Lecture 6: Embryology of Pharyngeal Apparatus Flashcards by Tosin Ibrahim

What does a pharyngeal apparatus consist of?

Pharyngeal Arches
Pharyngeal Pouches (internal structure - endoderm)
Pharyngeal Grooves/Clefts (ectoderm)

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What is the embryological origin of phayngreal arches?

Neural Crest Cells

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What pharyngeal arch do the primordial jaws arise from?

1^st Pharyngeal Arch

Neural Crest Cell

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What germ layer(s) contribue to the core of a pharyngeal arch?

What germ layer(s) externally cover the pharyngeal arch?

*Core:** Mesoderm and Mesenchyme (migratory NCC)
*Cover:** Ectoderm and Endoderm

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What is the function of pharyngeal arches?

Support lateral walls of primordial pharynx
Give rise to facial prominences of tissue that contribue to craniofacial development

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What germ layer plays an essential role in regulation the development of arches?

Pharyngeal Endoderm

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What germ layer gives rise to PA muscles?

Paraxial Mesoderm

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What germ layer gives rise to endothelium?

Lateral Plate Mesoderm

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What germ layer gives rise to extraocular muscles?

Prechordal Plate Mesoderm

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What structures will a typical pharyngeal arch contain?

Cartilagenous Rod: skeletal elements
Muscular Component: Paraxial and Prechordal derived
Cranial Nerves: sensory and/or motor components
Artery: from truncus arteriosus

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What pharyngeal arch does Meckel’s cartilage arise from?

Pharyngeal Arch 1

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What does the dorsal part of Meckel’s cartilage form?

What germ layer do these structures arise from?

Malleus and Incus bones

Neural Crest Cells

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What does the perichondrium of Meckel’s cartilage form?

Anterior ligament of Malleus bone

Sphenomandibular Ligament

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What does the ventral side of Meckel’s cartilage give rise to?

Primordium of mandible (transient structure)

Proper mandible will form laterally to it while Meckel’s cartilage disintegrates

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What is the cartilage of Pharyngeal Arch 2?

Riechert’s cartilage

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What does the dorsal part of Riechert’s cartilage form?

Stapes

Styloid Process of the Temporal Bone

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What arises from the ventral end of Riechert’s cartilage?

Lesser cornu/horn of Hyoid bone

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What arises from the perichondrium of the Riechert’s cartilage?

Stylohyoid L.

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What does the 3^rd Pharnygeal cartilage give rise to?

Greater horn/cornu of hyoid bone

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What does the hypopharyngeal eminence give rise to?

What PAs does it arise from?

Body of hyoid bone

Arises from Pharyngeal Arches 3 and 4: floor of embryonic pharanyx

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What pharyngeal arches do the laryngeal cartilages come from?

Pharyngeal Arches 4 and 6

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What pharyngeal arches do the epiglottis come from?

Pharyngeal Arch 4

Where does the aoritc arch system arise from?

Aortic Sac

What muscles does the 1^{<span>st</span>} pharyngeal arch give rise to?

What is it innervated by?

Muscles of Mastication (4)

Cranial Nerve V3

What muscles do the 2^nd pharygneal arch give rise to? What is it innervated by?

* Stapedius * Muscles of Facial Expression Cranial Nerve VII

What muscle does the 3^rd Pharyngeal Arch give rise to? What is it innervated by?

Stylopharyngeus Muscle Cranial Nerve IX

What muscles do the 4^th pharygneal arch give rise to? What is it innervated by?

* Cricothyroid M. * Levator veli palatini M. * Pharyngeal constrictor ms. Cranial Nerve X

What muscles do the 6^th pharygneal arch give rise to? What is it innervated by?

Intrinsic Muscles of Larynx Cranial Nerve X

What is the embryological origin of the External Acoustic Meatus?

1^st Pharyngeal Groove ## Footnote *Ectoderm*

What grooves lie in the cervical sinus?

Grooves 2-4

What is the fate of pharyngeal grooves 2-4?

Obliterated in Week 7

What part of the pharyngeal apparatus is a cervical/branchial cyst most likely to be a remnant of?

**REMNANTS OF 2ND GROOVE/CERVICAL SINUS** * Slowly enlarging, painless, free-lying cyst in neck * Inferior to **MANDIBLE** * Accumulation of fluid and cellular debris

How does the cervical sinus remant develop?

2nd PA groove and cervical sinus fail to obliterate and are covered with skin * Mucus discharge noted * Typically bilateral

How does an Internal cervical sinus anomaly occur?

Persistence of 2nd PA pouch * Opens into tonsillar sinus or near palatopharyngeal arch * Rare

What initiates formation of double-layered pharyngeal membranes?

Pouch endoderm contacts ectoderm of pharyngeal grooves

What is the embryological origin of the Tympanic Membrane?

1^st Pharyngeal Membrane ## Footnote *Mesenchyme*

What structures are developed from 1^st pharyngeal pouch?

* Tubotympanic Recess ----\> Pharyngotympanic Tube * Tympanic Cavity * Mastoid Antrum * Tympanic Membrane

What structures are developed from 2^nd pharyngeal pouch?

Tonsillar Sinus

Around week 20, mesenchyme of the 2^nd pharyngeal pouch will differentiate into what tissue?

Lymphoid tissue

What structures are developed from 3^rd pharyngeal pouch?

Inferior Parathyroid Gland Thymus (ventral portion)

What structures are developed from 4^th pharyngeal pouch?

Superior Parathyroid Gland Ultimobranchial body (NCC)

What does the ultimobranchial body give rise to?

**Parafollicular cells** (fuses with thyroid gland and disseminates)

What is the origin of the thymus?

3^rd Pharyngeal Pouch endoderm

What is the origin of the thyroid gland?

Endodermal thickening of Primordial Pharnyx

What structure connects the fetal tongue to the developing thyroid gland?

Thyroglossal duct ## Footnote *Transient*

What is the direct embryological origin of the foramen cecum?

Thyroglossal duct

Abnormal migration of NCC into the 1st pharyngeal arch can cause what facial features to be malformed?

Eyes, ears, mandible, palate

What autosomal dominant disorder leads to malar hypoplasia, deformed external ears and down-slanting palpebral fissures?

Treacher Collins syndrome (Mandibulofacial dysostosis)

Mutation in the what gene causes Treacher-Collin's sydrome? What does the mutation result in?

TCOF1 Cannot make TREACLE protein, involved in ribosome biogenesis, leads to increased apoptosis of cranial NCCs

What is the Pierre Robin Sequence?

PA Arch 1 abnormality 1. Small mandible (Micrognathia) =\> 2. Posterior displacement of the tongue =\> Obstruction of full closure of palate =\> 3. Bilateral cleft palate

Patients with Pierre Robin sequence likely had a defect in development of what embryonic structure?

1^st PA

In thyroid hemiagenesis, what lobe is more commonly absent?

Left Lobe

A patient with agenesis of the thyroid and parathyroid glands, shortened philtrum of the upper lip, low-set ears, nasal cleft, and a heart murmur may have what disorder?

DiGeorge Syndrome

What is the pathology behind Digeorge Syndrome?

Breakdown of signaling from pharyngreal arch endoderm to Neural crest cells