Embryology: Development of Head and Neck

Question 1

Where do the head regions of the embryo develop?

Answer 1

Around the cranial end of the neural tube (which extends to form the brain)

Question 2

Where are the face and neck derived from?

Answer 2

Inferior to the brain, the face and neck are derived from a series of pharyngeal arches that lie either side of the stomodaeum (future oral cavity)

Question 3

What do the arches give rise too?

Answer 3

Each arch gives rise to:

• a cranial nerve (V, VII, IX, X)
• cartilages, bones and muscles involved in swallowing, facial expression and speech
• artery

Question 4

When do the pharyngeal arches begin developing?

Answer 4

Early in the 4th week

Question 5

How many pairs of arches develop?

Answer 5

5 pairs of arches: 1,2,3,4 and 6 form a craniocaudal sequence as rounded swellings on the surface

5th arch is rudimentary and soon disappears

Question 6

What 3 things does each arch consist of?

Answer 6

1) core of Mesenchyme
2) outer covering of Ectoderm
3) Inner lining of Endoderm

Question 7

What is found between each arch on the OUTSIDE?

What is found between each arch on the INSIDE?

Answer 7

Outside: Clefts/grooves (lined by ectoderm)

• Eg. b/w 1st and 2nd pharyngeal arch is the 1st pharyngeal cleft/groove

Inside: Pouches (lined with endoderm)

• Eg. b/w 1st and 2nd pharyngeal arch is the 1st pharyngeal pouch

Question 8

What does the 1st pharyngeal arch seperate into?

What does this give rise to?

Answer 8

Separates into 2 prominences that give rise to upper and lower jaw:

• the maxillary prominence
• the mandibular prominence

Question 9

What is the nerve, muscle and cartilage associated with the 1st arch?

Answer 9

Nerve: Trigeminal nerve (CN V)

Muscle: mastication and swallowing muscles

Cartilage: malleus/Incus (ear bones)

Question 10

The 2nd pharangeal arch is known as the what?

Answer 10

The hyoid arch

Question 11

What is the nerve, muscle and cartilage associated with the 2nd arch?

Answer 11

Nerve: Facial nerve (CN VII)

Muscle: facial expression, cheek

Cartilages: most of the hyoid bone and stapes (ear bone)

Question 12

Where does the hyoid bone develop from?

Answer 12

Mostly the 2nd pharyngeal arch, with contributions from the 3rd pharyngeal arch

Question 13

What is the nerve, muscle and cartilage associated with the 3rd arch?

Answer 13

Nerve: Glossopharyngeal (CN IX)

Muscle: Stylopharyngeus (raises the pharynx in swallowing)

Cartilages: Hyoid bone

Question 14

The 4th pharyngeal arch froms what?

Answer 14

Thyroid Cartilage (anterior part of Larynx)

Question 15

What is the nerve, muscle and cartilage associated with the 4th arch?

Answer 15

Nerve: vagus nerve (CN X)

Muscles: pharyngeal muscles

Cartilages: Thyroid and Cricoid cartilages

Question 16

Which is the ONLY cleft that contributes to adult structure and what specific structure is it?

Answer 16

1st cleft contributes to the External Acoustic Meatus + Ear Drum

Question 17

Describe how the 2nd arch grows and what important structure is contained here

Answer 17

2nd arch grows rapidly as a FLAP, down and over the remaining clefts ➞ this flap contains the Platysma m

Question 18

What is Cervical Sinus?

Answer 18

Normally the second flap grows over the other pouches and joins with the 4th arch. Pouches 2-4 below usually fade

However, when remnants of lower cervical clefts (lined with ectoderm) remain beneath the flap of the 2nd arch it causes a collection of ectoderm ➞ Cervical Sinus

Question 19

What happens if a Cervical Sinus Enlarges?

Answer 19

can form a branchial cyst along the anterior border of the sternocleidomastoid muscle

cyst may get bigger as cellular debris/fluid accumulate

may open via a branchial sinus/ fistula externally or internally

Question 20

Describe how the 1st pouch grows

Answer 20

Endoderm of 1st Pouch ➞ expands eventually creating the Middle ear cavity and eustachian tube

Remember: The EAM (external acoustic meatus) & tympanic membrane formed by the ectoderm of the 1st cleft

Question 21

What if the 1st Pharyngeal Arch Fails to Develop Correctly?

Give 2 clinical syndromes and briefly explain each

Answer 21

Deformities involving the mandible, eyes and ears can result = First Arch Syndrome for example:

1) Treacher-Collins Syndrome (autosomal dominant): Mandible and facial bones are small, external ears malformed
2) Pierre-Robin Syndrome (autosomal dominant): Mandible is extremely small and palate is cleft

Question 22

What arch does the anterior (2/3) vs posterior (1/3) of the tongue develop from?

What CN supply sensory and taste to each part

Answer 22

1st arch swellings merge with each other to form anterior 2/3rds of tongue

• Sensory: CN V
• Taste: CN VII

3rd + 4th arches swell and merge to form the posterior 1/3rd of tongue

• Sensory: CN IX and CN X
• Taste: CN IX

Question 23

What is the V-shaped groove where the two unequal halves of the tongue fuse?

What is the midline of this structure known as?

Answer 23

Fuse at the Sulcus Terminalis

Midline depression of Sulcus Terminalis is the Foramen Caecum

Question 24

What structure marks the ORIGIN of the Thyroid Gland?

Answer 24

Foramen Caecum

Question 25

What week does development of the thyroid gland begin and how?

Answer 25

Appears in 4th Week 1) At the Foramen Caecum (point of origin) there is an invagination of floor between 1st and 2nd Pouches 2) grows as thyroid diverticulum and DESCENDS down neck (anterior) 3) during decent divides into 2 lobes

Question 26

What is the lengthening tube between the foramen caecum and gland known as?

Answer 26

Thyroglossal Duct

Question 27

Lable image of the pharyngeal arches below State what is indicated by X

Answer 27

X: where development of Thyroid Gland begins (b/w between 1st and 2nd Pouches)

Question 28

What normally happens to the Thyroglossal Duct? What happens if this doesn't occur?

Answer 28

Normally the thyroglossal duct degenerates However. anywhere along the route taken by the thyroglossal duct you can have ectopic thyroid tissue ➞ in the tongue or upper neck This can enlarge into a cyst that can perforate the midline skin of neck = Thyroglossal Fistula

Question 29

How could you differentiate a Thyroglossal Fistula from Goitre?

Answer 29

Goitre will only move when you swallow. A Thyroglossal Fistula will move when you swallow and when you stick tongue out (because it is still attached to the tongue which it its point of origin)

Question 30

When does development of the face begin and how?

Answer 30

Begins in 4th week with FIVE swellings forming around the stomodaeum (future mouth) These are: * 1 Frontonasal Prominence * Paired Maxillary Prominences * Paired Mandibular Prominences

Question 31

The face has 2 origins, where does the top vs lower part of the face come from?

Answer 31

Top part of face comes from the Frontonasal Prominence (NOT part of pharyngeal arches) Lower part of Face comes from the 1st pharyngeal arch

Question 32

What does each swelling become?

Answer 32

Frontonasal Prominence: midline structures (nose, forehead, central part of lip philtrum) Maxillary Prominences: cheek Mandibular Prominences: Mandible

Question 33

Describe the process of facial development by 5th Week

Answer 33

1) MAXILLARY prominences enlarge and grow medially 2) Lateral Nasal Placodes (eventual nostrils) appear on each side of frontonasal prominence 3) Nasal placodes move towards each other and merge midline forming = Intermaxillary segment 4) Maxillary prominences fuse with nasal areas

Question 34

How do muscles develop in the face? State nervous innervation to these muscles

Answer 34

The mesenchyme of the face is invaded by muscles derived from pharyngeal arches Muscles from 1st arch = Muscles of mastication innervated by CN V Muscles from 2nd arch = Muscles of facial expression innervated by CN VII

Question 35

What is an Oblique Facial Cleft?

Answer 35

When facial prominences fail to fuse If nose doesn't merge with maxillary prominence correctly it results in exposed nasolacrimal duct on surface (nasolacrimal duct: goes from corner of eye to nose) Requires surgery

Question 36

Failure/problems with facial prominences fusing can result in a 'large' or 'small' mouth. What are the medical words for these How can these cause problems?

Answer 36

Macrostomia (large mouth) Microstomia (small mouth) Can cause problems with suckling and feeding

Question 37

Palate develops from fusion of the primary and secondary palate, where do each of these derive from?

Answer 37

Primary palate from the Intermaxillary segment (A) Secondary palate formed by two Palatine Processes from the maxillary prominences (B) The two palates grow and fuse together at the midline \*\* C = soft palate

Question 38

What is the importance of the palate? What may occur when palates don’t fuse and list 3 consequences of this

Answer 38

Palate separates the nose and the mouth If palates dont fuse then **Cleft Lip** and **Cleft Palate** may occur which result in: * abnormal facial appearance * defective speech * trouble feeding These are common defects because development of the palate occurs very quickly (aprox 48hrs)

Question 39

What is cleft palate a result of and how would you treat? What are the two types?

Answer 39

Cleft palate results when two palatal shelves fail to meet and fuse with each other. Can very successfully be corrected with surgery after birth Types: * Anterior deformities include a cleft lip * Posterior deformities can include a cleft uvula

Question 40

Where does the eye initially develop from?

Answer 40

Outgrowth from forebrain towards ectoderm

Question 41

What give rise to the early eye? List 3 structures this then goes on to become?

Answer 41

Optic vesicle gives rise to the early eye, goes on to become: * The retina of the eye * The iris of the eye * The ciliary body of the eye

Question 42

Briefly describe development of the eye

Answer 42

1) outgrowth from forebrain towards ectoderm called optic vesicles 2) ectoderm forms a lens (localised thickening) opposite the optic vesicle 3) optic vesicle + lens become cup-shaped (invagination) 4) lens sinks beneath the surface ectoderm ➞ ectoderm closes over it ➞ becomes the cornea. 5) mesenchyme cells arrange themselves to form the choroid and sclera.

Question 43

Compare the initial position of the eye to the adult position and why

Answer 43

Initially the eyes lie on the sides of the head Growth of the maxillary prominences towards the midline shifts them to the front

Question 44

Where are muscles that move the eye derived from?

Answer 44

Derived from somites

Question 45

What is a Coloboma?

Answer 45

When choroidal fissure fails to close ➞ 'Keyhole in Iris'

Question 46

If a baby is born with opaque lens, what is this known as?

Answer 46

Congenital cataracts

Question 47

What is Anophthalmos?

Answer 47

Absence of eye ➞ when optic vesicle fails to develop