Prenatal Craniofacial Development Flashcards
Morphogenesis of the face is a complex
3D process involving (4)
patterning,
outgrowth, fusion and moulding of tissues
~— days, head & neck region comprises ~ half of embryo.
22days
During 4th week, ventral mesoderm condenses into a series
of
segmented bilaterally paired, mesenchymal swellings
pharyngeal (branchial) arches
> ultimately — pairs, arising in — order
5
cranial-caudal
Initially, five prominences (tissue masses) surround the
stomodeum:
Øfrontonasal prominence
Øfirst arch (> mandibular process)
Ømaxillary process (delayed appearance)
First (mandibular) arch
and maxillary processes
are both — structures
paired
> grow ventro-laterally
around the OM
The majority of the growth and morphological change
in facial tissue masses is driven by the
invading cranial
neural crest-derived mesenchyme.
Contrary to textbook literature, the paired first pharyngeal
arches only give rise to the
mandibular processes.
Maxillary processes originate as
separate (delayed)
swellings rostral to the mandibular processes.
Maxillary processes originate as separate (delayed)
swellings rostral to the mandibular processes.
> Invaded by distinct population of
late migrating CNCC
Formation and invagination of nasal placodes (thickening
of ectoderm) in
Frontonasal Prominence (FNP)
•Induction and invagination of nasal placodes similar to
neural plate induction and invagination
Two types of morphological processes for ‘joining’ tissues
merging
fusion
1st: Merging
joining of two masses already partly in contact
merging examples (2)
- Merging of Maxillary (MXP) & Lateral Nasal Processes (LNP)
* Medial merging of Mandibular Processes (MNP)
2nd: Fusion
joining of two separate tissue masses
fusion example
•Contact-dependent fusion between MNP and MXP/LNP
ØForms the primary palate and nares (nostrils)
Secondary palate (future roof of mouth)
arises later from the medial aspects of the
MXPs >
palatal shelves
After immigration of cranial neural crest cells, growth & morphogenesis of the face is controlled by
ongoing epithelial-
mesenchymal interactions
Cranial neural crest cells (CNCC) primarily determine
facial form/shape
The Ectoderm in Growth & Patterning
•defined boundaries of
expression in facial ectoderm
skipped
Neural expression of Shh/Fgf8 precedes (and is
required for) the ectodermal expression: (3)
•juxta-positioned FGF8/SHH
» frontonasal ectoderm zone (FEZ)
•transplanted ectoderm retains FGF8/SHH pattern of
expression from originating tissue.
•Secreted factors from the FEZ can direct outgrowth of
the underlying CNCC and determines D-V pattern of the FNP
Øbut precise shape depends upon pattern
information inherent in the neural crest.
A Dynamic Ectoderm (3)
support and direct outgrowth of the facial processes
guides differentiation of CNC
facilitating timely fusion of the facial processes
interrelationship between the
CNC and ectoderm
facilitating timely fusion of the facial processes
fusion establishes the
primary palate
goes on to form the alveolus and lip proper
Third most common birth defect (~1 in 700 births worldwide)
Cleft lip +/- cleft primary palate
Cleft lip +/- cleft primary palate
Embryological origin? (2)
ectodermal or CNCC
Cleft lip +/- cleft primary palate
What else may be affected?
Cleft lip +/- cleft primary palate
What is impact on ongoing development of the face
(2) and ultimately function (3)
growth, ossification
feeding,
breathing, oral hygiene
The Secondary Palate (2)
•Palatal shelves (condensations of CNCC mesenchyme)
•Secondary palate forms well after the primary palate/lip
(10th week vs 7th week)
Ossification of the secondary palate
(osteoblasts derived from CNCC) begins
just before
palatal development complete
formation of the secondary palate (3)
- Growth (either side of tongue)
- Elevation & rotation
- Medial growth & fusion at midline and with nasal septum
cleft secondary palate prevalence
~1 in 1000 births world-wide
cleft secondary palate can result from (3)
1) poor growth of shelves,
2) failed elevation, or
3) failed fusion
Critical that tongue lowers to allow the shelves to rotate to
a position
above the tongue
— (enlarged tongue) can
provide a physical barrier to
secondary palate closure
Macroglossia
Small or retrognathic mandible also associated with
failed palatal fusion (Pierre-Robin sequence)
> Tongue forced backward and elevated
Craniofacial cartilage first appears in the — and
shortly thereafter in the (2)
mandible
midface and cranial base
Craniofacial cartilage first appears in the mandible and
shortly thereafter in the midface and cranial base (2)
> Provides structural support for shaping the growing head
Precedes ossification [NB: in the face, few cartilages are replaced
by bone unlike the axial skeleton (ie. long bones)]
•Cartilage of PA1 (Meckel’s cartilage) arises during
7th week (human) > forms template for the mandible. (E11.5 in mice)
•PA1 also contributes to
two ear ossicles
> parts of malleus and incus (and ligaments)
BUT, like most facial
cartilages, most is not
converted to —.
bone
Rather… parts —, with caudal
aspects forming part of middle ear bones.
degenerate
Development of the Ear is formed independent from
embryonic structures
External and middle ear: (2)
- Derivatives of PA1 and PA2
- Intervening pharyngeal cleft
- Pharyngeal pouch
nner ear:
•Arises from
thickening of ectoderm
(otic placode) dorsal to PA3 at level of
hindbrain and subsequent
invagination (otic pit)
**Inner and outer ear malformations often associated with --- defects
mandibular
Knowledge of the embryological origins of
clinical presentations is important to appreciate
the totality of the patient’s problem as it may: (3)
- merit additional clinical considerations
- prompt more detailed assessment
- impact treatment plans
What might seem causally unrelated may in fact
be of clinical significance
> diagnosis
