transiently –
which is also an odontoblast product
– adjacent ABs align closely with each other – form junctional complexes
between them keeps them aligned
• these complexes encircle the cells at the distal end of the cell
(adjacent to the stratum intermedium)
• formed of fine actin filaments radiating from these complexes into
the cytoplasm

112. Secretory Stage
• cells acquire intense synthetic
and secretory activity
• enamel proteins are translated by
the RER, modified by the Golgi
and packaged into secretory
granules
– migrate to the distal Tome’s
processes
• secretion is constitutive – the
secretory granules are not stored
for long within the cells
• the contents of the secretory
granules are released against the
newly formed dentin along the
surface of the Tome’s process
• little time elapses between the
secretion of enamel and its
mineralization

113. • as the initial enamel layer forms – the ABs migrate away from the dentin
surface and develop a distal portion of Tome’s process – extension from
the existing proximal portion of Tome’s process
– the pTP extends from the distal junctional complex to the surface of
the enamel layer
– the dTP interdigitates into the enamel beyond the initial layer
– the cytoplasm of both processes is continuous with that of the body of
the AB
– so once the initial enamel layer forms the AB only has a pTP
– the dTP forms once the enamel forms into rods
– when the dTP forms – the enamel proteins are secreted at two sites
located at defined ports along the dTP
– the dTP lengthens as the enamel layer thickens
– it also becomes thinner as the rod grows in diameter
– eventuallyu squeezed out of existence

114. Figure 7-31 In three dimensions,
interrod (IR) enamel surrounds the
forming rod (R) and the distal
portion of Tomes’ process (dpTP);
this portion is the continuation of
the proximal portion (ppTP) into the
enamel layer. The interrod (IGS) and
rod (RGS) growth sites are
associated with membrane
infoldings (im) on the proximal and
distal portions of Tomes’ process,
respectively. These infoldings
represent the sites where secretory
granules (sg) release enamel
proteins extracellularly for growth in
length of enamel crystals and,
consequently, the thickening of
interrod and rod enamel.

115. Maturation Stage
• both tooth eruption – the enamel hardens
• change in the physiochemical properties of the
enamel
• actually because the pre-existing HA crystals of the
enamel grow in width and thickness and NOT
because new crystals are made
• Tome’s processes are not apparent at this stage
• the ABs are generally referred to as post-secretory
cells at this stage
– although they still secrete proteins
• made up of a transitional phase and the maturation
proper phase
– transitional phase – after the full thickness of
the enamel has formed
• the ABs undergo significant morphological
changes that prepares them for the
maturation of the enamel
• reduction of AB height and a decrease in
their volume and organelle content

116. – Maturation Proper Phase – ABs become involved in the removal of water
and organic material
• also undergo apoptosis so that approximately 25% of the ABs die
during the transitional phase and an additional 25% die during the
MP phase
• characterized by the modulation of the cells – cyclic creation, loss and
recreation of a highly ruffled apical surface and a smooth surface
• occurs in waves traveling across the crown of the developing tooth –
from least mature to most mature enamel regions
• can happen every 8 hours in some species
• significance is unknown – could be related to calcium transport and
completion of enamel mineralization
• the calcium ions are required for active crystal growth because the
cellular junctions at the ruffled end are more leaky
• these ABs show enhanced endocytic activity and numerous
lysosomes – for the withdrawl of enamel proteins from the maturing
enamel matrix
• BUT the main mechanism for organic matrix removal is the
production of bulk-degrading enzymes into fragments small enough
to be able to leave the enamel layer and be taken up by the AB

117. Ruffled and Smooth ameloblasts

118. Enamel proteins
• amelogenin – accumulate during the secretory stage
– undergo minor short-term and major long-term processing to form
smaller fragments
– these fragments form the bulk of the final organic matrix of maturing
enamel
– prevents crystals from fusing during their formation and must be
removed to permit crystal growth
– least concentrated at positions where the interrod and rod crystals
grow in length = enamel growth sites
– form aggregates called nanospheres that surround the enamel crystals
along their long axis

119. Non amelogenins
– enamalin – small degredation during the secretory stage which
decreases in the deeper zones of the enamel
• crystal nucleation and growth
– ameloblastin – undergoes rapid degredation – the intact protein is
found near the enamel-forming surface while the fragmented forms
are found in the deeper zones of the enamel
• promotes mineral formation and crystal elongation
• also known as amelin and sheathlin
• highest concentration can be found at the enamel growth sites
• secreted together with amelogenins

120. • sulfated glycoproteins – short half life in the enamel
• tuftelin – localizes specifically at the DEJ and participates in its
establishment
– not specific to enamel
• enzymes: metalloproteinases – e.g. MMP20 or enamelysin (short-term
breakdown), serine proteinases (bulk degredation), phosphatases
• dentin sialoprotein – transiently expressed

121. Mineral pathway
• introduction of minerals span the secretory and maturation phases
• calcium moves from the blood supply through the enamel organ to
reach the enamel
• a smooth tubular network has been described that is found in ABs and
opens onto the enamel
• this network is similar to the ER/sarcoplasmic reticulum
• calcium is likely to be routed from high-capacity stores associated with the
ER
• no matrix vesicle are associated with mineralization – as is the basis for
collagen-based mineralized tissues like bone
• almost an immediate formation of crystallites within the enamel secreted
against the dentin – so there is no pre-enamel

122. Enamel structural organization
• Striae of Retzius – in longitudinal sections they
are a series of dark lines extending from the DEJ
toward the tooth surface
– forms becuase of a weekly rhythm of enamel
production resulting in structural alterations of
the rods
– OR could be due to appositional deposition of
successive layers of enamel
• Cross striations – forms at 4um intervals across
the rods
– rhythmicity??
• Bands of Hunter and Schreger – optical
phenomenon produced by the changing
orientations of adjacent groups of rod
• Gnarled enamel
• Enamel tufts and lamellae – no clinical
significance
– like geological faults
– project from the DEJ for a short distance into the
enamel
– branched and contain greater concentrations of
enamel that the rest of the enamel
– abrupt changes in the directions of the rods
arising from the DEJ