Amelogenisis / dental implant courses by Indian dental academy

AMELOGENESIS
INDIAN DENTAL ACADEMY
Leader in continuing Dental Education
www.indiandentalacademy.com

• Tencate’s oral histology and embryology-8th
edition
• Oral histology-Berkovitz,3rd edition
• Orban’s oral histology,13th edition
• Oral development & histology ,James K.
Avery 2nd edition.
• Dental enamel, John wiley

Learning objectives
• At the end of the seminar learner should
be able to understand :
– Various stages of amelogenesis
– Morphology of ameloblast in various
stages of amelogenesis
– Modification of the matrix and
mineralization
– Clinical considerations

• Enamel is an ectodermally originated tissue
covering the anatomical crown of a tooth.
• It is formed by the enamel organ .
• Enamel organ is derived from a localized
proliferation of the oral epithelium.

EPITHELIAL ENAMEL ORGAN
At the stage preceding the
formation of hard structures, the
enamel organ, originating from
the stratified epithelium of the
primitive oral cavity, consist of
the four distinct layers –
• Outer enamel epithelium
• Stellate reticulum
• Stratum intermedium
• Inner enamel epithelium
(ameloblastic layer)

• Before enamel formation begins cells of IEE
assume a columnar shape and differentiate into
ameloblast that produces organic matrix.

• Ameloblasts are 4-5 μ in diameter and 40μ
high, attached to one other by junctional
complexes.
• Amelogenesis i.e.formation of enamel by
ameloblast is divided into 3 functional
stages.

Functional stages in the life cycle of ameloblast
Presecretory
stage
Morphogenetic
phase
Differentiation
phase
Secretory
stage
Maturation
stage
Transitional
phase
Maturation
proper

Presecretory
stage
Morphogenetic
phase
Differentiation
phase

MORPHOGENETIC STAGE:-
• Before the ameloblasts are fully
differentiated and produce enamel,
they interact with the adjacent
mesenchymal cells, determining the
shape of the dentinoenamel junction
and the crown.

• The cells are cuboidal or low
columnar, with large oval nuclei that
almost fill the cell body.
• The Golgi apparatus and the
centrioles- proximal end of the cells .
• Distal end (dentinal )
• Proximal /Basal end (Str. Intermedium)
• Mitochondria are evenly dispersed
throughout the cytoplasm.

Life
cycle of
amelo-
blast

Differentiation stage:
• As cell of IEE differentiate into
ameloblasts they elongate and their
nuclei shift proximally toward the
stratum intermedium.
• Golgi complex increase in volume and
migrates distally.
• Mitochondria cluster in proximal region

• Specialized attachment which encircle distal
and proximal extremities of cells.
• Fine actin filament from these complexes
radiates into cytoplasm of cells and can be
distinguish as terminal webs.
• Their function is to hold ameloblast together
and regulate the substance to enter and leave
the enamel.

Secretory stage:
• At this stage ameloblast reflects their
intense synthetic and secretory activity.
• Golgi complex and RER is present
extensively.
• Protein synthesized is processed in golgi
complex and packed into secretory granules
• These granules migrate into Tomes’ process.
• They are released against newly formed
mantle dentin .

• When enamel
formation begins
Tomes’ process
comprises only of
proximal portion.

• Ameloblast moves away from the dentin as enamel is
formed and develops distal portion of Tomes’ process
.

Secretion of enamel protein confine to
2 sites
Proximal part of
Tomes’ process
forms interrod
enamel
Distal part of Tomes’
process forms
enamel rod

• The interrod and rod growth sites are
associated with membrane infoldings on
proximal and distal portion of Tomes’
process .
• These infoldings represent the site where
secretory granules release enamel proteins
extracellularly.
• It leads to increase in length of crystals and
thickness of enamel layer.

• The distal portion of Tomes’ process lengthen as
enamel thickens & disappear .
• Creating a narrow space between rod and interrod
forming rod sheath.

• The enamel layer is composed of rod containing layer
sandwiched between rodless initial and final enamel.
• It is because initial and final enamel is formed by
proximal portion of Tomes’ process and rods are in
relation to distal portion of Tomes’ process.www.indiandentalacademy.com

MATURATION STAGE:-
TRANSITIONAL
PHASE
MATURATION
PROPER

• Enamel maturation (full mineralization) occurs
after most of the thickness of enamel matrix
has been formed in the occlusal or incised area.
• In the cervical parts of the crown, enamel
matrix formation is still progressing at this
time.
• During enamel maturation the ameloblasts are
slightly reduced in length and are closely
attached to enamel matrix.
MATURATION STAGE:-

Transition stage
• After immature enamel is formed ameloblasts
undergo significant morphologic changes in
preparation for their next role of maturing
enamel.
The changes seen during transitional
stage are :
•Reduction in height of ameloblasts
•Cessation of enamel secretion

•50% ameloblasts undergo apoptosis
•Organelles involved in protein synthesis undergo
autophagocytosis
•Development of ruffle border in the apical cytoplasm

Maturation proper
• In this phase there is bulk removal of
water and organic substance from
enamel , to allow introduction of
inorganic material.

Modulation
• Modulation is very dramatic
phenomenon shown by ameloblasts.
• It is cyclic creation, loss & recurrence
of highly invaginated ruffle ended
apical surface and alternate with
smooth bordered cells.

• It waves across the crown from least
matured region to most matured
region i.e. from the cervical region to
incisal /occlusal region.
• Ameloblast modulate rapidly – once
every 8 hrs – thereby yielding 3
complete modulations per day.

Smooth & ruffle ended ameloblasts
• Ruffle ended ameloblasts possess
proximal junction that are leaky and distal
junctions that are tight, whereas most
smooth ended ameloblasts have proximal
junction tight & the distal ones are leaky.

• Ruffle ended ameloblasts show considerable
endocytic activity & contain numerous
lysosomes, calcium binding protein &
membrane-associated calcium ATPases that
appear to promote the pumping Ca ions into the
maturing enamel.

• Smooth –ended ameloblast leak small
proteins ,show little endocytic activity ,
no ATPase activity .
• Interstitial fluids may leak into maturing
enamel and contribute to neutralization
of pH of enamel matrix.

• Ca+ pass actively through the ruffle
ended ameloblasts & passively through
the sides of smooth ended ameloblasts to
the mineralizing front.
• Ruffle ended ameloblasts secrete
bicarbonate ion to keep the mineralizing
front alkaline, prevent acidification &
helps to keep the mineralization process
to continue.

• Loss of organic matrix is observed
during enamel maturation.
• It is due to enzymes that degrade
extracellular matrix protein into
polypeptide fragments.

• As ameloblast begin first series of
modulation cycle , they deposit basal
lamina.
• It adheres to enamel surface by
hemidesmosomes.
• Basal lamina contains laminin-332 , which
is essential for formation of
hemidesmosomes.
• Its deficiency leads to focal enamel
hypoplasia.www.indiandentalacademy.com

• Amelotin(ameloblast secretory product)
is also associated with basal lamina.
• Basal lamina rich in glycoconjugates
which helps in adhesion and also
regulate the movement of substance into
and out of enamel layer.

PROTECTIVE PHASE:-
• When formation of full thickness of
enamel is completed , ameloblast
reduces in size.
• Cells of stratum intermedium, stellate
reticulum and OEE reorganizes such
that their differentiation is not possible.
• Blood vessels invaginate deeply into
these cells to form a convoluted
structure known as papillary layer.

Reduced enamel epithelium
• When fully matured enamel is
formed, ameloblast and papillary
layer regresses and form reduced
enamel epithelium.
• Which protects the Enamel until
the tooth erupts.

DESMOLYTIC STAGE
• The reduced enamel epithelium proliferates and
induce atrophy of the connective tissue separating it
from the oral epithelium, so that fusions of the two
epithelia can occur.

• It is probable that the epithelial cells
elaborate enzymes that are able to
destroy connective tissue fibers by
desmolysis.
• Outer layer of REE and cells of oral
epithelium proliferate into degenerated
connective tissue.
• It forms a mass of cells over the
erupting tooth called epithelial plug.

• Cell death in the middle of the
epithelial plug leads to formation of
epithelial lined canal through which
tooth erupts without haemorrhage.
• Premature degeneration of the reduced
enamel epithelium may prevent the
eruption of a tooth.

• On the basis of ultrastructure &
composition two processes are involved
in amelogenesis:
Organic matrix formation
Mineralization

FORMATION OF ORGANIC MATRIX
• Secretory activity begins after first layer of
dentin is laid down .
• Islands of enamel matrix deposited along the
predentin.
• Dentinoenamel membrane : a thin continuous
layer of enamel formed along the dentin,
which is aprismatic.

Enamel Proteins
Amelogenin
Ameloblastin
Enamelin
Tuftelin
Amelotin

• Amelogenin :
• Major enamel matrix protein.
• It undergoes extracellular degradation by
proteolytic enzyme like MMPs into smaller
low molecular weight fragments.
• Many isofoms of amelogenin are produced
with similar terminal amino acid sequence
but with differing sequence at central
portion.

• The genes coding for amelogenin is present on
both X and Y chromosomes.
•Most of the secreted amelogenin is removed
during maturation.

• Amelogenin form thixotrophic gels , they
can be easily squeezed out by pressure from
growing crystals.
• It forms minute nanospheres between which
enamel crystals are formed.Thus it maintains
space between crystals.
• Its absence leads to formation of hypoplastic
teeth.

• Ameloblastin and enamelin are non-
amelogenin .
• They also undergo extracellular
degradation by proteolytic enzymes
but at a rapid rate.
• They help in nucleation and crystal
growth.

• Tuftelin is localized to DEJ and involved in
cell signaling.
• Recently new protein amelotin suggestive to
help in enamel formation.

• Proteinases are involved in extracellular
degradation of enamel protein.
• Enamelysin (MMP20) , an enzyme from
matrix metalloprotinase family (MMP) is
involved in this.
• Another enzyme kallikrein 4 functions as
bulk digestive enzyme during maturation
stage.
• Loss of function of these enzymes
hypomineralized enamel.

• The organic component & water are
lost in mineralization.
• Over 90% of initially secreted protein
is lost.
• Remaining protein forms envelops
around individual crystals.

MINERALIZATION
• Two stages :
Immediate partial mineralization (30%)
Completion of mineralization

Immediate mineralization occurs in
enamel matrix as it is laid down.
Nucleation is initiated by the apatite
crystallites of dentin on which enamel
is laid down.
Initial mineral formed is octacalcium
phosphate which is unstable and
converted into hydroxyapatite.

Gradual completion of mineralization
starts from height of the crown &
progresses cervically
It begins at dentinal ends of rods

Each rod mineralizes from the depth
to the surface.
The sequence of mineralization of
rods is from cusps or incisal edge
toward the cervical line.
Rate of enamel formation is 4 μ/day
and it is more in permanent teeth.

• Characterized by growth of crystals
• Ribbon shaped crystals increase in
thickness.
• Gradually the organic matrix becomes
thinned & more widely spaced to make
room for growing crystals
• Crystals increase in size from 1.5 µm to 25
µm
• Crystal sizes increase further after tooth
eruption due to ionic exchange with saliva

• Ca reaches the matrix via enamel organ.
• It happens by transcellular route.

Intracellular movement
of Ca in ameloblast .
&
Transcellular route of Ca
transportation.

• Ca transport may be active by carriers
or passive by simple flow of Ca across
the cell .
• The layer of ameloblasts has limited
but controlled permeability to ions like
calcium & fluoride.
• Initial nucleation occurs in dentin,
which cross the DEJ.
• No matrix vesicles have been
demonstrated in enamel mineralization.

• Nanospheres [which are self assembled
amelogenin fragments] are formed
between the 1st crystals of enamel.
• Through this organization amelogenins
control initial enamel biomineralisation.
• Initially crystals grow by fusion of
nucleation sites but once prismatic
structure is achieved crystals enlarge
preferentially in length than in width.

The matrix can control crystal growth by 2
mechanisms
• By breaking down in a controlled pattern
to provide space for new crystal
deposition
• By modulating the effect of directly
inhibiting molecules.
• 1st Could be achieved by formation of
appropriately oriented microchannels with
the same dimensions as crystals & crystal
form at growing end of the prisms.

The degraded matrix proteins accumulate
in extracellular space around the
ameloblasts & by inhibiting further
activity, control & limit the thickness of
enamel deposited.

Development of prismatic structure
• The 1st formed enamel is laid by proximal
process of tomes’ process of ameloblasts.
• It is prismless & contains small crystals
(15nm x1.6nm).
• As the ameloblasts form pyramidal
process tomes' process at distal end & the
enamel formed thereafter is prismatic.

• The crystals in head of prisms are
approximately parallel to long axis of the
prism but in tail deviate 60 o .

Incremental lines
• Enamel is formed incrementally
with periods of alternating activity
& quiescence.
• This leads in structural markings
known as Incremental Lines.
• They are of two types-
– Short periods: Cross striation (diurnal)
– Long periods: Enamel striae (weekly)

CROSS STRIATIONS
• They are seen as lines
traversing the enamel at
right angle to their long
axis.
• They reflect diurnal
rhythm, relating to rate of
secretion by ameloblasts

• At low power cross striations appears lines
2.5-6μm apart , being closer together near
DEJ .
• Cross striations result as subtle changes in
the nature of the organic matrix &/or
crystallite orientation .
• The prisms are seem to narrow at cross
striation.

ENAMEL STRIAE
• These are the structural lines
appear as brownish bands in
ground section of enamel.
• These represent the
incremental pattern of the
enamel & are known as
Incremental lines of Retzius.

• In transverse sections they
run in circumferential
pattern like rings of tree.

• In logitudinal section
they surround the tip of
the dentin.
• In the middle &
cervical part they run
obliquely.
• The striae overlying the
cusp & incisal edge
don't reach the surface.

• The term incremental lines –reflect
variations in structure & mineralization,
either hypomineralization or
hypermineralization

• The rhythmic alteration of periods of
enamel matrix formation & rest can be
upset by metabolic disturbances .
• Prolonged rest period -broadening of
incremental lines, rendering them more
prominent.

• In human tooth there are
about 7-10 cross striations
between adjacent striae in
any individual.
• So the striae are formed at
about weekly interval.
• Average distance between
adjacent cross striations is
4μm & between enamel
striae in middle part of tooth
is 25-35μm.

• In cervical areas enamel is formed
slowly& cross striation may be only
about 2μm apart & enamel striae 15-
20μm apart.

Clinical
considerations

HYPOPLASIA :
• Defective matrix formation :
pitting , furrowing ,even
total absence of enamel

Causes :
1. Systemic: nutritional
deficiencies,
endocrinopathies, febrile
diseases & chemical
intoxication .

2. Local :Single tooth
• Infection of pulp which causes infection
of periapical tissues of a deciduous tooth
that subsequently causes defect in enamel
in permanent successor.
3. Genetic : Transmitted as a mendelian
dominant character.
• Entire enamel of both deciduous as well
as permanent is affected.
• The surface of crown is yellow-brown ,
smooth , glossy and hard.

HYPOCALCIFICATION
• Defects in matrix structure and mineral
deposition: opaque or chalky areas on
normally contoured enamel surface .
• Enamel matrix : soft & acid soluble .

Causes:
1.Systemic hypocalcification :
Mottled enamel.(White patch
of hypominerelized & altered
enamel)
It is endemic and limited in
areas where fluoride content
in water is more than 1.5 PPM
.
1-1.2 PPM of fluoride reduces
susceptibility to dental caries.

2.Local :Injury during maturation stage.
3.Genetic : normal enamel matrix but
defective maturation .

• Discolouration of teeth
due to tetracycline .

CONCLUSION:
• The process of amelogenesis involves
cells that secrete enamel proteins
• Which immediately participate in
mineralization (30%).
• Once the entire thickness of enamel is
formed , it then acquires additional
mineral coincident with bulk
removal of enamel protein and water

• Yield a unique layer of 95% minerals.
• This process is under cellular control .
• The associate cells undergo significant
morphological changes throughout
amelogenesis, reflecting their evolving
physiological activity .

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Amelogenisis / dental implant courses by Indian dental academy

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Amelogenisis / dental implant courses by Indian dental academy