Development of the sense organs: Eye and ear

Question 1

Where are the 3 places where sensory neurons can originate from? What are examples of these?

Answer 1

1) Neural crest (eg. sensory neurons of the DRG)
2) Neural tube (eg. retina)
3) Placodes (eg. otic (ear) placode)

Question 2

Where is the ‘eye field’ set up and how?

Answer 2

In the anterior NEURAL PLATE

Set up by anterior transcription factors, which upregulate other transcription factors in cells which are characteristic of the eye field

Question 3

What are the stages of eye formation in vertebrates?

Answer 3

1) Proliferation in the neural tube, causing a change in shape of the neural tube - forming the optic grooves
2) Once the neural tube has closed, the optic grooves from optic vesicles and contact the ectoderm
3) At the ectoderm contact, there is a thickening of the ectoderm - forming a LENS PLACODE
4) The lens placode invaginates and eventually pinches off to form the lens
5) The lens comes to sit in a double layered cup, formed by the optic vesicles

Question 4

When do the optic grooves form?

Answer 4

As the anterior neural tube proliferates

Question 5

Where does the lens come from?

Answer 5

The invagination of the lens placode, which pinches off to forma a circle of cells

Question 6

What happens to the optic vesicle as the lens placode invaginates?

Answer 6

Causes the optic vesicle to buckle in on its self - forming a double layered cup, where the inside layer is the retina

Question 7

Where does the retina come from and how is it formed?

Answer 7

Outgrowth of the ventral forebrain (diencenphalon)

Formed by the optic vesicle buckling in on its self to form a double layered cup

The inner layer of the cup is the retina

Question 8

Where do retinal neurons differentiate from?

Answer 8

The inner layer of the retinal cup

Question 9

Where do retinal neurons extend their axons?

Answer 9

Through the optic nerve, from the inner layer of the retinal cup

Question 10

Which part of the double layered optic cup is the retina?

Answer 10

The inner layer

Question 11

What do the outer layer cells of the optic cup become?

Answer 11

• Retinal pigment epithelium

Question 12

What does the retinal pigment epithelium produce?

Answer 12

Melanin

Question 13

What are the properties of the inner layer cells of the optic cup and what do they become?

Answer 13

Stem-cell population:

1) Can self-renew
2) Differentiate into ganglion cells, interneurons or sensitive cells in the neural retina

Question 14

What is PAX6?

Answer 14

A master eye regulator

A transcription factor which is conserved through evolution causes the outgrowth of the eye to form the retina

Question 15

What is PAX6 normally restricted to?

Answer 15

The optic vesicles

Question 16

What happens if ectopically express PAX6?

Answer 16

Induces a second retina

Question 17

How is the sensory retina built up?

Answer 17

In the same way as the neurons in the spinal chord:
- Build up may layers of differentiated neurons, while retaining a stem cell progenitor in the neuroepithelial layer (at the VZ)

• Made of different neuronal types, each born at a different time period

Question 18

What determines what neurons are born in the retina?

Answer 18

The time at which the wave of neurons is born

how long the cells has been sitting as a progenitor (stem) cell

Question 19

When do neurons begins to be born in the retina?

Answer 19

6-7 weeks, before this (5 weeks) there is no differentiation of cells

Question 20

What layer is formed in the 1st wave of neurons and what cells are formed?

Answer 20

• Innermost neuroblastic layer (closest to the eyeball)

- Ganglion cells

Question 21

What layer is formed in the 2nd wave of neurons and what cells are formed?

Answer 21

• Inner neuroblastic layer

- Bipolar cells

Question 22

What layer is formed in the 3rd wave of neurons and what cells are formed?

Answer 22

• Outermost neuroblastic layer (furthest away from the eyeball)
• Rods and cones

Question 23

Why is it important to know how long each neuron type remains as a progenitor?

Answer 23

• Can begin to manipulate pluripotent cells and direct them down particular routes in order to restore function

Question 24

What is the structure of the vertebrate eye?

Answer 24

Retina is a laminar structure made of many layers

Question 25

Where are sensory neurons derived from in the vertebrate eye?

Answer 25

From the neural plate

Question 26

Which TF does eye formation in the vertebrate require?

Answer 26

Pax-6

Question 27

What are placodes?

Answer 27

Embryonic structures that give rise to structure such as the eye, ear and olfactory epithelium

Question 28

Where do placodes originate from?

Answer 28

From the anterior neural plate border cells (NOT the neural crest)

Question 29

What are cranial placodes?

Answer 29

Thickened areas formed in the neural plate border, where there is contact between the neural plate and the ectoderm - at early development, before the neural plate elongates and the neural tube forms

Question 30

What is the neural plate border important in forming anteriorly?

Answer 30

Formation of cranial placodes

Question 31

How are the cranial placodes arranged?

Answer 31

With bilateral symmetry, at the neural plate border, until the neural tube forms, where they spread out

Question 32

What happens to the placodes as the neural plate forms the neural tube?

Answer 32

They spread out and form additional placodes

Question 33

What is the neural plate border important in forming posteriorly?

Answer 33

The neural crest (DRG, peripheral nervous system etc)

Question 34

What are the cranial placodes?

Which ones are formed when the original cranial placodes spread out?

Answer 34

• Olfactory
• Lens
• Trigeminal
• Otic

Formed when the cranial placodes spread out:

• Nodose
• Petrosal
• Geniculate

Question 35

How does the ear develop?

Answer 35

• From the otic placode, which folds to give a pit

- Pinches off from the surface ectoderm to form a vesicle

Question 36

What do the cells in the otic vesicle give rise to?

Answer 36

• Different structures of the ear (eg. hair cells, auditory sensory neurons)
• Dependant on where the cell lies in the otic placode

Question 37

What are neurogenic regions of the otic placode and what comes from this?

Answer 37

At the base of the vesicle

Sensory neurons come from this

Question 38

What splits the eye field into 2 bilateral regions?

Answer 38

Expression of shh which down regulates the genes in the middle