Head and Neck Development (Week 1--Trelease)

Question 1

When do the face and important structures of the head and neck develop from pharyngeal arch tissues?

Answer 1

Between 4th and 12th embryonic weeks

Question 2

What does the pharynx look like during the 4th and 5th week of development?

Answer 2

Primordial pharynx is bounded laterally by pharyngeal (branchial) arches

Question 3

Pharyngeal apparatus

Answer 3

Arches, grooves/clefts (separate arches externally), pouches (separate arches internally) make up the pharyngeal apparatus

Pharyngeal apparatus is transformed into face, lips, jaws, tongue, palate, pharynx, etc

Question 4

Pharyngeal arch components

Answer 4

Ectoderm

Nerve

Cartilage

Artery

Endoderm

Clefts (grooves)

Pouches

Question 5

What do neural crest cells do regarding pharyngeal arches?

Answer 5

Neural crest cells migrate into each arch and are the source of connective tissue components (cartilage, bone, ligaments, nerves)

Question 6

What arteries are derived from what pharyngeal arches?

Answer 6

Arch 1: mostly disappears, maxillary artery and part of external carotid artery remains

Arch 2: mostly disappears, hyoid artery and stapedial artery remain

Arch 3: forms common carotid and initial internal carotid artery

Arch 4: forms right subclavian artery, left arch of aorta

Question 7

Neural crest cells give rise to which ganglia and peripheral nerves?

Answer 7

Trigeminal ganglion

CN V (V1, V2, V3)

CN VII (facial nerve)

CN IX (glossopharyngeal nerve)

CN X (vagus nerve)

Question 8

Neural crest cells from which pharyngeal arches form which cargilages and eventually which bones?

Answer 8

Arch 1: Meckel’s cartilage –> maxilla, mandible, zygoma, part of temporal bone, malleus, incus, several ligaments

Arch 2: Reichert’s cartilage –> styloid process, upper part of body of hyoid bone, lesser horn of hyoid bone

Arch 3: cartilage –> lower part of body of hyoid bone, greater horn of hyoid bone

Arch 4-6: cartilage –> laryngeal cartilages (thyroid, cricoid, arytenoid, corniculate, cuneiform)

Question 9

What do the pharyngeal arch grooves/clefts and pouches (internal) turn into?

Answer 9

Arch 1: tubotympanic recess –> auditory (Eustachian) tube, middle ear cavity, mastoid sinus, inner part of tympanic membrane

Arch 2: tonsillar fossa and palatine tonsil (+ migrating mesoderm)

Arch 3: inferior parathyroid glands and thymus

Arch 4-6: superior parathyroid glands and ultimobranchial body, which fuses with thyroid and forms parafollicular C (calcitonin secreting) cells (lower arch 4)

Question 10

Do the pharyngeal grooves (external) and pharyngeal membranes all disappear?

Answer 10

All except for the first pair of pharyngeal grooves and membranes disappear

First pharyngeal groove –> external acoustic meati

First pharyngeal membrane –> tympanic membranes

Question 11

What forms the cervical sinus and cysts?

Answer 11

Ectodermally lined remnants of clefts 2-4 may persist as a cervical sinus, with cervical (or branchial) cysts

Cysts may drain to the surface or internally via fistulae

Question 12

What parts of the tongue develop from what pharyngeal arches?

Answer 12

Tuberculum impar (medial lingual swelling) from arch 1

Copula (hypobranchial eminence) from arches 2, 3, 4

Epiglottal swelling forms from arch 4

Lateral swellings overgrow tuberculum and fuse to form anterior 2/3 of tongue (still arch 1 innervation from V3)

Posterior 1/3 of tongue forms copula and arch 3 overgrows arch 2 so innervation from CN IX

Question 13

How do we explain CN XII motor innervation of the tongue?

Answer 13

Most tongue myoblasts thought to originate from occipital myotomes (although some may be developed in situ), so get innervation from CN XII

Question 14

How does the thyroid develop?

Answer 14

Epithelium proliferates in pharyngeal floor at position of later foramen cecum (between tuberculum impar and copula)

Developing thyroid tissue becomes bilobed and descends ventral to gut but remains connected to surface of tongue by thyroglossal duct

Thyroid lobes descend anterior to hyoid bone and laryngeal cartilages and reach final position anterior to trachea in week 7

Thyroid function begins around end of 4th month

Question 15

Thyroglossal cysts

Answer 15

Congenital thyroglossal cysts may appear anywhere along the thyroglossal duct, about half near hyoid body

Cysts can rupture, forming fistulae

Any aberrent thyroid tissue may be found along migratory course

Question 16

End of 4th week development of midface and jaw from arch 1

Answer 16

By end of 4th week, facial prominences have been formed by arch 1 and neural crest mesenchyme

Maxillary prominences lateral to stomodeum (primitive oral opening)

Mandibular prominences caudal to stomodeum

Frontonasal prominence formed by mesenchyme ventral to brain vesicles, rostral to stomodeum

Nasal (olfactory) placodes formed on frontonasal processes

Question 17

5th week development of nasal primordia

Answer 17

Nasal placodes invaginate to form nasal pits with nasal ridges

Lateral and medial nasal prominences

Question 18

6th and 7th week development of midface and jaw from arch 1

Answer 18

Maxillary prominences grow and push medial nasal prominences toward midline

Clefts between medial nasal prominences and maxillary prominences close, forming upper lip

Lateral nasal prominences do NOT contribute to upper lip

Fusion of mandibular processes across midline form lower lip and jaw

Nose forms from the 5 facial prominences (frontal prominence forms bridge, medial prominences form crest and tip, lateral prominences form alae)

Medial nasal prominences merge deeply to form upper jaw component (4 incisor/front teeth) and primary palatal component (altogether is intermaxillary segment)

Question 19

What is anteromedial primary palate formed from?

Answer 19

Intermaxillary segment (frontonasal process origin)

Question 20

What is main part of palate formed by?

Answer 20

2 internal shelf-like outgrowths called maxillary palatine shelves (at 6.5 weeks)

Initially slope obliquely to side of tongue

Become horizontal above tongue and begin to close to form secondary palate (7.5 weeks)

Fusion of palatine shelves and primary palate (10 weeks)

Question 21

Developmental anomalies of face and palate

Answer 21

Common

Mainly result of maldevelopment of neural crest tissue (deficiency in # of cells, incomplete migration into face, failure of induction)

Arrested development or failure of fusion of facial and palatal prominences, or both

Thought to involve a combination of genetic and environmental factors

Question 22

Cleft lip

Answer 22

Failure of merger of mesenchymal masses in medial nasal and maxillary prominence

(early on, around 7 weeks?!)

Question 23

Cleft palate

Answer 23

Failure of mesenchymal masses of palatine shelves (processes) to meet and to merge

(later on, 7-10 weeks is when they’re supposed to close?!)

Question 24

In general, what causes facial clefts?

Answer 24

Neural crest migration failure due to delay or low cell numbers

Multifactorial process (genes and environment)

Question 25

Which pharyngeal arches form which cranial nerves?

Answer 25

Arch 1: CN V

Arch 2: CN VII

Arch 3: CN IX

Arch 4 and 6: CN X