15. Amelogenesis Flashcards
Amelogenesis is a 2 step process, which are …
- when enamel first forms, it’s partially mineralised (around 30% by weight)
- enamel is fully formed (mature) and mineral contents increase to around 96%
At late bell stage, shortly after dentine formation, what happens in amelogenesis?
- inner enamel epithelium cells initiate morphologic changes in enamel organ
- ameloblasts move away from dentine after formation of first enamel layer
Ameloblast change their … and … during life cycle
- shape
- function
3 main functional stages of amelogenesis
- presecretory
- secretory
- maturation
6 stages of ameloblast life cycle in the 3 stages of amelogenesis
in presecretory
- morphogenetic stage
- histodifferentiation stage
in secretory
- initial secretory stage without Tomes process
- secretory stage (Tomes present)
in maturation
- ruffle-ended ameloblast
- smooth ended ameloblast
- protective stage
Explain ‘morphogenetic phase’
- shape of crown is established during bell stage
- IEE cells are short with centrally placed nuclei, basal lamina seen, dentine not yet mineralised
Explain histodifferentiation phase
- IEE cells differentiate into ameloblasts
- cells become taller (establishment of polarity)
- development of ability to synthesize proteins
- basal lamina fragmented
Explain ‘initial secretory stage’
- ameloblasts elongate and secrete enamel matrix
- form partially mineralised initial layer of enamel (rodless enamel layer)
- ameloblasts move away from dentine after formation of first enamel layer
- they do this to form secretory surface (later called proximal portion of Tome’s process) and to develop distal portion of it
Explain secretory stage
- ameloblasts elongate with distal portion of Tome’s process to form enamel rod and proximal protion of Tome’s process to form interrod enamel
- as enamel thickness increases and enamel rod grows width, distal portion elongates, becomes thinner and gets squeezed between rod and interrod enamel
- it creates a narrow space between them which fills with organic material forming rod sheath
- once outermost layer of enamel is being formed, ameloblast becomes shorter, loses distal portion of Tome’s and forms thin rodless enamel layer
2 proteins in enamel matrix
- enamelins
- amelogenins
Of the proteins in enamel matrix, …% are enamelins and …% are amelogenins
10
90
Role of enamelins
- first matrix element secreted
- due to hydrophilic and acidic nature, role in calcium and phosphate conc and crystal nucleation
Role of amelogenins
- secondary to enamelin secretion
- due to hydrophobic nature, 2 roles in mineralisation
Enamel biomineralisation in secretory phase
- assembly of enamel matrix occurs extracellularly and resides in structure of protein
- amelogenins and mineralized enamel ribbons secreted
- amelogenins form nanospheres
Enamel biomineralisation in maturation phase
- matrix disassembled during this phase
- collapse of amelogenin nanospheres and thickening of crystal
- ameloblasts pump calcium, phosphate and carbonate ions into matrix and remove water and degraded protein
In the bell stage, IEE layer has what shape cells?
IEE cells then …
Ameloblasts begin to … and actively deposit …
- short columnar undifferentiated
- differentiate into ameloblasts that induce differentiation of odontoblasts
- secrete enamel matrix
- enamel matrix
Explain post decretory ameloblast proteins
- proteins (enamelysin and kallikrein4) do post secretory processing and degradation of enamel proteins
- non-amelogenins (like enamelin and ameloblastine) at periphery of enamel prisms form enamel sheath
- basal lamina protein (amelotin) produced in maturation and present in junctional epithelium
The maturation stage sees a growth in … and …
- width
- thickness
of pre-existing cystals
2 phases of maturation stage
- transitional
- maturation
Explain transitional phase
- after enamel thickness formed
- decrease in height and volume of ameloblasts
- 50% of ameloblasts die
Explain maturation phase
- removal of water and proteins in matrix
- transport of ions
- cyclic modulation of ameloblasts (alterations in permeability of enamel organ)
How does cyclic modulation of ameloblasts occur in ruffle ended stage?
- leaky junctions between ameloblasts at proximal/basal end
- tight junctions at the distal/enamel end
- selective transfer of calcium ions across ameloblast layer
How does cyclic modulation of ameloblasts occur in smooth ended stage?
- leaky junctions distally
- proteins and water exit
- interstitial fluid leaks into maturing enamel between ameloblasts
- trace elements incorporated in interstitial fluid and enter enamel fluid in enamel layer
Explain movement of ions in maturation stage
- selective transfer of ions across ameloblast layer between interstitial and enamel fluid
- rate of calcium transfer is 4 times higher in maturation phase
- active transport of calcium out and passive transport of trace elements in
What 4 structures form in the maturation stage?
- papillary layer
- primary enamel cuticle
- reduced enamel epithelium
- Nasmyth’s membrane
What is the Nasmyth’s membrane?
- covers enamel of unerupted teeth
- leaky
Role of reduced enamel epithelium
- protects unerupted crown surface from resorption by osteoclasts and from deposition of cementum
- REE on Nasmyth’s membrane prevents bleeding
- forms junctional epithelium
Role of primary enamel cuticle
- acquired pellicle on exposed crown
Explain the gradient of mineralisation of enamel
- gradient increase from EDJ to surface
- and from occlusal to cervical region
Which are more mineralised? Primary or secondary?
Why?
- secondary
- more time in maturation phase
Throuhgout life, teeth have a dynamic what?
- dynamic enamel demineralisation/remineralisation cycle
- mature enamel is in chemical equilibrium with surrounding media
Fluoridation benefits
- water fluoridation (1ppm) means incorporating fluoride ion into HA crystal and becomes more resistant acid and reducing dental caries
Fluoridation risks
- long term excessive consumption of fluoride ions
- disturbs ameloblasts causing mottled enamel fluorosis
How does acid-etching work?
- etching enamel surface with acid (called enamel conditioning) beneficial for adhesive dental restorative materials
- appears matter and whiter
Explain abnormal enamel
- related to poor oral hygeine e.g white spot legions
- related to enamel formation or defects in all teeth (amelogenesis imperfecta -fluorosis etc) or in some teeth due to non-systematic or systematic causes affecting teeth developing like enamel hypoplasia or hypomineralisation
Enamel defects which affect all teeth are caused by what?
Give examples
- environmental and systemic factors like febrile disease, treatment with tetracycline antibiotics, chronic ingestion of fluoride ions/fluorosis
- genetic factors like genetic dysplasia (syndromic vs non-syndromic, pattern of inheritance - autosomal dominant, recessive or x-linked)
Explain amelogenesis imperfecta
- inherited defects
- can be hypoplastic, hypocalcific or hypomaturation
- associated with pulp calcification, delayed eruption and other issues
Explain molar incisor hypomineralisation
- affects teeth formed in 1st year of life (permanent first molars and incisors)
- molars worse affected than incisors
- associated with opacities and loss of enamel in severe cases - caries
- more common in children who take amoxicillin in first year of life, often for otitis media
- associated with hypomineralisation
Explain ‘chronological/linear enamel hypoplasia’
- disruption of enamel formation
- causes deep grooves to form on tooth surface
- caused mainly by poor nutrition, stressful event or high fever during tooth development
