pharyngeal arches

Question 1

bone and cartilage

Answer 1

o Bones which develop include facial bones as well as bones and cartilage in the visceral part of the neck.

o 1st arch
• Meckel’s cartilage:
• Proximal parts become Malleus and Incus bones by endochondral ossification.
• Middle part becomes sphenomandibular ligament.
• Distal part, the Mandibular part, induces the intramembraneous ossification around the cartilage, and then slowly disappears as the lower jaw develops.
meckels cartilage induces bone and then produces a tunnel for VAN to run through –> maxillary canal

o 2nd arch
• Reichert’s cartilage forms stapes, styolid process, stylohyoid ligament and lesser cornu of hyoid bone.

o 3rd arch
• Cartilage becomes the rest of the hyoid bone.

o 4th and 6th arches:
• Cartilage becomes cartilages of the larynx (thyroid and cricoid cartilage, etc.)
o Upper & lower Jaw bones are all membranous bone: maxilla, mandible

Question 2

aortic arch derivatives

Answer 2

o AA 1: degenerates.
o AA 2: degenerates.
o AA 3: Carotid arteries. (internal)
o AA 4: Left-arch of aorta (tiny sliver) and right-subclavian.
o AA 6: Pulmonary arches.
o These aortic arches are in the mesoderm of the pharyngeal arches.

Question 3

cranial nerve derivatives

Answer 3

•CN come from the embryonic brainstem, which is segmented into rhombomeres
ganglia of cranial nerves comes from the neural crest and ectodermal placodes
PA1 max: max. division of trigeminal n
• Anterior 2/3 of the tongue (lingual N.)
• Muscles of tongue develop later (hypoglossal N innervation)
• PA1 mand.: mandibular division of the trigeminal n.
• PA 2: facial n.
• PA 3: glossopharyngeal N (posterior 1/3 of tongue)
• PA 4/6: vagus N. (larynx and pharynx)

Question 4

muscle derivatives

Answer 4

• PA 1: muscles of mastication
• PA 2: muscles of facial expression
• PA3: stylopharyngeus M
• PA 4: cricothyroid and mm of pharynx
• PA 6: muscles of the larynx

Question 5

pharyngeal pouch derivatives

Answer 5

Pharyngeal pouch 1: middle ear and auditory tube. (between arches 1 and 2)
closing between arches 1 and 2=tympanic membrane
PP 2: Palatine tonsil
PP 3: Parathyroid, thymus
PP 4/5: Parathyroid, C-cells of thyroid
Pharyngeal groove 1: external auditory meatus
Behind 2nd arch, pharyngeal grooves fuse into a cervical sinus, which disappears (occasionally forms cyst)–>”branchial cleft cyst”
usually benign but can sometimes become infected