1. Amelogenesis involves the life cycle of ameloblasts from the pre-secretory to post-secretory phases as they form enamel. 2. In the secretory phase, ameloblasts deposit enamel matrix proteins and undergo partial mineralization, developing Tome's process which is responsible for enamel rod and interrod formation. 3. Enamel maturation then occurs, fully mineralizing the enamel from the dentin-enamel junction outward in a gradual process modulated by alternating ameloblast types.

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