Pharyngeal apparatus

Question 1

Parts of the pharyngeal apparatus

Answer 1

pharyngeal clefts
pharyngeal arches
pharyngeal pouches

Question 2

Pharyngeal clefts

Answer 2

1. External invagination of ectoderm.

2. 4 clefts

Question 3

pharyngeal arches

Answer 3

1. Core of mesenchyme (neural crest and paraxial mesoderm).
2. Neural crest will give rise to ct components; paraxial mesoderm gives rise to muscles.
3. Each arch has its own nerve (cranial nerve) and blood supply (aortic arch artery).
4. 6 pharyngeal arches initially develop; but the 5th regresses, leaving arches 1-4 and 6.

Question 4

Pharyngeal pouches

Answer 4

1. Internal evagination of pharyngeal endoderm.

2. 4 pouches

Question 5

Derivatives of Arch 1

Answer 5

1. Skeletal derivatives – viscerocranium (malleus, incus) and connective tissues of face
2. Muscular derivatives
   a. Muscles of mastication (temporalis, masseter, medial and lateral pterygoids)
   b. Also: anterior belly of digastric, mylohyoid, tensor tympani, tensor veli palatini.
   “MAST and MATT”
3. Nerve: trigeminal nerve (CN V3)
4. Oropharyngeal membrane develops between the two processes of first arch.
   a. Composed of an outer layer of ectoderm and an inner layer of endoderm.
   b. Ruptures by day 26 to allow communication between pharynx and oral cavity.

Question 6

Derivatives of Arch 2

Answer 6

1. Skeletal derivatives
   a. Stapes, styloid process of temporal bone
   b. Stylohyoid ligament; lesser horn and superior portion of body of hyoid
2. Muscular derivatives
   a. Muscles of facial expression
   b. Also: stylohyoid, posterior belly of digastric, stapedius
3. Nerve: facial nerve (CN VII)

Question 7

Derivatives of Arch 3

Answer 7

1. Skeletal derivatives: greater horn and inferior portion of body of hyoid bone.
2. Muscular derivatives: stylopharyngeus
3. Nerve: glossopharyngeal nerve (CN IX)

Question 8

Derivatives of Arch 4

Answer 8

1. Skeletal derivatives: with 6th arch, forms all laryngeal cartilages.
2. Muscular derivatives: muscles of pharynx (except stylopharyngeus); cricothyroid
3. Nerve: vagus nerve (CN X)

Question 9

Derivatives of Arch 5

Answer 9

Regresses

Question 10

Derivatives of Arch 6

Answer 10

1. Skeletal derivatives: with 4th arch, forms all laryngeal cartilages.
2. Muscular derivatives: laryngeal muscles (except cricothyroid); skeletal muscle of esophagus
3. Nerve: vagus nerve (CN X; recurrent laryngeal nerve).

Question 11

Derivatives of Pharyngeal Pouch 1

Answer 11

contributes to formation of the middle ear and auditory tube.

Question 12

Derivatives of Pharyngeal Pouch 2

Answer 12

the stroma of palatine tonsil; tonsillar fossa.

Question 13

Derivatives of Pharyngeal Pouch 3

Answer 13

1. Dorsal portion: inferior parathyroid glands

2. Ventral portion: thymus

Question 14

Derivatives of Pharyngeal Pouch 4

Answer 14

1. Dorsal portion: superior parathyroid glands

2. Ventral portion: ultimobranchial body (see development of thyroid gland below)

Question 15

Derivatives of Pharyngeal Clefts

Answer 15

A. First cleft contributes to formation of the external ear.

B. The second arch overgrows arches 3 and 4, burying clefts 2-4.

C. Remants of clefts 2-4 form the cervical sinus which is later obliterated.

Question 16

Thyroid gland development

Answer 16

A. Derived from a median endodermal thickening of pharyngeal floor.

B. The gland descends along the pharynx into the neck, remaining connected to tongue via the thyroglossal duct (degenerates by week 7).

C. Superior opening of the thyoglossal duct remains as the foramen cecum.

D. In about 50% of individuals a pyramidal lobe of the thyroid is present; this is a remnant of the distal thyroglossal duct.

E. Parafollicular (C cells) are derived from the ultimobranchial body of the 4th pouch.

Question 17

Tongue development

Answer 17

A. 1st pharyngeal arch form the body (anterior 2/3) of tongue (GSA from trigeminal n.)

B. 3rd arch forms the root (posterior 1/3) of tongue (GVA from glossopharyngeal n.)

C. 4th arch forms the epiglottic region of tongue (GVA from vagus n.)

D. Muscle derived from paraxial mesoderm of occipital somites

Question 18

Arches and the nerves that innervate their derivatives

Answer 18

1--> trigeminal n.
2--> facial n.
3--> glossopharyngeal n.
4--> vagus n.
5--> mostly regresses
6--> vagus n. (mainly recurrent laryngeal branch)

Question 19

Congenital Anomalies: Ectopic thyroid or parathyroid tissue

Answer 19

1. Remnants of thyroid tissue can remain along the course of migration.
2. The parathyroids also undergo a migration and are highly variable in their location.

Question 20

Congenital Anomalies: Lateral cervical cysts

Answer 20

Lateral cervical cysts form when the cervical sinus fails to degenerate. These cysts are found along the anterior border of the SCM.

Question 21

Congenital Anomalies: branchial fistulas

Answer 21

Branchial fistulas occur when second arch fails to overgrow the third and fourth arches. The cervical sinus retains a connection with the surface (external fistula). In rare cases, the lateral cervical cyst can open into the pharynx (internal fistula).

Question 22

Congenital Anomalies: Thyroglossal Cyst

Answer 22

1. Remnant of thyroglossal duct.

2. Always located on or near midline of the neck.

Question 23

Congenital Anomalies: Neural Crest Defects

Answer 23

1. Neural crest cells are important for the development of the pharyngeal arches.
2. Severe craniofacial defects result from disruption of neural crest cell migration or differentiation.
3. Ex: Treacher Collins Syndrome, Robin Sequence, DiGeorge Anomaly
4. DiGeorge Syndrome
   a. Deletion on long arm of Chromosome 22
   b. Cardiac abnormalities
   c. Craniofacial defects
   d. Thymic hypoplasia
   e. Parathyroid dysfunction – hypocalcemia due to parathyroid insufficiencies