Development of Pharyngeal Apparatus

Question 1

• consists of sets of pharyngeal arches, pouches, grooves (clefts), and membranes
• contributes to the formation of nasal cavities, oral cavity, tongue, larynx, pharynx, ear, and neck

Answer 1

pharyngeal apparatus

Question 2

Describe the development of the pharyngeal arches:

Answer 2

• develop early in the 4th week as neural crest cells (ectoderm) migrate into future head and neck
• 1st arch: primordial jaws, appear lateral to developing pharynx
• 2-4 arches: appear shortly after 1st as ridges on each side of future head and neck regions
• 5-6 arches: rudimentary and not visible on surface of embryo

Question 3

What structures of the face do the neural crest cells migrating from forebrain, midbrain, and hindbrain develop into?

Answer 3

• forebrain: frontal nasal prominence (forehead)
• midbrain and hindbrain: midfacial regions (cheekbones and upper jaw)
• hindbrain (some midbrain): lower part of face (lower jaw)
• hindbrain: neck

Question 4

What are the germ layer components of the pharyngeal arches and what do these layers give rise to?

Answer 4

• each arch has a mixed core of mesoderm and mesenchyme
  
  • mesoderm
    
    • arises from paraxial mesoderm
    • populates each arch to form PA musculature
  • mesenchyme
    
    • embryonic CT derived from migratory NCC
    • forms all CT in head, including dermis and smooth muscle
• lateral plate mesoderm
  
  • angioblasts differentiate into endothelium
• prechordal plate mesoderm
  
  • extraocular musculature
• externally covered by ectoderm
• internally covered by endoderm
  
  • ​plays essential role in regulating development of the arches

Question 5

What do the pharyngeal arches do shortly after they are formed?

Answer 5

• support lateral walls of the primitive pharynx
• give rise to facial prominences that contribute to craniofacial development

Question 6

What are the derivatives of PA1?

Answer 6

Meckel’s cartilage

• dorsal portion forms malleus and incus
• perichondrium > anterior L. of malleus and sphenomandibular L.
• ventral parts > primordium of the mandible
• bone forms laterally to Meckel’s, which disintegrates

Question 7

What are the derivatives of PA2?

Answer 7

Riechert’s cartilage

• dorsal region contributes to stapes and styloid process of temporal bone, remained disintegrates
• perichondrium > stylohyoid L.
• ventral end ossifies > forms lesser cornu/horn of hyoid bone

Question 8

What are the derivatives of PA3?

Answer 8

• ossifies > forms greater cornu of hyoid bone
• body of hyoid bone is formed by hypopharyngeal eminence (prominence in floor of embryonic pharynx (from PA3 and PA4)

Question 9

What are the derivatives of PA4 and PA6?

Answer 9

• 4th arch > laryngeal cartilages including epiglottis (NCC derived)
• 6th arch > laryngeal cartilages, but origin is uncertain

Question 10

What muscular structures do the arches give rise to?

Answer 10

most muscular components arise from paraxial mesoderm

• 1st arch
  
  • muscles of mastication (CN V)
  • also: mylohyoid M., tensor Ms., anterior belly of digastric M.
  • # chewonthat
• 2nd arch
  
  • stapedius and muscles of facial expression (CN VII)
  • # smileandsaycheese
• 3rd arch
  
  • stylopharyngeus (CN IX)
  • # elevateanddilate
• 4th arch
  
  • cricothyroid, pharyngeal constrictors (CN X)
  • pushitpushitrealgood
• 6th arch
  
  • intrinsic muscles of the larynx (CN X)
  • # soundoff
• extraocular muscles from prechordal plate
• tongue musculature from occipital myotomes

Question 11

What is the motor innervation of the pharyngeal arches?

Answer 11

• trigeminal N. (CN V): 1st arch​
  
  • principal sensory nerve of the head and neck
  • motor nerve for muscles of mastication
• facial N. (CN VII): 2nd arch
  
  • muscles of facial expression
• glossopharyngeal N. (CN IX): 3rd arch
  
  • stylopharyngeus M.
• vagus N. (CN X): 4-6th arches
  
  • 4th arch > superior laryngeal and recurrent laryngeal branches of CN X
  • constrictors of pharynx (4th) and intrinstic muscles of larynx (6th)

Question 12

• abnormal development of the components of the first arch
• malformation of eyes, ears, mandible, and palate, which together constitute this syndrome
• results from insufficient migration of NCC into the first arch during 4th week
• most common types: Treacher-Collins syndrome and Pierre Robin sequence

Answer 12

first arch syndrome

Question 13

• malar hypoplasia w/ down-slanting palpebral fissures, defects of lower eyelids, deformed external ears, and sometimes middle and internal ears
• autosomal dominant disorder: mutations in _______-_______-Franceschetti syndrome 1 gene (TCOF1)
• gene encodes for protein TREACLE, involved in ribosome biogenesis
• truncated proteins in this syndrome lead to increase in apoptosis of cranial NCC

Answer 13

Treacher-Collins syndrome (mandibulofacial dysotosis)

Question 14

• typically occurs de novo in most patients
• a/w hypoplasia of mandible, cleft palate, and defects of eyes and ears
• initiating defect in small mandible (micrognathia) results in posterior displacement of tongue > obstruction of full closure of palate > results in bilateral cleft palate

Answer 14

Pierre Robin sequence

Question 15

What are the nearby structures a/w pharyngeal arches?

Answer 15

• pharyngeal grooves: located externally between pharyngeal arches, covered w/ ectoderm
• pharyngeal pouches: located internally as part of the pharynx, covered w/ endoderm (1st pair lies between 1st and 2nd arches)
• pharyngeal membranes: located in between

Question 16

What are the derivatives of the 1st groove, 1st pouch, and 1st membrane?

Answer 16

• 1st groove: forms the external acoustic meatus
  
  • grooves 2-4 lies in cervical sinus, eventually obliterated
  • birth defects of 2nd groove are common
• 1st pouch
  
  • expands into tubotympanic recess > tympanic cavity and mastoid antrum
  • elongates to form pharyngotympanic tube
  • endoderm contacts 1st pharyngeal groove > contributes to tympanic membrane
• 1st membrane + intervening mesenchyme > tympanic membrane

*all “1st” structures contribute to ear formation*

Question 17

What are the derivatives of the 2nd pouch?

Answer 17

• portion forms tonsillar sinus
• endoderm forms tonsillar epithelium
• mesenchyme forms lymphoid nodules of palatine tonsil

Question 18

What are the derivatives of the 3rd pouch?

Answer 18

• dorsal portion differentiates into inferior parathyroid gland
• ventral portions form thymus
• both migrate caudally due to growth of brain and cardiac regions

Question 19

What are the derivatives of the 4th pouch?

Answer 19

• dorsal portion differentiates into superior parathyroid glands
• ultimobranchial body > fuses w/ thyroid gland and gives rise to parafollicular cells

Question 20

• failure of 2nd groove and cervical sinus to obliterate
• typically external, internal is rare
• detected due to discharge of mucus
• commonly a/w auricular sinuses

Answer 20

external cervical sinus branchial anomaly

Question 21

• persistence of 2nd pouch
• rare
• open into tonsillar sinus or near palatopharyngeal arch

Answer 21

internal cervical sinus branchial anomaly

Question 22

• remnants of cervical sinus and/or 2nd groove
• slowly enlarging, painless, free-lying cyst in the neck, inferior to angle of the mandible
• accumulation of fluid and cellular debris

Answer 22

cervical cysts

Question 23

• canal that opens into tonsillar sinus and external side of the neck
• persistence of parts of 2nd groove and pouch
• ascends through subcutaneous tissue and platysma to reach carotid sheath; passes between carotids and opens into tonsillar sinus

Answer 23

cervical fistula

Question 24

Describe the process of thyroid gland development:

Answer 24

• first endocrine gland to develop (~24 dpf) > thyroid primordium
• forms from an endodermal thickening in floor of primordial pharynx
• descends in the neck as tongue grows > ventral to hyoid bone and laryngeal cartilages
• connected to the tongue by a narrow tube, thyroglossal duct
• primordium is hollow, but will become solid, cellular mass
• divides into right/left lobes connected by the isthmus
• definitive shape by 7 weeks and thyroglossal duct will degenerate

Question 25

What are the 3 most common thyroid gland development anomalies?

Answer 25

• ectopic thyroid tissue (accessory, lingual, cervical): can form along the course of the duct
• sublingual thyroid gland: forms but does not descend in the neck
• thyroglossal duct cyst: form in tongue or anterior neck, critical to distinguish from ectopic thyroid prior to surgical removal

Question 26

What are congenital anomalies related to the thyroid development?

Answer 26

• agenesis of thyroid gland: absence of a thyroid gland or one of its lobes (rare)
• thyroid hemiagenesis: (unilateral failure of formation) left lobe is more commonly absent, mutations in the receptor for TSH are likely involved
• DiGeorge Syndrome: breakdown of signaling from PA ectoderm to NCC; agenesis of thymus and parathyroid glands; congenital hypoparathyroidism; congenital hypoparathyroidism; shortened philtrum of upper lip, low set and notched ears; nasal clefts, thyroid hypoplasia; cardiac abnormalities (defects of the aortic arch and heart)

Question 27

PA 1

• arch artery:
• skeletal elements:
• muscles:
• cranial nerve:
• ligaments:

Answer 27

PA 1

• arch artery: terminal branch of maxillary A.
• skeletal elements: derived from arch cartilages (originating from NCC):
• from maxillary cartilage: alisphenoid, incus
• from Meckel’s cartilage: malleus
• derived by direct ossification from arch dermal mesenchyme: maxilla, zygomatic, squamous portion of temporal bone, mandible (originate from NCC)
• muscles: muscles of mastication (temporalis, masseter, medial and lateral pterygoids), mylohyoid, anterior belly of digastric, tensor tympani, tensor veli palatini (originate from head mesoderm
• cranial nerve:
• maxillary and mandibular divisions of trigeminal N. (CN V)
• ophthalmic division (CN V) does not supply arch components
• ligaments:
• anterior ligament of malleus
• spheno-mandibular ligament

Question 28

PA 2

• arch artery:
• skeletal elements:
• muscles:
• cranial nerve:
• ligaments:

Answer 28

PA 2

• arch artery: stapedial artery (embryonic), caroticotympanic artery (adult)
• skeletal elements: stapes, styloid process, lesser horns and part of body of hyoid (derived from second-arch (Reichert’s) cartilage, originate from NCC)
• muscles: muscles of facial expression (orbicularis oculi, orbicularis oris, risorius, platysma, auricularis, frontalis, and buccinator), posterior belly of digastric, stylohyoid, stapedius (originate from head mesoderm)
• cranial nerve: facial N. (VII)
• ligaments: stylohyoid ligament

Question 29

PA 3

• arch artery:
• skeletal elements:
• muscles:
• cranial nerve:

Answer 29

PA 3

• arch artery: common carotid artery, root of internal carotid
• skeletal elements: lower rim and part of body of hyoid (derived from third-arch cartilage, originate from NCC)
• muscles: stylopharyngeus (originates from head mesoderm)
• cranial nerve: glosspharyngeal N. (CN IX)

Question 30

PA 4

• arch artery:
• skeletal elements:
• muscles:
• cranial nerve:

Answer 30

PA 4

• arch artery:
• arch of aorta (left side), right subclavian artery (right side)
• original sprouts of pulmonary arteries
• skeletal elements: thyroid and epiglottal laryngeal cartilages (derived from fourth-arch cartilage, originate from NCC)
• muscles: constrictors of pharynx, cricothyroid, levator veli palatini (originate from occipital somites)
• cranial nerve: superior laryngeal branch of vagus N. (CN X)

Question 31

PA 6

• arch artery:
• skeletal elements:
• muscles:
• cranial nerve:

Answer 31

PA 6

• arch artery: ductus arteriosus, roots of definitive pulmonary arteries
• skeletal elements: remaining laryngeal cartilages (derived from sixth-arch cartilage, uncertain whether they originate from neural crest or mesoderm)
• muscles: intrinsic muscles of larynx (except cricothyroid, originate from occipital somites)
• cranial nerve: recurrent laryngeal branch of vagus N. (CN X)