L17: Embryology of head and neck Flashcards
What is the embryo like in week 4 in accordance to the head and neck?
- no distinguishable facial features
- head and neck represent roughly 1/2 the length of the embryo
- the embryonic head and neck is a complex tissue system which involves the brain, CVS, and special sensory organs
How does the head and neck start to develop?
Segmental development
-each arch has a neurovascular plan and develops muscles and skeletal elements
Development of gut tube in the pharynx
-endoderm tube modification, glandular development (parathyroid, palatine tonsils, thyroid)
What are the pharyngeal arches?
Comprised of a system of mesenchymal proliferations in the neck region of the embryo
(get progressively smaller as you move caudally)
How many pharyngeal arches are there?
5
Numbered 1-6 (humans do not form the 5th)
What is the FNP?
Frontonasal prominence
-single, midline, unpaired structure
FNP and pharyngeal arches constitute the building blocks for the head and neck region
What is the structure of the pharyngeal arches?
-centre is mesenchyme
-surrounded by ectoderm to give skin surrounding these tissues
-endoderm lines the outside in contact with the gut tubule
For each arch, it has an associated artery, nerve and cartilage bar (rod shaped)What
What are the regions called that will develop into the special senses?
- nasal placode
- lens rudiment
- inner ear rudiment
What is found between each pharyngeal arch?
Pharyngeal cleft
What are the pharyngeal pouches?
Endoderm of the gut tube thats been blown up to line the arches
-like the cleft, but on the inside
What cranial nerves are associated with the arches?
CN 5, 7, 9, 10
Mixed sensory and motor functions
-supply the derivatives of the pharyngeal arches
What are the muscular derivatives of the pharyngeal arches?
Muscles of mastication: Ph Arch 1 derivative supplied by the 5th cranial nerve (trigeminal)
Muscles of facial expression: Ph Arch 2 derivative supplied by the 7th cranial nerve (facial nerve)
Stylopharyngeus: Arch 3
Cricothyroid/levator palatini/constrictors of the pharynx: Arch 4
Intrinsic muscles of pharynx: Arch 6
What do the cartilage bars of the arches go onto develop?
1: Meckle’s: malleus and incus and template for formation of mandible
2: Reichert’s: stapes and upper part of hyoid bone
3: remainder of hyoid bone
4/6: cartilages of the larynx
What do the arteries that supply the pharyngeal arches become?
Aortic sac lies in the floor of the pharynx and gives rise to:
1/2nd arch arteries disappear
3rd arch: becomes the internal carotid
4th arch: arch of aorta an brachiocephalic artery
6th arch: pulmonary arch
Which pharyngeal pouch is the largest and what does it form?
1st pharyngeal pouch
-becomes the tympanic cavity
What else does the endodermal lining form?
-palatine tonsils
-parathyroid glands
-thymus
Paired structures^
Which cleft remains open?
1st cleft
-becomes external acoustic meatus (allows sound in to reach tympanic membrane)
The 2nd arch grows down to cover the other clefts
-branchial cysts/fistulas can occur if this process doesn’t occur
Cyst: fluid filled remnant of a cleft
Fistula: abnormal opening of cleft draining serous fluid onto surface of neck
What key feature do you look for to determine a branchial cyst/fistula?
SCM
-cyst lies anterior to the SCM
What factors drive the development of the face?
- expansion of the cranial neural tube
- appearance of the complex tissue system associated with the cranial gut tube and the outflow of the developing heart
- development of the sense organs and the need to develop a way to separate respiratory tract from the GI tract
What is the facial primordia
Facial primordia (so large it looks like it has two prominences):
- 1st pharygeal arch
- FNP: surrounds ventro-lateral part of the forebrain
(primordia of eyes develop as well)
What are the components of the face embryologically?
- stomatodeum (depression between the brain and heart and develops into the mouth/anterior lobe of pituitary), no opening of the gut tube as it is closed by the buccopharyngeal membrane
- frontonasal prominence
- 1st arch: maxillary prominence and mandibular prominence
What does the FNP/maxillary prominence/mandibular prominence develop into?
FNP: forehead, bridge of nose, nose, philtrum
Maxillary: cheeks, lateral upper lip, lateral upper jaw
Mandibular: lower lip and jaw
These all need to fuse
How does the nose develop?
-nasal placodes appear on the FNP (a placode is a thickening of ectoderm)
-they sink to become the nasal pits as they get thicker
-medial and lateral prominences form on either side of the pits
(the eyes start to develop laterally to these)
-maxillary prominences grow medially, pushing the nasal prominences closer together in the midline
-maxillary prominences fuse with medial nasal prominences
-medial nasal prominences then fuse in the midline
(eyes also move medially)
How do you separate the nasal and oral cavities?
- nasal placodes dip into the FNP creating an opening: nostrils
- buccopharyngeal membrane will rupture
- opening now into the pharynx from the nose and the mouth
- intermaxillary segment separates these
- maxillary prominences give rise to 2 platal shelves which grown vertically downwards in the oral cavity on each side of the developing tongue
- mandible grows to allow tongue to drop allowing the palatal shelves to grow towards each other and fuse in the midline which separates the nasal cavity above it from the oral cavity below
- nasal septum develops as a midline down growth and fuses with the palatal shalves
What is the intermaxillary segment?
Fusion of the medial nasal prominences create this
-this then goes on to form the philtrum, upper jaw containing 4 incisors, and the primary palate
How does a lateral cleft lip form?
Failure of fusion of the medial nasal prominence and maxillary prominence
How does a cleft lip and palate form?
Combined with failure of the palatal shelves to meet in the midline
-can’t feed normally and speech may be impaired
How does the ear start to develop?
- external auditory meatus develops from the 1st Ph cleft
- middle ear cavity and ossicles from 1st pharyngeal pouch and cartilages of the 1st and 2nd cartilages
- auricles of the ear develop from proliferation within the 1st and 2nd arches surrounding the meatus
What is the positioning of the ears in development?
- external ears develop initially in the neck
- as mandible grows, the ears ascend to the side of the head to lie in line with the eyes
Why are the ears important in chromosomal abnormalities?
All common chromosomal abnormailities have associated external ear abnormalities
-hypoplasia/underdevelopment of lower jaw= low set ears
Can you consume alcohol in pregnancy?
There is no known safe level of alcohol consumption during pregnancy
- passes freely across the placenta and enters the fetal circulation
- alcohol is very toxic to developing neurones and neural crest and the facial skeleton is derived from neural crest cells
- neural crest migration and development of brain are sensitive to alcohol
What are the signs of fetal alcohol syndrome?
significant repeated exposure to alcohol during pregnancy
-small head
-low nasal bridge
-small eye openings
-flat midface
-short nose
-thin upper lip
-underdeveloped jaw
-smooth philtrum
-epicanthal folds
Neuro development is also affected
What is ARND?
Alcohol related neurodevelopmental disorder
-lower conc of alcohol exposure
-not associated with the classic facial abnormalities of FAS
Public health is vital to educate people on this
What is the location of the tongue?
Lie partly in the oral cavity and partly in the pharynx
What attaches your tongue to the floor of your mouth?
Lingual frenulum
-tongue is highly mobile as this is its only attachment to the mouth floor
What is the boundary between the anterior 2/3rds and posterior 1/3rd of the tongue?
Sulcus terminalis
- V shaped line
- point of V is called the foramen cecum
How does the tongue develop?
- receives a component from each of the pharyngeal arches
- primordia of tongue appear at same time as palate begins to form
- 2 lateral lingual swellings in Arch 1
- 3 median lingual swellings: Arch 1: Tuberculum impar, Arch 2 and 3 and 4: cupola, Arch 4: epiglottal swelling
- lateral lingual swellings over grow the tuberculum impar
- the 3rd arch component of the cupola over-grows the 2nd arch component
- extensive degeneration occurs freeing tongue from the floor of the oral cavity expect the lingual frenulum
What does it mean when an infant is tongue tied?
When the lingual frenulum is too short
- tongue not as mobile
- can cause difficulty in feeding
Where does the thyroid gland develop from?
Primordium of the thyroid gland appears in the floor of the pharynx between the tuberculum impar and the cupola
-final position is in the anterior neck
How does the thyroid descend?
- point of origin for the descent of the thyroid is later marked by the foramen cecum
- thyroid primordium bifurcates and descends as a bi-lobed diverticulum connected by the isthmus
- during its descent the thyroid gland remains connected to the tongue bu the thyroglossal duct
What is a pyramidal lobe?
Remnant of the thyroglossal duct
-in 50% of population
What are some thyroid abnormalities?
-thyroglossal cyst (if patency is not lost)
(in the midline)
-thyroglossal fistulae (abnormal opening)
-ectopic thyroid tissue
What structures develop from the 3rd and 4th pouches?
- parathyroid glands
- thymus
