Embryology 3 Development Of Midline Structures

Question 1

Where is the pituitary gland found in the skull?

Answer 1

Hypophyseal/ middle ranial fossa in sphenoid bone

Question 2

What are the two lobes of the pituitary gland called, what did they develop from and how and what tissue are they made from?

Answer 2

infundibulum -> Posterior - neuroendocrine (downward outgrowth of forebrain-> grows towards roof pharynx)

Rathke’s pouch -> Anterior - endocrine connected to portal hypophysis vessels (releasing hormones) (outpocketing of ectoderm of stomatodeum, evagination of roof of oropharynx, grows dorsally towards developing forebrain

Question 3

Where is the tongue found and what is it made up of?

Answer 3

Lies partly in oral cavity and partly in pharynx

Lingual frenulum - attaches tongue to floor of mouth

Intrinsic (move) and extrinsic (change shape) muscles

Sulcus terminalis - line separates anterior 2/3rds and posterior 1/3 tongue

Foramen cecum- middle of sulcus terminalis

Question 4

Describe how the tongue grows from the pharyngeal arches

Answer 4

Primordial of the tongue appear at about the same time as the palate begins to form. It receives a component from each of the pharyngeal arches.

PhArch 1 -> 2 lateral lingual swellings + 3 median LS+ tuberculum impar (middle of LLS)

PHArch 2/3/4 -> cupola

PHArch 4 -> epiglottal swelling

Slide 13

Then:

• LLS overgrowth tuberculum impar
• 3rd arch part of cupola overgrows 2nd arch part
• extensive degeneration frees tongue from floor of oral cavity (bar lingual frenulum)

Question 5

Explain how different structures growing from different pharyngeal arches explains sensory innervation to the tongue

Answer 5

Mucosa of anterior 2/3rds derived from PHA 1/3 -> general sensory CN5/9

Posterior 1/3 from PHA 3/4 -> general and special sensory CN9/10

Tastebuds develop papillae -> special sensory CN7

(PH1=5, 3=9, 4=10)

Question 6

What is chorda tympani and what does it supply?

Answer 6

The chorda tympani is a branch of the facial nerve that originates from the taste buds in the front of the tongue, runs through the middle ear, and carries taste messages to the brain.

Branch of CN7 of second arch but passes into first arch therefore passes through middle ear

Question 7

Explain motor innervation to the tongue

Answer 7

Both intrinsic and extrinsic muscles develop from myogenic precursors that migrate into developing tongue - Cn12

Question 8

Where does the thyroid gland begin development and where does it end up, how?

Answer 8

Primordium appears in floor of pharynx between tuberculum impar & cupola (this point later becomes foramen cecum) -> bifurcates & descends as a bi-lobed diverticulum connected by the isthmus (remains connected to tongue by thyroglossal duct) -> anterior neck

50% of ppl have an obvious remnant = pyramidal lobe

Question 9

Name some thyroid abnormalities

Answer 9

Thyroglossal cysts and fistulae (abnormal opening)

Ectopic thyroid tissue - anywhere along path of descent

Question 10

How do you get a cleft lip and cleft palate?

Answer 10

Palate is formed from palatal shelves which grow medially into oral cavity from maxillary prominence -> once mandible enlarged sufficiently to allow tongue to drop, palatal shelves meet in midline and fuse

Cleft lip and palate results from failure FNP to fuse with max p & failure palatal shelves to fuse

Question 11

What is first arch syndrome?

Answer 11

Spectrum of defects in development of eyes/ ears/ mandible/ palate

Neural crest cells (vulnerable to teratogenic insult) -> cartilage bars on each pharyngeal arch -> Skelton of face

Thought to result from failure of colonisation of 1st arch with neural crest cells e.g. Treacher Collins syndrome

Question 12

What is treacher-Collins syndrome?

Answer 12

A first arch syndrome

Characterised by hypoplasia mandible & facial bones

Inherited, autosomal dominant condition

Haploinsufficiency of treacle (serine/ alanine rich nucleolar phosphoprotein involved in rDNA gene transcription & processing of pre-ribosomal RNA ) -> insufficient ribosome biogenesis & impacts on NC migration

Question 13

What is Di-George syndrome?

Answer 13

Congenital thymic aplasia & absence of parathyroid glands

Variety of additional defects

CATCH22 - deletion of chromosome 22

Disruption development of 3/4th pharyngeal pouches

Abnormal development of neural crest cells

Question 14

What is CHARGE syndrome?

Answer 14

CHD7 heterozygous mutation (essential for production of multipotent Neural crest cells)

Genetic/ teratogenic insult

Coloboma
Heart defects
Chloanal Atresia
Retardation of growth & development 
Genital hypoplasia
Ear defects