Development of Pharyngeal Arches Flashcards
Describe how the pharyngeal arches begin to develop
After closure of the neuropores, the arches appear as tissue bulges in the cranial region separated by clefts.
Made up of 3 embryological layers;
- mesenchyme core (mesoderm and neural crest cells)
- internal endodermal pouch
- external ectodermal cleft
What are the components of the pharyngeal arches?
- muscle
- cartilage
- artery
- cranial nerve
Describe the origins of the cells that give rise to the pharyngeal arch components
- paraxial mesoderm: voluntary muscles, arteries, neurocranium, meninges, dorsal skin (arches 1-3)
- lateral plate mesoderm: laryngeal structures (arches 4 and 6)
- neural crest cells: mesenchyme, ganglia, nerves, viscerocranium
- ectodermal placodes: sensory neurons/glia
What are the components of the first pharyngeal arch?
- maxillary part of external carotid artery
- trigeminal nerve (CNV)
- maxilla, mandible, malleus, incus, zygomatic, (Meckel’s cartilage)
- muscles of mastication, tensor tympani, tensor veli tympani, mylohyoid, anterior belly of digastric
What are the components of the second pharyngeal arch?
- hyoid and stapedial arteries
- facial nerve (CNVII)
- stapes, styloid process, lesser horn and upper body of hyoid
- muscles of facial expression, stylohyoid, stapedius, posterior belly of digastric
What are the components of the third pharyngeal arch?
- common carotid and proximal internal carotid arteries
- glossopharyngeal nerve (CNIX)
- greater horn of hyoid and lower body of hyoid
What are the components of the 4th pharyngeal arch?
- left: aortic arch
- right: subclavian artery
- superior laryngeal nerve (of vagus n. (CNX))
- epiglottis, laryngeal cartilages, thyroid, cunieform, corniculate, arytendoids
- all muscles of pharynx except stylopharyngeus
- all muscles of palate except tensor veli palatini and cricothyroid
What are the components of the 6th pharyngeal arch?
- pulmonary arteries and ductus arteriosus
- recurrent laryngeal nerve (of vagus n. (CNX))
- all muscles of larynx
- cricoid cartilage
What are the derivatives of the pharyngeal arches?
1: primitive tympanic cavity
2: palatine tonsil/tonsillar fissure
3: inferior parathyroid glad (dorsal pouch), thymus (ventral pouch)
4: superior parathyroid gland (dorsal pouch), ultimobranchial body (ventral pouch) - produces parafollicular Ca2+ producing cells in thyroid
What are the derivatives of the pharyngeal clefts and its clinical relevance?
1: external auditory meatus and external part of tympanic membrane (dorsal), tympanic membrane (deep)
2nd cleft will proliferate and overgrow clefts 3 and 4 obliterating them.
2, 3, and 4 lose contact with the exterior forming the cervical sinus which usually disappears.
CLINICAL RELEVACE: no obliteration of clefts 3 and 4 by 2nd:
- causes cysts in adults
- causes fistulas/sinuses in children
- located anywhere along anterior border of SCM, cancer risk
Describe the process of molecular determination
- 3 streams of neural crest cells migrate from rhombomeres carrying the homeodomain to the pharyngeal arches
- homeodomain = OTX transcription factor and HOX gene
- these respond to local endodermal signals at the arches (FGF, BMP, PAX, SHH etc) which determines mesenchymal expression pattern and arch characteristics
Describe some craniofacial defects
Treacher Collins Syndrome (arches 1 and 2)
- abnormal eye shape
- micrognathia
- malformed ears
- conductive hearing loss
- underdeveloped zygoma
Di George Syndrome (arches 3 and 4)
- defective thymus and parathyroids
- CATCH 22: cardiac abnormality (esp. Teratology of Fallot), abnormal facies (cleft lip and palate, small jaw and upper lip, slanted eyes, low set abnormal folding ears)
- cleft palate
- hypocalcaemia
SHH:
- inadequate function = less pairing of structures, narrowing and fusion (eg. cylopia)
- excessive function = excessive duplication and widening of structures (eg. diprosopus)
Craniosynostosis Syndromes:
(FGFR2 and FGFR3 mutations)
* results in gain of function leading to activation of receptors
* activates bone calcification to cause premature fusion and decreases cell proliferation in mesenchyme at the suture
* also causes syndactyly
