Pharyngeal Arch Development Flashcards
What makes up the pharyngeal apparatus?
1) pharyngeal arches: 1-4, 6
2) pharyngeal grooves: 4 pairs
3) pharyngeal pouches: 4 or 5 pairs
4) pharyngeal membrane: between arches
- arches emerge at neural tube closure
- approx. 4 weeks
- contain all 3 layers
What are the layers located on the arch?
- ectoderm is on the outside
- endoderm is on the inside
- neural crest is between the ectoderm and endoderm
- mesoderm surrounds an artery, which is surrounded by the neural crest
- clefts are on the outside
- pouches are on the inside
What happens to the oropharyngeal membrane?
breaks down during week 4, connects stomodeum to primitive gut
What are the components of the pharyngeal arches?
- aortic arch
- carilaginous rod
- muscular component
- nerve
- during the 3rd week the primary tissue is mesenchyme, but as the embryo reaches the 4th week it becomes neural crest
What is derived from pharyngeal arch 1?
- Bone: mocker’s cartilage, maxilla, mandible, malleus, incus
- Muscles: muscles of mastication
- nerve: Trigeminal
- Arteries: Maxillary and Ext. carotid
What is derived from pharyngeal arch 2?
- Bone: Stapes, styloid process, less horn and upper portion of the body of hyoid
- Muscles: muscles of facial expression
- Nerve: facial nerve
- Arteries: stapedial
What is derived from pharyngeal arch 3?
- Bone: Greater horn and lower portion of body of the hyoid
- Muscles: Stylopharyngeus
- Nerve: Glossopharyngeal
- Arteries: Common and Int. carotid
What is derived from the 4th arch?
- Bone: Laryngeal cartilages
- Muscles: Constrictors of pharynx
- Nerve: Vagus and Sup. laryngeal
- Arteries: Aortic and subclavian
What is derived from the 6th arch?
- Muscles: intrinsic m. of larynx
- Nerve: Recurrent laryngeal
- Arteries: Pulmonary
How is the first arch structured and formed?
- mandibular process –> lower lip , lower face, lower cheek regions, chin, mandible, body of the tongue
- Maxillary process –> mid face, upper cheek regions, upper lip sides, Secondary palate, maxilla, zygomatic bones, secondary palate
What are some pharyngeal anomalies?
- First arch syndrome is congenital
- failure of neural crest to properly migrate into the first arch
1) Treacher-Collins: hypoplasia of arch derived facial bones; down-slanting of palpebral fissures, lower eyelid colhbomas, ear deformations, cleft palate and tooth defects
2) Pierre Robin: hypoplasia of the mandible (micrognathia), misplacement of tongue (glossoptosis) cleft palate, defects of the eye and ear, airway obstruction
What becomes of the grooves/clefts?
- Lined with ECTODERM
- Cleft one is usually the only one not obliterated
- clefts 2-4 persist during abnormalities
- Pharyngeal cleft 1: becomes external auditory meatus
- Abnormality
- Congenital auricular sinuses and cysts: small sinuses and cysts commonly found in a triangular area of skin ant. to the ear; may be recent of pharyngeal groove –> first or second arch defect
What is a lateral cervical sinus abnormality?
uncommon, open externally, failure of second groove or cervical sinus to obliterate
What is a internal branchial groove defect?
Rare, persistant, 2nd pouch, open into intratonsillar cleft (into pharynx)
What are the pharyngeal pouches?
- form as pits internally in the pharynx, between the arches
- pouch linings are endodermal
What does the first pharyngeal pouch give rise to?
- Middle Ear
- Distal: together with lining of 1st cleft, form tympanic membrane
- widens and forms tympanic cavity
- Proximal: stays narrow, forms auditory tube
What happens during middle ear development?
- Ossicles: 1st and 2nd arch cartilages, from neural crest
- tympanic membrane: 1st cleft/pouch, from neural crest
- tympanic cavity and internal auditory meatus: 1st pouch
What does the second pharyngeal pouch give rise to?
- Lining of the crypt in the palatine tonsils
- Tonsils by themselves are mostly composed of mesoderm
- pouch forms the crypts lining, which is epithelium and continuous with lining of mouth
What does the third pharyngeal pouch give rise to?
- Inferior parathyroid gland and the thymus
- Dorsal wing: inferior parathyroid glands
- Ventral wing: thymus
- Inf. parathyroid gland travels with the thymus before moving over to the thyroid
What does the fourth pharyngeal pouch give rise to?
- Superior parathyroid gland and the ultimobranchial body
- Dorsal wing: sup. parathyroid glands
- Ventral wing: ultimobranchial body –> embryonic structure, contains C-cells
What abnormalities arise from migration?
- Cervical thymus: cord of thymus persists in neck on path of descent
- Accessory Thymus: piece of thymus remaining in path of descent
What abnormalities arise from pharyngeal pouches?
- DiGeorge Syndrome: failure of 3rd and 4th pouches to differentiate into thymus, parathyroid
- neural crest defect
- Catch 22: Cardiac abnormality, thymus aplasia, cleft palate, hypo calcemia and thryoidism
- Tetralogy of Fallot: pulmonary stenosis (R. ventricular hypertrophy
What becomes of the pharyngeal membrane?
- form the epithelia of the grooves and pouches as they approach each other
- Oropharyngeal membrane: technically not a pharyngeal membrane. Marks site where ectoderm and endoderm meet. Glossopalantine arch marks location
- Tympanic membrane: derived from layers between first cleft and 1st pouch
- 1st ectoderm and endoderm only, then neural crest moves in
- remaining regions between arches do not remain as membranes –> obliterate during development
What are the importance of HOX genes during pharyngeal arch formation?
- regulate A-D identity
- Neural crest cells with different A-P ID as determined by hox expression, migrate from the rhombomeres into the pharyngeal arches
- Loss of Hox expression causes arch 2 to lose its A-P ID, arch 2 gives rise to duplicate arch 1 structures
