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MD4001 > Development of Pharyngeal Arches > Flashcards

Development of Pharyngeal Arches Flashcards

1
Q

Describe development of pharyngeal arches from the beginning of the third week of fetal life.

A

-The most distinctive feature in development of the head and neck is the presence of pharyngeal arches (= branchial arches)
-These arches appear in the fourth and fifth weeks of development and contribute to the characteristic external appearance of the embryo.
-Initially, they consist of bars of mesenchymal tissue separated by deep clefts known as pharyngeal clefts
-Simultaneously, with development of the arches and clefts, a number of outpocketings, the pharyngeal pouches, appear along the lateral walls of the pharynx, the most cranial part of the foregut.
-The pouches penetrate the surrounding mesenchyme but do not establish an open communication with the external clefts.
-Pharyngeal arches not only contribute to formation of the neck but also play an important role in formation of the face.
-At the end of the fourth week, the center of the face is formed by the stomodeum (24 days stomodeum is closed by oropharyngeal membrane, which slightly later ruptures), surrounded by the first pair of pharyngeal arches.
When the embryo is 42 days old, five mesenchymal prominences can be recognized: the mandibular prominences (first pharyngeal arch), caudal to the stomodeum; the maxillary prominences (includes upper and (larger) lower) (dorsal portion of the first pharyngeal arch), lateral to the stomodeum; and the frontonasal prominence, a slightly rounded elevation cranial to the stomodeum.
-Development of the face is later complemented by formation of the nasal prominences.

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2
Q

Describe the basic components of the pharyngeal arches and their origins.

A
  • Each pharyngeal arch consists of a core of mesenchymal tissue covered on the outside by surface ectoderm and on the inside by epithelium of endodermal origin.
  • In addition to mesenchyme derived from paraxial (forms mostly muscle tissue of the head, and some connective tissue/bone) and lateral plate (small contribution the head) mesoderm, the core of each arch receives substantial numbers of neural crest cells, which flow ventrally around ectodermal placodes into the arches to contribute to skeletal components of the face (mesenchyme, ganglia, nerves).
  • The original mesoderm of the arches gives rise to the musculature of the face and neck. Thus, each pharyngeal arch is characterized by its own muscular components.
  • The muscular components of each arch have their own cranial nerve, and wherever the muscle cells migrate, they carry their nerve component with them
  • In addition, each arch has its own arterial component
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3
Q

Describe the fate of the first pharyngeal arch.

A
  • The first pharyngeal arch consists of a dorsal portion, the maxillary process, which extends forward beneath the region of the eye, and a ventral portion, the mandibular process, which contains Meckel’s cartilage.
  • During further development, Meckel’s cartilage disappears except for two small portions at its dorsal end that persist and form the incus and malleus
  • Mesenchyme of the maxillary process gives rise to the premaxilla, maxilla, zygomatic bone, and part of the temporal bone through membranous ossification
  • The mandible is also formed by membranous ossification of mesenchymal tissue surrounding Meckel’s cartilage.
  • In addition, the first arch contributes to formation of the bones of the middle ear

Musculature of the first pharyngeal arch includes the muscles of mastication (temporalis, masseter, and pterygoids), anterior belly of the digastric, mylohyoid, tensor tympani, and tensor palatini.

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4
Q

What is the nerve supply to the muscle of the first arch ?

A

The nerve supply to the muscles of the first arch is provided by the mandibular branch of the trigeminal nerve. Because mesenchyme from the first arch also contributes to the dermis of the face, sensory supply to the skin of the face is provided by ophthalmic, maxillary, and mandibular branches of the trigeminal nerve.

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5
Q

Define ectodermal placode.

A

Thickening of ectoderm that help give rise to sensory neurons/ganglia.

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6
Q

Define somitomere.

A

Ill defined “segments” of paraxial mesoderm in the head region rostral to somites

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7
Q

Identify all the musculature derived from paraxial mesoderm (somitomere).

A

ARCH
1 - mastication + extras
2 - facial expression + extras
3 - stylopharyngeus (cranial somitomeres)
4-6 – constrictors, soft palate, lev v palatini, cricothyroid, larynx musculature, (somites)

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8
Q

Identify all the blood vessels derived from paraxial mesoderm (somitomere).

A

1- The artery of the first pharyngeal arch becomes the terminal portion of the maxillary artery, which is a branch of the external carotid

2- Stapedial artery (transitory) and hyoid artery

3- Common Carotid Arteries, first part of Internal Carotid Artery

4- Proximal Part of Sc artery on the right + Arch of aorta between origins of L common carotid and L Sc arteries

6- Pulmonary arteries, ductus arteriosus

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9
Q

Which cranial nerve is associated with each pharyngeal arch ?

A 
1- TriG (Skin of the face supplied by V1 V2 V3, muscles of mastication supplied by V3)
2- Facial 
3- Glossopharyngeal 
4- Superior laryngeal branch of vagus
5- Recurrent laryngeal branch of vagus
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10
Q

Identify the cartilaginous and bony elements associated with each pharyngeal arch.

A

1 (mandibular arch)- Malleus, Incus, Meckel’s cartilage, premaxilla, maxilla, zygomatic bone, and part of the temporal bone, mandible
2- (hyoid arch) Stapes, Styloid process, Stylohyoid ligament, lesser horn of hyoid bone
3- Greater horn of hyoid bone, Body of hyoid bone
4- Thyroid cartilage
6- Cricoid cartilage

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11
Q

State the origin of the mesenchyme making of the each arch.

A

Arches 1,2,3: Neural crest

Laryngeals: lateral plate mesoderm

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12
Q

Identify the origin of the mesenchyme making up each of the following:

  • Squamous part of temporal bone
  • Sphenoid
  • Parietal
  • Laryngeals
  • Petrous temporal
  • Occipitals
  • Hyoid
  • Nasal
  • Lacrimal
  • Zygomatic
  • Maxilla
  • Incisive
  • Mandible
A

NEURAL CREST

  • Nasal
  • Lacrimal
  • Zygomatic
  • Maxilla
  • Incisive
  • Mandible
  • Squamous part of temporal bone
  • Sphenoid
  • Frontal

PARAXIAL MESODERM

  • Petrous temporal
  • Occipitals
  • Parietal

LATERAL PLATE
-Laryngeals

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13
Q

Describe the fate of the second pharyngeal arch.

A

MUSCLES: Muscles of facial expression + Stylohyoid + Stapedius + Posterior belly of digastric
SKELETAL: Stapes + Styloid process + Stylohyoid ligament + Lesser horn of the hyoid + upper part body of hyoid

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14
Q

Describe the fate of the second pharyngeal arch.

A

MUSCLES: Stylopharyngeus
SKELETAL: Greater horn of the hyoid + lower part body of hyoid

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15
Q

Describe the fate of the fourth and sixth pharyngeal arches.

A

MUSCLES:

4th: All the muscles of the pharynx (except stylopharyngeus) + All the muscles of the palate (except TVP) + cricothyroid
6th: All the muscles of the larynx

SKELETAL 
Epiglottis 
Laryngeal cartilages 
Thyroid 
Cuneiform 
Corniculate 
Arytenoids
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16
Q

Describe the fate of each of the following in the formation of the head, neck and face:

  • Paraxial mesoderm
  • Lateral plate mesoderm
  • Neural crest
  • Ectodermal placodes
A

Paraxial mesoderm:
All voluntary muscles, arteries, neurocranium, meninges, dorsal skin

Lateral plate mesoderm:
Laryngeal cartilages; regional connective tissue

Neural crest (ventral flow around placodes and into arches):
Pharyngeal arch skeleton,
viscerocranium,
glandular connective tissue,
parts of the neurocranium, teeth, interact with ectodermal placodes for ganglia/sensory nerve formation

Ectodermal placodes:
with neural crest to form neurones of sensory ganglia
(V,VII, IX and X) also sensory apparatus (eyes/nose not shown)

17
Q

Distinguish between pharyngeal clefts, and pouches.

A

Pharyngeal clefts are external (ectoderm)
Pharyngeal pouches are internal, forming as invaginations of endoderm in the lateral walls of the pharynx

Each takes number from rostral arch

18
Q

Name the derivative of each pharyngeal pouch.

A

1 - Primitive tympanic cavity
2 - Palatine tonsil/tonsillar fissure
3 - Inferior Parathyroid gland and thymus (these cells migrate to become associated with thyroid gland)
4 - Parathyroid gland (superior) and Ultimobranchial body (parafollicular (C) cells of thyroid)

19
Q

Describe migration of the thymus, parathyroid glands and ultimobranchial body.

A
  • Both gland primordia (inferior parathyroid and thymus) lose their connection with the pharyngeal wall, and the thymus then migrates in a caudal and a medial direction, pulling the inferior parathyroid with it.
  • The parathyroid tissue of the third pouch finally comes to rest on the dorsal surface of the thyroid gland and forms the inferior parathyroid gland
  • When the parathyroid gland loses contact with the wall of the pharynx, it attaches itself to the dorsal surface of the caudally migrating thyroid as the superior parathyroid gland
20
Q

Identify possible abnormalities in migration of thymus, parathyroid glands.

A
  • Thymus can remain in neck (ectopic thymus)

* Inferior parathyroids can be variable in posi1on (ectopic parathyroid)

21
Q

Name the derivative of each pharyngeal cleft.

A
  • Dorsal part of the 1st cleft gives rise to the external auditory meatus and external part of the tympanic membrane. Deepest part gives rise to the tympanic membrane (ear drum)
  • 2nd arch proliferates and overgrows the 3rd and 4th arches
  • 2nd , 3rd and 4th clefts lose contact with the exterior forming cervical sinus
  • After losing connection with exterior the cervical sinus usually disappears.
22
Q

What are the commonest congenital diseases of the neck ?

A

Branchial cleft anomalies:
• Cysts (adults) and Fistulas/Sinuses (children), e.g. pre-auricular fistulas, lateral cervical cyst
• Anterior to sternocleidomastoid
• Diagnostic challenge presume cancerous until proven otherwise

23
Q

Describe molecular control of pharyngeal arch biogenesis.

A

Hox genes are key controllers of rostrocaudal pacerning in the head
• 3 streams of neural crest cells migrate mostly from segments of hindbrain
called rhombomeres
• Carry expression of homeodomain containing transcrip1on factor OTX and HOX genes to pharyngeal arches
• Provide guidance cues for sensory cranial nerves growing back from ganglia
• Guidance cues for motor elements of cranial nerves are currently unclear

Neural crest cells form skeletal elements of the arches under control of pouch endoderm signals
• Neural crest cells migrate into arch carrying (OTX/HOX) codes
• Neural crest cells respond to local patterns of endoderm signals (FGF, BMP, PAX, SHH)
• This then specifies mesenchymal expression pattern and arch characteristics

24
Q

What proportion of all birth defects do craniofacial defects make up ?

A

1/3 of birth defects

25
Q

What is the a common underlying problem in craniofacial defects ?

A

Many perturb signals directed at neural crest (migration, proliferation, differentiation)

26
Q

Identify examples of congenital craniofacial defects.

A
  • Treacher Collins Syndrome
  • Di George syndrome
  • Craniosynostosis syndromes (e.g. Apert and Crouzon syndromes)
  • Holoprosencephalon spectrum (inadequate SHH function)/Diporopus (excess SHH function)
27
Q

State the following for Treacher Collins Syndrome:

  • Genetic Inheritence Pattern
  • Underlying cause
  • Clinical features
A

TREACHER COLLINS SYNDROME

  • Autosomal dominant
  • First/second arch syndrome, with defective protein called Treacle (TCOF1 gene), and failure of formation/apoptosis of neural crest cells and migration into first and second pharyngeal arches
  • Abnormal eye shape, micrognathia, conductive hearing loss, underdeveloped zygoma, malformed ears
28
Q

State the following for Di George Syndrome:

  • Genetic Inheritence Pattern
  • Underlying cause
  • Clinical features
A

DI GEORGE SYNDROME (= congenital thymic hypoplasia):

  • Underlying cause: 22q11 deletion, third and fourth pharyngeal pouch syndrome where they fail to develop and thymus and parathyroids are effective
  • CATCH22 (cardiac abnormality esp Tetralogy of Fallot, abnormal faces, thymic aplasia, cleft palate, hypocalcaemia)

Abnormal faces frequently include cleft lip and palate, small jaw, small upper lip, eyes slanted upwards/downwards, low set abnormal folding ears

29
Q

Identify an example of molecule mechanism of craniofacial anomaly.

A

Sonic hedgehog (SHH) in craniofacial patterning – holoprosencephalon spectrum (i.e. “prosencephalon (the forebrain of the embryo) fails to develop into two hemispheres”)

SHH governs the width of facial features, so:

  • Inadequate SHH function: Narrowing and fusion (can be mild e.g. single median incisor, no philtrum, or severe e.g. cyclopia)
  • Excess SHH function: Widening structures duplicated (e.g. diprosopus i.e. craniofacial duplication)
30
Q

Define Craniosynostosis syndromes.

A

Craniosynostosis is defined as the premature fusion of one or more of the cranial sutures. It leads not only to secondary distortion of skull shape but to various complications including neurologic, ophthalmic and respiratory dysfunction.

Due to genetic causes, esp. mutations.

31
Q

Identify a possible cause of craniosynostosis.

A

FGFR2 and FGFR3 mutations and craniosynostosis

  • Mutations cause a gain of funcrion -> constitutive activation of receptors
  • FGF signalling activates bone calcification and reduces cell proliferation in the mesenchyme at the suture – generating premature fusions
  • Premature fusion (amongst other affects such as syndactaly) results
  • Mutations spontaneous in the paternal line increase with age
32
Q

Define Brachycephaly. What is this a feature of ?

A

“Shape of a skull shorter than typical for its species”

Feature of Craniosynostosis syndromes.

33
Q

Identify genes other than FGFRs which may be implicated in a Craniosynostosis syndrome.

A

TWIST transcription factor, EPHRIN-B1

MD4001 (74 decks)

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