L10: embryological development of head and neck structures

Question 1

Describe the pharyngeal arches

Answer 1

Sequence of ridges that form in the lateral walls of the embryonic pharynx, towards the cranial end of the neural tube (~4 weeks)
Each arch has a large mesenchyme core, covered by ectoderm on its external surface
Between each arch = pharyngeal groove & cleft
Internal surface is covered by endoderm – similar pattern of grooves but on inner surface they are known as pharyngeal pouches

Question 2

How many pharyngeal arches are there? Explain their structure

Answer 2

There are six, but the fifth doesn’t form in humans
First arch is the biggest and they get progressively smaller
Associated with each arch is a cartilage, nerve and artery
Cartilage bar of the 1st pharyngeal arch is the largest = Meckel’s cartilage -> gives rise to the mandible, malleus & incus bones
4th & 6th arches give rise to thyroid, arytenoids and cricoids

Question 3

Which cranial nerves are associated with each pharyngeal arch?

Answer 3

1 – trigeminal
2 – facial 
3 – glossopharyngeal 
4 – vagus (superior laryngeal nerve)
6 – vagus (recurrent laryngeal nerve)

Question 4

List the muscles derivatives of the pharyngeal arches

Answer 4

1 – muscles of mastication
2 – muscles of facial expression, stapedius
3 – stylopharyngeus
4-6 – constrictor muscles of pharynx & muscles of soft palate, muscles of the larynx + striated muscle of upper oesophagus

Question 5

List the derivatives of the pharyngeal pouches

Answer 5

1 – pharyngotympanic (eustachian) tube & middle ear cavity
2 – palatine tonsil
3 – inferior parathyroid & thymus
4 - superior parathyroid & C cells of thyroid

Question 6

Describe development of the face

Answer 6

Five building blocks – frontonasal prominence, two maxillary prominences & two mandibular prominences
First evidence of face development – appearance of a depression in the ectoderm = stomadaeum (site of future mouth)
Five prominences folds around this to form the face
Both maxillary and mandibular prominences are collectively the 1st pharyngeal arch
Eventually the prominences will fuse together to complete the face (failure = facial cleft & cleft lip)

Question 7

Describe what the different facial prominences form

Answer 7

Frontonasal prominence = forehead, bridge of the nose, upper eyelids & centre of the upper lip
Maxillary prominences = middle 1/3 of face, upper jaw & most of the lip and sides of the nose
Mandibular prominences = lower 1/3 of face – lower jaw & lip

Question 8

Describe the first part of the development of the nose

Answer 8

First evidence – appearance of two ectodermal thickenings (nasal placodes) on the frontonasal prominence
Nasal placodes invaginate and form deep pits, with the entrance to each pit being the future nostril
Deepening nasal pits are separated only by the oronasal membrane

Question 9

What happens after the oronasal membrane disappears?

Answer 9

Oral and nasal cavities become one continuous space

Subsequent separation requires the development of the palate

Question 10

Describe the development of the palate

Answer 10

Involves the maxillary prominences and the medial nasal prominences
Medial nasal prominences merge in the midline, separating nostril from the mouth & forms the philtrum of the upper lip + the primary palate
A palatal shelf grows from each maxillary prominence towards the midline – fuse with each other and with the primary palate
Fusion creates the secondary palate & separates the nasal cavity from the oral cavity

Question 11

Describe why clefts form

Answer 11

Clefts of the nose, lips or palate can occur due to the failure of fusion between structures of the developing face
Cleft lip – failure of the fusion of the medial nasal prominence and maxillary prominence
Cleft palate – the above occurs, along with the failure of the palatal shelves to meet in the midline

Question 12

Describe the diagnosis and treatment of clefts

Answer 12

May be diagnosed antenatally with ultrasound scans or after delivery
If left uncorrected will cause difficulties with feeding and speech development
Treatment: can be corrected successfully with surgery
-cleft lips typically around 3 months & palate repairs later at 9-12 months

Question 13

Describe why branchial cysts, sinuses and fistulas can form

Answer 13

Cyst = enclosed, sinus = communicates with skin, fistula = connects skin with pharynx
Obliteration of pharyngeal clefts can be disordered – 2nd arch grows down lower than pharyngeal cleft -> remnants of cavities can remain & present as a lump

Question 14

Describe how the ascent of the ears is affected in fetal alcohol syndrome

Answer 14

Ears are low set in the syndrome

Ingestion of alcohol has led to the arrest of cells in the developing ears

Question 15

Describe the innervation of the tongue

Answer 15

Complex innervation reflects complex development
Anterior 2/3 – general sensation from the mandibular branch of trigeminal, taste from facial
Posterior 1/3 – general sensation and taste from glossopharyngeal
Muscles arise from somites at the level of the hypoglossal nerve

Question 16

Describe the development of the thyroid gland

Answer 16

Thyroid diverticulum originates at the foramen cecum
-grows down and takes a group of cells with = thyroid gland
Thyroglossal duct connects the tongue to the thyroid gland – this should break down

Question 17

Describe pathology of the thyroid gland that arises due to incorrect development

Answer 17

Thyroglossal cyst – failure of the thyroglossal duct to break down
-diagnose: stick tongue out & cyst will elevate
Ectopic thyroid gland tissue can sometimes be found anywhere along the path of descent