Embryology of the Pharyngeal Apparatus Flashcards
What are the components of the pharyngeal apparatus?
Pharyngeal arches, pouches, grooves, and membranes
What does the pharyngeal apparatus form?
Nasal cavities, mouth, larynx, pharynx, neck
When do pharyngeal arches develop embryonically and from what cell type?
Fourth week from neural crest cells migrating into the future head/neck region
What is the oral cavity called in embryos?
Stomodeum
Initially, what cell sources compose the arches?
Externally - surface ectoderm
Internally - pharyngeal endoderm
Core - mesenchyme (mesoderm) from NCC [and then true mesoderm from paraxial mesoderm migrates in]
What do the arches do?
Establish blueprint for face and structural components for developing pharynx
What does the NCC-derived mesenchyme form in each arch?
All connective tissue in the head including dermis and smooth muscle of vasculature
What does paraxial mesoderm form?
Populates each arch to form paraxial musculature
What does lateral plate mesoderm form in each arch?
Angioblasts which differentiate into endothelium lining the blood vessels in the face
What does the prechordal plate mesoderm form in each arch?
Extraocular musculature
What are the four components of pharyngeal arches?
Cartilaginous rod, muscular component, cranial nerves, arch artery
What does the cartilage of the first pharyngeal arch do?
Back portion forms malleus and incus, also provides model for mandible to form. Eventually disintigrates and is replaced w/ bone to form mandible.
What does the cartilage of the second pharyngeal arch do?
Contributes to development of stapes and styloid process of temporal bone and lesser cornu of hyoid bone
What does the cartilage of the third pharyngeal arch do?
Ossifies to form greater cornu of hyoid bone
What does the cartilage of the fourth and sixth pharyngeal arches do?
4th - laryngeal cartilage and epiglottis
6th - laryngeal cartilage
What muscles come from the first arch?
Muscles of mastication (innervated by CN V)
What muscles come from the second arch?
Stapedius m and muscles of facial expression (innervated by CN VII)
What muscles come from the third arch?
Stylopharyngeus m (innervated by CN IX)
What muscles come from the fourth and sixth arches?
4th - cricothyroid m, levator veli palatini m, and constrictors of pharynx (innervated by CN X)
6th - intrinsic muscles of the larynx (innervated by CN X)
What nerve comes from the first arch?
Trigeminal nerve (CN V2 and V3 ONLY)
What nerve comes from the second arch?
Facial nerve (CN VII)
What nerve comes from the third arch?
Glossopharyngeal nerve (CN IX)
What nerve comes from the fourth and sixth arches?
Vagus nerve
What does the first groove form?
The external acoustic meatus
What do grooves 2-4 do?
Lie in the cervical sinus that closes off during development
What germ layer derivative lines the grooves?
Grooves are externally located so ectoderm lines them
What types of congenital malformations are associated with pharyngeal grooves?
Cervical cysts, cervical sinus, cervical fistula
What is a cervical cyst?
Remnant of cervical sinus that is covered with skin but still open internally. Are mobile and painless.
What is a cervical sinus?
Failure of 2nd or 3rd groove to close completely (aka cervical sinus doesn’t close all the way). Can be internal (rare) or external.
What is a cervical fistula?
Open connection between groove that remains patent to the environment (persistence of parts of 2nd groove and pouch)
What does the first pharyngeal membrane contribute to?
Tympanic membrane
What does the first pouch give rise to?
The tympanic cavity, mastoid antrum, and pharyngotympanic tube (eustachian tube). The deepest part of the pouch that comes in contact with the first pharyngeal membrane contributes to the tympanic membrane
What does the second pouch give rise to?
Tonsils - tonsillar sinus and lymphoid nodules of palatine tonsil
What does the third pouch give rise to?
Inferior parathyroid gland and thymus
What does the fourth pouch give rise to?
Superior parathyroid gland and ultimobranchial body (contributes to parafollicular cells of the thyroid gland)
How do the third and fourth pouches move during development?
Third pouch moves down, fourth pouch doesn’t move
Where does the thymus develop from?
The third pharyngeal pouch endoderm which is comprised of epithelial cords
What are the functions of the epithelial cords?
Grow into thymic lobules, become arranged around central point to form thymic corpuscles, and form an epithelial reticulum for housing lymhocytes
What happens to the thymus as it continues to develop from childhood to adulthood?
It involutes and largely becomes replaced by adipose tissue
What does the thyroid gland develop from?
The floor of the developing pharynx
How does the thyroid develop?
From the thyroid primordium which invaginates downward and descends past hyoid bone. Stays attached to developing pharynx via thyroglossal duct which then disintegrates.
What is first arch syndrome?
Congenital abnormality caused by underdevelopment of the first pharyngeal arch due to abnormal migration of neural crest cells into that area.
What is the most common type of first arch syndrome?
Treacher-Collilns syndrome. Autosomal dominant, truncation of TCOF1 gene that forms TREACLE protein which is involved in ribosome biogenesis. Therefore NCCs with this mutation have impaired ribosome formation/activity which causes them to die and decreases NCC population.
What is another type of first arch syndrome that was discussed?
Pierre Robin sequence. Hypoplasia of the mandible/chin, bilateral cleft palate
What is absence of the thyroid gland called?
Agenesis of thyroid gland
What is unilateral failure of formation of the thyroid gland called?
Thyroid hemiagenesis
What is DiGeorge syndrome?
Agenesis of thymus and parathyroid glands, cardiac abnormalities
