Embryology - pharyngeal arch notes

Pharyngeal arches
= branchial arches
Transient structure
Component of face and neck
Ventro-lateral side
Conserved in all vetebrate embryo
5 pairs
1, 2, 3, 4, 6
5th arch degenerates immediately
Appear at 4th and 5th week of gestation, rostral -caudal order
1st --->maxillary arch +mandibular arch
Flatten and fuse later
Pharyngeal clefts/grooves
(indentation outside)(ectoderm)(4 pairs)
Pharyngeal pouches
(indentations inside )(endoderm)(4 pairs)
Pharyngeal clefts
1st Pharyngeal clefts = between 1st and 2nd arches
…
2nd Pharyngeal pouch= between 2nd and 3rd arches
Component
1. Epithelium (endoderm +ectoderm)
i. Endoderm
1. Wall of pharyngeal gut
2. Rostral end of the foregut endoderm
3. Internal surface of pharyngeal arches (pharyngeal pouch)
4. Becomes epithelial lining of pharynx and several glands in adults
ii. Ectoderm
1. external of pharyngeal arches
2. Just outside of the arch---> neurogenic placode---> sensory neurons
Ex. Trigeminal placode (t)---> CN V
Geniculate placode (g)---> CN VII
Petrosal placode (p)---> CNIX
Nodose placode (n) --->CN X
Epibranchial placodes
2. Mesenchyme
 Neural crest-derived mesenchyme
 Ectomesenchyme/ mesectoderm
 Cranial NCC (neural crest cell)
 From forebrain, midbrain, and hindbrain
 Migrate in three streams from midbrain, and
hindbrain
 Hind brain is organized into 8 segments -->
rhobomeres (R)
 NCC from
 Mb, R1, R2 ---> 1st pharyngeal arch