Embryology: Development of Head and Neck Flashcards
Where do the head regions of the embryo develop?
Around the cranial end of the neural tube (which extends to form the brain)
Where are the face and neck derived from?
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)
What do the arches give rise too?
Each arch gives rise to:
- a cranial nerve (V, VII, IX, X)
- cartilages, bones and muscles involved in swallowing, facial expression and speech
- artery
When do the pharyngeal arches begin developing?
Early in the 4th week
How many pairs of arches develop?
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
What 3 things does each arch consist of?
1) core of Mesenchyme
2) outer covering of Ectoderm
3) Inner lining of Endoderm
What is found between each arch on the OUTSIDE?
What is found between each arch on the INSIDE?
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
What does the 1st pharyngeal arch seperate into?
What does this give rise to?
Separates into 2 prominences that give rise to upper and lower jaw:
- the maxillary prominence
- the mandibular prominence
What is the nerve, muscle and cartilage associated with the 1st arch?
Nerve: Trigeminal nerve (CN V)
Muscle: mastication and swallowing muscles
Cartilage: malleus/Incus (ear bones)
The 2nd pharangeal arch is known as the what?
The hyoid arch
What is the nerve, muscle and cartilage associated with the 2nd arch?
Nerve: Facial nerve (CN VII)
Muscle: facial expression, cheek
Cartilages: most of the hyoid bone and stapes (ear bone)
Where does the hyoid bone develop from?
Mostly the 2nd pharyngeal arch, with contributions from the 3rd pharyngeal arch
What is the nerve, muscle and cartilage associated with the 3rd arch?
Nerve: Glossopharyngeal (CN IX)
Muscle: Stylopharyngeus (raises the pharynx in swallowing)
Cartilages: Hyoid bone
The 4th pharyngeal arch froms what?
Thyroid Cartilage (anterior part of Larynx)
What is the nerve, muscle and cartilage associated with the 4th arch?
Nerve: vagus nerve (CN X)
Muscles: pharyngeal muscles
Cartilages: Thyroid and Cricoid cartilages
Which is the ONLY cleft that contributes to adult structure and what specific structure is it?
1st cleft contributes to the External Acoustic Meatus + Ear Drum
Describe how the 2nd arch grows and what important structure is contained here
2nd arch grows rapidly as a FLAP, down and over the remaining clefts ➞ this flap contains the Platysma m
What is Cervical Sinus?
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
What happens if a Cervical Sinus Enlarges?
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
Describe how the 1st pouch grows
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
What if the 1st Pharyngeal Arch Fails to Develop Correctly?
Give 2 clinical syndromes and briefly explain each
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
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
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
What is the V-shaped groove where the two unequal halves of the tongue fuse?
What is the midline of this structure known as?
Fuse at the Sulcus Terminalis
Midline depression of Sulcus Terminalis is the Foramen Caecum
What structure marks the ORIGIN of the Thyroid Gland?
Foramen Caecum
What week does development of the thyroid gland begin and how?
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
What is the lengthening tube between the foramen caecum and gland known as?
Thyroglossal Duct
Lable image of the pharyngeal arches below
State what is indicated by X
X: where development of Thyroid Gland begins (b/w between 1st and 2nd Pouches)
What normally happens to the Thyroglossal Duct?
What happens if this doesn’t occur?
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
How could you differentiate a Thyroglossal Fistula from Goitre?
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)
When does development of the face begin and how?
Begins in 4th week with FIVE swellings forming around the stomodaeum (future mouth)
These are:
- 1 Frontonasal Prominence
- Paired Maxillary Prominences
- Paired Mandibular Prominences
The face has 2 origins, where does the top vs lower part of the face come from?
Top part of face comes from the Frontonasal Prominence (NOT part of pharyngeal arches)
Lower part of Face comes from the 1st pharyngeal arch
What does each swelling become?
Frontonasal Prominence: midline structures (nose, forehead, central part of lip philtrum)
Maxillary Prominences: cheek
Mandibular Prominences: Mandible
Describe the process of facial development by 5th Week
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
How do muscles develop in the face?
State nervous innervation to these muscles
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
What is an Oblique Facial Cleft?
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
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?
Macrostomia (large mouth)
Microstomia (small mouth)
Can cause problems with suckling and feeding
Palate develops from fusion of the primary and secondary palate, where do each of these derive from?
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
What is the importance of the palate?
What may occur when palates don’t fuse and list 3 consequences of this
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)
What is cleft palate a result of and how would you treat?
What are the two types?
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
Where does the eye initially develop from?
Outgrowth from forebrain towards ectoderm
What give rise to the early eye?
List 3 structures this then goes on to become?
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
Briefly describe development of the eye
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.
Compare the initial position of the eye to the adult position and why
Initially the eyes lie on the sides of the head
Growth of the maxillary prominences towards the midline shifts them to the front
Where are muscles that move the eye derived from?
Derived from somites
What is a Coloboma?
When choroidal fissure fails to close ➞ ‘Keyhole in Iris’
If a baby is born with opaque lens, what is this known as?
Congenital cataracts
What is Anophthalmos?
Absence of eye ➞ when optic vesicle fails to develop
