Head and Neck Development

Question 1

Pharyngeal arches

Answer 1

• 5 arches (1, 2, 3, 4, and 6) separated by a pharyngeal cleft on the outside (ectoderm) and a pharyngeal pouch on the inside (endoderm)
• Each arch has an artery, a nerve, cartilage, and mesenchyme
• Each pharyngeal nerve (a cranial nerve) is mixed- it has motor and sensory components

Question 2

First pharyngeal arch

Answer 2

• Has 2 subdivisions: the maxillary process (innervated by V2) and the mandibular process (V3)
• The frontonasal process (FNP) is anterior midline of the first arch (but its own structure) and is innervated by V1 (opthalmic)

Question 3

Innervation of arches 2, 3, 4, and 6

Answer 3

• Arch 2 from VII
• Arch 3 from IX
• Both arches 4 and 6 from X

Question 4

Carniofacial muscles

Answer 4

• Derived from paraxial mesoderm, but not from somites
• Maintain their early innervation pattern once it is established
• Some migrate w/ the migrating arch, some do not
• But can always trace the arch a muscle rose from by its innervation

Question 5

Pharyngeal arch cartilage

Answer 5

• 1st arch cartilage: template for formation of mandible and maxilla
• 2nd arch cartilage: supporting elements for jaw
• 3rd-6th arch cartilage: supporting elements for neck and larynx

Question 6

Pharyngeal mesenchyme

Answer 6

• Mesoderm and neural crest origin
• Mesoderm is source of cranial vascular endothelium and striated muscle
• Neural crest is the source of the dura, most of the vascular smooth muscle
• Cranial bones (and vascular smooth muscle) come from both

Question 7

Formation of palate

Answer 7

• Mandibular processes fuse in the midline
• Maxillary processes do not fuse at first, but are initially separated by the distal end of the FNP- intermaxillary segment (FNP is origin of philtrum of upper lip)
• FNP forms the primary palate (anterior 4 incisors)
• The palatal shelves from the maxillary processes form the secondary palate
• The primary palate and the anterior portion of the secondary palate become ossified to form the hard palate
• The remaining non-ossified portion is the soft palate
• Cleft palate: failure in fusion btwn the palatal shelves
• Cleft lip: failure in fusion of intermaxillary segment w/ either or both maxillary processes

Question 8

Formation of nasal cavity

Answer 8

• First forms by invagination from the surface ectoderm
• Innervated by CN V
• Nasal septum grows down the roof of the cavity and fuses w/ the superior side of the palate to divide the cavity in 2

Question 9

Formation and innervation of the oral cavity

Answer 9

• Oropharyngeal membrane is the only location of a direct contact btwn ectoderm and endoderm (no mesoderm btwn)
• This transition of ectoderm to endoderm lies w/in the oral cavity, anterior to the 1st pouch
• This is also the transition of sensory innervation from CN V (ectoderm) to CN IX (endoderm)

Question 10

Formation and innervation of the tongue

Answer 10

• Anterior 2/3 of the tongue forms from tongue buds derived from the 1st arch (ectodermal, CN V)
• The 3rd arch displaces the 2nd arch to the outside, and then creates the posterior 1/3 of the tongue (endodermal, CN IX)
• The very posterior end of the tongue derived from the 4th arch (endodermal, CN X)
• These nerves are only for somatosensory axons, special sensory taste axons run through slightly different pathways
• Special sensory taste in the anterior 2/3 of the tongue is from CN VII (2nd arch)
• Special sensory taste in posterior 1/3 is the same as somatosensory (CN IX and CN X, from arch 3 and 4 respectively)

Question 11

Muscles of the tongue

Answer 11

• Derived from the anterior-most (occipital) cerivcal somites, which migrate into the tongue (not pharyngeal muscles- not from pharyngeal arches)
• Tongue muscles are innervated by CN XII prior to migration and carry the innervation w/ them

Question 12

Development of the ear 1

Answer 12

• External ear (auricle) forms from 3 pairs of swellings outside the 1st (mandibular) and 2nd arches
• Inner ear is derived from surface ectoderm called otic placode, which buds off and moves inward (now otic vesicle)
• Otic placode originates btwn arches 2 and 3 and thus is not pharyngeal
• The otic vesicle acquired CN VIII innervation, corresponding to its position btwn arches 2 and 3 (which are innervated by CN VII and IX, respectively)

Question 13

Development of the ear 2

Answer 13

• External auditory meatus is derived from the 1st pharyngeal cleft (ectoderm)
• The tympanic cavity and auditory tube and derived from the 1st pharyngeal pouch (endoderm)
• Ear drum forms at the interface of the 1st pouch and the 1st cleft, so it has both ectodermal (V3) and endodermal (IX) innervation
• Not related to oropharyngeal membrane, since the eardrum does contain mesoderm

Question 14

Development of the thyroid

Answer 14

• Forms from the dorsal surface of what will become the tongue, just behind the junction of the 1st and 3rd arches (thus is endodermal)
• It descends through the tongue and migrates to straddle the larynx
• The connection to the dorsal surface of the tongue is the thyroglossal duct, which atrophies as the thyroid reaches the larynx

Question 15

Fate of pharyngeal pouches and clefts

Answer 15

• 1st pouch and cleft are involved in ear development
• All other clefts are irrelevant
• 2nd pouch-> palatine tonsils
• 3rd pouch-> inferior parathyroids and thymus
• 4th pouch-> superior parathyroids and ultimobranchial bodies