Embryology - Pharyngeal Apparatus

Question 1

the pharyngeal apparatus - overview

Answer 1

*basis for the formation of the face, ear, jaw, neck, and pharynx
*several components:
-5 arches
-4 clefts
-4 pouches
-4 membranes

Question 2

pharyngeal apparatus - clefts

Answer 2

*made from ectoderm
*only the FIRST cleft remain at birth → lining of ear canal

Question 3

pharyngeal apparatus - arches

Answer 3

*made from mesoderm & neural crest cells
*gives rise to muscles, cartilage, bones, and blood vessels

Question 4

pharyngeal apparatus - pouches

Answer 4

*made from endoderm
*give rise to epithelial linings and glands

Question 5

pharyngeal arches & derivatives - overview

Answer 5

*arch 1:
-innervated by cranial nerve V (trigeminal)
-gives rise to lower 2/3 of face and part of the ear

*arch 2:
-innervated by cranial nerve VII (facial)
-gives rise to upper neck, part of the ear, and face muscles

*arch 3:
-innervated by cranial nerve IX (glossopharyngeal)
-gives rise to hyoid bone & cartilage and stylopharyngeus

*arches 4 & 6:
-innervated by cranial nerve V (vagus)
-give rise to larynx

Question 6

pharyngeal pouches & derivatives - overview

Answer 6

*pouch 1:
-middle ear (tympanic cavity)
-eustachian tube
-mastoid air cells

*pouch 2:
-palatine tonsils

*pouch 3:
-inferior parathyroids
-thymus

*pouch 4:
-superior parathyroids
-parafollicular (C) cells of thyroid

Question 7

pharyngeal clefts & derivatives - overview

Answer 7

*cleft 1:
-external auditory meatus

*clefts 2-4: regress

Question 8

pharyngeal membranes & derivatives - overview

Answer 8

*membrane 1:
-tympanic membrane

*membranes 2-4: obliterated

Question 9

development of ear based on pharyngeal apparatus

Answer 9

*external ear - derived from arches 1 & 2
*external auditory meatus - derived from cleft 1
*auditory tube - derived from pouch 1
*malleus & incus - derived from arch 1
*stapes - derived from arch 2

note - inner ear develops from ectoderm

Question 10

development of tongue based on pharyngeal apparatus

Answer 10

*anterior 2/3 of tongue - arch 1 (CN V)
*posterior 1/3 of tongue - arch 3 (CN IX)
*root of tongue - arch 4 (CN X)

note - arch 2 briefly associates with tongue, which is how CNVII carries taste sensation

Question 11

development of thyroid gland based on pharyngeal apparatus

Answer 11

*thyroid develops from tongue mesoderm and migrates to anterior throat through thyroglossal duct (not from pharyngeal apparatus)
*parafollicular (C) cells result from pouch 4

Question 12

aortic arch arteries associated with pharyngeal arches

Answer 12

*pharyngeal arch 1 → aortic arch 1 → MAXILLARY ARTERY
*pharyngeal arch 2 → aortic arch 2 → HYOID & STAPEDIAL ARTERIES
*pharyngeal arch 3 → aortic arch 3 → INTERNAL CAROTID ARTERY
*pharyngeal arch 4 → aortic arch 4 → R SUBCLAVIAN ARTERY & AORTA
*pharyngeal arch 6 → PULMONARY ARTERY & DUCTUS ARTERIOSIS

Question 13

branchial cleft cysts / cervical cysts

Answer 13

*result from persistence of pharyngeal grooves/clefts 2-4, which create the cervical sinus
*symptoms include:
-small pits, lumps, or skin tags at either side of the neck or just below the collarbone
-fluid drainage from a pit on the neck

Question 14

thyroglossal duct cysts

Answer 14

*formed when the thyroglossal duct fails to completely close (failure of thyroglossal duct atrophy)
*cyst moves with tongue elevation
*found in midline of neck

Question 15

cleft lip, cleft palate, and related conditions

Answer 15

*normal palate formation: fusion of the two medial nasal prominences in the midline (fusion driven via growth of the maxillary prominences)
*failure of this fusion results in some variation of cleft lip/palate

Question 16

Treacher Collins Syndrome

Answer 16

*a disorder of the 1st and 2nd pharyngeal arches
*mutation in gene TCOF1
*autosomal dominant; high penetrance
*features:
-hypoplasia of the zygomatic bones, maxilla, and mandible
-downslanting palpebral fissures
-coloboma of the microtia
-cleft palate, conductive hearing loss
-no intellectual disability

Question 17

DiGeorge Syndrome - overview

Answer 17

*caused by a deletion on the long arm (“q” arm) of chromosome 22
*leads to hypoplasia of the 3rd and 4th pharyngeal arches and pouches
*consequential effect on 1st and 2nd arches leading to cleft lip and palate

Question 18

DiGeorge Syndrome - clinical features

Answer 18

CATCH 22:
C - cardiac defects (VSD, tetralogy of fallot, truncus arteriosus, interrupted aortic arch)
A - abnormal facies (widely-spaced eyes, small jaw, low-set ears)
T - thymic hypoplasia
C - cleft lip/cleft palate
H - hypoparathyroidism / hypocalcemia
22 - chromosome 22