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
