Embryology: Derivatives of Pharyngeal Apparatus Flashcards
When do the PAs start developing?
4th week when neural crest cells migrate into the future head and neck
1st pair of arches, the ___ ___, appear lateral to the developing pharynx
Primordial jaws
What arches are rudimentary and not visible on the surface of the embryo the 4th week?
5th/6th arches – eventually disappear completely
Neural Crest Cells migrate from ___ (Prosencephalon), ___ (Mesencephalon), and ____ (Rhombencephalon) during the 4th week.
- Forebrain
- Midbrain
- Hindbrain
NCC from forebrain and midbrain form the ___ ___
Frontal Process (FNP)
NCC from the midbrain and hindbrain form the (1) ___ and the (2) ____
(1) Maxillary (MXP)
(2) Mandibular
What would happen if NCC from midbrain and hindbrain didn’t separate into Maxillary and Mandible?
You wouldn’t have a jaw
Each PA consists of a core of ___ and ____, covered externally by ___ and internally by ___
- Mesoderm and Mesenchyme
- Ectoderm
- Endoderm
Mesenchyme
Embryonic connective tissue derived from migratory NCC
Head Mesoderm
Mesoderm arising from Paraxial Mesoderm
Once formed, the PA:
(1)
(2)
(1) support the lateral walls of the Primitive Pharynx
(2) give rise to facial prominences that contribute to craniofacial development
Pharyngeal ___ plays an essential role in regulating the development of the arches.
Endoderm
What are the 4 PA components?
(1) NCC-Derived Mesenchyme
(2) Paraxial Mesoderm
(3) Lateral Plate Mesoderm
(4) Prechordal Plate Mesoderm
What does NCC-Derived Mesenchyme form?
All connective tissue in the head, including the dermis and smooth muscle
What does the Paraxial Mesoderm form?
It populates each arch to from PA musculature (the muscles that arise from each arch)
What does the Lateral Plate Mesoderm form?
Angioblasts that differentiate into endothelium (blood vessel formation)
What does the Prechordal Plate Mesoderm form?
Extraocular musculature (the muscles that move your eyes)
What 4 main elements do PAs house?
(1) Cartilaginous Rod: forms skeletal elements
(2) Muscular Component: forms muscles of the head/head
(3) Cranial Nerves: with sensory and/or motor components
(4) Arch Artery: form vasculature of the head/neck
What will the first arch cartilage form?
- Malleus and Incus
- Perichondrium -> anterior ligament of malleus and sphenomandibular ligament
- Meckel’s Cartilage -> primordium of mandible (bone forms lateral to this cartilage, which eventually disintegrates)
What is cartilage of the 2nd PA called?
Riechert’s Cartilage
What does the Second Arch Cartilage (Riechert’s cartilage) form?
- Contributes to stapes and styloid process of temporal bone; remainder disintegrates
- Perichondrium -> Stylohyloid Ligament
- Lesser cornu/horn and upper body of Hyoid Bone
What does the Third Arch Cartilage form?
- Greater cornu/horn and lower body of Hyoid Bone
What does the Fourth Arch Cartilage form?
Thyroid and Epiglottal cartilages (Epiglottis doesn’t form until 5th month)
When does the Epiglottis form?
Later in the 5th month
What does the Sixth Arch Cartilage form?
- Remaining Laryngeal cartilages (cuneiform, corniculate, arytenoid and cricoid cartilage)
(Uncertain if this cartilage originates from NCCs or mesoderm)
Most muscular components of the arches form from what type of mesoderm?
Paraxial
What muscles does the First Arch form?
ChewOnThat
- Muscles of mastication (Temporalis, Masseter, Pterygoids, Mylohyoid, Anterior Belly of Digastric, Tensor Veli Palatini, Tensor Tympani)
- Muscles are innervated by the Mandibular division of the Trigeminal N (CN V)
What muscles does the Second Arch form?
SmileAndSayCheese
- Muscles of facial expression (Buccinator, Auricular muscles, Occipitofrontalis, Posterior Belly of Digastric, Stylohyoid, Stapedius, Platysma)
- Muscles are innervated by the Facial N (CN VII)
What muscles does the Third Arch form?
ElevateAndDilate
- Stylopharyngeus
- Muscle is innervated by Glossopharyngeal N (CN IX)
What muscles does the Fourth Arch form?
PushItPushItRealGood
- Muscles: Pharyngeal constrictors, cricothyroid, Levator Veli Palatini
- Muscles are innervated by the Superior Laryngeal branch of the Vagus N (CN X)
What muscles does the Sixth Arch form?
SoundOff
- Muscles: Intrinsic Laryngeal Muscles (Cricoarytenoids, Thyroarytenoid)
- Muscles innervated by the Recurrent Laryngeal branch of Vagus N (CN X)
Extraocular muscles arise from the ___ ___
Prechordal Plate
- Levator Palpebrae Superioris, Superior/Medial/Inferior Recti, & Inferior Oblique all innervated by Oculomotor N (CN III)
- Superior Oblique innervated by Trochlear N (CN IV)
- Lateral Rectus innervated by Abducens N (CN VI)
Tongue musculature arises from ___ ___
Occipital Myotomes
(Muscles: Extrinsic and Intrinsic Lingual muscles except Palatoglossus; innervated by Hypoglossal N)
What nerve supplies the First PA?
Trigeminal N (CN V)
- Principal sensory nerve of the head and neck (V2: Maxillary Division)
- Motor nerve for the muscles of mastication (V3: Mandibular Division)
What nerve supplies the Second PA?
Facial N (CN VII)
What nerve supplies the Third PA?
Glossopharyngeal N (CN IX)
What nerve supplies the Fourth PA?
Superior Laryngeal Branch of Vagus N (CN X)
What nerve supplies the Sixth PA?
Recurrent Laryngeal Branch of Vagus N
Endothelium of the head vasculature and aortic arch arteries arise from what mesoderm?
Lateral Plate Mesoderm
Aortic Arch Arteries
Basket-like structure that initially consists of 5 pairs of arteries arising from the Aortic Sac
Arteries arising from which aortic arches are bilateral?
Arteries arising from 1st-3rd Aortic Arch Arteries are bilateral
Arteries arising from which aortic arches are asymmetrical?
4th and 6th
Aortic Arch Arteries can contribute to ____ ___ when they do not form as expected
Cardiovascular Anomalies
First Arch Syndrome
- Abnormal development of the components of the first arch
- Leads to malformations of the eyes, ears, mandible and palate
What is First Arch Syndrome a result of?
Insufficient migration (likely due to early apoptosis) of NCC into the 1st arch during the 4th week
Treacher-Collins Syndrome (Mandibulofacial Dysostosis)
- Malar Hypoplasis with down-slanting palpebral fissures, defects of lower eyelids, deformed external ears and sometimes middle and internal ears
- Autosomal dominant disorder
Treacher-Collins Syndrome is a result of what gene mutation?
Treacher-Collins Franceschetti Syndrome 1 Gene (TCOF1)
- encodes for the protein TREACLE which is involved in ribosome biogenesis
- truncated in TCS leading to increased apoptosis of cranial NCC
Pierre Robin Sequence
- Typically occurs de novo in most patients
- Associated with hypoplasia of the mandible, cleft palate, and defects of the eyes & ears
What is the initiating defect of Pierre Robin Sequence? What does this defect lead to?
Small mandible (micrognathia) -> posterior displacement of tongue -> obstruction of full closure of the palate -> bilateral cleft palate
Where are Pharyngeal Grooves located? What are they covered with?
- Located externally between Pharyngeal Arches
- Covered with Ectoderm
Where are Pharyngeal Pouches located? What are they covered with?
- Located internally as part of the pharynx
- Covered with Endoderm
First pair of Pharyngeal Grooves and Pouches are located between the __ and __ arches with ___ ___ located between them
- 1st and 2nd
- Pharyngeal Membranes
What does the First Pharyngeal Groove form?
External Acoustic Meatus
Pharyngeal Grooves 2-4 lie in the ___ ___ which is eventually obliterated by PA tissue closure of 2, 3 and 4
Cervical Sinus (called a sinus because still open)
Birth defects of what groove are most common?
2nd Pharyngeal Groove (occurs when cervical sinus doesn’t close)
The First Pharyngeal Pouch expands into what? What does this become?
Tubotympanic Recess -> Tympanic Cavity & Mastoid Antrum
What does the First Pharyngeal Pouch elongate to form?
Pharyngotympanic Tube
Endoderm from the First Pharyngeal Pouch contacts the First Pharyngeal Groove (contact of these two results in formation of the 1st Membrane) which contributes to:
Tympanic Membrane
1st Membrane + Intervening Mesenchyme (from middle of PA) forms:
Tympanic Membrane
Portion of the 2nd Pouch forms the:
Tonsillar Sinus
Endoderm from the 2nd Pouch forms the:
Tonsillar Epithelium
Mesenchyme from the 2nd Pouch forms the:
Lymphoid Nodules (that give rise to the Palatine Tonsil)
Dorsal portion of the 3rd Pouch differentiates into:
Inferior Parathyroid Gland
Ventral portion of the 3rd Pouch differentiates into:
Thymus
The Inferior Parathyroid Glands and Thymus migrate ___
Caudally
(due to the growth of the brain and cardiac regions)
Dorsal portion of the 4th Pouch differentiates into:
Superior Parathyroid Glands
Ventral portion of the 4th Pouch gives rise to the:
Ultimobranchial Body
What does the Ultimobranchial Body fuse with? What does it form?
Fuses with the Thyroid Gland and gives rise to Parafollicular Cells
Branchial Anomalies: External Cervical Sinus
- Failure of 2nd groove and cervical sinus to obliterate/combine
- Typically external; internal is rare
- Detected due to discharge of mucus (b/c lined with epithelium)
- Commonly associated with auricular sinuses (near Tragus of ear)
Branchial Anomalies: Internal Cervical Sinus
- Persistence of 2nd Pouch
- Rare
- Open into tonsillar sinus or near Palatopharyngeal Arch
- Also can be detected due to discharge of mucus (b/c lined with epithelium)
Branchial Anomalies: Cervical Cysts
- Remnants of cervical sinus and/or 2nd Groove; outer surface of cyst is encapsulated in cyst
- Slowly enlarging, painless, free-lying cyst in the neck, inferior to angle of mandible
- Over time can accumulate fluid and cellular debris (can causing bulging, problems swallowing, infection)
Branchial is same thing as Pharyngeal
Branchial Anomalies: Cervical Fistula
- Canal that opens into Tonsilar Sinus and external side of neck (essentially an opening in the skin of the neck that has a canal with an internal opening into the tonsillar sinus where the palatine tonsil is located)
- Persistence of parts of cervical sinus and tonsillar sinus (2nd pouch)
- Ascends through subcutaneous tissue and Platysma to reach carotid sheath; passes between carotids and opens into tonsillar sinus
- Would allow for leakage of food and liquids; prone to infections
When does Thyroid Gland start developing?
24 days after fertilization
(first endocrine gland to develop!)
What does the Thyroid Gland form from?
Endodermal thickening in the floor of the Primordial Pharynx
Thyroid Gland Development
(1) Endodermal thickening in floor of Primordial Pharynx -> Thyroid Primordium
(2) Descends in the neck (through the Foramen Cecum) as the tongue grows -> moves ventral to hyoid bone an dlaryngeal cartilages
(3) Once in new position, it is still connected to the tongue by a narrow tube called the Thyroglossal Duct
(4) Promordium is hollow but eventually becomes a solid cellular mass, that divides into right/left lobes conneccted by the isthmus
(5) Definitive shape by 7 weeks and thyroglossal duct with degenerate
Thyroid Gland Development Anomalies: Ectopic Thyroid Tissue
- If this anomaly occurs, the extra thyroid tissue is often in accessory, lingual or cervical regions
- Extra tissue forms along the course of the duct
Example of a Thyroid Gland Development Anomaly
Sublingual Thyroid Gland forms but does not descend in the neck, just sits below the tongue
Thyroid Gland Development Anomalies: Thyroglossal Duct Cyst
- Forms in tongue or anterior neck
- Critical to distinguish from ectopic thyroid prior to surgical removal
- Results from persistent Thyroglossal Duct
- Can cause problems swallowing
Congenital Anomalies of Thyroid Gland: Agenesis
Absence of a thyroid gland or one of its lobes (rare)
Congenital Anomalies of Thyroid Gland: Thyroid Hemiagenesis
- Unilateral failure of formation
- Left lobe more commonly absent
- Mutations in the receptor for the thyroid-stimulating hormone are likely involved
DiGeorge Syndrome
- Breakdown of signaling from PA endoderm to NCC
- Often caused by chromosomal deletion which impairs signaling from endodermal lining in the pouches – w/o signaling then NCC doesn’t know what to do
- Associated signs/sxs: agenesis of thymus and parathyroid glands, congenital hypoparathyroidism, shortened philtrum of upper lip and low-set and notched ears, nasal clefts, thyroid hypoplasia, cardiac abnormalities (defects in aortic arch and heart)
