Odontogenesis Flashcards
Odontogenesis
The process of tooth development
Primary dentition features
20 teeth, develops during the prenatal period (in the uterus)
Permanent dentition feature
32 teeth that succeed the primary teeth
Histogenesis
Differentiation of undifferentiated cells to make new tissues
Mineralised dental tissues
Enamel, denting and cementum and unmineralised dental tissues - dental pulp and periodontium
Dental laminate
Contributes to development of teeth
Vestibular lamina
Contributes to the vestibule of the mouth, delineating the lips and cheek from the teeth formation of sulcus
Initiation stage
First stage of tooth development
Bud stage
Second stage simple, spherical to ovoid, epithelial condensation poorly morphodifferentiated and histodifferentiated.
Cap stage
Third stage of tooth development in which the tooth germ grows into a cap shape
Bell stage
Fourth stage, in which differentiation occurs to furthest extent and starts to resemble a bell
Crown stage
Some text books count as a stage-some call it late bell stage- this is with fully developed tooth
External enamel epithelium
Outer layer of cuboidal cells that limits the enamel organ
Stellate reticulum
Of epithelial origin but behave like mesenchymal cells - synthesis of collagen I, II, and III
Stratum Intermedium
Two or three layers of cells sitting above IEE. Express alkaline phosphatase (for transport) like SR but not IEE
Inner enamel epithelium
Columnar shaped cells rich in RNA. Connect by desmosomes to each other.
Ectomesenchyme
Interesting group of cells whose origin is very controversial, some think neural crest others mesodermal somites.
What happens first (week 6)?
Condensation of ectomesenchyme within the mesenchyme. Causing the formation of primary epithelial band within the ectoderm.
What results in the condensation of ectomesenchyme?
Lef-1 which switches on fibroblast growth factor-8 (FGF-8) which interacts with the ectomesenchyme. This is because the ectomesenchyme have receptors for this GF.
What does the primary epithelial give rise to?
Dental lamina and Vestibular lamina
How is the sulcus formed?
Primary epithelial band causes the vestibular lamina to grow and then it breaks down (apoptosis) forming the vestibule between the teeth and lips/cheek.
Week 7: Dental and vestibular lamina
Differentiation between the two sides
Week 9: Dental and vestibular lamina
Apoptosis in the vestibular lamina
Week 12: Vestibular lamina
Labial/buccal sulcus (vestibule) has been formed as a result of apoptosis.
Three phases of tooth development
Initiation, morphogenesis, and histogenesis
Epithelium (ectoderm) give rise to…
Enamel, hyaline layer of the root
Mesenchyme (ectomesenchyme) give rise to…
Dentine, pulp, cementum, periodontium (PDL & bone)
Which tissue initiates tooth development?
Epithelium initially has the odontogenic potential, then the ectomesenchyme
Which components form the tooth germ?
Enamel organ, dental papilla and dental follicle
Odontogenesis: week 8
Bud stage: Dental lamina is growing down into the mesenchyme.
At this point, very little/no histodifferentiation or morphogenesis.
Odontogenesis: week 11
Cap stage: enamel organ is starting to look cap-shaped, still attached to the enamel organ. The beginning of histodifferentiation.
Odontogenesis: week 14
Bell stage: Dental lamina is beginning to shrivel. Dental papilla and dental follicle is distinguishable.
4 distinct layers seen during bell stage which make up the “enamel organ”
- Stellate reticulum
- Stratum intermedium
- Outer enamel epithelium
- Inner enamel epithelium
What is the function of dental papilla?
Forms odontoblasts (dentine) and pulp
Dental follicle function
Forms cementum, PDL and bone
Features of inner enamel epithelium
- Basal lamina conforms to future ADJ
- Columnar cells at cusp tips
- Role: forms ameloblasts —> enamel
Features of stratum intermedium
- Flattened cells
- Role: synthesis and transport to and from IEE
Features of Stellate reticulum
- Star-shaped
- Fluid filled extra cellular space
- Osmotic gradient due to extra cellular GAGs
- Role: maintenance of shape and protection
Features of outer enamel epithelium
- Cuboidal cells
- Role: maintenance of shape and exchange
Odontogenesis: week 17
Late-bell stage: dental lamina has started to disintegrate (leaving cell nests/rests these can have cystic potential)
Transitory structure that may be seen
- Enamel knot (signalling centre - condensation of BMP, Shh etc)
- Enamel niche (2D -hole, 3D -depression)
- Enamel septum (aid change in shape from cap-bell)
Control: initiation stage
Signal from dental epithelium (Shh, BMP4, FGF8 have been found to diffuse into the mesenchyme)
Control: bud-stage
Initial epithelial signalling has switched on genes such as Msx1 and Pax9 —> now we see the mesenchymal cells signalling back to the epithelial cells (see Msx2, p21, BMP 2 being switched on).
Control: cap stage
Enamel knot signalling - signalling results in a certain amount of cell death, this activates FGF 4/9 increasing proliferation of other cells in towards the mesenchyme.
What knockout genes could you use to prove the control of the early stages of odontogenesis (initiation, bud and cap stage)?
Msx1
What study showed that you must have two specific things present for tooth development to initiate?
Study showed that Msx1 and Msx2 (the controllers of tooth development) were only activated when dental epithelium and dental mesenchyme were present (i.e. couldn’t have a non-dental mesenchyme or non-dental epithelium).
Key genes: initiation
Bmp2/4, FGF8, Msx1/2, Shh
Key genes: proliferation condensation
Bmp2/4, DLX1, Pax9, syndecan
Key genes: morphogenesis
Msx1/2, collagens
Why is extra cellular matrix important in tooth development?
Important for cell adhesion, migration, provides an area for the cells to proliferate, polarisation (going in the correct direction), allows differentiation of cells to take place.
What components of the extra cellular matrix are important for tooth development?
Expression of type I, III, IV collagens, laminin, various proteoglycans - basement membrane of developing tooth.
Integrins
Cell receptors which bind to extra cellular matrix
Examples: alpha V, beta 5 (which bind to tenascin, fibronectin).
What happens to integrins during tooth development?
Changes in expression
Intact basement membrane
Necessary for tooth development. Contains type 4 Collagen, tenascin and syndecan.
Mesenchyme needs to be able to bind to this so it can then differentiate into odontoblasts (after this differentiation, the basement membrane is broken down and the cells can interact directly with the ameloblasts to form dentine).
Syndecan
Mainly expressed in the mesenchyme where it is in contact with the epithelium (also expressed in the epithelium).
Tenascin
Expressed in the mesenchyme, more expressed on the buccal than the labial side.
Where are they expressed? Collagen IV, fibronectin and tenascin
Collagen IV: basement membrane
Fibronectin: In the ECM of mesenchyme (in both the follicle and the papilla)
Tenascin: just found in the dental papilla
BMP4
- Bone morphogenic protein 4: a type of Cytokine.
- Found to be expressed in the dental lamina at day 11.
- Also found to be expressed in the dental mesenchyme at day 12.5 and 13.
- Studies has shown its influence on tooth development (if you add BMP4 to knockout MSX1 mice you still get the early stages of tooth development going ahead).
Week 14
Crown formation
Signalling from bell to crown stage
- Cervical loop cells divide (root formation)
- Signalling from dental papilla to inner enamel epithelial cells (causing them to mature - elongate and polarise)
- Inner enamel epithelial cells will then signal back to the dental papilla (causing these to differentiate and they will invade into the acellular zone)
- DP will then signal back to IEE (mature…AGAIN into preameloblasts).
Dental papilla cells mature to…
Odontoblasts which form the predentine —> dentine.
Inner enamel epithelial cells mature to…
Pre ameloblasts —> ameloblasts which forms the enamel.
What is happening at A and B?
A - IEE cells showing that as signalling occurs (moving upwards), the cells have started to differentiate, much more columnar and nucleus has moved towards the stratum intermedium. Now pre ameloblasts
B - differentiating into odontoblasts
A - Odontoblasts
B - Predentine
C - Pre-ameloblasts
A - Ameloblasts
B - Stratum intermedium
C - Stellate reticulum
A - Dental papilla
B - inner enamel epithelium
C - Stratum intermedium
D - Stratum reticulum
E - Outer enamel epithelium
F - Dental follicle
G - cervical loop
What is the cervical loop?
Where the outer enamel epithelial cells meet the inner enamel epithelial cells. Begins to differentiate the dental follicle from the dental papilla.
What does the signal from the dental papilla to the IEE at the cervical loop do?
Controls number of cell divisions
How does the cervical loop develop?
How do odontoblasts mature?
Ameloblasts matures first, sends out a signal and the peripheral ectomesenchymal cells in the papilla are going to start to divide, some will migrate down underneath what is going to become the odontoblast layer as signalling is occurring.
What happens to the basement membrane underneath odontoblasts?
Starts to get broken down and replaces with predentine.
Once ameloblasts have “done their job” how does their lifecycle come to an end?
Nucleus moves down (as it’s secreting less enamel), now acts as protection for the enamel layer as it goes through towards the end of its life
Ameloblasts: Pre secretory
Cytodifferentiation, morphodifferentiation, resorption of basal lamina, epi-mes interaction
Ameloblasts: secretory
Initial layer of a prismatic enamel formed, develop tomes processes, matrix secretion to final thickness, initiation and mineralisation, crystallite elongation, matrix degradation, development of prismatic structure .
Ameloblasts: Transition
Ameloblasts shorten - 50% die, vascular invagination, reformation of basal layer, cessation of matrix secretion, matrix degradation and selective matrix withdrawal.
Ameloblasts: Maturation
Cycling of ruffled and smooth ameloblasts, final degradation and withdrawal of matrix, crystal growth and final third mineralised
Ameloblasts: Post-maturation
Enamel organ degenerates, enamel coverings established eruption, exposure to oral cavity
Predentine
Is the organic matrix that makes up dentine before its calcified
Dentine
Mineralised predentine
Enamel
Different proteins from dentine. The amelogenins which are multiple splice variants making up 90-95% Enamelins (or non-amelogenins) making up 5-10%.
BMP4: layers of expression
Ameloblasts layer and odontoblast layer only!
What stage does morphogenesis occur?
Late cap stage or early bell stage
Tooth shape is determined by…
Differential rate of cell division/maturation
NOT by FORCES within the papilla by BY a SIGNAL from the papilla.
Studies proving how morphogenesis is controlled…
Incisor region epithelium (at bud stage) placed on molar region ectomesenchyme will produce a molar tooth ( and visa versa).
How do the cusps develop?
Inner enamel epithelial cell maturation. Multiple cusps (i.e molars) will have multiple points of maturation.
Pitx2
This gene must be switched on for tooth development to occur. The only place this is expressed is in the dental epithelium.
The tooth shape is driven by…
Transcription factors; depending on which area of the jaw you are in will depend on which transcription factor is in excess.
Expression of ____ drives incisor shape development.
Msx1
Expression of _____ drives molar shape development.
Barx-1
Absence of Dlx-1 and 2 results in…
Absence of maxillary molars
What is the most important gene for determining where teeth will form?
Pitx2
Shh (sonic hedgehog gene)
Found in excess in the dental epithelium, doesn’t really affect activity in the ectomesenchyme.
Wnt
Gets switched on at a late point during the bud to cap stage.
Plays a role within the ectomesenchyme.
Can control arresting of tooth morphogenesis and therefore controls boundaries of tooth development.
Overstimulation of wnt results in loads of extra teeth forming in the oral epithelium (not dental).
Reduced enamel epithelium
Forms when enamel formation is complete.
Reduced ameloblasts, OEE, SR and SI - these whither away and start to protect the enamel.
Secretes proteases - aid CT degradation and bone resorption as the tooth erupts.
Three key roles of the reduced enamel epithelium
- Protection of enamel surface from; resorption, prevention of cementum formation.
- Provide an epithelial lined pathway for eruption.
- Forms initial junction all epithelium.
Successional dental laminae
Projection of epithelium that will go on to form the permanent tooth. Would typically form on the lingual side (if it formed on labial it would be a tooth gland lamina).
Anodontia
INITIATION STAGE ERROR: All teeth missing
Oligodontia
INITIATION STAGE ERROR: More than 6 teeth missing (errors with Msx1, Pax9 and Axin2)
Hypodontia
INITIATION STAGE ERROR: Tooth loss except third molars (Msx1, Pax9)
Supernumerary tooth/teeth
INITIATION STAGE ERROR: Extra layers of teeth being developed
Microdontia
BUD STAGE ERROR: Small teeth
Macrodontia
BUD STAGE ERROR: large teeth
Dens in dente
CAP STAGE ERROR: enamel organ invaginates into the dental papilla. Deep lingual pit forms.
Fusion and Gemination
CAP STAGE ERROR: requires ortho treatment.
Fusion: the union of two adjacent teeth
Gemination: One tooth trying to divide (one pulp cavity).
Journey: Epithelium
Ameloblasts —> enamel
Journey: Dental papilla
Odontoblasts —> dentine
Journey: Dental follicle
Cementoblasts —> cementum
Root development is driven by…
Hertwig’s epithelial root sheath
What is Hertwig’s epithelial root sheath?
Apical development of cervical loop region OEE and IEE.
HERS; function
Defines root morphology
Inductive influence
Root dentinogenesis
Cementogenesis (indirect)
- it proliferates apically and disintegrates cervically.
Dental follicle: origin and function
Origin: ectomesenchyme
Function: gives rise to cells of tooth support (cementoblasts, fibroblasts, osteoblasts)
Three layers of the dental follicle
- Inner investing layer ( vascular; become cementoblasts)
- Loose connective tissue layer
- Outer layer - lines alveolus with bone
What initiates the formation of the PDL, cementoblasts and the pulp?
From the HERS signal (most likely a Shh or BMP)
Root formation: stage one
HERS induces odontoblast formation
Root formation: stage two
Pre-dentine then dentine form HERS basal lamina breaks down
What is the difference between root dentine and coronal dentine?
The way in which it is laid down
Coronal dentine; pre dentine fibrils are thicker and more densely packed and they’re often arranged parallel to the odontoblast process. The odontoblast processes remain at the site of the future ADJ/DEJ. Differentiated odontoblasts are columnar.
Root dentine; odontoblast processes retreat within the cell bodies away from the basal lamina. Processes are atubular - forms the granular layer of Tomes. Differentiated odontoblasts aren cuboidal. Phosphoproteins are low in quality and quantity here. Low levels of the alpha chains of collagen type 1.
Root formation: stage 3
HERS cells secrete enamel matrix proteins (amelogenins) on to predentine surface.
Root development 4: stage 4
Enamel matrix proteins mineralise to form the Hyaline Layer (of Hopewell Smith).
Root formation: stage 5
HERS disintegrates
Root formation: stage 6
Induction of follicle cells: migrate to root surface and form cementoblasts
Root formation: stage 7
Cementoblasts secrete cementum matrix and retreat outwards
Root formation: stage 8
Cementum matrix mineralises entrapping PDL fibres
Root formation: stage 9
Remnants of HERS form cell rests of Malassez in PDL - cystic potential.
Hyaline layer (of Hopewell - smith)
- Between denting and cementum
- Highly mineralised
- 10 microns thick
- Origin: matrix from enamel matrix proteins (epithelial cells) and ectomesenchymal cells as well.
Role of Hyaline layer
- Induces cementoblast formation
- Cements cementum to tooth
Cementogenesis
The formation of cementum (the calcified connective tissue that covers the roots of the teeth, from the root sheath).
Types of cementum
Primary - acellular
Secondary - cellular
Cementoblasts
Deposit matrix (collagen, GAGs,)
Acellular or cellular if in response to trauma.
Mineralises collagen fibres and PDL collagen fibres and then become trapped in the cementum and mineralise.
Derived from dental follicle.
Gubernacular canals
Area between apex of deciduous and permanent tooth germ. Connective tissue through the alveolus bone.
Alveolar bone
Not much difference in composition between alveolar bone and cementum.
HERS cells cause osteogenic differentiation - form bone cells.
PDL
CT between cementum and the AB.
Spindle-shaped fibroblasts, functionally different from the fibroblasts found in gingival tissues.
Produce masses of collagen fibres (as well as hyaluronan, growth factors such as TGFbeta1 and EGF.
A- Transeptal
B - Alveolo-gingival
C - Inter-radicular
D - Oblique
E - Apical
F - Dentinogingival
G - Circumferential
H - Alvelolar crest
I - Horizontal
What role could collagen fibres in the PDL have in eruption?
Push/pull force ?
What is the intermediate plexus?
Collagen fibres that link the sharpey’s fibres in the bone with the sharpey’s fibres in the cementum.
How does more than 1 root form?
Papilla will be signalling down to the HERS cells causing it to proliferate into the papilla. Controlled by FGFs
Summary of crown formation
Summary of root formation
How does dentine mineralise?
- Matrix vesicles
- Heterogeneous nucleation - template molecules.
Mantle dentine formation
Matrix vesicles and heterogeneous nucleation
Circumpulpal dentine formation
Heterogenous nucleation.
What is the first mineralise tissue in the tooth?
Dentine
Protrusion movements - determined by
Incisor-relationship
What muscles retrude and protrude the jaw?
Retruding - Temporalis
Protruding - lateral pterygoid
Which muscle is going to dictates a lateral movement of the jaw to one side?
Medial pterygoid muscle
