Amelogenesis-Dr.Arun Mohan

1. Amelogenesis
Dr. Arun Mohan
Dept. Of Oral Pathology
Royal Dental College

2. Life cycle of ameloblasts
Pre-secretory phase
◦ Morphogenic stage
◦ Organizing stage
Secretory phase
◦ Formative stage
Post-secretory phase
◦ Maturative stage
◦ Protective stage
◦ Desmolytic stage

3. Morphogenic stage
 Before production of enamel – determine
shape of the crown
 Cells are short columnar with large oval
nuclei almost filling up the cells
 Golgi bodies and centrioles will be at the
proximal ends and mitochondria will be
dispersed throughout the cytoplasm

4.  Proximal and distal terminal bars will develop
- intercellular attachments
- formed by thickening of cell membranes,
caused due to condensation of underlying
cytoplasm
 Cells are separated from d.papilla by a delicate
basal lamina
 Adjacent d.papilla is a light, cell-free zone

5. Organizing stage/Differentiation
stage
 Differentiates into ameloblasts
- becomes tall columnar (40µ in length)
- reversal of polarity (nucleus shifts to proximal
end)
- golgi apparatus and centrioles shift to distal
part
- mitochondria concentrate in the proximal
cytoplasm
 In this stage ameloblasts exert an organizing
influence on adjacent dental papilla cells – and

6.  Towards the end of this stage odontoblasts
secrete dentin
 Deposition of dentin helps ameloblasts attain
secretory function – reciprocal induction
 Source of nutrition changes to dental sac –
more capillaries in dental sac and collapse of
stellate reticulum
 Basal lamina disintegrates

7. Pre-secretory phase

8. Life cycle of ameloblasts
Pre-secretory phase
◦ Morphogenic stage
◦ Organizing stage
Secretory phase
◦ Formative stage
Post-secretory phase
◦ Maturative stage
◦ Protective stage
◦ Desmolytic stage

9. Formative stage
Deposition of enamel matrix (proteins)
and partial mineralization

10.  In this stage amelobalsts are fully differentiated
and and are structurally suited for secretion
- cells have abundant mitochondria, RERs,
free ribosomes, well developed golgi bodies
- basal cytoplasm has abundant secretory
granules packed with enamel proteins
- abundant microtubules are present which
helps in movement of secretory granules to
basal plasma membrane

11.  Secretory granules filled with
synthesised enamel proteins – move
towards basal plasma membrane and
release the proteins by exocytosis

12.  Proximal and distal terminal bars –
provides attachment between cells
 Distal terminal bars prevents entry of
Calcium ions and other molecules
from ECF

13.  In the initial stage of secretory stage, basal
region is flat.
 After initial deposition of enamel (aprismatic
enamel), a conical cytoplasmic process
develops at the base
– Tome’s process
 Cytoplasm of Tome’s process contain secretory
granules, microtubules and few mitochondria

15.  Tome’s process is responsible for
enamel rods and interrod enamel
 It is lost before the last phase of
secretion
- just before deposition of surface
aprismatic enamel

16. Life cycle of ameloblasts
Pre-secretory phase
◦ Morphogenic stage
◦ Organizing stage
Secretory phase
◦ Formative stage
Post-secretory phase
◦ Maturative stage
◦ Protective stage
◦ Desmolytic stage

17. Maturative stage
 Enamel maturation (full mineralization), occurs in
incisal and occlusal areas first and then extends to
cervical region.
 During maturative stage ameloblasts- reduce in
height, volume and organelle content
 They have dual functions
- Ruffle bordered : microvilli in their distal
extrimities
- mineralization
- Smooth bordered : reabsorbs protein and water

18.  Stratum intermedium lose their
cuboidal shape and becomes spindle
shaped

19. Protective stage
 Basal plasma membrane loses ruffled ended
borders.
 Secrete a protein similar to basal lamina onto
the surface of newly formed enamel –
Nasmyth’s membrane
 Ameloblasts develop hemidesmosomal
attachments to dental lamina

20.  After deposition of full thickness of epithelium,
ameloblasts cease to exist as a well- defined
layer
 It becomes indistinguishable from OEE and
stratum intermedium – form a 2-3 layered
stratified epithelial covering of enamel – Reduce
Enamel Epithelium
 REE protects mature enamel from coming into
contact with CT ( - thus prevents resorption of
enamel and deposition of cementum over
enamel), till the tooth erupts

22. Desmolytic stage
 REE induces atrophy of CT separating it
from oral epithelium - facilitating eruption of
tooth
 REE secretes enzymes like collagenases
 REE proliferates and fuses with the oral
epithelum to form a solid plug of epithelial
cells – its central cells degenerate and to
from a canal through which bloodless
eruption of tooth takes palce

25. Amelogenesis
 Enamel matrix deposition
 Development of Tome’s process
 Mineralization

26. Enamel matrix deposition
 The secretory ameloblasts which are
structurally suited for synthesis and secretion
of enamel proteins, start secretory function
after a layer of dentin is deposited

28.  After a layer of dentin is deposited,
ameloblasts synthesize and secrete enamel
through the distal part.
 The secretory granules packed with enamel
proteins fuses with the basal plasma
membrane and release the matrix protein
against the newly formed dentin by a process
called exocytosis.
 Begins in cusp tips and progresses outward
and cervically
 With progress of matrix deposition –

30.  In the early stages the matrix contains 20-
30% proteins – amelogenin and non-
amelogenin (enamelin, tuftelin,
ameloblastin)
 The proportion of proteins gradually
decreases during mineralization
 Tuftelin– hypothesized to induce nucleation
 Amelogenins found in intercrystalline
spaces – hypothesized to control crystal
growth

31. Development of Tome’s
process
 After deposition of about
30µ thickness of matrix is
deposited – ameloblasts
develop a conical process
at the base – Tome’s
process
 It extends beyond the
distal terminal bars
 Matrix deposition and
mineralization takes place
only through this area – in

32.  The distal terminal bars have fine radiating
actin filaments extending into cytoplasm –
forming a septa which partially separates
Tome’s process from rest
 Mostly contains secretory granules,
microfilaments, mitochondria and some
Golgi bodies

33.  Secretions from proximal part of
secretory end, occur early and form
interrod enamel
 Secretions from distal part (Tome’s
process) secrete rod enamel
(perpendicular to interrod enamel) into
the pit like outline formed by interrod
enamel

37.  Head of one enamel rod is formed by
one ameloblast and tail is formed by
three surrounding amelobalsts
 Thus each enamel rod is formed by 4
ameloblasts and each ameloblast
contributes to 4 enamel rods

40. Mineralization
 Ameloblasts are involved in both secretion
of enamel matrix and its mineralization
 Ca2+ ions circulation ECF
actively transported to cell
attached to Calcium binding proteins
transported to distal cytoplasm
Ca2+ ions are extruded through secretory
end
by active process

41.  2 stages :
- Immediate partial mineralization
- Maturation

42. Immediate partial
mineralization
 In formative stage, immediately after matrix is laid
down
 25-30% of eventual total mineral content
 Mineralization begins in DEJ – rich in Tuftelin
- crystals perpendicular to DEJ
 After nucleation ameloblasts secrete matrix rich in
amelogenin – regulate crystal size
 First forms octacalcium – unstable – immediately
converts to HA

43. Maturation
 Gradual completion of
mineralization
 Each rod matures
from DEJ to surface
and the sequence of
maturing rods is from
cusps/incisal edges
towards cervical line
 Thus, occlusal
regions mature ahead
of cervical regions

44.  Growth of crystals formed in primary
mineralization stage – regulated by enamel
proteins, mainly amelogenin
( proteolytic enzymes cleave amelogenin
protein and thus helps in growth of individual
crystals)
 During maturation, crystal thickness increases
from 1.5µ to 25µ
 Rate of deposition - 4µ/day

45. Ameloblast modulation
 As maturation progresses, proteins and
water needs to be reabsorbed and broken
down by ameloblasts – to create space for
growing crystals
 Leads to modulation – alternating cycles of
ruffle-ended and smooth-ended
ameloblasts

46.  Ruffle ended ameloblasts have microvilli
projections in basal membrane, leaky
proximal and tight distal terminal bars –
contains lysosomes, and vesicles containing
matrix proteins
 Smooth ended ameloblasts have smooth
basal membrane, tight proximal and leaky
distal terminal bars

47.  Ruffle-ended cells are believed to actively secerte
Ca ions and release various proteases that break
down enamel proteins
 Broken down matrix remnants are reabsorbed or
leaks through the space in b/w smooth-ended
ameloblasts

48.  Thus about 90 % of enamel proteins
secreted are reabsorbed – process
unique to enamel formation
 Finally forms enamel with 96% mineral
content