Pharyngeal Arch embryology Flashcards
What is the pharyngeal apparatus?
Consists of the pharyngeal arches, pouches, grooves and membranes
Contribute to the formation of nasal cavities, mouth, larynx, pharynx and neck
What week do pharyngeal arches develop?
Which germ layer does this come from?
4th week
Neural crests, migrating into the future head and neck
Sequence of development of the arches
The 1st pair, the primordial jaws appear lateral to the developing pharynx = is the future head and neck
Arches 2-4 then appear on each side
Arches 5-6 are rudimentary and not visible on embryological surface, Arch 5 disintegrates
Core of each arch is made of….
External lining of each arch is made of….
Internal lining of each arch is made of …
Mesoderm and mesenchyme
Ectoderm
Endoderm
The core mesenchyme is derived from
Migratory neural crest cells and mesoderm cells from the prechordal plate and the paraxial mesoderm respectively
Pharyngeal arches function and gives rise to ….
Support the lateral walls of the primordial pharynx and give rise to prominences of tissue (e.g., laryngeal prominence)
NCC derived mesenchyme forms
All connective tissue in the head, including the dermis and smooth muscle (vasculature)
Paraxial mesoderm forms
Pharyngeal arch musculature
Lateral plate mesoderm forms
Angioblasts that differentiate into endothelium
Prechordal plate mesoderm forms
Extraocular musculature (Eye muscles)
Structures contained in the pharyngeal arches
Cartilagenous rod, muscular component, cranial nerve, arch artery
Cartilagenous rod forms …
Muscular component forms ….
Skeletal elements
Head and neck muscles
Arch artery arises from …
Course
Truncus arteriosus
Passes around the primordial pharynx to enter the dorsal aorta
PA1 cartilage is called
Meckel’s cartilage
Dorsal portion of the meckel’s cartilage forms ….
Ventral portion of the meckel’s cartilage forms ….
Perichondrium of the meckel’s cartilage forms ….
Malleus and Incus
Primordium of the mandible
Anterior ligament of malleus and the sphenomandibular ligament
What eventually happens to Meckel’s and Riechert cartilage?
Disintegrates and replaced by bone
PA2 cartilage is called
Riechert’s cartilage
Part of the dorsal region of riechert’s cartilage forms ….
Ventral region …
Perichondrium …
- stapes and styloid process of temporal bone, while the rest will disintegrate
- ossifies and forms the lesser cornu/horn of the hyoid
- becomes the stylohyoid ligament
PA3 (Third arch cartilage) will…
Ossify and forms the greater cornu of the hyoid
Hypopharyngeal eminence (floor of the embryonic pharynx - coming from PA3 and PA4) will form…
Body of the hyoid bone
4th and 6th cartilage will form
4th: laryngeal cartilage, including the epiglottis (NCC derived)
6th arch: laryngeal cartilage of unk origin
First arch muscular components forms which muscles?
Second arch muscular components forms which muscles?
Third arch?
Fourth arch?
Sixth arch?
- Muscles of mastication (CN V, chewing)
- stapedius and muscles of facial expression (CN VII)
- stylopharyngeus (elevate and dilate, innervated by CN IX)
- cricothyroid, levator veli palatini and constrictors of pharynx (pushes food down, CN X)
- intrinsic laryngeal muscles (CN X, make sound)
Which PA is each nerve associated with? CN V CN VII CN IX CN X
- PA1 (basically sensory n. of head and neck)
- PA2
- PA3
- PA4-6 (4th arch specifically supplied by the superior laryngeal branch and recurrent laryngeal branch)
Which branch of CN V is not associated with PA1?
V1 (ophthalmic)
What are the aortic arch arteries?
Basket like structure of arteries. initially five pairs arising from the aortic sac of the dorsal aorta)
Remodeled in development to form the great arteries of head and neck
What are pharyngeal grooves?
Located between the pharyngeal arches and are lined by ectoderm
First groove will form ….
How about grooves 2-4?
External acoustic meatus
Fuse together to form the cervical sinus, which disintegrate during development
Which groove is most commonly affected in birth defects?
2nd groove. Cervical sinus forms caudal to this
Cervical cysts:
Cause
Clinical
Remnants of cervical sinus and/or 2nd groove, collect cell debris and form cysts in the neck,
Usually painless. Cysts form inferior to angle of mandible
Cervical sinus:
Cause
Clinical
2nd PA groove and cervical sinus (a hole thing that disintegrates during development) did not close and obliterate completely so you have an opening
Detected due to mucosal discharge. Commonly associated with auricular sinuses
Cervical fistula:
Cause
Clinical
2nd groove and pouch persistence
Abnormal canal that opens into the tonsillar sinus on the side of the neck
1st pharyngeal membrane forms…
Tympanic membrane
1st pouch forms
Contacts the 1st groove which also contributes to the tympanic membrane
Tympanic cavity and mastoid antrum from the tubotympanic recess
Elongates to form the pharyngotympanic tube
2nd pouch forms
Tonsillar sinus
Endoderm forms the tonsillar epithelium
Mesenchyme forms the lymphoid nodules of the palatine tonsil
3rd pouch forms…
Dorsal portion:
Ventral portion:
- inferior parathyroid
- thymus
(these caudally migrate to accommodate brain and heart development and end up below the derivatives of the 4th pouch)
4th pouch forms
Dorsal portion:
Superior Parathyroid glands (4 doesn’t move down, so it ends up superior)
Ultimobranchial body fuses with thyroid gland to give rise to parafollicular cells
Describe the development of the thymus:
PA3 endoderm > thymic primordium. 3 epithelial cords
Functions of the epithelial cords:
Grow into surrounding mesenchyme
Arrange around a central point and form the thymic corpuscles
Form an epithelial reticulum to house lymphocytes
Thymus and aging
Active in childhood but involutes and mostly becomes adipose in adulthood
Development of the thyroid:
Develops 24 days post fertilization from the floor of the developing pharynx > thyroid primordium > invaginates downward and passes ventrally to the hyoid and laryngeal cartilage
Connected to the tongue via the thyroglossal duct, which will later disintegrate
First pharyngeal arch syndrome:
Cause
Clinical
Caused by insufficient migration of NCC into 1st arch during the 1st week
Malformation of eyes, ears, mandible and palate = first arch syndrome
Treacher Collins syndrome (mandibulofacial dysostosis)
Cause
Clinical
Autosomal dominant, caused by mutations in TCOF1 gene. Encodes TREACLE involved in ribosome biogenesis. This protein is truncated in TCS causing apoptosis of the neural crest cells > abnormal migration
Pierre Robin sequence:
Hypoplasia of mandible, cleft palate and defects of eyes and ears
Initiating defect is small mandible (micrognathia) which results in posterior displacement of tongue and obstruction of palate closure. Results bilateral cleft palate
Agenesis of the thyroid gland:
Absence of thyroid gland or one of its lobes. Rare
Thyroid hemiagenesis
Unilateral failure formation, left lobe commonly absent. Possibly caused by mutations in TSH receptor
DiGeorge Syndrome:
No signaling of PA endoderm to NCC to migrate. Thymus and parathyroid glands do not develop. No T cells.
Clinical: congenital hypoparathyroidism, shortened philtrum of upper lip, low set and notched ears, nasal clefts and thyroid hypoplasia. Also cardiac abnormalities due to defects in aortic arch and heart
