1.2 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 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
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)
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
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
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
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.
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
