6.0 Embryology of the Pharyngeal Apparatus

Question 1

What muscles come from the fourth arch?

What cranial nerve innervates them?

Answer 1

Cricothyroid, levator veli palatini and the constrictors of the pharynx.

CN X

Question 2

What is a cervical cyst?

Answer 2

An opening behind the mandible created by the failure of the second pharyngeal groove (of the cervical sinus) to close completely.

They are typically asymptomatid until they get larger and fill with fluid and cellular debris.

Question 3

What general structures are found in each pharyngeal arch?

Answer 3

Question 4

What forms the body of the hyoid bone?

Answer 4

Hypopharyngeal eminence (from PA 3 and PA 4)

Question 5

What is responsible for the formation of the maxillary and mandibular components of pharyngeal arch 1?

Answer 5

Neural crest cell migration into the area.

Question 6

What is the “cervical sinus?”

Answer 6

A depression in ectoderm caused by PA 2 overgrowing PA 3 and PA 4. This leads to a common pharyngeal groove “sinus” for pharyngeal grooves 2,3, and 4.

Question 7

What muscles come from the second arch?

What cranial nerve innervates them?

Answer 7

Stapedius and the muscles of facial expression.

CN VII

Question 8

From what does the thyroid gland develop?

Answer 8

A median thickening in the endoderm of the primordial pharynx.

Question 9

What is Treacher-Collins syndrome?

What is the cause?

Answer 9

(Mandibulofacial dysostosis)

Treacher-Collins syndrome (mandibulofacial dysostosis)

• Malar hypoplasia w/ down-slanting palpebral fissures, defects
of lower eyelids, deformed external ears, & sometimes middle
& internal ears
• Autosomal dominant disorder; mutations in Treacher Collins–
Franceschetti syndrome 1 gene (TCOF1)
• Encodes for the protein TREACLE, involved in ribosome
biogenesis
• Truncated in TCS leading to ↑↑ apoptosis of cranial NCC

Question 10

What is Meckel’s cartilage?

What is it’s function?

Answer 10

The cartilage of pharyngeal arch 1.

Used as a model for osteoblasts to form the mandible. After the mandible is created, Meckel’s cartilgae will degenerate.

Also, two nodules break off and serve as a precursor from the malleus and incus.

Question 11

What is formed by the cartilage of the third pharyngeal arch?

Answer 11

Greater cornu of the hyoid bone.

Question 12

The majority of the developing pharyngeal arches are what type of tissue?

Answer 12

Neural crest cells.

Question 13

What muscles come from the sixth arch?

What cranial nerve innervates them?

Answer 13

Intrinsic muscles of the larynx.

CN X

Question 14

What muscle comes from the third arch?

What cranial nerve innervates it?

Answer 14

Stylopharyngeus.

CN IX

Question 15

What are the first structures the first pharyngeal arch develops into?

Answer 15

The maxillary and mandibular prominences.

Question 16

What embryological structure do the external carotid arteries come from?

Answer 16

Common carotid.

Question 17

What is formed from the 1st pharyngeal membrane?

Answer 17

The tympanic membrane

Question 18

What all is derived from the first pharyngeal pouch?

Answer 18

Forms the tubotympanic recess.

This recess becomes both the pharyngotympanic tube (proximally) as well as the tympanic cavity, the mastoid antrum, and a contribution to the tympanic membrane (distally).

Question 19

What is DiGeorge syndrome?

Answer 19

• Agenesis of thymus & parathyroid glands
• Congenital hypoparathyroidism
• Shortened philtrum of upper lip, low-set &
notched ears
• Nasal clefts, thyroid hypoplasia
• Cardiac abnormalities (defects of the
aortic arch & heart)

CATCH22: Cardiac abnormality, Abnormal face, Thymus hypoplasia, cleft palate, hypocalcemia, 22nd chromosome problem”

Question 20

What is the Pierre Robin sequence?

Answer 20

Pierre Robin sequence:

• Typically occurs de novo in most patients
• Associated with hypoplasia of the mandible, cleft
palate, & defects of the eyes & ears
• Initiating defect is a small mandible
(micrognathia) → results in posterior
displacement of the tongue → obstruction of full
closure of the palate → results bilateral cleft
palate

PRS = Palate (cleft), Retracted Tongue, Small mandible

Question 21

What forms the stylohyoid ligament?

Answer 21

Riechert’s cartilage from PA 2.

Question 22

Where does Riechert’s cartilage come from, and what does it make?

Answer 22

Riechert’s cartilage comes from pharyngeal arch 2, and makes the stapes and styloid process of the temporal bone. It also makes the lesser cornu of the hyoid, and the ligament in between its structures - the stylohyoid ligament.

Question 23

What do we call the adult remnant of the thyroglossal duct?

Answer 23

The foramen cecum.

Question 24

What all is derived from the fourth pharyngeal pouch?

Answer 24

This forms a dorsal bulbar part and a ventral hollow part (just like arch 3).

The dorsal part becomes the superior parathyroid gland (not to be confused by the more superior pouch number 3 forming the inferior parathyroid gland).

The hollow part becomes the ultimopharyngeal body, which fuses with the thyroid gland and disseminates to form the parafollicular cells.

Question 25

What is formed from the perichondrium of Meckel’s cartilage?

Answer 25

Anterior ligament of the malleus.

Sphenomandibular ligament.

Question 26

What muscles come from the first arch?

What cranial nerve innervates them?

Answer 26

Muscles of mastication.

CN V

Question 27

Where do the muscular components of the pharyngeal arches come from generally?

Answer 27

Prechordal and paraxial mesoderm.

Question 28

What is formed by the fourth and sixth arch cartilages?

Answer 28

Laryngeal cartilages.

Question 29

What forms the lesser cornu, greater cornu, and body of the hyoid?

Answer 29

Lesser cornu = Reichert’s cartilage (PA 2)

Greater Cornu = Third arch cartilage

Body of Hyoid = Hypopharyngeal eminence (PA 3 and PA 4)

Question 30

What all is derived from the second pharyngeal pouch?

Answer 30

Part of the cavity of the pouch forms the tonsillar sinus.

Endoderm of the pouch proliferates and breaks down, forming the tonsillar crypts in the center, and the epithelium of the tonsils and crypts on the outside.

Mesenchyme of the pouch forms lymphoid tissue and will organize into lymphatic nodules of the palantine tonsil.

Question 31

What are the general roles of the neural crest cells, the paraxial mesoderm, the lateral plate mesoderm, and the pharyngeal endoderm in the developing pharyngeal arches?

Answer 31

Neural crest cells: make the maxillary and mandibular prominences, as well as the connective tissue (bone, dermis) and smooth muscle.

Paraxial mesoderm is called the myogenic mesoderm, because it forms the muscle primordium in each arch.

Lateral plate mesoderm gives rise to angioblasts which form endothelium.

Pharyngeal endoderm plays a regulatory role.

Question 32

What all is derived from the third pharyngeal pouch?

Answer 32

There are two parts: the dorsal bulbar part and the ventral hollow part.

The dorsal bulbar part (more superior) forms the inferior parathyroid gland.

The hollow part (more ventral and inferior) forms the thymus.

Question 33

What forms the stapes and styloid process of the temporal bone?

Answer 33

Reichert’s cartilage from PA 2.