Development of the Eyes and Ears

Question 1

What is a placode?

Answer 1

Thickened ectodermal patch on the developing head

Question 2

What is the pharyngeal apparatus?

Answer 2

Series of external ridges and furrows with corresponding internal pouches

Question 3

What are the parts of the ear and general functions?

Answer 3

Outer ear - collects sound

Middle ear - conducts sound

Inner ear - coverts sound

Question 4

Functions of the ear?

Answer 4

Hearing and balance

Question 5

How does the membranous labyrinth develop?

Answer 5

Otic placodes on the posterior surface of the head thicken and invaginate to form auditory vesicles

Otic vesicle elongates and coils to give the cochlea, saccule.

Dorsal part of the otic vesicle gives rise to the utricle, semilunar canals and endolymphatic duct

These structures form the membranous labyrinth

Question 6

What is the middle ear derived from?

Answer 6

The first pharyngeal pouch

Cartilage bars of first and second pharyngeal arches

Question 7

What does the first pharyngeal pouch give rise to and how?

Answer 7

Expands distally to create the tympanic cavity

Remains narrow proximally to give the Eustachian tube

Question 8

Where are the middle ear ossicles derived from?

Answer 8

Malleus and incus - cartilage of first pharyngeal arch (Meckel’s cartilage)

Stapes - cartilage of second pharyngeal arch (Reichert’s cartilage)

Question 9

What does the cartilage of the first and second pharyngeal arches give rise to?

Answer 9

First (Meckel’s) - malleus, incus, mandible

Second (Reichert’s) - stapes and hyoid bone

Question 10

What does the external auditory meatus develop from?

Answer 10

The first pharyngeal cleft

Question 11

What do the auricles of the ear develop from?

Answer 11

Proliferations within the first and second pharyngeal arches - auricular hillocks

Question 12

How do the ears change position with development?

Answer 12

External ear initially lies in the neck

As mandible grows, the ears ascend to the side of the head to lie in line with the eyes

Question 13

Innervation of the muscles acting on the middle ear ossicles?

Answer 13

Reflect their arch derivation

• tensor tympani: mandibular branch of trigeminal nerve
• stapedius: facial nerve

Question 14

What can cause congenital deafness?

Answer 14

Middle ear deafness - first and second pharyngeal arch problems

Inner ear deafness - maldevelopment of the organ of Conti caused by teratogenic agents eg rubella

Question 15

Sensory innervation to the auricle?

Answer 15

Anterior to external meatus - auriculotemporal (V3)

Rest of the auricle - great auricular nerve (cervical plexus)

Also some facial and vagus nerve

Question 16

Sensory innervation of the external acoustic meatus?

Answer 16

Vagus nerve

Question 17

Sensory innervation of the tympanic membrane?

Answer 17

External side:

• posteriorly - auricular branch of vagus nerve
• anteriorly - auriculotemporal branch of V3

Internal side:

-glosspharyngeal nerve

Question 18

When does development of the eye begin?

Answer 18

Week 4

Question 19

How does the eye begin development?

Answer 19

Begins as out-pocketings of the forebrain (optic vesicles) which grow out to make contact with the overlying ectoderm

Lens begins as optic placodes, which the forebrain will contact

Question 20

What happens once the optic vesicles have grown out to make contact with the lens placodes?

Answer 20

The lens placodes invaginate and pinch off to form the double-walled optic cup

Question 21

What initially separates the two layers (inner and outer) of the optic cup?

Answer 21

The intra-retinal space - soon disappears and the two layers appose each other

Question 22

What does the choroid fissure allow?

Answer 22

Passage of the hyaloid artery into the inner chamber of the eye

Question 23

What happens if the choroid fissure fails to close?

Answer 23

Coloboma

Question 24

What forms the lens placode and where does it eventually move to?

Answer 24

Surface ectodermal cells elongate to form the placodes

They then invaginate and develop into the lens vesicle

The lens vesicle loses contact with the surface ectoderm and lies in the anterior mouth of the optic cup

Question 25

What happens when the lens vesicle sits in the mouth of the optic cup?

Answer 25

Cells on its posterior wall elongate to meet the anterior wall to form the lens

Question 26

What does the proximal portion of the hyaloid artery become?

Answer 26

The central artery of the retina

Question 27

What structures does the optic cup give rise to?

Answer 27

The retina - comprised of neural (inner) and pigmented (outer) layers

Iris - a contractile diaphragm with a central aperture

Ciliary body - muscular and vascular structure connecting the choroid to the lens

Question 28

When is the optic stalk transformed into the optic nerve?

Answer 28

When the choroid fissure on its ventral surface closes

Question 29

What surrounds the optic nerve?

Answer 29

A continuation of the choroid and sclera, the pia, arachnoid and dura layers

Question 30

How do the extraocular muscles develop?

Answer 30

Preotic myotomes which develop in the region of the eye

Question 31

How does the position of the eyes change?

Answer 31

Primordia positioned on the side of the head

As facial prominences grow, the eyes move to the front of the face to give binocular vision

Question 32

What is congenital cataracts? Caused by?

Answer 32

Opacity of the lens

Genetic or a result of exposure to a teratogen eg rubella

Question 33

What happens in a detached retina?

Answer 33

When the two retinal layers separate

Question 34

Symptoms of a detached retina?

Answer 34

Floaters, sudden flashes of light