Chp 7- Enamel 2.pdf bds 1 year oral biology ten cates

Oral biology ten cates enamel

1. BY
Dr Maliha Shahbaz

2. Learning Outcomes
Define Amelogenesis

Enumerate different morphological and functional
phases/stages which an ameloblasts passes through
during amelogenesis

Explain morphogenetic, histodifferentiation and
secretory phases of amelogenesis in terms of
histological appearance and dimensions of cells

3. Introduction
•Process by which enamel is formed incrementally by
ameloblasts is known as Amelogenesis .

•Two step process:
•Secretory phase: Initially the enamel formed is only
partially mineralized to about 30%.
•Maturation phase: After full thickness of enamel layer
has been formed, organic matrix and water is gradually
removed; minerals are added and crystals grow wider
and thicker to attain greater than 96% mineral content.

4. Stages/Phases of Amelogenesis
Amelogenesis has been described in as many as six
phases but generally is subdivided into three main
functional stages referred to as the
1. Presecretory stage (Morphogenetic phase,
Organization and histodifferentiation phase)
2. Secretory Stage
3. Maturation Stage (Transition Phase and maturation
Proper, protection phase)

5. Life Cycle of Ameloblast

6. 1- Morphogenetic phase
2-Histodifferentiation phase

7. Presecretory stage
The presecretory stage includes all activities of
the future ameloblast before the secretion of the
main component of the enamel matrix.
This stage has two principal features:
1.Differentiation of the pre-ameloblasts
2.Formation and subsequent resorption of a
basal lamina.

8. 1. Morphogenetic Phase
Early bell stage
Shape of crown determined
Basal lamina present
Cuboidal/low columnar cells of IEE
Uniform in length and arranged in a
single row
Large central nucleus
Mitotic activity present in IEE
Proximal junctional complex
present (proximal end - towards the
stratum intermedium)


Proximal end
Distal end
nucleus
JC
BL

9. poorly developed Golgi
elements in the proximal
portion of the cells (facing
the stratum intermedium),
Mitochondria and other
cytoplasmic components
evenly scattered throughout
the cell

10. 2. Histodifferentiation Phase
Late bell stage
Mitosis ceases
Tall columnar cells (40 µm long, 5µm
wide)
Proximal located nucleus (Polarized)
Distal located RER and Golgi complex
Basal lamina disintegrates
Distal Junctional complex appears.
Tightly hold the ameloblasts and
determine at different stages that what
may, and may not, pass between them to
enter or leave the enamel.
Secretion of enamel and dentine proteins
(transient) - - - - - - - - - - - - - - - -

11. The Ameloblast becomes a
polarized cell, with the
majority of its organelles like
Golgi complex and RER
situated in the cell body
distal to the nucleus
(supranuclear cytoplasm)

Mitochondria cluster in the
infranuclear compartment
(proximally), with only a few
scattered through the rest of
the cell.

Distal
Junctional
complex
Proximal
junctional
complex
BL disintegrates

12. Morphogenetic and
Histodifferentiation Phases
odontoblasts

13. Secretory phase
At late bell stage of tooth development
Ameloblast cells present as Taller columnar
cells (50µm long, 5-10 µm wide)
Electron microscope E/M
Golgi apparatus; developed and condensed
occupying a major part of the central core
Mitochondria clustered in the proximal
region
RER and vesicles increased in number
reflecting their intense synthetic and
secretory activities.
Cell attachment.. Proximal and distal
junctional complexes

14. The cells acquire intense
synthetic and secretory
activity
mRNA for enamel proteins
are translated by rough
endoplasmic reticulum
Modified by Golgi complex
packed into secretory
granules
These granules migrate to
distal extremity of the cell,
into Tomes Process

15. Secretory Phase
Enamel proteins and water (organic
matrix) are secreted through secretory
vesicles at the distal end of the cells
against the newly formed mantle dentin.
This matrix is immediately mineralized by
deposition of hydroxy apatite crystals
So an un-mineralized enamel matrix is
never seen in enamel – mineralization is
30% at this stage.
Initially crystallites of hydroxy apatite are
thin (1.5 nm thick), needle like and much
smaller as compared to mature enamel
crystals
Enamel thickness completely formed
during this phase.





dentine

16. Early Apposition
©Copyright 2007, Thomas G. Hollinger, Gainesville, Fl

17. Secretory phase
Ameloblasts as they move away
from the forming enamel
develop Tomes’ process at the
distal secretory end (i.e.
towards the enamel)
Initially the tomes’ process of
ameloblast comprise only of
proximal portion. Thus initial
thin layer of enamel doesn’t
contain enamel rods (aprismatic
enamel).
As the ameloblasts migrate
migrate away, they develop the
distal portion of tomes’ process.

18. Proximal portion
of tomes’
process
Distal portion
of tomes’
process
The shape of the Tomes process of ameloblasts is responsible for the prismatic
appearance of Enamel
Proximal portion: from junctional complex to surface of enamel layer.
•result in formation of interrod enamel
Distal portion: penetrates into and interdigitates with enamel beyond initial layer.
•Result in formation of enamel rods

19. •Formation of interrod is always a step ahead as it delimits the cavity into
which rod enamel is formed.
•Both sites have same composition, but they differ only in orientation of
their crystallites.

20. Scanning electron micrograph of the surface of a developing human tooth
from which ameloblasts have been removed. The surface consists of a
series of pits previously filled by Tomes’ processes, the walls of which are
formed by interrod enamel

21. Secretory Phase
Prismatic enamel (having rods and inter-
rods) formed making up the bulk of the
enamel
Final enamel is also aprismatic (i.e.
without rods and inter rods arrangement
because of absence of tomes process)
It means that prismatic enamel is
sandwiched between aprismaric enamel

Aprismatic enamel (initial
and final enamel increment
Prismatic enamel (in
middle, bulk of enamel)

22. Initial
enamel
Dentin
Final
enamel
SEM images: The (A) first (initial) and (B) last (final) enamel
layers are aprismatic; that is, they do not contain rods.
A
B
goof
Prismaticsandwichedinmiddle
initial
A
prismatics
final
layer
that's
why
cover
outside

23. Secretory Phase
Capillaries of Dental follicle
come closer to proximal end
of ameloblasts during
secretory phase without
disrupting the basal lamina
of outer aspect of enamel
organ to form papillary
layer.

Enamel is formed
incrementally

25. Interrod Enamel
Rod Enamel
Pre Enamel (rodless)

26. Early mineralization

27. Secretory Phase
Enamel proteins secreted are
amelogenins and non-amelogenins
(ameloblastin, enamalins)

Role of ameloblastin: causes
adhesion of secretory ameloblasts
to mineralized enamel surface

28. Functional Stages in the Life Cycle of Ameloblasts


1.Morphogenetic stage
2.Histodifferentiation stage
3.Initial secretory stage ( no Tomes’ process)
4.Secretory stage (Tomes’ process)
5.Ruffled-ended ameloblast of maturative stage
6.Smooth-ended ameloblast of maturative stage
7.Protective stage/Post maturation stage

29. Learning Outcomes
Describe important histological and functional
changes in ameloblasts during transition and
maturation phases

Define Nasmyth’s membrane, primary enamel cuticle,
modulation

30. Maturation Phase

Amelogenesis is a slow process.
 takes about 5 years to complete.

Maturation phase is the longest phase of Amelogenesis.
It takes about 2/3
rd
of the time taken in amelogenesis.

31. Maturation Phase
The enamel which is deposited in secretory phase has a
high content of water and enamel proteins (about 70%)
and a low content of minerals (about 30%)

The process that converts this young, immature enamel
into fully mineralized and mature enamel is known as
Maturation.

Crystal growth during the maturation stage occurs at the
expense of matrix proteins and enamel fluid that are
largely absent from mature enamel.

PP9

32. Can be divided into:
1.Transition Phase
2.Maturation Proper

33. Transition Phase
A brief period in which the ameloblasts change from a
secretory form to a maturation form is known as transition
phase

Enamel formative proteins (amelogenins and non
amelogenins) secretion stops

Withdrawal of tomes’ process.

Ameloblasts become short and the cell organelles and
volume decreases.

34. Transition Phase
Programmed cell death occur known as Apoptosis (in
which 25% ameloblasts die in transition phase and
another 25% die in maturation proper = total 50% cells
die)

Apoptosis is defined as programmed cell death
without any physical insult, injury, or any external
causative factor. The cell itself sense the need to die
and it occurs without any inflammatory response. The
adjacent cells and environment remains unaffected

35. Apoptosis
The Bcl-2 family of proteins,
comprising antiapoptotic
and proapoptotic proteins
is a major regulator of
apoptosis.

36. Necrosis
Trauma
Cells and organelles swell
Cell lysis and Invasion of
phagocytic cells leading to
Inflammation

37. Transition Phase – at the end
At the end of transition phase and beginning of maturation
proper  Basal lamina reforms

The basal lamina in this case is unique because it lacks
Collagen type IV and it is facing a mineralized surface and
attached with it by means of hemidesmosomes which have
laminin-322 proteins (previously known as laminin-5),and
also Amelotin (which is secreted by ameloblasts)

No Tomes process is present at the distal end of
Ameloblasts. It is flat again

Microvilli appear on cells of Outer enamel epithelium,
Stratum intermedium and Stellate reticulum

38. Maturation Proper
Matrix proteins and water needs to be removed so that
space is created for the growth of the crystals

During the maturation phase already formed
hydroxyapatite crystals (of secretory phase) grow in
width and thickness but new crystals are not
deposited.

Attachment proteins are only secreted (amelotin and
apin) which keeps the maturation ameloblasts and
basal lamina attached with the mineralized surface

39. Maturation Proper
•MAIN EVENTS:
•Degradation of enamel proteins
by proteinases (protein
degrading enzyme -Kellikerin 4)
•Withdrawal of great amounts of
degraded proteins &water
from enamel
•Rapid influx of mineralizing
salts (calcium and phosphate
ions)
•Enamel mineralization increased
from 30% to 96%


PP
Q1

40. Maturation Proper
Modulation between ruffle ended
and smooth ended ameloblasts
Modulation is a reversible change
in cell activity and morphology.
Difference in permeability of
junctional complexes

Significance of the
modulations:
maintaining an environment that
allows accretion of mineral content
and loss of organic matrix, in part
through alterations in permeability
of the enamel organ.




Ruffled Smooth
D
P
They can modulate once
every 8 hours)

41. Ruffle ended Ameloblasts
Ruffled ended ameloblasts have tight
distal junction and leaky proximal
junction

Cells spend 8o% of its lifetime in this
form

show considerable endocytotic
activity and they absorb protein
break down product.

The route by which calcium moves
from blood vessels to enamel is via
ruffle ended ameloblasts. (because
their distal ends are tight).
Leaky
Tight
Imp
Doonly
major
differences

42. Ruffle Ended ameloblast
Contain:
numerous lysosomes,
calcium-binding proteins, and
membrane-associated calcium-
adenosinetriphosphatases or Calcium
ATPase (promote the pumping of calcium
ions into the maturing enamel).

SO.. Active secretion of matrix degrading
enzymes (proteinases) AND
incorporation of mineral ions into
crystals occurs mainly in relation to the
ruffle-ended cells

Some protein fragments from the enamel
layer also may be taken up by endocytosis
across the membrane infoldings of the
ruffled border.

43. Smooth Ended Ameloblasts
Smooth ended ameloblasts have leaky
distal junctions and tight proximal
junction
Cell spends 20% of its lifetime in this form

Have almost no membrane calcium-
ATPase activity (thus don’t pump Ca ions),

Show little endocytotic activity, thus
Polypeptide fragments leaving the enamel
likely pass between the leaky distal
junctions of smooth-ended cells and
diffuse laterally among the ameloblasts to
be taken up along their basolateral
surfaces.

leaky
Tight
Imp

44. Ruffled ended ameloblasts secrete lysosomes, calcium and phosphate ions into the
maturing enamel through the ruffled border

Smooth ended ameloblasts permit exit of degraded proteins and water from the maturing
enamel through the leaky distal junctions

46. Significance of modulation
Ruffle ended Ameloblast
The acidification associated with
ongoing mineral accretion during
maturation causes ruffle-ended
ameloblasts to produce
bicarbonate ions.
This process continuously alkalizes
the enamel fluid to prevent:
reverse demineralization of
the growing crystallites and
maintain pH conditions
optimized for functioning of
the matrix degrading enzymes,
which prefer slightly acidic to
near neutral conditions
Smooth ended Ameloblast
Low pH triggers the modulation of
smooth ended amelobalsts
Interstitial fluids that may leak into the
maturing enamel during the smooth-
ended phase may also contribute to
neutralizing the pH of the enamel fluid.
Bulk-Degrading enzymes act
extracellularly to digest various matrix
proteins into fragments small enough to
leave the enamel.

Polypeptide fragments leave the
enamel through distal (leaky) junctions
of smooth ended ameloblasts and
diffuse laterally among the ameloblasts
to be taken up along their basolateral
surfaces.

48. Post maturation Phase
Flattened ameloblasts
Primary enamel cuticle (amorphous protein layer 1 um
thick) secreted by post maturation ameloblasts
Reduced enamel epithelium forms (comprising of
remains of enamel organ)
Reduced enamel epithelium and the primary enamel
cuticle together are known as Nasmyth’s Membrane
and during eruption this membrane protects the
enamel surface. This is an investing layer associated
with the crowns of un-erupted teeth only.

50. Functional Stages in the Life Cycle of Ameloblasts


1.Morphogenetic stage
2.Histodifferentiation stage
3.Initial secretory stage ( no Tomes’ process)
4.Secretory stage (Tomes’ process)
5.Ruffled-ended ameloblast of maturative stage
6.Smooth-ended ameloblast of maturative stage
7.Protective stage/Post maturation stage

51. ASSIGNMENT
Enumerate the life cycle of ameloblast. Describe each
phase with diagrams.

Draw them in your practical copy.