Development Of The Pharynx

Question 1

Describe the structure of a pharyngeal arch.

Answer 1

• consists of core of mesenchyme
• outside is covered with ectoderm
• inside is lined with endoderm
• separated from other arches by a groove on the outside and a pouch on the inside
• intervening tissues between groove and pouch consists of endoderm and ectoderm and is subject to perforation

Question 2

What are the pharyngeal arch components?

Answer 2

• artery
• cranial nerve
• skeletal component

Question 3

What is the main organizer for pharyngeal arch development?

Answer 3

-foregut endoderm

Question 4

What is the craniocaudal segmentation determined by?

Answer 4

-Hox genes

Question 5

Outline the processes used for pharyngeal arch formation.

Answer 5

Pharyngeal endoderm Tbx-1 -> FGF-8 -> pharyngeal arch formation

Question 6

How many pharyngeal arches are there?

Answer 6

• primitively there are 6 pairs

- in humans there are 4 pairs of arches and 4 grooves that separate from each other

Question 7

What are the derivatives of the first arch?

Answer 7

-mandibular arch

-Meckel’s cartilage
+main component of embryonic lower jaw
+distal part is reabsorbed via apoptosis
+sphenomandibular ligament
+malleus
+incus

Question 8

What is the nerve associated with the first arch?

Answer 8

Trigeminal nerve

Question 9

How does the first arch develop?

Answer 9

• neural crest derived from rhombomeres 1-2 and midbrain

- differentiation associated with Otx-2

Question 10

What are the derivatives of the second arch?

Answer 10

-hyoid arch

-Reichert’s cartilage
+body and lesser horn of hyoid
+stapes
+styloid process and stylohyoid ligament

-muscles of facial expression

Question 11

What nerve is associated with second arch?

Answer 11

-facial nerve

Question 12

How is the second arch developed?

Answer 12

-influenced by Hoxa2

Question 13

What nerves are associated with the third and fourth arches?

Answer 13

-glossopharyngeal and vagus nerves

Question 14

What are the derivatives of the third and fourth arches?

Answer 14

-part of hyoid bone and upper pharynx (3rd)
+greater horn of hyoid

• stylopharyngeus muscle (3rd)
• muscles and cartilages of larynx and lower pharynx (4th)

Question 15

What is the derivative of the 1st groove?

Answer 15

-external auditory meatus

Question 16

What is the derivative of the 2nd and 3rd grooves?

Answer 16

-cervical sinus

Question 17

What are the derivatives of the first pouch?

Answer 17

• tympanic cavity of middle ear

- auditory tube

Question 18

What are the derivatives of the second pouch?

Answer 18

-fossa of palatine tonsils

Question 19

What are the derivatives of the third pouch?

Answer 19

• inferior parathyroids -> high conc Shh

- thymus -> high conc of BMP-4

Question 20

What are the derivatives of the fourth pouch?

Answer 20

-superior parathyroids

-postbrachial (ultimobrachial) bod
+become incorporated into thyroid as parafollicular cells which secrete calcitonin

Question 21

What does the thyroid begin as?

Answer 21

-begins as a node (anlage) of cells located on the ventral surface of the pharynx between the first and second pairs of pharyngeal arches

Question 22

How is the anlage signaled?

Answer 22

• induced via signals from the underlying mesenchyme

- the cells express transcription factors Hhex, Nkx2-1, Pax8, adn Foxe-1

Question 23

How does the thyroid anlage grow?

Answer 23

• grows down into the underlying mesenchyme as the thyroid diverticulum
• diverticulum remains connected temporarily to the floor of the pharynx via the thyroglossal duct
• the opening of this duct persists as a small pit at the base of the tongue called the foramen cecum

Question 24

What is the infundibular process and what does it form?

Answer 24

• ventral downgrowth from floor of diencephalon

- forms neurohypophysis (posterior pituitary)

Question 25

What is Rathke’s pouch and what does it form?

Answer 25

• midline upgrowth from roof of stomodeum
• stimulated by BMP-4 followed by FGF-8 from floor of diencephalon

-forms a cup like extension distally that attaches to the infundibular process
+inner layer adheres to neurohypophysis and forms pars intermedia

\+outer layer is separated from pars intermedia by narrow lumen called the residual lumen

+outer cup will form the anterior pituitary

Question 26

Where are the lateral lingual swellings? When do they form?

Answer 26

-5 weeks

-located in ventral areas of first pair of pharyngeal arches
+located between sarcolemma and basal lamina

Question 27

Where does the tuberculum impar form? When?

Answer 27

• 5 weeks

- ventral swelling between first and second pairs of arches

Question 28

Where does the cupula form? When?

Answer 28

• 5 weeks

- ventral swelling between second and third pairs of arches

Question 29

Where is the foramen cecum located?

Answer 29

-between the tuberculum impar and the copula

Question 30

What is micrognathia?

Answer 30

-condition in which the jaw is undersized

Question 31

What is agnathia?

Answer 31

-absence of a portion or the entirety of one or both jaws

Question 32

What is Treacher Collins syndrome?

Answer 32

-autosomal dominant disorder characterized by craniofacial deformities such as absent cheekbones

Question 33

What is ankyloglossia?

Answer 33

• aka tongue tie

- unusually short, thick lingual frenulum