S11) Development of Midline Structures

Question 1

Describe the structure and location of the pituitary gland

Answer 1

• Location: closely related to the hypothalamus
• Structure:

I. Anterior lobe – endocrine secretion

II. Posterior lobe – neuroendocrine secretion

Question 2

Describe the embryological origins of the pituitary gland

Answer 2

• Ectoderm → Rathke’s pouch → Anterior pituitary
• Neurectoderm → Infundibulum → Posterior pituitary

Question 3

What is Rathke’s pouch?

Answer 3

• Rathke’s pouch is an out-pocketing of ectoderm of the stomatodeum
• It is formed from an evagination of the roof of the (oro-) pharynx and grows dorsally towards the developing forebrain

Question 4

What is the infundibulum?

Answer 4

The infundibulum is a downward out-growth of the forebrain which grows towards the roof of the pharynx

Question 5

What is the sulcus terminalis?

Answer 5

The sulcus terminalis is a V-shaped shallow groove that separates the dorsum of the tongue into a superior oral surface, and a posterior pharyngeal surface

Question 6

What is the foramen cecum?

Answer 6

• The foramen cecum is a shallow depression in the posterior midline of the tongue which marks beginning of the terminal sulcus
• It is also the point of attachment of the thyroglossal duct and is formed during the descent of the thyroid diverticulum in embryonic development

Question 7

Describe the embryonic origins of the tongue

Answer 7

The tongue receives a component from each of the pharyngeal arches

Question 8

When does the tongue begin to form?

Answer 8

Primordia of the tongue appear at about the same time as the palate begins to form

Question 9

Describe the second stage in the development of the tongue (lingual swellings)

Answer 9

• 2 lateral lingual swellings → Ph Arch 1
• 3 median lingual swellings:

I. Ph Arch 1 → tuberculum impar

II. Ph Arch 2,3&4 → cupola

III. Ph Arch 4 → epiglottal swelling

Question 10

Describe the final stage in the development of the tongue (overgrowth & lingual frenulum)

Answer 10

• Lateral lingual swellings over-grow tuberculum impar
• 3rd arch component of the cupola over-grows the 2nd arch component
• Extensive degeneration occurs, freeing tongue from oral cavity floor (lingual frenulum)

Question 11

What is the lingual frenulum?

Answer 11

The lingual frenulum is a small fold of mucous membrane extending from the floor of the mouth to the midline of the underside of the tongue

Question 12

Describe the embryonic development & sensory innervation of the anterior and posterior parts of the tongue

Answer 12

• Ph Arch 1&3: mucosa of anterior 2/3s

⇒ General & special sensory innervation CN V & IX

• Ph Arch 3&4: mucosa of posterior 1/3

⇒ General & special sensory innervation CN IX & X

Question 13

Describe the embryonic development & sensory innervation of the taste buds

Answer 13

Taste buds develop in papillae

⇒ Special sensory innervation CN VII

Question 14

Describe the embryonic development & motor innervation of muscles of the tongue

Answer 14

Both intrinsic & extrinsic muscles of the tongue develop from myogenic precursors that which migrate into the developing tongue

⇒ Motor innervation CN XII

Question 15

Where does the initial embryonic development of the thyroid gland occur?

Answer 15

Primordium of the thyroid gland appears in the floor of the pharynx between the tuberculum impar and the cupola

Question 16

Describe the descent of the thyroid gland

Answer 16

• Thyroid gland bifurcates and descends as a bi-lobed diverticulum connected by the isthmus
• Thyroid remains connected to the tongue by the thyroglossal duct

Question 17

What marks the point of origin for the descent of the thyroid?

Answer 17

Point of origin for the descent of the thyroid is marked by foramen cecum

Question 18

The thyroid gland is a midline diverticulum in floor of pharynx.

Where does the thyroid gland descend to?

Answer 18

Thyroid gland migrates anterior to pharyngeal gut, hyoid bone & laryngeal cartilages

Question 19

Identify some thyroid abnormalities that have embryonic origins

Answer 19

• Thyroglossal cysts & fistulae
• Ectopic thyroid tissue

Question 20

What is Treacher-Collins Syndrome?

Answer 20

• Treacher Collins Syndrome is an inherited, autosomal dominant condition characterised by hypoplasia of mandible & facial bones
• It is thought to result from the failure of colonisation of Ph Arch 1 with neural crest cells

Question 21

What is Di-George Syndrome?

Answer 21

• Di-George Syndrome is a condition characterised by congenital thymic aplasia & absence of parathyroid glands due to a deletion on chromosome 22
• It is caused by the disrupted development of the 3rd and 4th pharyngeal pouches and the abnormal development of neural crest

Question 22

What is CHARGE Syndrome?

Answer 22

• CHARGE Syndrome is a condition caused by a heterozygous mutation of the CHD7 protein which is essential for the production of multipotent neural crest
• It presents with coloboma, heart defects, choanal atresia, growth & developmental retardation, genital hypoplasia and ear defects