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Topic 1: The Origin of Ectomesenchyme & Its Role in Tooth Development
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Embryonic stage
Phases
Processes
Pre• Fertilized egg
implantation
↓
Phase
(After rapid divisions)
morula
↓
(After fluid accumulation & cells realign)
blastocyst
• Blastocyst: consists of trophoblast &
embryoblast

Embryonic
Phase
(2nd week)

•
•

Formation of bilaminar germ disc (day 8
of gestation)
2-layered disc:
o Epiblast layer: faces amniotic
cavity & columnar cells
o Hypoblast layer: faces yolk sac &
cuboidal cells

Pictures

Fetal period
3rd – 9th month
Embryo to fetus
Maturation to
existing structures

Embryonic
Phase
(3rd week)

•
•

•

Formation of
notochord

•
•

•

Gastrulation: Formation of trilaminar
germ disc
3-layered germ layers:
o Ectoderm
o Mesoderm
o Endoderm
Processes:
Increased proliferation of cells &
thickening in midline area
↓
Primitive streak (ends as primitive
node / pit)
↓
Forms 3 germ layers & notochord
↓
Epiblast cells migrate & displace
hypoblast cells of yolk sac: forms
endoderm
↓
Epiblast cells pack space between
ectoderm & endoderm: forms
mesoderm
↓
Remaining epiblast cells: forms
ectoderm
Beneath primitive streak
Epiblast cells enter node & migrate
between ectoderm & endoderm
(midline) towards cranial region: forms
notochord
Notochord:
basis for axial skeleton & CNS
development & intervertebral disc

Neurulation

•
•

•

•

•

•

•

Differentiate ectoderm to form brain &
spinal cord
Thicken ectodermal layer
↓
Neural plate
↓
(further growth & thickening +
deepen & invaginate centrally)
Neural groove
↓
(deepens further + forms raised
margin)
Neural fold
↓
Neural folds fuse
↓
A neural tube separates from
ectoderm
↓
Ectodermal cells at crest of neural
tube gets separated from
epithelium:
Neural crest cells
3 mesodermal aggregates:
o Paraxial mesoderm (break into
Somatomeres & Somites)
o Intermediate mesoderm
o Lateral plate mesoderm
Neural crest cells fx:
Migrate & differentiate within
developing embryo
NCC migrate to 1st branchial arch below
oral epithelium:
Known as ectomesenchyme
Ectomesenchyme cells develop into:
Dentine, pulp & tooth supporting
structures (cementum, PDL & alveolar
bone)
Mesenchyme =
Embryonic CT & it is derived from
mesoderm

•

•

Embryonic
Phase
(4th week)

•
•

•
•

•

•
•

Teacher Collins syndrome:
Failure of full facial development bc
NCC fail to migrate properly to facial
region
Depletion of NCC
↓
Prevents proper tooth development
Embryonic folding into embryo
Folding occurs at:
o Cephalic end
o Caudal end
o Lateral side
Head fold (at cephalic end):
Forms primitive stomodeum
Results of folding:
Positions of embryonic layers take on a
more recognizable placement for future
development of embryo
After folding:
o Ectoderm encapsulates embryo
↓
Forms surface epithelium
o Mesoderm fills in area between
ectoderm & endoderm except 2
embryonic membranes:
buccopharyngeal membrane &
cloacal membrane)
Stomodeum = Primitive mouth
Originates as a depressed area of
surface ectoderm & foregut endoderm
In early embryo:
Stomodeum is sealed off from primitive
gut by buccopharyngeal membrane,
separating stomodeum from primitive
pharynx & foregut
↓
th
4 week of prenatal:
Buccopharyngeal membrane
disintegrate + stomodeum is connected
to foregut
↓
Primitive mouth is deepens & enlarged
↓
Future development:
Stomodeum gives rise to oral cavity

Topic 2: Tooth Development
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Genes
Questions
What controls initiation of
tooth development?

What controls position &
no
...


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Hypothetical models explaining how different tooth shapes are determined
Theories
Field model

Descriptions
• Genes reside within ectomesenchyme in distinct but graded fields for
each tooth family
• Each fields express differing patterning homeobox genes
• Genes in oral epithelium provides signalling
• Coded pattern in ectomesenchyme might be expressed following an
epithelial signal
Clone model
• Each tooth class is derived from a clone of ectomesenchymal cells
programme by epithelium
• Ectomesenchyme assumes dominant role in crown formation
• For tooth initiation: both theories can be combined

5
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d
...
d
...
d
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d
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Reduced Enamel Epithelium (REE)
o Overlies a developing tooth
o 2 layers:
o A layer of ameloblasts
o Adjacent layer to IEE (SI, SR & EEE)
o As REE degenerate: Tooth is revealed by erupting into oral cavity

7
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Parts
Parts
Cervical loop

Fxs & Descriptions
• Fx: For root development
• Most cervical portion of enamel
organ
• Consists of only IEE & EEE
• Epithelial cells of it proliferate
↓
Forms Hertwig’s Epithelial Root
Sheath (HERS)

Hertwig’s
Epithelial Root
Sheath (HERS)

•

•

Fxs:
o To determine shape of
root
o To determine no
...
Disturbances during root development
Types of
disturbances
Accessory root
canal

Descriptions
•

•

Enamel pearl

•
•

Dilaceration

•

•
•
Supernumerary •
roots
(Accessory
root)
•

•

Cause:
due to a break in continuation of
HERS
o HERS encircles a blood
vessel
o Lack of fusion of epithelial
diaphragms
↓
Causes no deposition of dentine
Usually at apical 1/3 of root

A small spherical enamel formed
on root surface
Cause:
caused by HERS which accidentally
contained SI & SR
↓
Forms ameloblasts at root area
Cause:
due to distortion of HERS caused
by injury / pressure during root
development so root is distorted
In any tooth
Cause problems during extraction
& endodontic treatment
Cause:
Due to trauma, pressure or
metabolic disease that affects
HERS
In any teeth
(Mainly permanent 3rd molar;
rare in incisors)
Cause problems during extraction
& endodontic treatment

Pictures

Topic 3: Growth and Development of Craniofacial Structures
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Patterning genes
o
o

Fx: for very early embryonic development, cell differentiation & general body pattern
Patterning genes from 3rd rhombomere are different compared to anterior region of 3rd
rhombomere

3
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but 5th & 6th are transient

Parts
Branchial cleft /
groove

Descriptions
Ectodermal cleft between adjacent arches

Branchial arch

Each arch has own cartilage, nerve, muscle &
artery

Branchial pouch

Endodermal outpocketing between adjacent
arches

Branchial
membrane

Site of contact of groove (ectoderm) and
puch (endoderm)

Pictures

4
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Development of bone
o 3 types:
o Intramembranous ossification
o Endochondral ossification
o Sutural growth

o Bone development of skull:
o Cranial vault, most of the face & jaws:
intramembranous ossification
o Base of skull:
endochondral ossification
Types of bone
Descriptions & Processes
formation
Intramembranous • Forms from CT
bone formation
• At cranial vault, maxilla & mandible
• Processes:
Selected centrally located
mesenchymal cells cluster
differentiate into osteoblasts
↓
Forms an ossification centre (appears
in fibrous CT membrane)
↓
Osteoid is secreted by osteoblasts
within fibrous membrane &
mineralized within a few days
+
Trapped osteoblasts become
osteocytes
•

Woven bone
(Bone structure is disorganized & very
cellular)
↓
Immature bone
(Less cellular, more organized,
formation of primary osteons)
↓
Mature lamellar bone
(Tightly packed osteons create an
organic bone matrix)

Pictures

Endochondral
bone formation

•
•
•

Sutural bone
growth

•
•
•
•

Replacement of cartilage by bone
At condyle, coronoid & base of skull
Processes:
Cartilage cells differentiate from
condensed mesenchymal cells
↓
Forms perichondrium
↓
Raid growth of cartilage by:
o Interstitial growth
o Appositional growth
↓
Chondroblasts die, creating space
penetrated by blood vessels, allowing
migration osteoblast to secrete
osteoid
Sutures:
fibrous joint between bones
Fx: allows skull & face to
accommodate growing organs
(e
...
: eyes & brain)
Made up of:
Outer fibrous layer + cambium

Topic 4: Growth and Development of Orofacial Structures
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Development of palate

Palates
Processes
Primary palates Fusion of medial nasal processes
(5th – 6th week) forms intermaxillary segments /
primary palate
Secondary
palates
(7th – 12th
week)

•

7th week: palatal shelves on each
side of tongue
↓
Tongue depresses & palatal
shelves elevate until complete
fusion of palatal shelves by 12th
week
↓
For complete fusion of palatal
shelves: epithelium covering
palatal shelves degenerate
↓

Pictures

Epithelial cells undergo
transformation & become
mesenchymal cells
•

Pressure for fusion of palatal
shelves:
Glycosaminoglycan content of
palatal shelves which are
hydrophilic attracts water
↓
Swelling & rapid expansion of
shelves
↓
Increase vascularity
↓
Increase tissue fluid pressure
↓
Rapid mitotic growth of
mesenchyme of palatal shelves

•

What can go wrong with palatal development?
Orofacial clefts

3

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