Enamel

INTRODUCTION

• Hardest calcified (Mineralized) tissue of the body covering the anatomic crown of the tooth.
• Ectodermal in origin.

• Cells forming enamel are – AMELOBLASTS

• It is non-living tissue- Acellular and Avascular tissue.

• Does not have capacity to repair and regenerate
• Does not contain collagen

• Function: To form a resistant covering of the teeth rendering them suitable for mastication.

PHYSICAL PROPERTIES
1.Thickness – variable
- maximum thickness is seen on the cusps of premolars and molars (2 to 2.5mm)
- minimum at the neck of the tooth (cervical region) almost a knife edge
2. Colour – varies from grayish white to yellowish white
- enamel is translucent and allows the yellow colour of the dentin visible through it.
- Yellow teeth – thin, transluscent enamel - Grayish teeth – more
opaque enamel
- Cervical region – slightly yellowish (enamel is thinnest)
- Incisal areas – bluish tinge (consists only of double layer of enamel, no dentin )
3. Hardness – hardest calcified tissue of the body due high content of mineral salts and their crystalline arrangement
- owing to its hardness, enamel is brittle.
4. Permeability – semi-permeable, allows passage of certain molecules
5. Specific gravity – 2.8
STRUCTURE of enamel

• Enamel rod [enamel prism]

• Gnarled enamel
• Incremental lines (lines of retzius, Cross Striations, Neonatal Lines, Imbrication Lines of Pickerill/Perikymata)
• Hunter-Schreger bands
• Enamel tufts
• Enamel lamella
• Enamel spindles
• Dentinoenamel junction
• Enamel Cuticle

ENAMEL ROD [ENAMEL PRISM]

Basic structural unit of enamel.

Absent in innermost and outermost layers (Aprismatic enamel).

• Shape – cylindrical
• Numbers – varies from 5 million (in lower lateral incisor) to 12 million (in upper first molar)
• Extent – from dentinoenamel junction (DEJ) to outer surface of enamel
• Course - wavy tortutous course throughout its length
 • Diameter – average 4microns.
- Narrow at DEJ and widens gradually towards the surface [1:2]
• Direction of rods
- oriented at right angles to the dentin surface - middle third of the crown – horizontal
- near incisal edge and cusp tips – starts becoming oblique - at the incisal edge and cusp tip – almost straight
- cervical third of deciduous teeth – horizontal
- cervical third of permanent teeth – deviate in apical direction
- in pit and fissure region – the rods converge in their outward course

Permanent tooth

Deciduous tooth
• Cross-sectional appearance - LIGHT MICROSCOPY

Clear crystalline appearance.

Hexagonal /round/oval. Resemble fish scales.
• Ultrastructure
In cross section - Key hole shape, having head and tail.

Head is pointed incisal/occlusally and tail pointed towards cervical

• Head formed by enamel rod.

• Tail formed by Interrod substance.
• Rods – 5microns in breadth and 9 microns in length
• Prisms are interlocked with one another, i.e. the enamel rods and
interrod enamel are arranged in such a way that the heads abuts against
the tail of adjacent rods.

• Hydroxyapetite crystals

• Rods and interrod substance are tightly packed hydroxyapatite crystals.

• Direction of hydroxyapatite crystals are different in Rods and interrod substance.
• Head area [enamel rod] – crystals are parallel to the long axis of the rod
• Tail area [interrod substance] –crystals flare laterally, with an angulation of 65°-90° to the long axis of the prism.
• ROD SHEATH-

There is abrupt change in direction of crystals in one rod to another.

Crystals are not tightly packed on the rod surface. The intercrystalline space provides more space for oraganic
matrix.
This accounts for the rod sheath

GNARLED ENAMEL

• Enamel rods follow a wavy course from DEJ to enamel

surface
• In the region of cusps or incisal edges, especially nearer to dentin,
the bundles of rods appear to interwine more irregularly. This
optical appearance of enamel is called as Gnarled enamel.
• Significance – increased strength of enamel for withstanding the strong
masticatory forces
STRUCTURAL LINES

• Indicates rhythmic deposition of enamel

• Reflect variations in structure and mineralization (either hypomineralized or hypermineralized) that occur during
enamel growth.
1. Cross Striations
2. Incremental Lines of Retzius
3. Imbrication Lines of Pickerill/Perikymata
4. Neonatal Lines

Cross Striations

• Each enamel rod is built up of segments separated by dark lines that gives it a striated appearance

• These cross striations are seen at an uniform length of 4µm. They are seen in longitudinal ground section

• The rods are segmented because the enamel matrix is formed in rhythmic manner.
• They are more pronounced in enamel that is insufficiently calcified

• Represent daily deposition of enamel. Enamel forms at a rate of approximately 4µm per day
• Rods in these areas show varicosities and variation in composition
• They represents area of higher organic content & less inorganic content.
 Incremental lines/ Striae of Retizus

• Appear as series of brownish bands in ground section

• Direction of striae of retzius

Longitudinal Ground Section-run obliquely from DEJ to the outer surface of enamel with occlusal deviation.
- they surround the tip of the dentin in cuspal and incisal regions
Transverse Ground Section-concentric circles.

• Exact nature unknown. Have been attributed to - periodic bending of enamel rods
- Variation in the basic organic structure. - Physiologic calcification rhythm.

• The lines of retizus was shown to represent a 6 to 11 day rhythm (weekly rhythm) in enamel formation

• They indicates the Resting phase – Decreased formation of crystals → increase organic content.

Significance –
• If present in moderate intensity, are considered normal
• Metabolic disturbances which can disturb the rhythmic pattern of enamel formation leading to prolonged periods. In such
abnormal condition the incremental lines are broadened and more prominent.
 Imbrication Lines of Pickerill/Perikymata

• These are the transverse wave-like groove, believed to be the external manifestations of striae of retzius
• They are more in cervical region (30 perikymata / mm). Decreases in number near occlusal or incisal edge (10 perikymata
/ mm).

Neonatal Line

• In deciduous teeth and permanent first molars enamel is deposited partly before birth & partly after birth.
• Incremental line separating prenatal enamel from postnatal enamel is called Neonatal line
• Neonatal line is accentuated because of disturbance in formation of enamel occurring at the time of birth due to abrupt change in
environment.
• Prenatal enamel is more homogeneous than the postnatal enamel due to more constant surroundings and good nutritional supply.
 HUNTER-SCHREGER BANDS

• Alternate dark and light bands[strips] extending from DEJ towards the enamel surface and ending
at some distance from the outer enamel surface.
• Best seen in longitudinal ground section under oblique reflected light.

• Optical phenomena due to change in the direction between groups of enamel rods.
• Group of Rods cut longitudinally produce a dark bands (called PARAZONES)
• Rods cut transversely produce a light bands (called DIAZONES)
• Angle between parazones & diazones = 40⁰.

• Significance – regarded as functional adaptation minimizing the risk of cleavage in the axial direction
 ENAMEL TUFTS

• Narrow ribbon like structure, arises at DEJ and reach into the enamel about
th rd
1/5 of 1/3 of its thickness.
• Resemble tufts of grass when viewed in G/S
• they are formed as result of abrupt changes in the direction of groups of enamel
rods that arise from different regions of scalloped DEJ

• They are hypomineralized and contain greater concentrations of

enamel protein (enamelin) than the rest of enamel

• Tufts are best seen in transverse ground section.

• Significance – their presence are a consequence or an adaptation, to the

spatial condition in enamel
 ENAMEL LAMELLAE

• Thin leaf like structure that extend from the surface of the teeth
towards DEJ and may extend to and
sometime penetrate into the dentin.

• less common than tufts and are best

visualized in transverse ground sections.

• They develop in the plane of tension.

• When rods cross such a plane, a short segment of the rod may not calcify
• If the crack develops in unerupted tooth - it may be filled by surrounding cells(either cells of enamel organ or connective tissue)
a. if filled by cells of enamel organ – a hornified cuticle may form
b. if filled by cells of connective tissue – cementum may form
• If the crack develops in erupted tooth - it may be filled by organic substance from the oral cavity (saliva)

• Thus, Enamel Lamellae are of Three types

Type A – composed of poorly calcified enamel rods. - Restricted to enamel.

Type B – consists of degenerated cells,

- may extend to dentin.

Type C – filled with organic matter derived from saliva

– formed after eruption of tooth & - may extend into dentin.

• In G/S lamellae can be confused with cracks caused by grinding. To distinguish between these two structures G/S are subjected
to decalcification. Lamellae will persist and cracks disappear.

• Significance –
- they may be sites of weakness in enamel
- may form a road of entry for the bacteria causing caries

ENAMEL SPINDLES

• Before enamel forms, some developing odontoblast processes extend into the ameloblast layer and, when enamel formation
begins, become trapped to form enamel spindles
• They are thickened at their ends, hence termed as enamel spindles
• The direction of odontoblast processes and the spindles in the enamel corresponds to the original direction of ameloblasts (i.e. at
right angles to the dentin surface)
• Since the enamel rods are formed at an angle to the axis of the ameloblast, the direction of enamel rod and spindles are divergent
• In G/S of dried teeth, organic content disintegrate and is replaced by air appear and thus appear dark in transmitted light.

• They are found mainly in the cusp tip regions.

• Organic content higher than surrounding enamel.

• They are hypomineralized structure containing less calcium and phosphorus.

DENTINO-ENAMEL JUNCTION

• Junction between the dentin and enamel. • The surface of dentin at DEJ pitted.
• Shallow depressions of dentin fits rounded projections of the enamel.
• Thus in section, DEJ appears scalloped with convexity facing the dentin.
• DEJ in primary teeth is straight
• Significance -This relation assures a firm hold of enamel cap on the dentin.
DENTINO-ENAMEL JUNCTION
 ENAMEL CUTICLE & PELLICLE

ENAMEL CUTICLE also known as Nasmyth’s membrane/primary enamel cuticle.

• A delicate membrane that covers the entire crown of the newly erupted tooth.
• Soon removed by mastication
• It is the Basal lamina secreted by the ameloblast [reduced enamel epithelium] after complete formation of crown

ENAMEL PELLICLE:
• Seen in erupted teeth
• Derived from salivary proteins.
• Reforms within hours after an enamel mechanically cleaned.
• Within a day or two, the pellicle is Colonized by
microorganisms—Dental plaque.
 AGE CHANGES

• Attrition – wear of occlusal surface and proximal contact points

• Loss of rod ends and flattening of perikymata • Discolaration – teeth becomes darker
• Reduced permeability
• Decrease in water content
• Subsurface mineralization - Increase in fluoride content
• Increased resistance to caries
 • Is the process of formation of enamel.
• Cells responsible---ameloblasts.
• Ameloblasts Undergo morphological & physiological changes related to the function of ameloblast. This is described as Life
cycle of ameloblast.
• Amelogenesis involves two processes :
a. Formation of organic matrix
b. Mineralization of the matrix

Life cycle of ameloblast

Divided into six stages: 1. Morphogenic stag

e 2. Organization stage
3. Formative stage
4. Maturative stage
5. Protective stage
6. desmolytic stage

1. Pre-secretory stage:
• Morphogenic stage.
• Organizing/Differentiating stage.
2. Secretory stage:
• Formative stage
3. Post-secretory stages:
• Maturative stage
• Protective stage
• Desmolytic stage.
 1. Morphogenic stage

• In this stage the shape of DEJ and the crown is determined. This takes place by interaction of IEE with the adjacent mesenchymal
cells.
• Morphology of the preameloblast [IEE] # Cell shape - short &columnar
# Nuclei - large oval nuclei. # Cell organelles -
- Golgi apparatus & centrioles—proximal end of the cell.
- Mitochondria—evenly distributed throughout the cytoplasm.
Proximal
Distal
Proximal/basal
dentinal end
ameloblast, at of which
enamel is
formed end facing SI Distal
• Morphology During Ameloblast differentiation # Mitochondria moves to the basal region.
# Terminal bars develop.
Represents Points of close contact b/w cells
They are thickening of opposing cell membranes associated with condensations of underlying cytoplasm.

Terminal bar

# The adjacent pulpal layer is cell free, narrow and

light zone containing cytoplasmic processes of superficial cells
2. Organizing Stage

• In this stage following occurs:

a. Epithelial Mesenchymal Interaction
b. Morphology of cells: Reversal of functional polarity of cells
c. Reversal of nutritional source

a. Epithelial Mesenchymal Interaction

IEE cells

Interacts with

Dental papilla cells

Differentiates into Odontoblasts

Forms dentin

Clear cell free zone between IEE and dental papilla cells disappear
b. Morphology of cells
# cell shape : tall columnar
# Nucleus : proximal part of the cell (reversal of polarity)
# cell organelles: Reversal of functional polarity of cells by shifting of centrioles and golgi complex to distal end.
- mitochondria – concentrate in proximal end
c. Reversal of nutritional source
- Differentiated odontoblasts lays down the first layer of dentin which is very critical event in life cycle of IEE
- Initially IEE derives its nutritional supply from the blood vessels in dental papilla
- When dentin forms, it cuts down the nutritional supply of ameloblasts from Dental papilla
- Then on they are supplied by blood vessels surrounding the OEE
- This reversal of nutritional supply is characterized by: proliferation of capillaries of dental sac
reduction and gradual disappearance in stellate reticulum
 3. Formative Stage

• In this stage formation of enamel matrix occurs

• This starts only after the first layer of dentin is formed.

• Appearance of ameloblast indicates its intense secretory activity

Cell shape - Ameloblasts retain almost the same length Nucleus – at proximal end (reversed polarity)
organelles – abundant mitochondria, golgi complex, RER, secretory granules, ribosomes

• Development of Distal Terminal Bar at distal end.

• Junctional complex
- In secretory stage, ameloblasts have two terminal bars: proximal terminal bars and distal terminal bars
- Fine actin filaments radiates from these terminal bars into the cytoplasm, forming webs. These are called as junctional
complexes.
- Function : controls the substances which passes from ameloblasts into the enamel

• Tomes’ Process: conical projections that develop at distal end of Ameloblast. The distal terminal bars separate the tomes’
process from cell proper
 Proximal
Junctional
complex
Proximal
terminal bar

Distal terminal
bar
Distal Junctional complex
Tomes’ process

Initial Secretory stage ameloblast

Secretory stage ameloblast
 4. MATURATIVE STAGE

• Ameloblasts enters the maturative stage only after most of the thickness of enamel matrix has been formed.
• In this stage full mineralization (enamel maturation) occurs
- removal of water and organic material from the enamel to allow introduction of additional inorganic material
• It begins in the occlusal or incisal area and progresses cervically
• 50% of ameloblasts undergo programmed cell death (apoptosis)
• Ameloblasts – reduces in length
- volume of cell organelles decreases
•Ameloblasts undergo Modulation
A process by which their distal surfaces alternate between a Smooth-ended border and a ruffled-ended
border[microvilli].

Smooth ended
Rough ended Ameloblast
Ameloblast
 Ameloblasts Modulation

Ruffled ended Ameloblasts Smooth ended Ameloblasts

distal junctional complex - tight proximal junctional distal junctional complex - leaky proximal junctional
complex - leaky complex -tight

•Promotes calcium entry into enamel

•Secretes bicarbonate ions into forming enamel – maintains Removal of H O + protein
2
alkaline environment necessary for mineral deposition
 5. PROTECTIVE STAGE

• After complete formation and mineralization of enamel, the ameloblasts, Stratum intermedium cells and OEE becomes flattened
and indistinguishable from each other
• These cell layers forms a stratified epithelial covering of the enamel, called as Reduced enamel epithelium (REE).
• Function of REE:

It covers and protects the newly formed enamel by separating it from the connective tissue until the tooth erupts.
if the connective tissue comes in contact with the enamel, anomalies may develop. enamel may undergo resorption or
may get covered by a layer of cementum.
 6. DESMOLYTIC STAGE

REE

Proliferates and secretes enzymes

These enzymes destroys the connective tissue fibers by desmolysis

This induces atrophy of the connective tissue separating REE from oral epithelium
 Amelogenesis

• Involves two processes A. Formation of matrix

B. Mineralization of formed matrix

Both processes occurs simultaneously. As soon as a layer of matrix is formed, it undergoes mineralization.

A. Formation of enamel matrix

• Following events occurs

- Secretion of enamel proteins
- Development of Tomes’ processes and formation of rod and inter-rod enamel

• Secretion of enamel proteins

- occurs after first layer of dentin is formed
- initially enamel matrix (enamel proteins) is deposited in the form of islands along the predentin
- eventually as the deposition proceeds, a thin continuous layer of
enamel is formed
along the dentin.
- this initial increment of enamel is Aprismatic
 Various proteins in the enamel matrix

Ameloblastin and enamelin

Tuftelin
Amelogenins

Helps in nucleation and growth of

crystals
Functional role in maintaining spaces Localised near DEJ. Involved in
between the crystals cell signalling
• Development of Tomes’ process and formation of rod and interrod enamel

As the initial layer of enamel is formed, ameloblasts migrate away from the dentin surface by leaving a conical projection into the
matrix. These are called as Tomes’ process.

Tomes process are fundamental in the formation of rod and interrod

 Secretion of enamel proteins occurs from two sites of Tomes’ process

Second site
First site

secretions from part of Tomes’ process Secretion from face of Tomes’ process
close to junctional complex

Forms interrod enamel

Fills the pits formed by interrod enamel

Thus a pit is formed by interrod enamel into

which resides the distal portion of
Tomes’ process This forms the enamel rod
• After almost full thickness of enamel is formed, the ameloblasts becomes shorter and losses is Tomes’ process. Thus the final
enamel increment (outermost enamel layer) is also Aprismatic.

B. Mineralization of formed matrix

• Takes place in two stages:

a)immediate partial mineralization
b)Maturation
• Source of minerals
calcium reaches through the circulation to tissue fluid. Calcium from tissue fluid is actively transported into ameloblasts.
Within ameloblasts, calcium binds to the calcium binding protein. This complex of Ca – Ca binding protein moves
towards the tomes’ process cytoplasm. Within tomes’ process ionic calcium is released and secreted into the formed
matrix.

a) Immediate partial mineralization/primary phase

- The enamel matrix undergoes partial mineralization, immediately after it is laid down.
- 25 – 30% of total mineral content is deposited during this phase.
- Nucleation is initiated by apatite crystals of dentin over which enamel is laid down

b) Maturation
- Gradual completion of mineralization
- Starts from the height of the crown and progresses cervically
- During this stage there is increase in the size of crystals seen in primary phase
- Re-absorption of water and organic matrix takes place
- Starts at dentinal ends of rods and progresses to outer side