Development Of The Pharyngeal Apparatus Flashcards
Which structures make up the pharyngeal apparatus?
Pharyngeal arches (PA), pouches, grooves (clefts) and membranes
What does the pharyngeal apparatus contribute to?
Formation of nasal cavities, oral cavity, tongue, larynx, pharynx, ear and neck
When do pharyngeal arches develop?
Early in the 4th week as neural crest cells that migrate into the future head and neck
Explain the appearance of the 1st-6th PAs
1st pair (primordial jaws) appear lateral to developing pharynx
Arches 2-4 soon appear as ridges on each side of the future head and neck regions
5/6th arches are rudimentary and not visible on the surface of the embryo
Neural crest cells migrate from which structures?
Forebrain, midbrain and hindbrain (during the 4th week)
What does each PA consist of?
A core of mesoderm and mesenchyme
Each PA is covered by what?
Externally by ectoderm and internally by endoderm
What is mesenchyme?
Embryonic CT derived from migratory NCC
What is head mesoderm?
Mesoderm arising from paraxial mesoderm
Once formed the pharyngeal arches support and give rise to what?
Support the lateral walls of the primitive pharynx
Give rise to facial prominences that contribute to craniofacial mesoderm
What is the stomodeum?
Oral cavity
What are the 4 components of a pharyngeal arch?
NCC-derived mesenchyme, paraxial mesoderm, lateral plate mesoderm and prechordal plate mesoderm
What does NCC-derived mesenchyme form?
Forms all CT in the head including the dermis and smooth muscle
What is the function of paraxial mesoderm in PAs?
Populates each arch to form PA musculature
What is the function of lateral plate mesoderm within PAs?
Angioblasts differentiate into endothelium
What does the prechordal plate mesoderm form within the PAs?
Extraocular musculature
What role does pharyngeal endoderm play?
Plays an essential role in regulating the development of the PA
What are the 4 elements that the PA house?
Cartilaginous rods, muscular component, CNs and arch artery
What does the cartilaginous rod of the PAs form?
Skeletal elements
What does the muscular component of a PA form?
Muscles of the head/neck
The CNs in PA have what components?
Sensory and/or motor components
What does the arch artery within the PA form?
Vasculature of the head/neck
What is Meckel’s cartilage?
Cartilage of the 1st PA
The dorsal portion of the Meckel’s cartilage forms what?
Malleus and incus
Describe Meckel’s cartilage
Bone forms laterally to Meckel’s which disintegrates
What does the ventral part of Meckel’s cartilage form?
The primordium of the mandible
What does the perichondrium of Meckel’s cartilage give rise to?
Anterior ligament of malleus and sphenomandibular L
What is Rierchert’s cartilage?
Cartilage of PA2
What does the dorsal region of Riechert’s cartilage contribute to?
Stapes and styloid of the temporal bone, remainder disintegrates
What does the perichondrium of Riechert’s cartilage form?
Stylohyoid ligament
What does the ventral portion of Riechert’s cartilage form after ossification?
Lesser Cornu/horn of hyoid bone
What does the third arch cartilage become after it ossifies?
Forms greater Cornu of hyoid bone
The body of the hyoid bone is formed by what?
Hypopharyngeal eminence
Prominence in floor of embryonic pharynx (from PA3 and 4)
What does the 4th arch cartilage give rise to?
Laryngeal cartilages, including the epiglottis (NCC derived)
What does the 6th PA cartilage give rise to?
Laryngeal cartilages but origin is uncertain
Most muscular components of the PAs are derived from what?
Paraxial mesoderm
What are the muscular components of the first PA?
Muscles of mastication, mylohyoid, anterior belly of digastric, tensor tympani and tensor veli palatini (CN V)
What are the muscular components of the second PA?
Muscles of facial expression (CN VII)
Posterior belly of digastric, stylohyoid and stapedius
What are the muscular components of the 3rd PA?
Stylopharyngeus M (CN IX)
What are the muscular components of the 4th PA?
Cricothyroid, levator veli palatini and pharyngeal constrictors (CN X)
What are the muscular components of the 6th PA?
Intrinsic muscles of the larynx except for the cricothyroid
CN X
What do extraocular muscles arise from?
Prechordal plate
Includes levator palpebrae superioris, superior, medial and inferior rectus, inferior oblique, superior oblique and lateral rectus
What does tongue musculature arise from?
Occipital myotomes
What does the trigeminal nerve (CN V) supply?
The first PA
Principal sensory nerve of the head and neck
Motor nerve for the muscles of mastication
What does the facial nerve (CN VII) supply?
Second PA (muscles of facial expression)
What does the glossopharyngeal nerve (CN IX) supply?
3rd PA (stylopharyngeus M)
What does the vagus nerve (CN X) supply?
4th-6th PA
4th arch -> supplied by superior laryngeal
6th arch -> supplied by recurrent laryngeal branches of CN X
Constrictors of pharynx (4th) and intrinsic muscles of larynx (6th)
What is first arch syndrome?
Abnormal development of the components of the first arch
Malformation of the eyes, mandible and palate
Results from insufficient migration of NCC into the 1st PA during the 4th week
What is Treacher-Collins syndrome (mandibulofacial dysostosis?
Malar hypoplasia w/ down-slanting palpebral fissures, defects of lower eyelids, deformed external ears and sometimes middle and internal ears
Explain the genetics of Treacher-Collins syndrome
Autosomal dominant disorder caused by mutation in Treacher-Collins-Franseschetti syndrome 1 gene (TCOF1) which encodes for the protein TREACLE invovled in ribosome biogenesis
Truncated protein leading to increased apoptosis of cranial NCC
What are two conditions that are an example of first arch syndrome?
Treacher-Collins syndrome and Pierre Robin sequence
What is Pierre Robin sequence?
Occurs de novo in most pts
Associated with hypoplasia of the mandible, cleft palate and defects of the eyes and ears
Explain the defect in Pierre Robin and how it leads to cleft palate
Initiating defect is a small mandible (micrognathia) -> results in posterior displacement of the tongue -> obstruction of full closure of the palate -> results in bilateral cleft palate
Where are pharyngeal grooves located?
Externally between pharyngeal arches covered with ectoderm
Where are pharyngeal pouches located?
Internally as part of the pharynx covered with endoderm
1st pair lies between the 1st and 2nd arches
Where are pharyngeal membranes located?
Between the pharyngeal grooves and pouches
What will the 1st pharyngeal groove form?
The external acoustic meatus
Where do grooves 2-4 lie?
In the cervical sinus which is eventually obliterated
Birth defects in which groove are common?
In the 2nd groove
The 1st pouch expands into the tubotympanic recess which becomes what?
Tympanic cavity and mastoid antrum
The 1st pouch also elongates to form what?
Pharyngotympanic tube
The endoderm of the 1st pouch contacts what structure?
The 1st pharyngeal groove and contributes to the tympanic membrane
What forms the tympanic membrane?
1st membrane + intervening mesenchyme
What does the 2nd pouch form?
Portion of it forms tonsillar sinus
Endoderm will form tonsillar epithelium
Mesenchyme will form lymphoid nodules of palatine tonsils
What does the 3rd pouch form?
Dorsal portion differentiates into inferior parathyroid gland
Ventral portion forms the thymus
Both migrate caudally due to growth of brain and cardiac regions
What are examples of branchial anomalies?
External and internal cervical sinus, cervical cysts and cervical fistula
What is external cervical sinus?
Failure of the 2nd groove and cervical sinus to obliterate
Typically external (internal is rare)
Detected due to discharge of mucus
Commonly associated with auricular sinuses
What is internal cervical sinus?
Persistence of the 2nd pouch
Rare
Open into tonsillar sinus or near palatopharyngeal arch
What does the 4th pouch form?
Dorsal portion differentiates into superior parathyroid glands
Remainder forms the ultimobranchial body which fuses with thyroid gland and gives rise to parafollicular cells
What are cervical cysts?
Remnants of cervical conus and/or 2nd groove
Slowly enlarging, painless, free-lying cyst in the neck, inferior to the angle of the mandible
Accumulation of fluid and cellular debris occurs
What is a cervical fistula?
Canal that opens into tonsillar sinus and external side of the neck
Persistence of parts of the 2nd groove and pouch
Ascends through subcutaneous tissue and platysma to reach the carotid sheath -> Passes between carotids and opens into tonsillar sinus
What is the first endocrine gland to develop?
Thyroid primordium (~24 dpf) and forms from an endodermal thickening in the floor of the primordial pharynx
As the tongue grows what structure descends into the neck?
The thyroid primordium and moves ventral to the hyoid bone and laryngeal cartilages
The thyroid primordium is connected to the tongue by what?
A narrow tube called the thyroglossal duct
Describe the development of the thyroid gland
Thyroid primordium is hollow but will become a solid cellular mass
Divides into right/left lobes connected by the isthmus
Definitive shape by 7 weeks and thryoglossal duct will degenerate
What anomalies are associated with thyroid gland development?
Ectopic thyroid tissue (accessory, lingual and cervical), sublingual thyroid gland and thyroglossal duct cyst
Congenital: Agenesis of the thyroid gland, thyroid hemiagenesis and DiGeorge syndrome
What is ectopic thyroid tissue?
Can form along the course of the duct
Can be accessory, lingual or cervical
What is a sublingual thyroid gland?
Gland forms but does not descend in the neck
What is a thyroglossal duct cyst?
Forms in the tongue or anterior neck
Critical to distinguish from ectopic thyroid prior to surgical removal
What is agenesis of the 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
Mutations in the receptor for thyroid-stimulating hormone (TSH) are likely involved
What is DiGeorge syndrome?
Breakdown of signaling from PA endoderm to NCC Agenesis of thymus and parathyroid glands
Develop congenital hypoparathyroidism
What are characteristics of DiGeorge syndrome?
Shortened philtrum of upper lip, low set and notched ears
Nasal clefts and thyroid hypoplasia
Cardiac abnormalities (defects in the aortic arch and heart)
