Case 19- Embryology

Question 1

Head and Neck- what does the pharyngeal surface ectoderm give rise to

Answer 1

Contributes to the development of the ear and palate

Question 2

Head and Neck- what does the pharyngeal (lateral plate) mesoderm give rise to

Answer 2

Gives rise to skeletal structures of the head and neck, to the tongue and forms the pharyngeal arch arteries. From the Somatic mesoderm

Question 3

Head and neck- what does the pharyngeal endoderm give rise to

Answer 3

Contributes to the development of the pharyngeal organs including the thymus

Question 4

Head and Neck- What does the neural crest ectoderm give rise

Answer 4

Contributes to the bones of the face and jaw (frontal, sphenoid and temporal bone), to the cranial nerves, and to the remodelling of pharyngeal arteries. Face, jaw, neck and palate

Question 5

Head and Neck- what does the Paraxial mesoderm give rise

Answer 5

Contributes to the bones of the posterior skull (Parietal and Occipital bone)

Question 6

What does the pharyngeal arches give rise to

Answer 6

Maxilla, Mandible, bones of middle ear, muscles of mastication, maxillary artery, trigeminal nerve, styloid process, muscles of facial expression, facial nerve, hyoid bone

Question 7

What do the pharyngeal clefts give rise to

Answer 7

External auditory meatus

Question 8

What do the pharyngeal membranes give rise to

Answer 8

Tympanic membrane

Question 9

What do the Pharyngeal pouches form

Answer 9

Tympanic cavity and auditory tube, palatine pouch, inferior parathyroid gland and thymus, superior parathyroid gland

Question 10

Pharyngeal arches

Answer 10

1) External structures in the embryonic neck region and appear from week 4 of development
2) There are five pharyngeal arches (1,2,3,4 and 6) arch 5 is transient and does not form any adult structures.

Question 11

Core of the pharyngeal arch

Answer 11

The central core of each pharyngeal arch consists of mesoderm and neural crest. The arch core contains a skeletal element the central rod (give rise to craniofacial bones and cartilages), a neural element (give rise to cranial nerves) and a vascular element (give rise to pharyngeal arch arteries). Next to the Central skeletal rod (cartilage surrounded by muscle) is the Pharyngeal arch artery and a Cranial nerve which is made from neural crest cells. Each nerve in the central arch is a different cranial nerve.

Question 12

Pharyngeal cleft

Answer 12

The external recesses between each pharyngeal arch- consists of ectoderm

Question 13

Pharyngeal pouch

Answer 13

Internally, the endoderm lined corresponding recesses

Question 14

Skeletal structures formed from the different pharyngeal arches

Answer 14

• 1st (Meckel’s cartilage)- Maxillary and Mandibular bone, Mallleus and Incus
• 2nd- Stapes, Styloid process, Stylohyoid ligament, lesser horn of hyoid bone
• 3rd- Hyoid bone
• 4th- Thyroid cartilage
• 6th- Cricoid cartilage

Question 15

Motor innervation from the pharyngeal arches

Answer 15

• 1st arch- Trigeminal nerve (V), muscles of mastication, gives rise to Ophthalmic, Maxillary and Mandibular branches
• 2nd arch- Facial nerve (VII), muscles of facial expression
• 3rd arch- Glossopharyngeal nerve (IX), controls the Stylopharyngeus muscle which controls swallowing
• 4th arch- Superior laryngeal branch of the Vagus nerve (X), controls the muscles of the pharynx and soft palate
• 6th arch- Recurrent laryngeal branch of the Vagus nerve (X) which controls the muscle of the larynx

Question 16

When are the pharyngeal arch arteries remodelled into adult vessels

Answer 16

Occurs from 28 to 44 days of development. The neural crest cells migrate in

Question 17

What does the first pharyngeal cleft and membrane go on to form

Answer 17

The first pharyngeal cleft goes on to form the external auditory meatus
The first pharyngeal membrane forms the tympanic membrane

Question 18

What do the different pharyngeal pouches go on to form

Answer 18

• 1st pharyngeal pouch forms the tympanic cavity and auditory tube
• 2nd pharyngeal pouch forms the Palatine tonsil
• 3rd pharyngeal pouch forms the inferior Parathyroid gland and Thymus
• 4th pharyngeal pouch forms the Superior Parathyroid gland

Question 19

Brachial cyst (from the pharyngeal cleft)

Answer 19

Due to non-removal of the 2nd cleft/cervical sinus, it then becomes filled with fluid. The cyst remains stationary on swallowing.

Question 20

Formation of the face

Answer 20

In the face, the left and right maxillary processes fuse with the frontonasal process at the midline, forming the philtrum. The left and right mandibular processes fuse at the midline to form the lower lip. Originally the Maxilla and Mandible are separated due to the division of the first pharyngeal arch. The nose is formed when the Ectodermal nasal placode covers the frontonasal process

Question 21

Formation of the Palate

Answer 21

The palate forms in the superior oral cavity at right-angles to the facial processes. In the palate the left and right palatal processes fuse with the primary palate at the midline. The primary/primitive palate is a continuation of the maxillary processes and nasal septum.

Question 22

Formation of the tongue

Answer 22

The tongue forms in the inferior oral cavity from 1st, 2nd, 3rd and 4th pharyngeal arches, (ventral to dorsal respectively). This accounts for the differences in cranial nerve supply to different parts of the adult tongue. The second arch regresses. In the 1st arch it is supplied by the lingual nerve which is a branch of the mandibular division of the trigeminal nerve.

Question 23

Formation of a cleft lip

Answer 23

Due to failure in fusion of the right and left maxillary processes with the two frontonasal processes. If there is failure in one line of fusion its unilateral, when its in both lines of fusion its bilateral

Question 24

Formation of a cleft palate

Answer 24

Due to failure in fusion of the left and right palatal processes with the primary palate. There are lines of fusion between the left palatal process and the primary palate, between the right palatal process and the primary palate, and along the midline of the secondary palate. Can be unilateral or bilateral. Midline cleft palate occurs when the fusion fails in the secondary palate

Question 25

Migration and fusion of facial processes

Answer 25

• Migration of maxillary and mandibular processes
• Fusion of mandibular processes
• Fusion of frontonasal and maxillary processes

Question 26

What do pharyngeal arch defects cause both craniofacial and cardiovascular abnormalities?

Answer 26

Due to neural crest cells

Question 27

Treacher Collin’s syndrome

Answer 27

• 1st and 2nd pharyngeal arch neural crest cell defect (TCOF1)
• Underdeveloped bones of the face
• 1 in 50,000
• Face: bilateral hypoplasia of the zygomatic bone and mandible, external ear malformations, facial clefts
• Eyes: colobomas (congenital scars), downward sloping palpebral fissures, supraorbital rim formation, sparse eyelashes

Question 28

Pierre Robin sequence (PRS)

Answer 28

Neural crest cell genetic defect (SOX9). Set of craniofacial abnormalities. 1 in 8500-1 in 14,000
Called a sequence because abnormalities occur in an order
1. mandible – micrognathia (small mandible)
2. tongue - glossoptosis (displacement)
3. pharynx - airway obstruction and cleft palate

Question 29

22q11.2 deletion syndrome: DiGeorge syndrome, velocardiofacial syndrome

Answer 29

• Neural crest cell genetic defect (deletion on chromosome 22)
• Variable features - craniofacial dysmorphy and cardiovascular defects
• 1 in 4,000 - may be more common due to undiagnosed cases
• Face: micrognathia, small mouth, smooth philtrum, ears rotated. Problems with Maxillary/Mandibular processes- 1st pharyngeal arch.
• Cardiovascular abnormalities - 4th and 6th pharyngeal arch arteries
• Developmental delay - learning difficulties and mental illness
• Defects in the immune system- thymus, 3rd pharyngeal pouch

Question 30

Cleft lip and palate

Answer 30

• Sometimes can just have cleft lip or can have both
• Multiple genetic and environmental causes
• Can be syndromic or non syndromic
• One of the most common craniofacial abnormality
• 1 in 750
• Neural crest and pharyngeal ectoderm contribute to palatal structures
• Can cause problems with neonatal feeding and physical abnormalities
• Repaired with surgery