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 - arches

Answer 2

*solid structures, each made from mesoderm & neural crest cells, containing an artery, cartilage, muscle, and a nerve
*gives rise to muscles, cartilage, bones, and blood vessels

Question 3

pharyngeal apparatus - clefts

Answer 3

*external invaginations (recesses) of the body surface between neighboring arches
*made from ectoderm
*only the FIRST cleft remains at birth → lining of ear canal

Question 4

pharyngeal apparatus - pouches

Answer 4

*internal invaginations of the pharynx between neighboring arches
*made from endoderm
*give rise to epithelial linings and glands

Question 5

pharyngeal apparatus - membranes

Answer 5

*sites where cleft ectoderm and pouch mesoderm meet between neighboring arches

Question 6

components of the pharyngeal apparatus from outside to inside, & their origins: CAP

Answer 6

*C - cleft = ectoderm (outermost)
*A - arches = mesoderm + neural crest cells
*P - pouches = endoderm (innermost)

Question 7

pharyngeal arches & derivatives - overview

Answer 7

*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 8

first pharyngeal arch

Answer 8

*most cranial arch
*forms: lower 2/3 of face, oral & nasal cavities, and parts of the ear
*bones: maxilla, zygomatic bones, 2 middle ear bones (malleus & incus), mandible, squamous portion of temporal bone
*muscles: muscles of mastication (temporalis, masseter, medial and lateral pterygoids), tensor tympani, tensor veli palatini
*innervation: CN V, trigeminal nerve (maxillary process by CN V2, mandibular process by CN V3)

Question 9

second pharyngeal arch

Answer 9

*aka hyoid arch
*forms the upper neck
*bones: styloid process of temporal bone, one middle ear bone (stapes), parts of hyoid bone, stylohyoid ligament
*muscles: stapedius stylohyoid, posterior belly of the digastric
*innervation: CN VII, facial nerve

Question 10

third pharyngeal arch

Answer 10

*stylopharyngeus muscle
*parts of hyoid bone
*innervation: CN IX, glossopharyngeal nerve

Question 11

fourth & sixth pharyngeal arches

Answer 11

*forms laryngeal cartilages (thyroid, cricoid arytenoid, corniculate, and cuneiform cartilages)
*pharyngeal muscles (constrictors) and other larynx structures
*innervation: CN X, vagus nerve
-superior laryngeal nerve innervates 4th arch-derived laryngeal muscles
-recurrent laryngeal nerve innervates 6th arch-derived laryngeal muscles

Question 12

pharyngeal pouches & derivatives - overview

Answer 12

*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 13

mnemonic for pharyngeal pouches

Answer 13

*ear, tonsils, bottom-to-top:
1st pouch = ear
2nd pouch = tonsils
3rd pouch = “bottom” (inferior parathyroids); “to” = thymus
4th pouch = “top” (superior parathyroids); aka parafollicular (C-cells) of thyroid

Question 14

pharyngeal clefts & derivatives - overview

Answer 14

*cleft 1:
-external auditory meatus

*clefts 2-4: regress

Question 15

pharyngeal membranes & derivatives - overview

Answer 15

*membrane 1:
-tympanic membrane of ear

*membranes 2-4: obliterated

Question 16

development of ear based on pharyngeal apparatus

Answer 16

*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 17

development of tongue based on pharyngeal apparatus

Answer 17

*anterior 2/3 of tongue - arch 1 (CN V provides general sensation; taste carried by chorda tympani, a branch of CN VII)
*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 18

development of thyroid gland based on pharyngeal apparatus

Answer 18

*thyroid develops from tongue mesoderm and migrates to anterior throat through thyroglossal duct (not from pharyngeal apparatus)
*thyroglossal duct normally atrophies once thyroid reaches its permanent location
*parafollicular (C) cells result from pouch 4

Question 19

aortic arch arteries associated with pharyngeal arches

Answer 19

*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 20

branchial cleft cysts / cervical cysts

Answer 20

*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 21

thyroglossal duct cysts

Answer 21

*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 22

cleft lip, cleft palate, and related conditions

Answer 22

*normal palate formation: fusion of the two medial nasal prominences in the midline (fusion driven via growth of the maxillary prominences)
*failure of fusion of 2 medial nasal prominences results in some variation of cleft lip/palate

Question 23

Treacher Collins Syndrome

Answer 23

*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 24

DiGeorge Syndrome - overview

Answer 24

*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 25

DiGeorge Syndrome - clinical features

Answer 25

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