Development

Question 1

Describe the embryo in week 4

Answer 1

• Early week 4
• FACE: no distinguishing external features
• BUT: head and neck represent ~½
length of embryo

Question 2

What do the pharyngeal aches form

Answer 2

• The embryonic head and neck • Complex tissue system • Involves many systems of the body
– notably:
• brain • CVS – heart & great vessels • special sensory organs
• Segmental development
– Each arch has a neurovascular plan
• develops muscles & skeletal elements

Question 3

What glands do the gut use form

Answer 3

Development of the gut tube in the pharynx
– Endoderm tube
• Modification • Glandular development
– Parathyroids, thymus, palatine tonsils – Thyroid

Question 4

What are the pharyngeal arches

Answer 4

• Comprised of a system of mesenchymal proliferations in the neck region of the embryo
• 5 in total, numbered 1 to 6
– (i.e. 5th does not form in
humans) - get smaller causally
• Together with FNP (frontonasal prominence) constitute the building blocks for the head and neck region

Question 5

What are the pharyngeal pouches

Answer 5

Dip on inside lined y endoderm

Question 6

Which cns are associated wwith the pharyngeal apparatus

Answer 6

• CN V, CN VII, CN IX & CN X 
• Mixed sensory & motor functions 
• Supply the derivatives of the pharyngeal arches 
• CN XI (cranial accessory) and CN XII have relationship wit  pharyngeal arch system
1- V
2 - VII
3 - IX
4-6 - X

Question 7

What are the muscular derivatives of each arch

Answer 7

• Muscles of the face
– Muscles of mastication are Ph Arch 1
derivatives – Muscles of facial expression are Ph Arch
2 derivatives
• 3rd arch
– stylopharyngeus 
• 4th arch
– Cricothyroid
– Levator palatini
– Constrictors of the pharynx 
• 6th arch
– Intrinsic muscles of the larynx

Question 8

Descrbe the cartilages of tht arches

Answer 8

Each of the pharyngeal arches develops a neural crest - derived cartilage bar
– Ph A 1 – Meckel’s: malleus & incus plus a template for formation of the mandible
– Ph A 2 – Reichert’s : stapes plus upper part hyoid bone
– Ph A 3 – remainder of hyoid bone
– Ph A 4 & 6
• Cartilages of the larynx

Question 9

What do the pharyngeal pouches develop into

Answer 9

• Endoderm lined pockets in the pharynx
• First pouch is the largest and becomes the tympanic cavity
• Development results in formation of palatine tonsils (Population of endoderm lining by lymphoidd cells - gives immune responsive tissue ) and parathyroid glands and the thymus (2 different lobes??)

Question 10

What do the clefts develop into

Answer 10

• 1st cleft is all that remains
– Becoming the external acoustic meatus
• 2nd arch grows down to cover others, obliterating all other clefts
• Branchial cysts (enclosed fluid filled sac, remnant of one of the clefts) or fistulae (abnormal connection) can occur if there are remnants

Question 11

What drives the development of the fac

Answer 11

• Expansion of the cranial neural tube - gives brain
• Appearance of a complex tissue system (PA) associated with:
– the cranial gut tube
– the outflow of the developing heart
• Development of the sense organs
– & the need to separate the respiratory tract from the GI tract

Question 12

What are the embryologial components of the fac

Answer 12

• The components of the face are:
– Stomatodeum
• buccopharyngeal membrane (ruptures - to give opening into oral cavity)
– Frontonasal prominence (Surrounds ventro-lateral part of the forebrain)
– 1st Pharyngeal Arch
• Maxillary prominence
• Mandibular prominence

Question 13

Which features do the fdp, maxillary and mandibular part of arch 1 form

Answer 13

FNP
Forehead
Bridge of nose
Nose
Philtrum 

Maxillary
Cheeks
Lateral upper lip
Lateral upper
jaw 

Mandibular
Lower lip & jaw

Question 14

What is a placode

Answer 14

Thickenings of ectoderm which represent beginning of development of organ of special sense

Question 15

Describe the development of the nose

Answer 15

• Nasal placodes appear on frontonasal prominence
• Then sink to become the nasal pits
• Medial and lateral nasal
prominences form on either side
of the pits
maxillary prominences grow medially, pushing the nasal prominences closer together in the midline
• Maxillary prominences fuse with medial nasal prominences
• Medial nasal prominences then fuse in the midline

Question 16

How are the nasal and oral cavities separated

Answer 16

• Fusion of medial nasal prominences creates the intermaxillary segment
– Labial component: philtrum
– Upper jaw: 4 incisors
– Palate: primary palate
• Main part of definitive palate is secondary palate
– Derived from palatal shelves derived from maxillary prominences

Question 17

Describe the palatal shelves and development of the tonge

Answer 17

• Maxillary prominence give rise to two palatal shelves
• These grow vertically downwards into the oral cavity on each side of
developing tongue
The mandible grows large enough to allow the tongue to “drop”
The palatal shelves then grow towards each other and fuse in the midline
The nasal septum develops as a midline down-growth & ultimately fuses with palatal shelves

Question 18

How to cleft lip and palate form

Answer 18

• Lateral cleft lip
– Failure of fusion of medial nasal prominence & maxillary prominence
• Cleft lip & cleft palate
– Combined with failure of palatal shelves to meet in midline

Question 19

Decsribe the development of ears

Answer 19

• External ears develop
initially in the neck
• As mandible grows to fill cavity; the ears ascend to the side of the head to lie in line with the eyes
• All common chromosomal abnormalities have associated external ear anomalies

Question 20

Describe fetal alcohol syndrome

Answer 20

• There is no known safe level of alcohol consumption during pregnancy
• Facial skeleton derived from neural crest cells populating the pharyngeal arches
• Neural crest migration as well as development of the brain are known to be extremely sensitive to alcohol
• Incidence of FAS & ARND = 1 / 100 births
Placenta does not act as a true barrier