Week 15 Head/Neck Development

Question 1

What muscle tissues do the pre-otic somitomeres become?

Answer 1

differentiate into the muscle tissues of the orbit.

Question 2

What 3 nerves are derived from Preotic Somitomeres?

Answer 2

Oculmotor CN III
Trochlear CN IV
Abducens CN VI

Question 3

What tissues and nerve are derived from Postotic somites?

Answer 3

tissues of the tongue.

Innervated by Hypoglossal nerve XII

Question 4

What nerve is derived from Somitic mesenchyme?

Answer 4

Accessory Spinal nerve CN XI

Question 5

Describe placodes generally.

Name each of the 3 placodes and what nerve/special sense they derive.

Answer 5

Placodes are thickening of the ectoderm in specific locations (called placodes) initiate the differentiation into the special sensory functions of olfaction, vision, and hearing/balance.

1. Olfactory placode: smell CN I
2. Optic placode: vision CN II
3. Otic placode: inner ear organs CN VIII

Question 6

Recall the order of the 6 Pharyngeal arches (these carry mixed innervation; motor and sensory)

Answer 6

1. CN V Trigeminal nerve
2. CN VII Facial nerve
3. CN IX Glossopharyngeal nerve
4. CN X Vagus nerve
5. Degrades
6. CN X Vagus nerve

• Everything these nerves innervate are derivatives of their same arches.
• Each arch has an artery, nerve, and cartilage/bone formed by invading neural crest cells.

Question 7

Describe the Pharyngeal Grooves and Pharyngeal Pouches

Answer 7

Pharyngeal Groove (External): Ectoderm
Pharyngeal Pouch (Internal): Endoderm

Question 8

Describe derivatives of Arch 1

Answer 8

1. Arch 1: The first arch derives the jaw complex, this includes the Maxilla and Mandible.

i. Nerve: CN V
ii. Muscles: Muscles of mastication, tensor tympani, tensor veli palatini, mylohyoid, and anterior belly of digastric
iii. Artery: aspects of external carotid and maxillary arteries
iv. Cartilage: Meckel cartilage –part of ear development, specifically the Incus and Malleus bones. Initially forming the early blueprint for mandibular development, this aspect of the mandibular primordium disappears as the mandible ossifies from surrounding mesenchyme via intermembranous ossification, leaving only the resulting sphenomandibular ligament.
v. Bone: It begins to differentiate into mandibular and maxillary prominances
1. Maxillary prominence: forms the maxilla, the zygomatic, and part of the vomer
2. Mandibular prominence: forms the mandible and squamous portion of the temporal bone

Question 9

Describe derivatives of Arch 2

Answer 9

i. Nerve: CN VII
ii. Muscles: Muscles of facial expression, stapedius, stylohyoid, and posterior digastric.
iii. Artery: stapedial arteries
iv. Cartilage: Reichert cartilage –part of ear development, results in the stapes and styloid process, and the lesser cornu and superior body of the hyoid. During the development, the stylohyoid ligament grows between the lesser cornu and styloid process as the distance between the two increases.

Question 10

Describe derivatives of Arch 3

Answer 10

i. Nerve: CN IX
ii. Muscles: Stylopharyngeus
iii. Artery: common and internal carotid arteries
iv. Cartilage: Greater cornu and inferior aspect of the hyoid body

Question 11

Describe derivatives of Arch 4/6

Answer 11

i. Nerve: CN X -4 forms the bulk of the vagus while 6 is responsible for the recurrent laryngeal branch.
ii. Muscles: From 4 are the levator veli palatini, pharyngeal constrictors, and the cricothyroid. From 6 are the other intrinsic muscles of the larynx which are innervated from the recurrent laryngeal branches.
iii. Artery:
1. 4th right –right subclavian artery, left –aortic arch
2. 6th right –right pulmonary artery, left –left pulmonary artery and ductus arteriosus. The difference in sides of the 6th arch arteries demonstrates why the recurrent laryngeal nerves differ as well, in addition to why they get pulled down in the first place.
iv. Cartilage: the components from 4 and 6 fuse to form the laryngeal cartilages. The epiglottis can from the mesenchyme of the 3rd 4th arches.

Question 12

1. What becomes of the 1st groove. The other grooves?

2. Potential complications?

Answer 12

1. 1st groove becomes the external acoustic meatus, the rest become gills…. And in humans obliterate.
2. Branchial Cysts: occasionally these grooves do not disappear and cervical or branchial cysts can form, filling with fluid, or releasing the fluid to the external surface.

Question 13

What does the 1st pouch become?

Answer 13

1. The 1st pouch becomes the tympanic membrane affiliated with the first groove externally. The endoderm around this location lines the tympanic cavity including the auditory and mastoid antrum and is innervated by the third arch nerve (CN IX).

Question 14

What does the 2nd pouch become?

Answer 14

2. The 2nd pouch is obliterated with the exception of the palatine tonsillar fossa between the palatoglassal and palatopharyngeal arches, eventually contributing to the palatine tonsil.

Question 15

What does the 3rd pouch become?

Answer 15

3. The 3rd pouch eventually differentiates into the inferior parathyroid glands and the thymus.

Question 16

What does the 4th pouch become?

Answer 16

4. The 4th pouch forms the superior parathyroid glands (the inferior follow the thymus from the 3rd pouch)

Question 17

Describe Pharyngeal arch syndrome.

Answer 17

occasionally the cartilages and other derivatives that degrade under normal developmental patterns fail resulting in congenital abnormalities of the eyes, ears, mandible and palate, generally due to insufficient migration of neural crest cells into the arches during the 4th week.

i. Treacher Collins syndrome: underdevelopment of the zygomatic bones including abnormalities to the external middle and inner ear.
ii. Pierre Robin sequence: general disorder including under development of the mandible, cleft palate, and defects of the eye and ear

Question 18

The thyroid is an endodermal outgrowth from what structure?
Describe its descent.
Identify potential pathologies with this process.

Answer 18

2nd pharyngeal arch
Descends in the neck, passing anteriorly to hyoid and larynx.
If it fails to descend properly it can remain in the oropharynx on the posterior tongue (lingual thyroid).

Question 19

Describe the development of the tongue.

Answer 19

1. A series of lingual swellings begin to diverticulate out from ventromedial parts of the first two pairs of arches forming the tongue mucosa (hence why CN V and CN VII both provide aspects of innervation to the anterior 2/3 of the tongue)
2. The copula and hypopharyngeal eminence forms below the lingual swellings from the 2nd, 3rd, and 4th arch pairs, hence why innervation inferiorly arises from IX and X in the posterior 1/3 down to epiglottis. The fusion of the copula and hypopharyngeal eminence from the terminal sulcus. The crossing of IX to innervate the vallate papillae appears to migrate later in development.
3. The muscles of the tongue arise posteriorly from the postotic occipital somites and migrate into the tongue, pulling in CN XII.

Question 20

Describe the general development of the salivary glands.

Answer 20

connective tissue and innervation of the glands develops from the neural crest of the relevant arch while the gland tissue itself develops through proliferation of the oral epithelium.

1. Parotid gland –develop as buds arising from oral ectodermal lining from the third arch neural crest (hence, CN IX)
2. Submandibular and sublingual. Similar trajectory, via the second arch neural crest.