aortic arch and branchial apparatus

Question 1

What are the aortic arches? What are the aortic arch derivatives?

Answer 1

aortic arches arise in sequence from the aortic sac. they begin symmetrically, but don’t necessarily stay symmetrical
1st aortic arch: maxillary artery
2nd aortic arch: hyoid and stapedial artery
3rd aortic arch: common carotid and proximal internal carotid
4th aortic arch: on left, aortic arch btw the left common carotid and the left subclavian. on right, forms part of the right subclavian
6th aortic arch: proximal parts of the pulmonary artery. on the left, also forms part of the ductus arterios.s

Question 2

embryology of the recurrent laryngeal nerves

Answer 2

on the right, the recurrent laryngeal nerves (which first descend to supply the 6th branchial arch and then move back up to go the larynx) loops around the right subclavian artery (just next to where it comes off of the brachiocephalic trunk). on the left, the recurrent laryngeal can’t move back up because of the ductus arteriosus, so it hooks around the ductus arteriosus (btw the pulm artery and the aortic arch)

Question 3

What are pharyngeal arches, pouches, and clefts? When do they appear?

Answer 3

appear in week 4-5
pharyngeal arches: bars of tissue derived from mesoderm and neural crest that are separated by deep pharyngeal clefts. Clefts are derived from ectoderm and are also called branchial grooves.
pharyngeal pouches are derived from endoderm
CAP covers inside to outside: clefts are are ectoderm, arches are mesoderm and neural crest, pouches are endoderm.

Question 4

What are the branchial cleft derivatives? Relevant pathology?

Answer 4

first cleft becomes the external auditory meatus
2-4 form tempoary cervical sinuses, which are obliterated by proliferation of the 2nd arch mesenchyme.
a persistent cervical sinus is a branchial cleft cyst in the lateral neck.

Question 5

1st branchial arch derivatives and pathology

Answer 5

branchial arches have cratilage, muscles, and nerves.

nerves: CN V2 and V3.
cartilage: meckel’s cartilage: mandible, malleus/inucs, sphenomandibular ligament
muscles: muscles of mastication (temporalis, masseter, lateral and medial pterygoids), mylohyoid, anterior belly of digastric, tensor tympani (dampens sounds from chewing) and tensor veli palatini (tenses the soft palate)
abnormality: treacher collins syndrome: 1st arch neural crest fails to migrate. causes mandibular hypoplasia and facial abnormalities, as well as hearing and/or vision loss in some patients. usually autosomal dominant

Question 6

2nd branchial arch derivative and pathology

Answer 6

nerve: CN VII
this is the reichert cartilage: forms the stapes, styloid process, lesser horn of hyoid, stylohyoid ligament.
forms the muscles of facial expression, stapedius, stylohyoid, platysma, belly of digastric
congenital pharyngocutaneous fistula: persistence of cleft and pouch causes a fistula btw the tonsillar area and the lateral neck

Question 7

3rd branchial arch derivatives and pathology

Answer 7

nerve: CN IX (stylopharyngeus)
cartilage: greater horn of the hyoid
muscles: stylopharyngeus
arches 3 and 4 make the posterior 1/3 of the tongue

Question 8

4-6 branchial arch derivatives

Answer 8

5 makes no major contributions
arch 4: CN X superior laryngeal branch (swallowing only)
arch 6: CN X recurrent laryngeal branch (speaks)
cartilage: thyroid, cricoid, arytenoids, corniculate, cuneiform
muscles: 4th arch does pharyngeal constrictors, cricothyroid, levator veli palatini
6th arch does all intrinsic muscles of the larynx except the cricothyroid.

Question 9

derivatives of the 1st branchial pouch

Answer 9

develops into the middle ear cavity, eustachian tube, and mastoid air cells
1st pouch contributes to the endoderm lined structures of the ear
(ear, tonsils, bottom to top)

Question 10

derivatives of the 2nd branchial pouch

Answer 10

epithelial lining of the palantine tonsil

Question 11

derivatives of the 3rd branchial pouch

Answer 11

dorsal wings: develop into the inferior parathyroids. ventral wings develop into the thymus
(3 structures: thymus and right and left inferior parathyroids)
3rd pouch derivatives end up below the 4th pouch derivatives.

Question 12

derivatives of the 4th branchial pouch

Answer 12

dorsal wings develop into the superior parathyroids

Question 13

pathology of the 3rd and 4th branchial pouch

Answer 13

DiGeorge syndrome: T cell deficiency from thymic aplasia and hypocalcemia from failure of parathyroid development. associated with cardiac defects (conotruncal abnormalities)

Question 14

MEN2A

Answer 14

mutation of germline RET cells that manifests as

1. parathyroid tumor (derived from the 3rd and 4th pharyngeal pouches)
2. medullary thyroid cancer from the parafollicular cells (associated with 4th and 5th pharyngeal pouches
3. pheochromocytomas (tumors of the adrenal medulla)

Question 15

cleft lip and cleft palate

Answer 15

cleft lip: failure of the maxillary processes to fuse with the median nasal processes (primary palate formation)
cleft palate: failure of the lateral palantine processes to fuse with each other, the nasal septum, or the median palatine process (secondary palate formation).