Embryology

Question 1

Describe the structure of pharyngeal arches. (2)

Answer 1

Mesenchyme core, covered by ectoderm externally and endoderm internally.

Question 2

Describe the structure of the cranial end of the neural tube. (5)

Answer 2

A sequence of 5 ridges labelled pharyngeal arches 1, 2, 3, 4 and 6. Between them externally are the pharyngeal clefts, and each arch has a nerve, and artery and a cartilage bar associated with it. Internally, pharyngeal pouches exist between the arches.

Question 3

Describe the artery, nerve and derivatives (muscles/sensory/bone) of arch 1. (6)

Answer 3

Common carotid
Trigeminal
Muscles of mastication
Maxillae, zygoma from the maxillary portion
Meckel’s cartilage - forms the mandible from the mandibular portion.

Question 4

Describe the artery, nerve and derivatives (muscles/sensory/bone) of arch 2. (5)

Answer 4

Common carotid
Facial
Muscles of facial expression, stapedius
Reichert’s cartilage - forms superior hyoid bone.

Question 5

Describe the artery, nerve and derivatives (muscles/sensory/bone) of arch 3. (4)

Answer 5

Common carotid
Glossopharyngeal
Stylopharyngeus muscle
Inferior border of the hyoid bone.

Question 6

Describe the artery, nerve and derivatives (muscles/sensory/bone) of arch 4 alone. (5)

Answer 6

Aorta (left) and subclavian (right)
Vagus
Epiglottis and soft palate
Cricothyroid

Question 7

Describe the artery, nerve and derivatives (muscles/sensory/bone) of arch 6 alone. (3)

Answer 7

Pulmonary arteries
Vagus
Laryngeal intrinsic muscles.

Question 8

Describe the derivatives shared by arches 4 and 6. (2)

Answer 8

Parts of the thyroid gland

Laryngeal cartilage.

Question 9

Explain why the recurrent laryngeal nerve loops under the aorta on the left and the subclavian artery on the right. (3)

Answer 9

Because these are the blood supply to the 4th pharyngeal arch which is supplied by the vagus nerve. As the head and neck have grown and things have changed position, the vessels have elongated this potion of the vagus nerve.

Question 10

Explain why the facial nerve innervates the muscles of facial expression but none of the muscles of mastication. (4)

Answer 10

Because the muscles of facial expression are derived from the second pharyngeal arch, which is innervated by the facial nerve. The muscles of mastication are derived from the first pharyngeal arch, which is innervated by the Trigeminal nerve, not the facial.

Question 11

Describe the derivatives of the pouch 1. (2)

Answer 11

Phagotympanic (Eustacian) tube

Middle ear cavity

Question 12

Describe the derivatives of the pouch 2 (1)

Answer 12

Palatine tonsils.

Question 13

Describe the derivatives of the pouch 3. (2)

Answer 13

Inferior parathyroid

Thymus

Question 14

Describe the derivatives of the pouch 4. (2)

Answer 14

Superior parathyroid

C cells of thyroid (rest is from arches 4+6).

Question 15

Describe what makes up the developing face. (3)

Answer 15

The frontonasal prominence, 2 maxillary prominences and 2 mandibular prominences. A depression in the ectoderm called the stomadaeum appears, which will become the mouth, and the 5 prominences will fold around this.

Question 16

Describe the derivatives of the 3 types of prominence that make up the face. (6)

Answer 16

Frontonasal - forehead, bridge of nose, upper eyelids, centre of upper lip.
Maxillary - middle 1/3 of the face, sides of the nose, upper jaw and lip.
Mandibular - lower 1/3 of the face including the lower lip and jaw.

Question 17

Describe what happens to the pharyngeal clefts. (2)

Answer 17

The second pharyngeal arch will grow down, covering all of the clefts and arches. The first cleft becomes the external auditory meatus of the ear.

Question 18

Describe what will happen if the second pharyngeal arch fails to fuse properly with the clefts that it is covering. (2)

Answer 18

Branchial cyst - remnants of the space where the cleft used to be that has become fluid filled.

Question 19

Describe the formation of the nose. (4)

Answer 19

Two nasal placodes form on either side of the frontonasal prominence, which invaginate to form the nasal pits (future nostrils), leaving a horseshoe shaped ridge around them.
The medial sides of this horseshoe fuse with maxillary prominences and then together at the midline.

Question 20

Describe the formation of the palate. (3)

Answer 20

The movement of the nasal placodes to fuse at the midline pushes the maxillary prominences outwards, and forms the maxillary structure we see in adults, the philtrum of the upper lip and the primary palate.
Internally, two palatal shelves grow from the maxillary prominence and fuse with each other and the primary palate at the midline, separating the oral and nasal cavities.

Question 21

Explain the differences between a cleft lip and a cleft palate. (3)

Answer 21

Lip = failure of fusion between medial nasal prominences and the maxillary prominence.
Palate = failure of the palatal shelves to meet.
Environmental and genetic causes.

Question 22

Describe foetal alcohol syndrome. (3)

Answer 22

Damage to the neural crest cells in the pharayngeal arches which are responsible for the production of the facial skeleton. Causes a flat face, low ears, underdeveloped jaw and small head.

Question 23

Describe ARND. (2)

Answer 23

Alcohol related neurodevelopmental delay. The less severe version of foetal alcohol syndrome.

Question 24

Describe the development of the pituitary gland. (6)

Answer 24

The pituitary gland has two distinct tissue types - endocrine and neuroendocrine - which are shown from their different origins.
The posterior pituitary and the pituitary stalk begin as the infundibulum, an outgrowth of the forebrain towards the pharynx. This fuses with the origins of the anterior pituitary, which begins as Rathke’s pouch, an ectoderm upgrowth. As the sphenoid bone ossifies, it pinches off the attachment that Rathke’s pouch has to the ectoderm.

Question 25

Describe the development of the tongue. (3)

Answer 25

Develops from pharyngeal arches 1, 2, 3 and 5, which is why it’s motor and sensory supply reflects this. It begins as 2 lateral swellings within the 1st arch, and 3 median lingual swellings within 2, 3 and 5.

Question 26

Describe how the tongue is freed from the floor of the mouth and describe a pathology if this does not happen correctly. (2)

Answer 26

Apoptosis apart from the lingual frenulum.

Tongue tied babies still have lots of tissue connecting their tongue to the floor of the mouth.

Question 27

Describe the sensory and motor innervation of the tongue. (7)

Answer 27

General sensory anterior 2/3: Trigeminal (chorda tympani and lingual branches of Vc)
General sensory posterior 1/3: glossopharyngeal
Special sensory anterior 2/3: facial
Special sensory posterior 1/3: glossopharyngeal
Motor: mostly hypoglossal with a bit of vagus.

Question 28

Describe Treacher-Collins syndrome. (1)

Answer 28

Hypoplasia of the mandible and facial bones from neural crest cell migration issues.

Question 29

Describe DiGeorge syndrome (1)

Answer 29

Deletion on chromosome 22 leads to disruption of 3rd and 4th pouches, so thymus and parathyroid glands fucked.