Embryology

Question 1

What are the pharyngeal arches?

Answer 1

The embryonic head and neck. They are ridges that form in the lateral wall of the embryonic pharynx at the cranial end of the neural tube.

Question 2

What is each pharyngeal arch composed of?

Answer 2

A mesenchyme core with some neural crest cells that migrate in surrounded by ectoderm. Each arch has an associated nerve, artery and cartilage.

Question 3

What is between each pharyngeal arch?

Answer 3

A pharyngeal crest/groove. All but the first one disappears(->external auditory meatus).

Question 4

What do the endoderm pouches of the pharyngeal arches facing the gut tube go on to form?

Answer 4

Parathyroids (3rd and 4th), tonsils and thymus.

Question 5

How many pharyngeal arches are there and which is the biggest?

Answer 5

6 but the 5th does not form in humans. 1st is the biggest and they get smaller

Question 6

What cranial nerves supply the derivatives of the pharyngeal arches?

Answer 6

CN V, VII, IX, X

Question 7

What is the facial skeleton derived from?

Answer 7

Frontonasal prominence and 1st pharyngeal arch

Question 8

Outline muscular derivatives from each pharyngeal arch (from mesoderm)

Answer 8

1st - muscles of mastication (CN V)
2nd - muscles of facial expression (CN VII)
3rd - stylopharyngeus (CN IX)
4th and 6th - larynx muscles (CN X)

Question 9

Outline skeletal derivatives from each pharyngeal arch

Answer 9

1st - Meckel’s cartilage bar -> mandible, malleus and incus (middle ear)
2nd - Reichert’s cartilage bar -> stapes, styloid process, hyoid bone
3rd - small portion of hyoid bone
4th and 6th - cartilage of larynx -> thyroid, arytenoid and cricoid cartilage

Question 10

What nerve is looped around the left arch of the aorta and right subclavian artery?

Answer 10

Recurrent laryngeal nerve of vagus

Question 11

From what structures does the face develop from?

Answer 11

Frontonasal prominence - forehead, bridge of nose, philtrum (upper lip groove)
2 maxillary prominences - cheeks, lateral upper lip and jaw
2 mandibular prominences - lower lip and jaw
In place by end of 4th week

Question 12

What are neural crest cells?

Answer 12

A specialised population of cells that originates within the neuroectoderm
A “fourth germ lineage”
They migrate and contribute to a variety of H&N structures

Question 13

Where is the facial skeleton derived from?

Answer 13

Neural crest of 1st pharyngeal arch

Question 14

What is the depression in the surface of an early embryo that develops into the mouth?

Answer 14

Stomodeum

Question 15

Describe the development of the nose

Answer 15

Nasal placodes appear on frontonasal prominence
They sink to become the nasal pits
Medial and lateral nasal prominences form either side of the pits
Maxillary prominences grow medially, pushing the nasal prominences towards the midline
Medial nasal prominences fuse in the midline. Maxillary prominences fuse with medial nasal prominences.

Question 16

Describe the separation of the nasal and oral cavities

Answer 16

Fusion of medial nasal prominences creates the intermaxillary segment - gives contributions to philtrum, upper jaw and palate. Main part of definitive palate is secondary primary palate, derived from maxillary prominences. They initially appear as palatal shelves under tongue. The mandible grows to allow the tongue to drop and the palatal shelves the fuse in the midline. The nasal septum the fuses with palatal shelves.

Question 17

Describe the different types of cleft lip and palate and the problems they result in

Answer 17

Lateral cleft lip - failure of fusion of medial nasal prominence and maxillary prominence
Cleft lip and palate - combined with failure of palatal Chelsea to meet in the midline
Problems suckling and speaking

Question 18

Describe the development of the eyes

Answer 18

Out pocketings of forebrain (optic vesicles). They grow to make contact with overlying ectoderm - optic/lens placodes. Lens placode then invaginates and pinches off to give rise to the lens.
Eye primordia positioned on the side of the head then moves to the front as facial prominences grow.

Question 19

What is the external ear derived from?

Answer 19

External auditory meatus - 1st pharyngeal cleft

Auricles - 1st and 2nd pharyngeal arches

Question 20

Describe the embryological positioning of the ears

Answer 20

External ears initially in the neck. Ascend to the side as mandible grows. All common chromosomal abnormalities have external ear abnormalities.

Question 21

Describe the development of the internal ear

Answer 21

Otic placodes invaginate and form a membranous labyrinth

Question 22

Describe feral alcohol syndrome

Answer 22

Neural crest migration and brain development are particularly sensitive. Distinct facial abnormalities can result

Question 23

How does a branchial cyst develop?

Answer 23

Incomplete fusion of second pharyngeal arch over the other arches/grooves allows the persistence of the cervical sinus. Can swell after infection -> smooth non tender mass along anterior border of SCM.

Question 24

What is a placode?

Answer 24

An area of ectoderm that thickens and differentiates to give rise to sensory structures

Question 25

Why is the pituitary gland unusual embryologically?

Answer 25

It has two distinct tissue types - neurocrine and neuroendocrine. It has ectoderm and neuroectoderm origins.

Question 26

Describe the development of the pituitary gland

Answer 26

Neuroendoderm - It begins as a downward outgrowth of the forebrain (infundibulum) towards the pharynx. Ectoderm - invagination of the ectoderm of the stomatodeum (Rathke's pouch) which grows upwards towards the developing forebrain.with ossification of the sphenoid bone Rathke's pouch is pinched off and joins to the infundibulum.

Question 27

Describe the development of the tongue

Answer 27

Develops from tissue from all 5 pharyngeal arches, beginning as 2 lateral swellings within 1st arch and 3 median lingual swellings within arches 1-4. This explains the tongues unusual pattern of sensory innervation. It is freed from the floor by cell degeneration leaving only the lingual frenulum. Muscles of the tongue develop from myogenic precursors that migrate into the developing tongue.

Question 28

Describe the development of the thyroid gland

Answer 28

Early development begins in the floor of the pharynx between two early components of the developing tongue. It's point of origin is marked by the foramen cecum. It descends down the neck and bifurcates into a bi-lobed diverticulum joined by the isthmus. It remains connected to the tongue by a patent tube called the thyroglossal duct. If this persists thyroglossal cyst and fistula can result. Presents as lump that moves of protrusion of tongue and swallowing.

Question 29

What is 1st arch syndrome?

Answer 29

Spectrum of defects in development of face thought to result from a failure of colonisation of the 1st arch with neural crest cells. E.g. Treacher-Collins syndrome is hypoplasia of mandible and facial bones, inherited autosomal dominant condition.