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
Where does the thyroglossal duct extend between?
What can the thyroglossal duct persist as and how common is this?
Extends between the tongue and thyroid.
The remnant of the distal end of the thyroglossal duct may persist in about 50% as the pyramidal lobe
How might you differentiate a goitre from a thyroglossal cyst/fistula and why?
Since the thyroid originates from the posterior tongue, if the patient sticks their tongue out (or swallows) the thyroglossal cyst/fistula should move.
This is because a solid cord of cells representing the remnant of the thyroglossal duct may persist, and so the connection with the tongue is maintained.
When and how does the face start to develop?
4th week with FIVE swellings forming around the stomodaeum (Future mouth)
List the five swellings that initiate the development of the face from anterior-posterior, which arches do they derive from if any?
1 Frontonasal prominence: forehead, bridge of nose, upper eyelid and centre of upper lip
doesn’t derive from a pharyngeal arch
Derived from 1st pharyngeal arch:
Paired maxillary prominences - becomes middle third of face, maxilla (upper jaw), sides of nose, most of lip
Paired mandibular prominences - becomes lower third of face; mandible and lip
What happens to the maxillary and frontonasal prominences by the 5th week?
*What forms the intermaxillary segment? *including what forms the future nostrils
Maxillary prominences enlarge and grow medially
On lateral side of frontonasal prominence forms lateral nasal placodes which form the eventual nostrils
The intermaxillary segment is formed by the nasal placodes moving together and merging
Why do facial clefts and cleft lips generally occur?
If the facial prominences fail to fuse together
What must happen to form the upper jaw and palate?
Maxillary prominences fuse with nasal areas
What happens if the facial prominences fail to fuse?
How might this present?
Can form an oblique facial cleft - the nose didn’t merge with the maxillary prominence correctly which results in an exposed nasolacrimal duct
Individuals may also have microstomia (small mouth) or macrostomia (large mouth)
What do the primary and secondary palate derive from?
Primary: from the intermaxillary segment
Secondary: formed by two palatine processes (lateral palatine processes) from the maxillary prominence
Why is it easier for a cleft palate to occur?
The fusion of the primary and secondary palates at the midline is so rapid that deformities such as cleft palate may easier occur
What are the consequences of an untreated cleft palate for an individual? When are cleft palates usually surgically corrected?
Abnormal facial appearance, defective speech and trouble feeding - usually corrected by 3 months
Name one example of an anterior and posterior deformity that may occur when the palates don’t fuse
Anterior: cleft lip
Posterior: cleft uvula
What does the eye develop from?
Outgrowth of the forebrain which interacts with the surface ectoderm known as the optic vesicle
What forms the optic vesicle and what does it give rise to as it grows on the outside?
An outgrowth forming on either side of the diencephalon
The retina, iris and ciliary body of the eye
What triggers the formation of the lens and cornea and how (specifically) is each formed?
forms the lens of the eye? What then happens to form the eye’s inevitable structure?
The optic vesicle releases signalling molecules to stimulate changes in the ectoderm to form the lens and cornea
Lens: ectoderm thickens opposite the optic vesicle
The optic vesicle and lens then become cup-shaped (forming an invagination)
Cornea: Once the optic vesicle and lens have become cup-shaped, the lens sinks beneath the surface ectoderm. The ectoderm closes over it and becomes the cornea
What term defines a single midline eye?
Cyclopia
Which cells are responsible for arranging themselves to form the choroid and sclera?
Mesenchyme cells (arrange themselves around the developing lens and retina)
What are the muscles responsible for moving the eye derived from?
Somites
Where are the eyes initially and how do the eyes move as they develop?
Initially they lie on the sides of the head, but as the maxillary prominence grows towards the midline the eyes shift to the front
What happens in Coloboma? How does this appear?
The choroidal/retinal fissure fails to close during 6th week, appears as a ‘keyhole in the iris’
What happens in anophthalmos?
If the optic vesicles fail to develop there will be an absence of the eye
Which deformity of the eye doesn’t cause vision problems?
Coloboma
Why does congenital cataracts cause vision problems?
Lenses are opaque
Where is the mesenchyme within each pharyngeal arch derived from?
Mostly from neural crest cells which migrate into the arches from the developing nervous system (and some mesodermal)
Mesoderm: muscle
Neural crest cells: bone or cartilage
Which vessels does the 1st, 2nd and 3rd pharyngeal arches give rise to?
Common, external and internal carotid arteries and breaches
Which arches form the hypobranchial eminence and what does it become?
3rd and 4th; becomes the epiglottis
What forms the entrance to each nostril?
The ‘arms’ of the medial and lateral nasal prominence s
What does the oronasal membrane separate and what happens once it disappears?
The oronasal membrane originally separates the deepening nasal pits and the ventral stomodaeum. As it disappears the oral and nasal cavities become a continuous space
How does the palate develop and what does it inevitably separate?
Development involves the…
A) medial nasal prominences: these merge in the midline to separate the nostrils from the mouth and form the philtrum of the upper lip and a small midline component of the palate (primary palate)
B) A palatal shelf grows from each maxillary prominence towards the midline. They fuse with each other and the primary palate
This fusion creates the secondary palate and separates the nasal and oral cavity!
What are the two major groups of cleft lips and cleft palates? When are most cleft palates identified?
- Anterior cleft anomalies
- Posterior cleft anomalies
Most identified at routine 20 week scan or soon after birth
Which chromosomal syndrome involves a cleft lip and/or palate?
Trisomy 13
Which type of cleft does genetics play a more important role?
Genetic factors more important in cleft lip (with or without a cleft palate) than a cleft palate alone
Where anatomically are the thyroid and the parathyroid glands?
Thyroid: anterior to trachea and deep to strap muscles (infrahyoid muscles)
Parathyroid: posterior aspect of thyroid
What happens if the thyroglossal duct fails to degenerate and what might occur as a result?
If portions of the thyroglossal duct persist and remain patent a thyroglossal cyst may form. If enlarged the cyst can perforate the midline skin of neck - forming a thyroglossal fistula
What happens if the thyroid fails to descend normally?
May give rise to ectopic thyroid tissue
Where does ectopic thyroid tissue most commonly form?
The tongue (lingual thyroid)
Where do thyroglossal cysts most commonly occur and how might they present?
Most common sites are at the base of the tongue and just inferior to the hyoid bone
Typically presents with abnormal lump in midline of the neck
Name two things that might accidentally occur as a result of a thyroidectomy and how might this present?
- Accidental removal/damage to parathyroid: may present with disturbances in calcium and phosphorus metabolism
- Damage to profuse blood supply
Which part of the thyroid gland must be incised during a tracheostomy and where is it?
The isthmus; which connects the R and L lobes and overlies the 2nd and 3rd tracheal cartilages
How might an enlarged thyroid gland present?
Can compress the recurrent laryngeal nerve - affecting movements of the vocal cords and resulting in hoarseness
Name the two parts of the parathyroid gland
- Adenohypophysis/anterior lobe:
2. Neurohypophysis/posterior lobe:
Describe the embryonic origin of the neurohypophysis and its connection throughout adult life
The infundibulum/downgrowth forms from a part of the diencephalon (the part that inevitably becomes the floor of the hypothalamus) and extends downwards towards the roof of the developing oral cavity. This connection between the neurohypophysis and brain is maintained as the pituitary stalk, from which nerve fibre tracts develop and grow down from the hypothalamus
While the infundibulum is forming, an outpouching called Rathke’s pouch forms from the roof of the oral cavity and grows up to meet it. Rathke’s pouch loses its connection with the roof of the mouth and comes to lie anteriorly to the infundibulum, wrapping around the pituitary stalk
What do the cells of Rathke’s pouch differentiate into?
Endocrine cells of the anterior pituitary
What network of blood vessels functionally links the anterior lobe of the parathyroid to the hypothalamus and posterior lobe?
Hypophyseal portal system
What might persistent remnants of Rathke’s pouch form?
Cysts
Which embryonic structure(s) is the majority of the palate derived from?
The largest part of the palate is formed by the secondary palate, which is embryologically derived from the lateral palatine processes.
