8. Branchial Arches

Question 1

When do branchial arches first appear, and when are they differentiated to terminal structures?

Answer 1

Week 4; Week 7

Question 2

Describe the steps in development of the branchial arches

Answer 2

Development proceeds cranially to caudally

1. Neural Crest Migration (start of W4): neural crest cells migrate into developing arches - form cartilaginous rods to make skeleton and c.t. of face
2. Terminal Fate of Arches (during W5): arch 2 enlarges and overgrows posterior arches - closing grooves of arch 2,3,4/6 - entrapped grooves condense to CERVICAL SINUS - obliterates by W7

Question 3

Adult Derivatives of Ectoderm, Endoderm, Ectomesenchyme, Mesoderm

Where do the bones of the face develop from?

Answer 3

Ectoderm: Epidermis and sensory neurons of epibranchial ganglia
Endoderm: epithelial lining of pharynx, taste buds, thyroid, parathyroid, thymus
Ectomesenchyme: sketal bone, c.t. of arches
Mesoderm: musculature, endothelial cells

Facial bones: ARCH 1 MESENCHYME (intramembranous ossification)
Mandible: grows around (not from) Arch 1 cartilage rod (Meckel’s cartilage)
Also maxilla, zygoma, palatine, pterygoid plates of sphenoid, squamous/tympanic parts of temporal

Question 4

Derivatives of each arch cartilage

Answer 4

1: Maeckel’s Cartilage - malleus, incus, sphenomandibular ligament (skull base to mandible)
2. Reichert’s Cartilage - stapes, styloid process, lesser cornu/upper body of hyoid bone, stylohyoid ligament
3. Greater cornu/lower body of hyoid bone
4/6: laryngeal cartilages

Question 5

Derivatives of each arch muscle

Answer 5

1: muscles of mastication, mylohyoid, ant belly digastric, tensor tympani, tensor veli palatini (soft palate)
2: SFM (migrates around Arch 1 muscles), stapedius, stylohyoid, post belly digastric
3: stylopharyngeus (tiny, non-functional)
4/6: intrinsic laryngeal muscles, pharyngeal constrictors, striated muscles of esophagus, cricothyroid m., levator veli palatini, palatoglossus

Question 6

Derivatives of each arch artery

Answer 6

1,2: disappears
3: part of common/internal carotids
4: proximal R subclavian and part of aortic arch
6: Right side - degenerates - why Right recurrent layrngeal nerve hooks under Arch 4’s proximal r. subclavian a.
Left side - persists as Ductus Arteriosus and Pulmonic Trunk - why L recurrent laryngeal nerve hooks under Arch 6’s ligamentum arteriosum

Question 7

Derivatives of each arch external groove

Answer 7

1: external acoustic meatus (outer ear) - ant part is Arch 1, post part is cephalic part of Arch 2
Other arches become cervical sinus and are obliterated

Question 8

Derivatives of each arch internal pouch

Answer 8

1: inner layer of TM, auditory tube epithelial lining, middle ear cavity, mastoid air cells
2: palatine tonsils (crypts and clefts)
3: inferior parathyroid gland, thymus epithelium
4/6: superior parathyroid gland, ultimobranchial body - eventually incorporated into thyroid gland

Question 9

Derivatives of each arch nerve (+ pretrematic) and remaining sensory innervation

Answer 9

1: CN V (Pretrematic: Opthalmic Division, V1)
2: CN VII (Pretrematic: Chorda tympani)
3: CN IX (Pretrematic: Tympanic N.)
4: CN X

Sensory:
External Surface: CN 7,9,10 innervate small sensory zones near external ear
Internal Surface: CN 7 - small bit on soft palate
CN 5,9 - sensory to tongue

Question 10

Branchial cyst vs. sinus vs. fistula vs. vestiges

Answer 10

Cyst: cystic remnants of 2nd pharyngeal groove, develop along ant border of SCM

Sinus: openings along ant border of SCM due to failure of 2nd pharyngeal groove/cervical sinus to obliterate

Fistula: Canal opening internally to tonsillar sinus & externally onto side of neck; persistence of 2nd pharyngeal groove AND pouch

Vestiges: cartilaginous/bony remnants of branchial arch cartilages which do not fully transform/disappear, ant to inf third of SCM muscle

Question 11

Preauricular Pits/Dimples/Cysts/Sinuses

Answer 11

Abnormal transformation of auricular hillocks, NOT related to first cleft sinuses

Question 12

DiGeorge Syndrome

Answer 12

Failure of 3/4th arch pouches to differentiate to thymus/parathyroid glands

Question 13

First (Mandibular) Arch Syndrome

Answer 13

Malformations of mandible, palate, ears

Insufficient migration of neural crest cells into 1st arch during W4

Question 14

Treacher-Collins Syndrome (what is it, signs)

Answer 14

Zygomatic Underdevelopment, Downward slanting palpebral fissures, lower eyelid defects, deformed external ears, abnormalities of middle/inner ears

Question 15

Pierre-Robin Syndrome (what is it)

Answer 15

Mandibular Underdevelopment, Cleft Palate, Eye/Ear defects

Question 16

Fetal Alcohol Syndrome (cause, signs)

Answer 16

Ethanol teratogenicity - promotes spontaneous cartilage formation
Short palpebral fissures, flat midface, short nose, indistinct philtrum, thin upper lip