Embryology of the Pharyngeal Apparatus Flashcards
When do the pharyngeal arches develop? What are they derived from?
4th week
Neural crest
How are the pharyngeal arches organized (layer wise)?
Arch is lined externally by ectoderm and internally by endoderm
Core is initially mesenchyme from Neural Crest but then true mesoderm migrates in
What is the role of NCC-derived mesenchyme in the pharyngeal arch?
form all connective tissue in the head (including dermis)
smooth muscle of vasculature
What is the role of paraxial mesoderm in the pharyngeal arch?
populate each arch to from PA musculature
What is the role of lateral plate mesoderm in the pharyngeal arch?
angioblast which differentiate into endothelium to line the blood vessels in the face
What is the role of prechordal plate mesoderm in pharyngeal arches?
extraocular musculature (orbicularis oculi)
What kind of cartilage is found in PA1? What is its role in development?
Meckel’s cartilage
Dorsal portion forms malleus and incus
Ventral= primordium of mandible
Placeholder of lower jaw –> forms mandible (laterally) via inter-membrane ossification
Perichondrium –> anterior ligament of malleus and sphenomandibular ligament
What kind of cartilage is in PA2? What does the dorsal and ventral regions become?
Riechert’s cartilage
Dorsal= stapes and styloid process of temporal bone Ventral= ossifies (intermembraneous) and forms lesser cornu aka horn of hyoid
What is the role of third arch cartilage?
Forms greater cornu of hyoid via cartilaginous ossification
Where is the body of hyoid formed from?
PA 3 and 4
formed by prominence in embryonic pharynx aka hypopharyngeal eminence
What do the 4th and 6th arch cartilages contribute to?
Laryngeal cartilage
- specifically 4th is NCC derived and includes the epiglottis
Where are the muscular components of the PAs derived from?
Paraxial mesoderm and prechordal plate
What muscle(s) come from the first arch?
Muscle of mastication (CN V) #chewonthat
What muscle(s) come from the second arch?
Stapedius m. and muscles of fascial expression (CN VII) #smileandsaycheese
What muscle(s) come from the third arch?
Stylopharyngeus (CN IX) #elevateanddialte
What muscle(s) come from the fourth arch?
Circothyroid Levatator veli palatini Constrictors of the pharynx CN X #pushitpushitrealgood
What muscle(s) come from the sixth arch?
Intrinsic muscles of the larynx (CN X) #soundoff
What nerve supplies PA1?
The trigeminal nerve: just V2 and V3
- principal sensory nerve of head and neck
- motor nerve for muscles of mastication
What nerve supplies PA2?
Facial nerve (CN VII) muscles of fascial expression
What nerve supples PA3?
Glossopharyngeal (CN IX)
Stylopharyngeus m.
What nerve supplies PA4?
superior laryngeal branch of Vagus N. (CN X) and its recurrent laryngeal branch
What nerve supplies PA6?
Recurrent (inferior) laryngeal of the Vagus N. (CN X)
Where do the arch arteries come from?
The aortic sac which is a basket like structure arising from the dorsal aorta
- remodeled into great arteries of the thorax, head and neck
Where are pharyngeal grooves located? What are they covered with?
Externally, between pharyngeal arches
Ectoderm
Where does the first groove lie?
External acoustic metaus
What do grooves 2-4 lie in?
the cervical sinus which closes off during development
Where are pharyngeal pouches? What are they derived from?
Lines pharynx internally
Pharyngeal endoderm
How do cervical cysts arise? What are the symptoms?
Remnants of cervical sinus and/or 2nd groove
Slow enlarging painless cyst in neck inferior to angle of mandible\, filled with fluid and cellular debris from desquamation of their epithelial lining
How do cervical sinus anomalies arise? What are the symptoms?
Failure of 2nd PA groove and cervical sinus to obliterate
Discharge of mucus
Normally bilateral and associated with auricular sinuses
How do cervical fistulas arise? What are the symptoms?
Abnormal canal that opens into the tonsillar sinus due to persistence of parts of 2nd groove and pouch
What are pharyngeal membranes?
Where pouch endoderm comes into contact with groove ectoderm
- quickly infiltrated by mesenchyme
What does the 1st pharyngeal membrane form?
Tympanic mebrane
What does the endoderm of the 1st pouch form?
Expands into tubotympanic recess –> Tympanic cavity and mastoid antrum
Elongates to form pharyngotympanic tube
Contacts 1st groove contributing to tympanic membrane
What does the 2nd pouch form?
Tonsillar sinus
Endoderm proliferate and forms tonsillar epithelium
Mesenchyme become lymphoid nodules of palatine tonsil
TONSILS
What does the 3rd pouch form?
Dorsal= inferior parathyroid Ventral= thymus
Caudally migrates aka pulls structures down
What does the 4th pouch form?
Dorsal= superior parathyroid glands
Ultimobranchial body fuses with thyroid gland and gives rise to parafollicular cells
Where are epithelial cords derived from? What are their 3 functions?
PA3 endoderm which form the thymic primordium
- grow into surrounding mesenchyme –> thymic lobules
- Become arranged around a central point –> thymic corpuscles
- Form epithelial reticulum for housing lymphocytes
Where is the thyroid gland derived from?
NOT ARCH DERIVATIVE
Comes from developing floor of the pharynx: median endodermal thickening
Descends in neck with tongue growth and passes ventral to developing hyoid and laryngeal cartilage, connected by thyroglossal duct
- only gland moving
What is first arch syndrome (generally)?
Insufficient migration of NCC into the 1st arch during the 4th week
What is Treacher-Collins syndrome?
type of first arch syndrome
Autosomal dominant, mutation in TCOF1 which is involved in ribosome biogenesis
- truncation leads to increased apoptosis of cranial NCC
Malar hypoplasia down slanting palperbal fissures, deformed external ear, defects of lower eyelid
What is Pierre Robin syndrome?
First arch syndrome, Occurs de novo
hypoplasia
Initiating defect= small mandible –> posterior displacement of tongue –> obstruction of closure of the palate –> bilateral cleft palate
- difficulties feeding
What is agenesis of thyroid gland?
Complete absence of a thyroid gland or one of its lobes
What is thyroid hemiagenesis?
Unilateral failure of formation
Left lobe more commonly absent
What is DiGeorge Syndrome?
Breakdown of signlaing from PA enoderm to NCC –> agenesis of thymus and parathyroid glands
CONGENITAL HYPOPARATYROIDISM
Cardiac abnormalities
Nasal clefts, thyroid hyperplasia
Shortened philtrum of upper lip, low set and notched ears
What is the foramen cecum a remanent of?
Thyroid gland
Little bump on tongue
