NB4-1 - Pharyngeal Arches and their Clinical Relevance

Question 1

What does the pharyneal apparatus consist of? When does it appear? From what does it develop?

Answer 1

The pharyngeal apparatus begins to develop around the 4th week in utero from neural crest cells that have migrated into the mesenchyme of the future head and neck region. It consits of pharyngeal arches, pouches, grooves, and membranes.

Question 2

What is each pharyngeal arch composed of and what separates these arches?

Answer 2

Each arch consists of a core of mesoderm covered by ecotderm externally and endoderm internally. Each arch contains cartilage, muscle, nerve, and vasculature. Each arch is separated by deep pharyngeal grooves and pouches.

Question 3

Answer 3

Question 4

Describe what the pharyngeal grooves, pouches, and membranes do and what they consist of.

Answer 4

• The grooves separate the arches externally and are lined by ectoderm
• The pouches separate the arches internally and are lined by endoderm
• The membranes are the areas between arches where the ectoderm and endoderm meet

Question 5

What does each component of the pharyngeal arch do?

Answer 5

• The vasculature serves to supply and drain the primordial pharynx of blood
• The cartilaginous rod of the arch acts as a type of skeleton
• The muscular component will give rise to the muscles of the head and neck
• The nerve supplies the mucosa and muscles of the arch

Question 6

From what vessel do the arteries of the arches arise? Where do they terminate

Answer 6

The arch arteries arise from the truncus arteriosus, course around the primordial pharynx and terminate at the dorsal aorta.

Question 7

How many arches are there and which ones have strange fates?

Answer 7

There are 6 arches.

Arch 4 merges with arch 6

Arch 5 often doesn’t develop and if it does it’s rudimentary and doesn’t persist for very long

Question 8

What is the cartilage of the 1st pharyngeal arch AKA. Describe what the 1st pharyngeal arch cartilage becomes.

Answer 8

The 1st arch cartilage (aka Meckel’s cartilage) will give the mandible its shape but does not ossify into the mandibular bone. It will obliterate except for the very tip which will give rise to the maleus, incus, anterior ligament of the malleus, and the spine of the sphenoid bone

Question 9

Describe what the 2nd pharyngeal arch cartilage becomes.

Answer 9

It will develop into the stapes, styloid process, sylohyoid ligament, and lesser comu of hyoid bone (most superior part of hyoid bone)

Question 10

Describe what the 3rd pharyngeal arch cartilage becomes.

Answer 10

The body of the hyoid bone

Question 11

Describe what the 4th and 6th pharyngeal arch cartilages becomes.

Answer 11

The thyroid, cricoid, and arytenoid cartilages

Question 12

List the structures the 1st arch muscles develop into.

Answer 12

• Temporalis
• Masseter
• Mylohyoid

Question 13

List the structures the 2nd arch muscles develop into.

Answer 13

• Frontalis
• Orbicularis oculi
• Buccinator
• Orbicularis oris
• Occipitalis
• Anterior & Posterior Bellies of Digastric Muscles
• Platysma
• Auricularis

Question 14

List the structures the 3rd arch muscles develop into.

Answer 14

• Stylopharyngeus

Question 15

List the structures the 4th and 6th arch muscles develop into.

Answer 15

Pharyngeal muscles

Question 16

List the structures the 1st arch nerves develop into.

Answer 16

• The V2 maxillary division of CN5
• The V3 mandibular division of CN5

Question 17

List the structures the 2nd arch nerves develop into.

Answer 17

CN7 - Facial Nerve

Question 18

List the structures the 3rd arch nerves develop into.

Answer 18

CN9 - glossopharyngeal

Question 19

List the structures the 4th & 6th arch nerves develop into.

Answer 19

CNX - Vagus

Question 20

What will the mesenchyme of the 1st pharyngeal arch develop into?

Answer 20

It will give rise to auricle hillocks 1, 2, & 3 which will form the proximal part of the auricles (aka pinnas)

Question 21

What will the mesenchyme of the 2nd pharyngeal arch develop into?

Answer 21

It will develop into the aurickle hillocks 4, 5, & 6 which will form the distal portions of the auricles (pinnas).

Question 22

Answer 22

B

Question 23

Which arteries do the vessels of each pharyngeal arch develop inot?

Answer 23

1st Arch - Maxillary Artery

2nd Arch - Stapedial (atrophies before birth)

3rd Arch - Common and Internal Carotid

4th Arch - Arch of the Aorta & Right Subclavian

6th Arch - Ductus Arteriosus & Pulmonary Arteries

Question 24

Answer 24

Question 25

What do the pharyngeal grooves develop into? Explain how this happens.

Answer 25

The 2nd pharyngeal arch enlarges laterally and downwards and eventually fuses with the 6th pharyngeal arch. This process causes the 2nd, 3rd, and 4th pharyngeal grooves to merge into the cervical sinus which later obliviates. Only the 1st pharyngeal groove persists which becomes the external acoustic meatus.

Question 26

What will the pharyngeal pouches develop into?

Answer 26

• 1st pouch - tubtympanic recess which will become the middle ear cavity (pharyngotympanic tube & mastoid antrum)
• 2nd pouch - the palatine fossa, which the palatine tonsils will grow into, obliterating it.
• 3rd pouch - inferior parathyroid glands and thymus
• 4th pouch - superior parathyroid glands and ultimopharyngeal body (gives rise to the parafollicular cells of the thyroid gland)

Question 27

What will the pharyngeal membranes devleop into?

Answer 27

• 1st Membrane - tympanic membrane

All other membranes obliterate

Question 28

What is and what causes Treacher Collins Syndrome? List the most common symptoms of this syndrome.

Answer 28

Treacher Collins Syndrome is a type of First Arch Syndrome caused by a failure of neural crest cells to migrate into the first pharyngeal arch. Its symptoms are as follows:

• Micrognathia (underdeveloped jaw)
• ~50% of patients have a cleft palate
• Underdeveloped zygomas, which also causes sagging cheeks and drooping eyelids
• Malformed pinna
• Malformed maleus and incus causing conductive hearing loss

Refer to Image

Question 29

What is Pierre Robin Sequence?

Answer 29

It is a series of events that commonly occurs with First Arch Syndrome

1. Micrognathia or retrognathia (posteriorly set jaw) which leads to…
2. Glossoptosis - tongue pulled back into pharynx causing airway obstruction and blockage of palatal shelves which leads to…
3. Cleft Palate

Question 30

What is the important take away lesson from Pierre Robin Sequence?

Answer 30

A first arch syndrome does NOT directly cause glossoptosis or a cleft palate. It causes the micrognathia/retrognathia which then causes the glossoptosis and cleft palate.

Question 31

What can malformation of the auricle hillocks lead to ?

Answer 31

Auricle sinuses and cysts

Refer to image

Question 32

Describe what a cervical sinus, cervical fistula, and cervical cyst are and how they form.

Answer 32

• A persistent cervical sinus occus when part of the cervical sinus remains leaving a blind ended tube. There are external cervical sinuses that are open to the outside environment and internal cervical sinuses that are open to the pharyngeal pouches
• A cervical fistula occurs when an internal or external cervical sinus join or extend all the way to the pharyngeal pouches or outside environment. There will often be clear discharge coming from the hole in the neck
• A cervical cyst occurs when the cervical sinus persists despite it closing off to the outside environment.

Question 33

Answer 33

A

Question 34

Pharyngeal arches 3 and 4 are most commonly affected by which disease? Describe the cause and symptoms of this disease

Answer 34

DiGeorge Syndrome - Catch 22

Caused by a deletion in the long arm of chromosome 22 which is often caused by exposure to retinoids & alcohol during pregnancy and maternal diabetes mellitus. Which leads to faulty migration of neural crest cells, including into the 3rd and 4th pharyngeal arches. Its symptoms are Cardiac anomalies, Abnormal face, Thymic hypoplasia/aplasia (leading to infections), Cleft palate, Hypoparathyroidism (leading to hypocalcemia), and psychiatric disorders.

Question 35

Describe how and when the thyroid gland develops?

Answer 35

It develops around day 24 as an epithelial proliferation of the gut tube in the foramen cecum at the base of the tongue (not a pouch structure). It will continue to grow in size and descend anterior to the foregut down to its adult position around C5-T1.

Question 36

What is the thyroglossal duct?

Answer 36

The path the thyroid gland takes from its point of origin to its final resting place

Question 37

List and describe the thyroid gland developmental anomalies we need to know.

Answer 37

• Ectopic thyroid tissue - could be located anywhere along the thyroglossal duct, Tissue will likely secrete thyroid hormone
• Cystic remnant of thyroglossal duct - occurs if duct fails to obliterate
• Thyroglossal fistula - occurs if thyroglossal duct fails to obliterate and opens to the external environment

Question 38

How can you tell the difference between a thyroglossal or cervical fistula/cyst.

Answer 38

Thyroglossal cysts and fistulas are always in the midline while cervical cysts and fistula are not. Also, thyroglossal cysts are frequently attached to the base of the tongue in one way or another. This means if you ask the patient to swallow or stick their tongue out, the thyroglossal cyst will rise while the cervical cyst will not.