Development of the Pharyngeal Apparatus Flashcards
What makes up the pharyngeal apparatus?
Pharyngeal arches, grooves, pouches, and membranes
What do the parts of the pharyngeal apparatus develop into?
Nasal cavity, oral cavity, tongue, larynx, pharynx, ear, and neck
During what week of development do pharyngeal arches form?
Early in the 4th week
What type of cells help create the pharyngeal arches?
Neural crest cells
What is each arch comprised of and where is it located?
Endoderm on the inside, ectoderm on the outside, and mesoderm/mesenchyme in the middle
What is mesenchyme?
Embryonic CT derived from neural crest cells
What is head mesoderm?
Mesoderm arising from paraxial mesoderm
Once formed, the pharyngeal arches:
1) Support the lateral walls of the primative pharynx
2) Give rise to the facial prominences that contribute to craniofacial development
What are the components of the pharyngeal arches?
NCC-derived mesenchyme - forms all connective tissue in the head, including the dermis and smooth muscle
Paraxial mesoderm - populates each arch to form PA musculature
Lateral plate mesoderm - angioblasts differentiate into endothelium
Prechordal plate mesoderm - extraocular mesoderm
What is a major role of the pharyngeal endoderm?
Regulating the development of the pharyngeal arches
What elements are housed in the pharyngeal arches? NOT ON THIS TEST
Cartilaginous rods that form the skeletal components
Muscular components that forms muscles of the head and neck
Cranial nerves with sensory and motor components
Arch artery that form the vasculature of the head and neck
What is the cartilage of the 1st PA?
Meckel’s cartilage
What will Meckel’s cartilage eventually form?
Dorsal portion will form the malleus and incus
Perichondrium -> anterior ligament of the malleus and sphenomandibular ligament
Ventral part -> Primordium of the mandible
What eventually happens to Meckel’s cartilage?
The bone will form laterally to the cartilage and the cartilage will disintegrate
What is the cartilage of the 2nd PA?
Riechert’s cartilage
What will Riechert’s cartilage eventually form?
Dorsal region contributes to the stapes and styloid process of the temporal bone, the rest of it disintegrates
Perichondrium -> stylohyoid ligament
Ventral end ossifies -> forms lesser horn/cornu of the hyoid bone
What does the third arch cartilage form?
Ossifies -> Forms greater cornu of hyoid bone
Body of the hyoid bone is formed by the hypopharyngeal eminence
What do the 4th and 6th PA’s form?
The laryngeal cartilages
What does the 4th PA specifically form?
The epiglottis
Where do most muscular component of the head and neck arise?
The paraxial mesoderm
Where arches are associated with which muslces?
First arch - muscles of mastication
Second arch - stapedius and muscles of facial expression
Third arch - stylopharyngeus
Fourth arch - cricothyroid, pharyngeal constrictors
Sixth arch - intrinsic muscles of the larynx
Where do the extraocular muscles arise from?
The prechordal plate
Where does the tongue musculature arise from?
The occipital myotomes
What nerves supply each arch?
First arch - trigeminal nerve (V)
Second arch - facial nerve (VII)
Third arch - glossopharyngeal nerve (IX)
Fourth and Sixth arches - vagus nerve (X)
What are examples of first arch syndromes?
Treacher-Collins Syndrome (mandibulofacial dysostosis)
Pierre Robin sequence
What is Treacher-Collins Syndrome?
Autosomal dominant disorder due to TCOF1 mutation leading to increased apoptosis of NCCs.
Presents with malar hypoplasia, defects of lower eyelid, deformed external ears, sometimes deformed middle and lower ear
What is Pierre Robin sequence?
Typically occurs de novo in patients.
Presents with hypoplasia of the mandible, cleft palate, and defects of the eyes and ears.
Where are pharyngeal grooves locations and what are they covered of?
Located externally between pharyngeal arches and covered with ectoderm.
Where are pharyngeal pouches located and what are they covered of?
Located internally as part of the pharynx and covered with endoderm.
Where are the pharyngeal membranes located?
In between the pharyngeal arches.
What does the first groove form?
The external acoustic meatus
What happens to grooves 2-4?
The lie in the cervical sinus and are eventually obliterated. Birth defects of the second groove are common.
What happens to the first pouch?
Expands into the tubotympanic recess -> tympanic cavity and mastoid antrum. It also elongates to form the eustachian tube.
What happens when the endoderm of the 1st pouch contact the 1st pharyngeal groove?
It contributes to the formation of the tympanic membrane.
What forms the main part of the tympanic membrane?
The 1st membrane and the intervening mesenchyme.
What happens to the second pouch?
A portion forms the tonsillar sinus
The endoderm will for the tonsillar epithelium
The mesenchyme will form lymphoid nodules of palatine tonsil
What happens to the third pouch?
Dorsal portion differentiates into inferior parathyroid gland
Ventral portions form the thymus
Both migrate caudally due to growth of brain and cardiac regions
What happens to the fourth pouch?
Dorsal portion differentiates into superior parathyroid glands
Ultimobranchial body -> fuses with thyroid gland and gives rise to parafollicular cells
What are some branchial anomalies?
External cervical sinus (more common) and internal cervical sinus, cervical cysts, and cervical fistula
What causes an external cervical sinus?
Failure of the 2nd groove and cervical sinus to obliterate.
How is an external cervical sinus detected?
Due to discharge of mucus
What causes a cervical cyst?
Remnants of cervical sinus and/or 2nd groove.
What makes up a cervical cyst?
It is an accumulation of fluid and cellular debris.
What are the characteristics of a cervical cyst?
It slowly enlarges, is painless, and free lying in the neck, inferior to the angle of the mandible.
What causes a cervical fistula?
The persistence of parts of the 2nd groove and pouch.
What is a cervical fistula?
A canal that opens into the tonsillar sinus and external side of the neck.
What is the first endocrine gland to develop.
The thyroid primordium (thyroid gland)
Where does the thyroid grow and where does it end up in the body?
The thyroid forms in the floor of the primordial pharynx (back of the tongue) and descends down into the neck (ventral to hyoid bone and laryngeal cartilages) as the tongue grows. It is connected to the tongue by the thyroglossal duct (narrow tube).
What is ectopic thyroid tissue?
An anomaly where the thyroid can form along the course of the duct.
What is a sublingual thyroid gland?
A thyroid gland that forms but does not descend into the neck.
What is a throglossal duct cyst?
A cyst that forms in the tongue or anterior neck - critical to distinguish from ectopic thyroid prior to surgical removal.
What is agenesis of thyroid gland?
Absence of a thyroid gland or one of its lobes. (Rare)
What is thyroid hemiagenesis?
Unilateral failure of formation - left lobe is more commonly absent.
What are some problems associated with DiGeorge Syndrome?
Breakdown of signaling from PA endoderm to NCC.
Agenesis of thymus and parathyroid glands.
Congenital hypoparathyroidism.
Shortened philtrum of upper lip, low-set and notched ears
Nasal clefts, thyroid hypoplasia
Cardiac abnormalities
