25-09-23 - Development of pharyngeal arches Flashcards
Learning outcomes
- Describe development of pharyngeal arches from the beginning of the third week of fetal life
- Describe the basic components of the pharyngeal arches and their origins
- Appreciate the location and further development of pharyngeal pouches internally and clefts externally
- Appreciate that neural crest cells populate and give rise to skeletal elements of the arches under control of pouch endoderm
- Describe the fates of each pharyngeal arch, pouch and cleft
- Describe the formation of the cervical sinuses and explain how they may give rise to cervical cysts and branchial fistulas
- Understand the potential role of signalling factors in development of the face
What are 3 other names for pharyngeal apparatus?
What is a synonym for pharyngeal?
What are the main numbers for pharyngeal arches?
- 3 other names for pharyngeal apparatus:
1) Pharyngeal arches
2) Pharyngeal clefts
3) Pharyngeal pouches - Pharyngeal and branchial = same meaning in the context of the above
- You may also see: – “visceral arch” or “pharyngeal groove in place of cleft”
- The main pharyngeal arches are 1, 2, 3, 4 and 6 but some sources call arch 6 arch 5.
When does the pharyngeal apparatus appear?
What structures do they give rise to?
What does the pharyngeal apparatus consist of?
What 3 things does each arch have?
- Pharyngeal apparatus appears appear within early week 4, forming cranial-caudally
- The pharyngeal apparatus give rise to structures in developing head and neck
- It is made from contributions from all germ layers and neural crest
- Each arch has an associated:
1) Artery
2) Cranial nerve
3) Cartilage
What is the stomodeum?
What 3 structures is the stomodeum between?
What is the stomodeum sealed by?
When does this rupture?
What is this the site of?
- The stomodeum is the original oral cavity
- 3 structures the stomodeum is between:
1) Mandibular prominence (arch 1)
2) Maxillary prominences
3) Frontonasal prominences - The stomodeum is sealed by buccopharyngeal membrane
- The buccopharyngeal membrane ruptures at about day 30, forming the primitive mouth
- This is the site of the endoderm meeting the ectoderm
What are the 4 pharyngeal apparatus germ layers?
- 4 pharyngeal apparatus germ layers:
1) Ectoderm lining externally (clefts)
2) Endoderm lining internally (pouches)
3) Mesoderm – from lateral plate (primitive connective tissue) and paraxial mesoderm
4) Neural crest – cartilage and cranial nerves
Where is there migration of neural crest cells from?
What are Epipharyngeal placodes? What is their role?
- There is migration of neural crest cells from fore-, mid- and hindbrain
- Epipharyngeal placodes are ectodermal thickenings
- They contribute to the cranial nerve ganglia
Germ layer origins of hard tissues of cranium.
What 3 structures does the neural crest form?
What 4 structures does the paraxial mesoderm form?
What structure does the lateral plate mesoderm form?
- Germ layer origins of hard tissues of cranium
- 3 structures the neural crest forms:
1) Viscerocranium
2) Frontal
3) Squamous temporal - 4 structures the paraxial mesoderm forms:
1) Parietal
2) Petrous Temporal
4) Occipital - The lateral plate mesoderm forms the laryngeal cartilages
What pharyngeal arches are associated with which cranial nerves? (in picture)
Label the cartilages formed by each pharyngeal arch (in picture)
Describe the pharyngeal arch arterial derivatives (in picture)
Pharyngeal arch summaries:
* Arch
* Hard tissues
* Cranial Nerve
* Artery
* Muscles
1st pharyngeal pouch and cleft.
What structure does pouch 1 form?
What does it come in contact with?
What 4 structures does the tubotympanic recess (1st pharyngeal arch) form?
What is Cleft 1 overgrown by?
What forms the external auditory meatus?
- 1st pharyngeal pouch and cleft
- Pouch 1 forms the the tubotympanic recess
- It comes in contact with the lining of the first pharyngeal cleft
- 4 structures the tubotympanic recess (1st pharyngeal arch) forms:
1) Eustachian tube
2) Tubal tonsil
3) Middle ear cavity
4) Tympanic membrane - Cleft 1 is overgrown by arch 2 (which forms external ear)
- The external auditory meatus if formed by an invagination of surface ectoderm
What 2 structures are formed from the 2nd pharyngeal pouch?
- The 2nd pharyngeal pouch forms the palatine tonsil and tonsillar fossa (tonsil is initially endoderm and mesoderm, lymphatic tissue infiltrates months 3-5)
What 2 structures are formed by pharyngeal pouches 2 and 3?
What can Failure of correct migration lead to?
- Pharyngeal pouch 3 forms the:
1) Inferior parathyroid gland
2) Thymus (migrates) - Pharyngeal pouch 4 forms the:
1) Superior parathyroid gland
2) Ultimopharyngeal body (parafollicular/C cells of thyroid) - Failure of correct migration can lead to ectopic thymus or parathyroid glands
What occurs over time in pharyngeal arch 2?
What do remnants of clefts 2, 3, and 4 form?
- Pharyngeal arch 2 proliferates, growing and merging with the epicardial ridge
- Remnants of clefts 2, 3 and 4 become incorporated into a cervical sinus – usually this disappears
What can happen if the cervical sinus persists?
Where is the cysts/fistula located?
How large is it?
What is the difference between a cyst and a fistula?
How common are lateral cervical cysts in children?
What may occur secondarily?
What is the treatment for this?
- Cervical sinus may persist and form a branchial (lateral cervical) cyst or fistula anterior to the SCM
- Not noticeable at first but enlarges in childhood
- Cyst - no connection to internal or external surface
- Fistula – connects either externally or internally
- 20% of neck masses in children are lateral cervical cysts
- Secondary infection may occur
- Resection or conservative management is needed
Molecular signalling.
Where do neural crest cells originate?
Where do neural crest cells migrate?
What different genes are expressed?
What is the role of these genes?
Describe the migration patterns of neural crest cells to each arch.
What do neural crest cells respond to?
How does this affect arch characteristics?
- Molecular signalling
- Neural crest cells originate from hindbrain region divisions called rhombomeres
- Neural crest cells migrate into arch carrying (OTX/HOX) codes
- There is different expression of OTX2 and HOX genes in different places
- These genes provide axonal cues for the development of neurones and connection from ganglia to hindbrain
- Migration of neural crest cells to arches occurs in patterns:
1) R1/2 to Arch 1
2) R4 to Arch 2
3) R6/7 to Arches 3, 4 and 6 - Neural crest cells respond to local patterns of endoderm signals (FGF8, BMP7, PAX1, SHH)
- This then specifies mesenchymal expression pattern and arch characteristics
That type of condition is Treacher Collins Syndrome?
What is another name for it?
What is it caused by?
What are 5 signs/features of Treacher Collins Syndrome?
- Treacher Collins Syndrome is an autosomal dominant (1/10,000)
- It is also known as First/second arch syndrome
- It is caused by a defective protein called Treacle (TCOF1 gene)
- This leads to failure of formation/apoptosis of neural crest cells and migration into first and second pharyngeal arches
- 5 signs/features of Treacher Collins Syndrome:
1) Abnormal eye shape
2) Micrognathia
3) Conductive hearing loss (ossicles)
* 1st and 2nd arch give rise to ossicles
* Without these, sound signals cant be transmitted from the tympanic membrane to the cochlea
4) Underdeveloped zygoma
5) Malformed ears
What deletion causes Di George syndrome?
What disorder is Di George syndrome considered to be a part of?
What structures fail to develop in this syndrome?
What are 5 signs/features of Di George syndrome?
- Di George syndrome is caused by 22q11 deletion
- Di George syndrome is considered to be part of Catch 22 disorder
- In Di George syndrome, 3rd and 4th pouches fail to develop leading to the thymus and parathyroids being defective
- 5 signs/features of Di George syndrome:
1) Cardiac abnormality (especially tetralogy of Fallot)
2) Abnormal facies
* CLP, micrognathia, small upper lip, eyes slanted upward/downward, low-set abnormal folding ears.
3) Thymic aplasia – infection prone
4) Cleft palate
5) Hypocalcaemia
What is Craniosynostosis?
What causes this?
- Craniosynostosis is a range of syndromes where cranial sutures close prematurely
- This is caused by FGF receptor(s) (FGFR3, 2, 1) mutations causing imbalance in mesenchymal proliferation at sutures, leading to premature closure
How does SHH (sonic hedgehog protein) affect facial width?
- Cyclopia – inadequate SHH (sonic hedgehog protein) function
- Diprosopus – excessive SHH function
Multichoice question 1 (in picture)
SWA question (in picture)
