Embryology - Pharyngeal Apparatus Flashcards
the pharyngeal apparatus - overview
*basis for the formation of the face, ear, jaw, neck, and pharynx
*several components:
-5 arches
-4 clefts
-4 pouches
-4 membranes
pharyngeal apparatus - arches
*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
pharyngeal apparatus - clefts
*external invaginations (recesses) of the body surface between neighboring arches
*made from ectoderm
*only the FIRST cleft remains at birth → lining of ear canal
pharyngeal apparatus - pouches
*internal invaginations of the pharynx between neighboring arches
*made from endoderm
*give rise to epithelial linings and glands
pharyngeal apparatus - membranes
*sites where cleft ectoderm and pouch endoderm meet between neighboring arches
components of the pharyngeal apparatus from outside to inside, & their origins: CAP
*C - cleft = ectoderm (outermost)
*A - arches = mesoderm + neural crest cells
*P - pouches = endoderm (innermost)
pharyngeal arches & derivatives - overview
*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
first pharyngeal arch
*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)
second pharyngeal arch
*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: muscles of facial expression, stapedius, stylohyoid, posterior belly of the digastric
*innervation: CN VII, facial nerve
third pharyngeal arch
*stylopharyngeus muscle
*parts of hyoid bone
*innervation: CN IX, glossopharyngeal nerve
fourth & sixth pharyngeal arches
*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
pharyngeal pouches & derivatives - overview
MNEMONIC: “Ear, Tonsils, Bottom-To Top” = middle ear (1), tonsils (2), inferior parathyroids & thymus (3), superior parathyroids (4)
*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
mnemonic for pharyngeal pouches
*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
pharyngeal clefts & derivatives - overview
*cleft 1:
-external auditory meatus
*clefts 2-4: regress
pharyngeal membranes & derivatives - overview
*membrane 1:
-tympanic membrane of ear
*membranes 2-4: obliterated
development of ear based on pharyngeal apparatus
*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
development of tongue based on pharyngeal apparatus
*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
development of thyroid gland based on pharyngeal apparatus
*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
aortic arch arteries associated with pharyngeal arches
*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
branchial cleft cysts / cervical cysts
*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
thyroglossal duct cysts
*formed when the thyroglossal duct fails to completely close (failure of thyroglossal duct atrophy)
*cyst moves with tongue elevation
*found in midline of neck
cleft lip, cleft palate, and related conditions
*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
*cleft LIP: failure of fusion of intermaxillary segment with maxillary process
*cleft palate: failure of fusion of the 2 lateral palatine shelves or failure of fusion of lateral palatine shelf with nasal septum and/or primary palate
Treacher Collins Syndrome
*a disorder of the 1st and 2nd pharyngeal arches / neural crest cells
*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
DiGeorge Syndrome - overview
*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
DiGeorge Syndrome - clinical features
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