Development of pharyngeal arches and face Flashcards
The embryonic germ layers
Ectoderm Neural tube Dorsal mesoderm Ventral mesoderm Endoderm
Ectoderm forms
Epidermis
Glands
Dental enamel
Neural tube forms
CNS and motor nerves
Dorsal mesoderm forms
Muscle and CT
Bone
Melanocytes
Sensory and autonomic nerves
Ventral mesoderm forms
BVs and cells
Endoderm forms
Gut lining and glands
At 3.5 weeks post-conception
Tail Hindlimb bud Pericardial sac Somites (mesodermal) Forelimb bud Occipital somite 3 pharyngeal / brachial arches Mandibular arch Head and buccopharyngeal membrane SEE PICTURE
`Buccopharyngeal membrane function
Composed of ectoderm and endoderm
Separates foregut from stomodeum
Naming of parts (look at diagram)
Frontonasal process (developing forebrain)
Optic placode (thickening of specialised epithelium)
Stomodeum (primitive mouth)
Pharyngeal arches (gill slits)
Site of primitive hearts
The pharyngeal arches (parts)
6 arches but never see all
Pairs that fuse in the middle
Arches 4, 5 and 6 fuse to form single arch
Arches separated externally by clefts
Internally, arches separated by pouches that correspond with clefts
Structure of pharyngeal arches (look at diagram)
Outer surface covered by ectoderm
Inner surface lined with endoderm
Packed with ectomesenchyme
Each pharyngeal arch has
Artery, vein and cranial nerve
Skeletal element
Muscle block (probably from mesoderm and thus migrated into the arch)
Lamprey
Shows similar structure to developing pharyngeal arches
Pattern formation
Migration of nerve fibres from neural tube into arches is consistent and predictable
They will always innervate specific muscles and regions of skin and mucosa
Cranial nerves
1st arch: trigeminal
2nd arch: facial nerve
3rd arch: glossopharyngeal
4th arch: vagus nerve
Hox gene
Patterns of Hox gene expression within midbrain and hindbrain set up identity
As cells migrate from neural tube they keep this pattern
Regulates movement and differentiation of cells
Rhombomere
Transiently divided segment of developing neural tube
Growth of the arches
Things that were close in development e.g. 4th pharyngeal arch and heart become far away, explains why some nerves have strange courses
Pharyngeal arch muscles: arch 1
Muscles of mastication
Some suprahyoids
Tensor veli palatini
Supplied by third division of CN V
Pharyngeal arch muscles: arch 2
Muscles of facial expression
Some suprahyoids
Stapedius
All supplied by CN VII
Pharyngeal arch muscles: arch 3
One trivial muscle (stylopharyngeus)
Supplied by CN IX
Pharyngeal arch muscles: arch 4
Pharyngeal constrictors
Muscles of soft palate and larynx
All supplied by CN X
What does Meckel’s cartilage form (1st arch)
Two of th ear ossicles: incus and malleus
2nd arch cartilage
Styloid process, stylohyoid ligament, upper part of hyoid bone (greater cornu)
3rd arch cartilage
Hyoid bone
4th arch cartilage
Upper part of thyroid cartilage
5th arch cartilage
Lower thyroid cartilage
Cricoid cartilage
Pharyngeal clefts
Between arch 1 and 2: auditory tube and middle ear, external auditory meatus, tympanic membrane
Middle ear cavity contains malleus, incus, stapes
All other clefts grow over
Pharyngeal pouches
2: palatine tonsil
3: inferior parathyroids
4: superior parathyroids, thyroid cells secreting calcitonin and thymus
Development of tongue
Motor: hypoglossal (CN XII)
Taste: chorda tympani (CN V) in first 2/3, glossopharyngeal in posterior 1/3
Sensation: Lingual and glossopharyngeal
Which part of tongue derived from which arch
Lingual: 1st Chorda tympani: 2nd Glossopharyngeal: 3rd Hypoglossal: occipital somites 2nd arch forms tastebuds
Arch malformations
Majority affect 1st arch Clearest impact on skeletal structures -hypotrophic mandible -conductive hearing loss (incus and malleus) -malformed external ear Can be part of a syndrome
Incidence of cleft lip and palate
c. 800-1000 live births per year in UK
Males>females
Very rare familial cleft carried through X chromosome
Window for palate to fuse
Several hours as embryo
Parts of embryo at 4 weeks
Frontonasal process Mandibular arch Somites Primitive tail Heart tube (atrium/ ventricle) Mouth is simple slit (stomideum), no nasal cavity
Formation of nasal cavity and primary palate
(look at images on lecture)
Nasal placode invaginates (pushes into) into embryo, gradually growing backwards
Cave develops into nasal pit
Frontonasal process becomes more ridged
Medial nasal processes develops in middle
Lateral nasal process develop on outside
Tissue in middle forms primary nasal septum which gradually grows forward
Nasal cavity becomes deeper and deeper
Nasal placode becomes olfactory epithelium later
What/ where is primary palate
Just behind central incisors in mature anatomy
Just behind filtrum
What/ where is primary palate
The primary palate begins the separation of the nasal and oral cavities anteriorly
Just behind central incisors in mature anatomy
Just behind filtrum
Nasal placode
Future of developing nose Specialised epithelium Where nasal cavities grow from Front of frontonasal process Thickened epithelium above
Difference in location of primary and secondary nasal septum
Secondary is further back
Primitive nasal cavity
The invagination of the nasal placodes forms nasal pits
Maxillary processes
Extend from 6 weeks from posterior aspect of mandibular arch and form lateral borders of mouth
Maxillary processes
Extend from 6 weeks from posterior aspect of mandibular arch and form lateral borders of mouth
Processes of the maxillary arches
Two pairs of processes grow mesially from the maxillary arches
Superior is tectospetal process, grows across and then downwards (becomes secondary nasal septum eventually)
Inferior is the palatine process, grows downwards towards developing tongue which fills oronasal space at this point
Up to about 8 weeks
Look at animation on slides
Growth of palatine processes
Dependent on several factors, e.g.
-FGF10 expressed in ectomesenchyme and acts on FGF2R in ectodermal covering
FGF10 -/- mutation leads to no or reduced growth of palatine processes
Palatal elevation
At 8 weeks
Embryo develops a cervical flexure, lifting head away from cardiac bulge
Mandible widens allowing tongue to drop into floor of mouth (no longer in oronasal process)
Palatal processes now have no restraint and elevate into horizontal position
Palatal elevation - intrinsic factors
In tongue experimentally depressed prematurely, palatine processes do not elevate until close to correct time
Hyaluronan accumulates around ectomesenchyme within processes
Bonds large amounts of water
> turgor
Agents that disrupt GAG synthesis tend to inhibit palate elevation
Fusion of palatine processes
At 10 weeks ALL processes have grown to point where they make contact and fuse
Fusion of palatine processes with
-each other (forming hard palate)
-primary palate (soft?)
-nasal septum
Takes 2 weeks
Fusion requires breakdown of ectodermal covering of processes to allow underlying ectomesenchyme to merge (like fingers)
Ectomesenchyme releases signals that institute programmed cell death (apoptosis) in ectodermal cells
Mammal/ bird/ reptilian recombinations show that ectomesenchyme determines fate of ectoderm
Ectoderm on palatine processes expresses gene (fusion of palatine processes)
Msxl –> genetic cascade releases BMP4 into ectomesenchyme –> ectomesenchyme releases BMP2 –> ectodermal apoptosis
Palatine processes fusion complete at
12 weeks
Palate 12 weeks -16 weeks
Anterior 2/3 palate invaded by bone (hard palate
Posterior 1/3 invaded by muscle from 4th pharyngeal arch to form muscles of soft palate
Differentiation of bone
Can leave epithelial cell rests
Frontonasal process compressed between maxillary processes to form
Protruding nose
Fusion of maxillary and mandibular processes to form
Mouth
Macrostomia
Maxillary and mandibular processes fail to fuse
Clefts
May occur between any internal or external processes
- normal with fusion lines
- unilateral cleft lip
- unilateral cleft lip and palate
- bilateral cleft of primary palate
- midline cleft of secondary palate
- unilateral cleft of primary palate with extension
Contributory factors of clefts
Heavy smoking
Heavy alcohol consumption
Folic acid deficiency
(in early stages of pregnancy)
