Pharyngeal apparatus Flashcards
Parts of the pharyngeal apparatus
pharyngeal clefts
pharyngeal arches
pharyngeal pouches
Pharyngeal clefts
- External invagination of ectoderm.
2. 4 clefts
pharyngeal arches
- Core of mesenchyme (neural crest and paraxial mesoderm).
- Neural crest will give rise to ct components; paraxial mesoderm gives rise to muscles.
- Each arch has its own nerve (cranial nerve) and blood supply (aortic arch artery).
- 6 pharyngeal arches initially develop; but the 5th regresses, leaving arches 1-4 and 6.
Pharyngeal pouches
- Internal evagination of pharyngeal endoderm.
2. 4 pouches
Derivatives of Arch 1
- Skeletal derivatives – viscerocranium (malleus, incus) and connective tissues of face
- Muscular derivatives
a. Muscles of mastication (temporalis, masseter, medial and lateral pterygoids)
b. Also: anterior belly of digastric, mylohyoid, tensor tympani, tensor veli palatini.
“MAST and MATT” - Nerve: trigeminal nerve (CN V3)
- Oropharyngeal membrane develops between the two processes of first arch.
a. Composed of an outer layer of ectoderm and an inner layer of endoderm.
b. Ruptures by day 26 to allow communication between pharynx and oral cavity.
Derivatives of Arch 2
- Skeletal derivatives
a. Stapes, styloid process of temporal bone
b. Stylohyoid ligament; lesser horn and superior portion of body of hyoid - Muscular derivatives
a. Muscles of facial expression
b. Also: stylohyoid, posterior belly of digastric, stapedius - Nerve: facial nerve (CN VII)
Derivatives of Arch 3
- Skeletal derivatives: greater horn and inferior portion of body of hyoid bone.
- Muscular derivatives: stylopharyngeus
- Nerve: glossopharyngeal nerve (CN IX)
Derivatives of Arch 4
- Skeletal derivatives: with 6th arch, forms all laryngeal cartilages.
- Muscular derivatives: muscles of pharynx (except stylopharyngeus); cricothyroid
- Nerve: vagus nerve (CN X)
Derivatives of Arch 5
Regresses
Derivatives of Arch 6
- Skeletal derivatives: with 4th arch, forms all laryngeal cartilages.
- Muscular derivatives: laryngeal muscles (except cricothyroid); skeletal muscle of esophagus
- Nerve: vagus nerve (CN X; recurrent laryngeal nerve).
Derivatives of Pharyngeal Pouch 1
contributes to formation of the middle ear and auditory tube.
Derivatives of Pharyngeal Pouch 2
the stroma of palatine tonsil; tonsillar fossa.
Derivatives of Pharyngeal Pouch 3
- Dorsal portion: inferior parathyroid glands
2. Ventral portion: thymus
Derivatives of Pharyngeal Pouch 4
- Dorsal portion: superior parathyroid glands
2. Ventral portion: ultimobranchial body (see development of thyroid gland below)
Derivatives of Pharyngeal Clefts
A. First cleft contributes to formation of the external ear.
B. The second arch overgrows arches 3 and 4, burying clefts 2-4.
C. Remants of clefts 2-4 form the cervical sinus which is later obliterated.
Thyroid gland development
A. Derived from a median endodermal thickening of pharyngeal floor.
B. The gland descends along the pharynx into the neck, remaining connected to tongue via the thyroglossal duct (degenerates by week 7).
C. Superior opening of the thyoglossal duct remains as the foramen cecum.
D. In about 50% of individuals a pyramidal lobe of the thyroid is present; this is a remnant of the distal thyroglossal duct.
E. Parafollicular (C cells) are derived from the ultimobranchial body of the 4th pouch.
Tongue development
A. 1st pharyngeal arch form the body (anterior 2/3) of tongue (GSA from trigeminal n.)
B. 3rd arch forms the root (posterior 1/3) of tongue (GVA from glossopharyngeal n.)
C. 4th arch forms the epiglottic region of tongue (GVA from vagus n.)
D. Muscle derived from paraxial mesoderm of occipital somites
Arches and the nerves that innervate their derivatives
1--> trigeminal n. 2--> facial n. 3--> glossopharyngeal n. 4--> vagus n. 5--> mostly regresses 6--> vagus n. (mainly recurrent laryngeal branch)
Congenital Anomalies: Ectopic thyroid or parathyroid tissue
- Remnants of thyroid tissue can remain along the course of migration.
- The parathyroids also undergo a migration and are highly variable in their location.
Congenital Anomalies: Lateral cervical cysts
Lateral cervical cysts form when the cervical sinus fails to degenerate. These cysts are found along the anterior border of the SCM.
Congenital Anomalies: branchial fistulas
Branchial fistulas occur when second arch fails to overgrow the third and fourth arches. The cervical sinus retains a connection with the surface (external fistula). In rare cases, the lateral cervical cyst can open into the pharynx (internal fistula).
Congenital Anomalies: Thyroglossal Cyst
- Remnant of thyroglossal duct.
2. Always located on or near midline of the neck.
Congenital Anomalies: Neural Crest Defects
- Neural crest cells are important for the development of the pharyngeal arches.
- Severe craniofacial defects result from disruption of neural crest cell migration or differentiation.
- Ex: Treacher Collins Syndrome, Robin Sequence, DiGeorge Anomaly
- DiGeorge Syndrome
a. Deletion on long arm of Chromosome 22
b. Cardiac abnormalities
c. Craniofacial defects
d. Thymic hypoplasia
e. Parathyroid dysfunction – hypocalcemia due to parathyroid insufficiencies