Development of head and neck Flashcards
What structures are the face and neck derived from? Where are these structures and what exists within them?
*what do they give rise to generally?
The pharyngeal arches which lie on either side of the stomodaeum (future oral cavity) in the lateral walls of the embryonic pharynx
*arches give rise to cartilages, bones and muscles involved in swallowing, facial expression and speech
When do the pharyngeal arches begin developing?
4th week
How many pharyngeal arches are there and which are the largest?
5 pairs of arches: 1,2,3,4,6
1st and 2nd are the largest
5th arch disappears (merges with the fourth)
What lines each pharyngeal arch and what does each contain?
Each pharyngeal arch has an artery, vein, cranial nerve and a core of mesenchyme
Lined by an outer covering of ectoderm and inner lining of endoderm
What separates the outside and the inside of the pharyngeal arches and what lines these structures? Where do they meet?
Outside: separated by clefts/grooves lined with ectoderm
Inside: separated by pouches lined with endoderm
The clefts and pouches meet end-on and are separated only by a thin sheet called the pharyngeal membrane (made of ectoderm on our side and endoderm on inside)
Which skeletal elements and nerve does the 1st pharyngeal arch give rise to?
Skeletal elements: Meckel’s cartilage (gives rise to mandible)
- mandible
- malleus and incus (2/3 of middle ear bones)
- sphenomandibular ligament
Trigeminal n CN
Which muscles does the 1st pharyngeal arch give rise to?
Muscles of mastication
- mylohyoid
- anterior belly of digastrics
- tensor tympani
- tensor veil palantine
(Separates into 2 prominences; the maxillary and mandibular prominences which gives rise to the upper and lower jaw = muscles of mastication)
Which skeletal elements and which nerve does the 2nd pharyngeal arch give rise to?
*what else is the 2nd pharyngeal arch referred to and why?
Also called the ‘hyoid arch’ as it gives rise to a part of hyoid bone
- stapes (1/3 of middle ear bones)
- styloid process
- stylohyoid ligament
Associated with facial n CN VII
What muscles does the 2nd pharyngeal arch give rise to?
Muscles for facial expression and the cheek
- stapedius
- styolohyoid
- posterior belly of digastrics
Which skeletal elements does the 3rd pharyngeal arch give rise to and what nerve is it associated with?
Greater Cornu and contributes to the hyoid bone
Associated with the glossopharyngeal n CN IX
What muscle does the 3rd pharyngeal arch give rise to and what does it do?
Stylopharyngeus; small muscle contributing to swallowing
Which cartilages does the 4th-6th pharyngeal arches give rise to?
Cartilages of the larynx:
- Thyroid cartilage (anterior part of larynx, hyaline cartilage)
- Cricoid cartilage (slightly below)
Which muscles does the 4th-6th pharyngeal arches give rise to?
Muscles of larynx
Constrictor muscles of pharynx
Striated muscle of upper esophagus
Levator veli palatine
Which nerve and which vessels does the 4th pharyngeal arch associated with?
X Vagus
Aortic arch and part of R subclavian artery
Which cleft is most important and why?
The 1st cleft as all clefts disappear EXCEPT the 1st cleft (between 1st and 2nd pharyngeal arches) as it contributes to adult structure and becomes the external acoustic meatus and ear drum
Which arch is associated with the platysma muscle and why?
2nd arch which grows rapidly as a FLAP that contains the platysma muscle
What happens normally as the 2nd arch grows? What might happen that is abnormal?
Normally a flap of tissue from the 2nd arch (Second pharyngeal groove) grows down and covers the 3-6 arches and cleft by merging with the 3rd and 4th pharyngeal grooves (or clefts), creating a space known as the cervical sinus which normally degenerates/closes
Sometimes the cervical sinus persists/the gap doesn’t close; so that the remnants of the lower clefts lined with ectoderm stay beneath the flap of the 2nd arch
What happens if a cervical sinus becomes enlarged and where would this happen? When does this tend to happen?
Adolescence
Can form a branchial cyst along the anterior border of sternocleidomastoid -> this cyst may grow as cellular debris/fluid accumulated and eventually open becoming a branchial sinus/fistula externally or internally
Which adult structures are the following pouches responsible for? A) pouch 1 B) pouch 2 C) pouch 3 D) pouch 4
A) Joins with the cleft and expands to eventually create the adult middle ear cavity and eustachian tube
B) crypts of palatine tonsil
C) dorsal part: inferior parathyroid
Ventral part: thymus
D) dorsal part: superior parathyroid
Ventral part: forms the ultimobranchial body which fuses with the thyroid diverticula and gives rise to C (calcitonin producing) cells of thyroid
Which structures are abnormal if the 1st pouch fails to properly develop? What is this called?
Called 1st arch syndrome:
Deformities in the maxilla and mandible, eyes and ears
Which structures are formed from the ectoderm of the 1st cleft?
External acoustic meatus and the tympanic membrane
Name two genetic conditions involving abnormal development of the 1st pouch. What is characteristic of each of them and what is their inheritance pattern?
Both are autosomal dominant
1. Treacher-Collins syndrome: mandible and facial bones are small, external ears malformed
- Pierre-robin syndrome: mandible very small and palate is cleft
Which arches are involved in the development of the tongue and which parts of the tongue do they give rise to?
Pharyngeal arches give rise to the CT and mucosa of the tongue (not extrinsic/intrinsic muscles)
1st arch swellings merge to form anterior 2/3 of tongue
3rd + 4th arches swell and merge to form posterior 1/3 of tongue
Where are the following structures found on an adult?
A) sulcus terminalis
B) foramen caecum
A) The V-shaped groove on the tongue where the two unequal halves of arches (1st + 3rd & 4th) fuse
B) midline depression of sulcus terminalis
What marks the point of origin for the thyroid gland?
Foramen caecum of the tongue
Which nerves supply sensory and taste innervation to the posterior 1/3 of the tongue?
Sensory: CN IX and X
Taste: CN IX
Which nerves supply sensory and taste innervation to the anterior 2/3 of the tongue?
Sensory: CN V
Taste: CN VII
Describe the embryology of the thyroid gland, including its
- time and point of origin
- development
- time, appearance and place of final structure
Appears in 4th week as a proliferation of cells at the back of the tongue, known as foramen caecum.
These cells form an invagination (outgrowth) of the floor between the 1st and 2nd pouches called the thyroid diverticulum - which descends down the neck and leaves a residual thyroglossal duct connecting the developing thyroid cells and the foramen caecum.
After reaching their final destination just inferior to the thyroid cartilage, the diverticulum solidifies and takes the shape of its adult bilobed structure.
The thyroglossal duct should degenerate and the thyroid should be fully formed and descended by the 12th week in utero!
How does the thyroid grow and travel as it develops?
The thyroid diverticula descends down the midline of the neck from the foramen caecum and becomes bi-lobed at its distal end, forming the thyroglossal duct
Which structures does the thyroid gland descend anterior to and when should the thyroid reach its final position in the neck?
Descends anterior to the hyoid bone and larynx
Should reach its final position by week 7