Embryology of the Pharyngeal Apparatus

Question 1

When do the pharyngeal arches develop? What are they derived from?

Answer 1

4th week

Neural crest

Question 2

How are the pharyngeal arches organized (layer wise)?

Answer 2

Arch is lined externally by ectoderm and internally by endoderm
Core is initially mesenchyme from Neural Crest but then true mesoderm migrates in

Question 3

What is the role of NCC-derived mesenchyme in the pharyngeal arch?

Answer 3

form all connective tissue in the head (including dermis)

smooth muscle of vasculature

Question 4

What is the role of paraxial mesoderm in the pharyngeal arch?

Answer 4

populate each arch to from PA musculature

Question 5

What is the role of lateral plate mesoderm in the pharyngeal arch?

Answer 5

angioblast which differentiate into endothelium to line the blood vessels in the face

Question 6

What is the role of prechordal plate mesoderm in pharyngeal arches?

Answer 6

extraocular musculature (orbicularis oculi)

Question 7

What kind of cartilage is found in PA1? What is its role in development?

Answer 7

Meckel’s cartilage

Dorsal portion forms malleus and incus

Ventral= primordium of mandible

Placeholder of lower jaw –> forms mandible (laterally) via inter-membrane ossification

Perichondrium –> anterior ligament of malleus and sphenomandibular ligament

Question 8

What kind of cartilage is in PA2? What does the dorsal and ventral regions become?

Answer 8

Riechert’s cartilage

Dorsal= stapes and styloid process of temporal bone
Ventral= ossifies (intermembraneous) and forms lesser cornu aka horn of hyoid

Question 9

What is the role of third arch cartilage?

Answer 9

Forms greater cornu of hyoid via cartilaginous ossification

Question 10

Where is the body of hyoid formed from?

Answer 10

PA 3 and 4

formed by prominence in embryonic pharynx aka hypopharyngeal eminence

Question 11

What do the 4th and 6th arch cartilages contribute to?

Answer 11

Laryngeal cartilage

- specifically 4th is NCC derived and includes the epiglottis

Question 12

Where are the muscular components of the PAs derived from?

Answer 12

Paraxial mesoderm and prechordal plate

Question 13

What muscle(s) come from the first arch?

Answer 13

Muscle of mastication (CN V) #chewonthat

Question 14

What muscle(s) come from the second arch?

Answer 14

Stapedius m. and muscles of fascial expression (CN VII) #smileandsaycheese

Question 15

What muscle(s) come from the third arch?

Answer 15

Stylopharyngeus (CN IX) #elevateanddialte

Question 16

What muscle(s) come from the fourth arch?

Answer 16

Circothyroid
Levatator veli palatini
Constrictors of the pharynx 
CN X
#pushitpushitrealgood

Question 17

What muscle(s) come from the sixth arch?

Answer 17

Intrinsic muscles of the larynx (CN X)
#soundoff

Question 18

What nerve supplies PA1?

Answer 18

The trigeminal nerve: just V2 and V3

• principal sensory nerve of head and neck
• motor nerve for muscles of mastication

Question 19

What nerve supplies PA2?

Answer 19

Facial nerve (CN VII) muscles of fascial expression

Question 20

What nerve supples PA3?

Answer 20

Glossopharyngeal (CN IX)

Stylopharyngeus m.

Question 21

What nerve supplies PA4?

Answer 21

superior laryngeal branch of Vagus N. (CN X) and its recurrent laryngeal branch

Question 22

What nerve supplies PA6?

Answer 22

Recurrent (inferior) laryngeal of the Vagus N. (CN X)

Question 23

Where do the arch arteries come from?

Answer 23

The aortic sac which is a basket like structure arising from the dorsal aorta
- remodeled into great arteries of the thorax, head and neck

Question 24

Where are pharyngeal grooves located? What are they covered with?

Answer 24

Externally, between pharyngeal arches

Ectoderm

Question 25

Where does the first groove lie?

Answer 25

External acoustic metaus

Question 26

What do grooves 2-4 lie in?

Answer 26

the cervical sinus which closes off during development

Question 27

Where are pharyngeal pouches? What are they derived from?

Answer 27

Lines pharynx internally Pharyngeal endoderm

Question 28

How do cervical cysts arise? What are the symptoms?

Answer 28

Remnants of cervical sinus and/or 2nd groove Slow enlarging painless cyst in neck inferior to angle of mandible\, filled with fluid and cellular debris from desquamation of their epithelial lining

Question 29

How do cervical sinus anomalies arise? What are the symptoms?

Answer 29

Failure of 2nd PA groove and cervical sinus to obliterate Discharge of mucus Normally bilateral and associated with auricular sinuses

Question 30

How do cervical fistulas arise? What are the symptoms?

Answer 30

Abnormal canal that opens into the tonsillar sinus due to persistence of parts of 2nd groove and pouch

Question 31

What are pharyngeal membranes?

Answer 31

Where pouch endoderm comes into contact with groove ectoderm - quickly infiltrated by mesenchyme

Question 32

What does the 1st pharyngeal membrane form?

Answer 32

Tympanic mebrane

Question 33

What does the endoderm of the 1st pouch form?

Answer 33

Expands into tubotympanic recess --> Tympanic cavity and mastoid antrum Elongates to form pharyngotympanic tube Contacts 1st groove contributing to tympanic membrane

Question 34

What does the 2nd pouch form?

Answer 34

Tonsillar sinus Endoderm proliferate and forms tonsillar epithelium Mesenchyme become lymphoid nodules of palatine tonsil TONSILS

Question 35

What does the 3rd pouch form?

Answer 35

``` Dorsal= inferior parathyroid Ventral= thymus ``` Caudally migrates aka pulls structures down

Question 36

What does the 4th pouch form?

Answer 36

Dorsal= superior parathyroid glands Ultimobranchial body fuses with thyroid gland and gives rise to parafollicular cells

Question 37

Where are epithelial cords derived from? What are their 3 functions?

Answer 37

PA3 endoderm which form the thymic primordium 1. grow into surrounding mesenchyme --> thymic lobules 2. Become arranged around a central point --> thymic corpuscles 3. Form epithelial reticulum for housing lymphocytes

Question 38

Where is the thyroid gland derived from?

Answer 38

NOT ARCH DERIVATIVE Comes from developing floor of the pharynx: median endodermal thickening Descends in neck with tongue growth and passes ventral to developing hyoid and laryngeal cartilage, connected by thyroglossal duct - only gland moving

Question 39

What is first arch syndrome (generally)?

Answer 39

Insufficient migration of NCC into the 1st arch during the 4th week

Question 40

What is Treacher-Collins syndrome?

Answer 40

type of first arch syndrome Autosomal dominant, mutation in TCOF1 which is involved in ribosome biogenesis - truncation leads to increased apoptosis of cranial NCC Malar hypoplasia down slanting palperbal fissures, deformed external ear, defects of lower eyelid

Question 41

What is Pierre Robin syndrome?

Answer 41

First arch syndrome, Occurs de novo hypoplasia Initiating defect= small mandible --> posterior displacement of tongue --> obstruction of closure of the palate --> bilateral cleft palate - difficulties feeding

Question 42

What is agenesis of thyroid gland?

Answer 42

Complete absence of a thyroid gland or one of its lobes

Question 43

What is thyroid hemiagenesis?

Answer 43

Unilateral failure of formation | Left lobe more commonly absent

Question 44

What is DiGeorge Syndrome?

Answer 44

Breakdown of signlaing from PA enoderm to NCC --> agenesis of thymus and parathyroid glands CONGENITAL HYPOPARATYROIDISM Cardiac abnormalities Nasal clefts, thyroid hyperplasia Shortened philtrum of upper lip, low set and notched ears

Question 45

What is the foramen cecum a remanent of?

Answer 45

Thyroid gland Little bump on tongue