Eye development

Question 1

What do photoreceptors do?

Answer 1

cells that are responsible for detecting light

Question 2

What do pigment receptors do?

Answer 2

provide trophic support to the photoreceptors to protect from damage and to avoid dispersion

Question 3

Is there diversity in the the morphology of the eye?

Answer 3

Yes

Question 4

What did Darwin suggest the eye consisted off?

Answer 4

Pigment cell

+

Photoreceptor cell

He noticed the eye was composed of two cell types: the photoreceptors and the pigment cells.

He hypothesised that this was an indication of a common origin of all eyes and the prototypic eye that would have been present in our first ancestor with eyes would have been formed by a photoreceptor cell protected by pigments

Many years later this prototypic eye was found and described in the planarians….

Question 5

What does the planarian eye consist of?

Answer 5

It consists of photoreceptors protected by a cap of pigment cells

Question 6

What are dinoflagellates?

Answer 6

• This is a unicellular organism (one cell surrounded by a membrane with organelles inside and a flagella).

Question 7

How does the dinoflagellates move?

Answer 7

It moves by following visual stimuli. This is because it has a structure on the cell membrane that is able to detect light. This structure looks very similar to a camera eye of a vertebrae.

Question 8

Describe the eye structure in dinoflagellates

Answer 8

• This structure is essentially an organelle and is formed by a cell membrane folding over itself many times. Within this membrane there is high levels of rhodopsin (the molecule that perceives light), which is present in photoreceptors.
• This organelles also have a crystalline body in front of the folded structure full of rhodopsin which is working as a lens and allowing the organism to focus the light they perceive in the environment to the folded rhodopsin light sensing structure

Question 9

What TF is needed in eye formation

Answer 9

Pax6

Question 10

What does mutations in Pax6 cause?

Answer 10

Eyeless - no eyes formed

Question 11

Are there lots of homologues of eyes

Answer 11

Yes

Question 12

How did researchers induce the formation of an ectopic eye?

Answer 12

• The group that discovered Pax6 in drosophila hypothesised that inducing Pax6 expression somewhere else in the body may lead to eye formation
• They used a system that allows them to manipulate gene expression
• The one they used was the Gal4/UAS system. Gal4 is a transcriptional activator that comes from yeast that can bind to enhancer sequences in the genome called UAS. Gal4 and UAS do not exist in drosophila, but we can use them to engineer flies that will drive whatever gene we put under the control of UAS which is controlled by Gal4
• So if we have a tissue in which we can express Gal4 and we have UAS driving the expression of another gene such as eyeless then we can induce the expression of the gene in the initial tissue
• The researchers had Gal4 drivers that were driving expression of Gal4 to the legs or other regions of the body and they combined these drivers with UAS and eyeless. When combined they were inducing eyeless expression in the legs and other regions, forming ectopic eyes across the body
• They also found that mouse Pax6 gene worked in drosophila to induce eye formation

Question 13

Is Pax6 the only gene that drives eye formation

Answer 13

No

refer to image

Question 14

What is Rx and is it found in drosophila?

Answer 14

• Rx is an essential eye master gene in vertebrates, along with Pax6.
• However this gene is not important in drosophila
• Drosophila rx is expressed in the cephalic embryonic primordia, but it does not seem to be required for eye formation

Question 15

What are the steps in eye formation?

Answer 15

The cells that will give rise to a mature eye start as a group of neuroepithelial cells, located at the most anterior portion of the neural plate. Through a series of complex morphogenetic rearrangements and inductive events, this group of cells ends up transforming into the optic cup, a hemispheric structure that will give rise to the retina and retinal pigment epithelium. Other tissues will assemble around the optic cup as development progresses, to give rise to the mature and differentiated eye.

Eye formation can be divided in the following steps:
• eye field specification
• optic vesicle evagination
• optic vesicle patterning
• optic cup folding#
• retinal and retinal pigment epithelium (RPE) differentiation

Question 16

Describe the parts of the eye

What does the CNS form?

Where are the photoreceptors?

Where do the nerves pass and form?

What does the retinal pigment do and cover?

What is the lens derived from and do?

What does the cornea do and derived from?

What are blood vessels derived from?

What do mast cells do?

Answer 16

• They are essentially the same that will form in the vertebrae eye
• There are two tissues that are derived from the CNS called the retina and the pigmented epithelium
• The photoreceptors are located in the retina, in the most external layer of the retina. Other neuronal cells are also present in the retina. The axons of the neurones in the retina become collected into the optic nerve which exists the eye along the optic stalk of the embryo to innervate the regions of the brain that are involved with visual input
• The pigmented epithelium covers the retina, which has trophic functions and protective functions for the eye.
• The lumen of the eye is continuous with the lumen of the brain and the neuroepithelium
• The mature eye is also formed by the assembly of other tissues that are derived from the ectoderm, the mesoderm and endoderm
• The lens is an ectodermal derivative that will collect and focus the light into the retina or the cornea which protects the eye, also an ectodermal derivative
• Blood vessels are derived from the ectoderm and mesoderm.
• Mast cells are also recruited for eye movement and focusing of the lens

Question 17

In mice, when does the optic vesicle start evaginating?

Answer 17

• The optic vesicles start evaginating prior to the closing of the neural tube

Question 18

What does the optic vesicles form?

What is the lumen of the vesicles in contact with?

Answer 18

• The optic vesicles form a tight neural epithelium that is seen easily at the bottom
• There is a large lumen which is in direct contact with the folding neural tube
• There is also close contact the overlaying ectoderm and optic vesicle. The optic vesicle starts to invaginate we can also see how the lens vesicle starts differentiating

Question 19

What specifies the eye field?

Where are these expressed?

Answer 19

• Promoted by a group of transcription factors known as Eye Field Specification TFs (EFTFs)
• All of these are expressed in the anterior neural plate and the region at which all of them are expressed at the same time will become the neural plate
• Part of the neural plate will become specified as the eye field
• Tightly linked to AP patterning of the neural plate

Question 20

Where does the eye field become specified?

Answer 20

• The eye field becomes specified in the anterior most portion of the neural plate – Otx positive region. The eye field can only become specified if there is Otx expressed in the tissue

Question 21

What demarcates the eye field?

Answer 21

• The combinatorial expression of EFTFs in a OTX-positive region of the neural plate demarcates the eye field, which will then differentiate to form the two optic vesicles.
• Cells in the eye field run across the midline

Question 22

What happens when the eye field is specified?

Does it specify the optic stalk?

Answer 22

• Once the eye field is specified, it is split in two domains by signals released at the underlying midline – Shh

Shh represses Pax6 at the midline, leading to two separate “eye fields”

This leads to the separation of the eye field to form two field on either side of the midline. It also allows specification of the optic stalk

Question 23

What other mechanisms are involved in splitting of the eye?

Answer 23

• Other mechanisms are also involved in splitting the eye field in lower vertebrates
o Physical separation by anterior movement of midline tissues which will subdivide the domain
o Active movement of eye field cells away from the midline

Question 24

Following optic vesicle evagination what structures can be be seen in zebrafish and mouse?

Answer 24

The zebrafish and mouse eye both look very different, however there are a number of similarities:

1. Neural epithelium is found in both the zebrafish and mouse on the apical side and basial side
2. In the mouse the lumen of the optic vesicles are very large but in the zebrafish the lumen is almost gone, the apical sides of the dorsal and ventral halves of the optic vesicle are touching each other

Question 25

Once the optic vesicle is formed how are the different regions specified for forming different structures of the eye? Regions

Answer 25

* After the optic vesicle has become patterned, signals will pattern the three main regions of the eye: the optic stalk, neural retina and retinal pigment epithelium. * This formation is formed from the proximal region to distal region. * The optic stalk is proximal and the reginal pigment epithelium is distal On image

Question 26

How can boundaries be generate in the optic vesicle?

Answer 26

* Shh from the midline promotes optic stalk by inducing the expression of Pax2 and Vax1/2 * Shh also represses Pax6 expression to more distal parts of the optic vesicle * Pax2 and Pax6 repress each other and maintain the optic stalk/neural retina boundary * Fgfs from the overlying ectoderm induce Vsx2 * Vsx2 and Pax6a induce neural retina fate * Wnts and BMPs from the extraocular mesenchyme promote retinal pigment epithelium fate by inducing Otx2 and Mitf * Mitf and Vsx2 repress each other and maintain the neural retina/retina pigment epithelium boundary * Many of the retina and retina pigment epithelium genes are initially expressed throughout the optic vesicle * NR versus RPE specification involves a gradual restriction of fate * Gradual restriction of fate is effected by inductive events, and by cell-intrinsic events.

Question 27

What will the optic vesicle form

Answer 27

Optic cup

Question 28

Following division of the different regions of the optic vesicle what does the RPE do?

Answer 28

* RPE cells flatten, become pigmented and spread at the back of the retina * They have essential functions to maintain the homeostasis of the retina

Question 29

What are the 5 functions of the retinal epithelium?

Answer 29

1. Formation of the outer blood-retinal barrier 2. Transepithelial transport of nutrients and waste 3. Transport/storage of retinoids allows rhodopsin to detect light 4. Phagocytosis and degradation of outer photoreceptor segments if they become damaged by phototoxicity 5. Protection against excessive light

Question 30

How do RPE cells enter the retina? What does this drive?

Answer 30

• RPE cells flattening and stretching • “rim” involution – cells at the distal tip will turn around and move into the retina. They do this by docking there apical sides and moving there basal sides so they become incorporated into the retinal cells. • The retinal cells become very elongated along the apical-basal axis • Neural retinal cells basal constriction These changes drive optic cup folding

Question 31

How does the lens develop following formation of the optic cup? What forms the cornea?

Answer 31

* The lens becomes invaginated from the overlying ectoderm and forms the lens vesicle * The overlying ectoderm that status outside will differentiate to give rise to the cornea which is a protective hard layer in the most anterior portion of the eye * The optic cup, lens and cornea develop in parallel and interact with each other

Question 32

When does the lens ectoderm becomes specified? Where is it specified - what is it called? What triggers optic cup folding and invagination of the lens ectoderm to form the lens vesicle?

Answer 32

* The lens ectoderm becomes specified very early on at the same stage at which the eye field becomes specified within the neural plate * The lens plate becomes specified at the edge between the neural plate and the ectoderm in a region called in the preplacodal ectoderm. * A number of signals are released by the lens ectoderm and the optic vesicle. This triggers the changes leading to the folding of the optic cup and the invagination of the lens ectoderm to form the lens vesicle

Question 33

What is the final stage in eye development?

Answer 33

The last process during eye formation that we will explore in a bit of detail is retinal differentiation. In the first slide in the recording below, you can see a schematic of the differentiated retina. This is a laminated structure, composed of six types of neurons (retinal ganglion cells, amacrine cells, horizontal cells, bipolar cells, cone photoreceptors and rod photoreceptors) and one type of glial cell (müller glia).

Question 34

Describe the organisation of the retina

Answer 34

* Between layers are synpatic junctions between the axons | * Photoreceptors = cones and rods

Question 35

How does neurogenesis proceed in waves? What does each wave form? What is wave progression controlled by?

Answer 35

* Several waves of neurogenesis sweep through the retina. The retinal progenitors that confer the retina will stop proliferating and will exit the cell cycle and start differentiating which occurs in waves. * Each wave generates a different type of neuron(s) * Wave progression is controlled by a feedback loop between a signaling molecule and a TF * The best understood wave is the one that generates retinal ganglion cells (RGCs) * Retinal ganglion cells sweep round the retina

Question 36

Describe the first wave of neurogenesis: RGCs

Answer 36

* Shh induces cell cycle exit and triggers RGC differentiation * RGC progenitors express Ath5 (induced by Shh) * Differentiated RGCs express Shh

Question 37

What is the competence model?

Answer 37

* All of the neurones that derive from the retinal progenitors seem to appear in a sequential way similar to the RGCs * This led to a model to explain how neurogenesis was happening in the retina called the competence model * This model states that retinal progenitor cells within the retina are all equivalent. They then give rise to a more differentiated form. * Retinal progenitors within the retina seem to behave in this way, but over time there competence to give rise to different types of cells will change. This change in competence will lead to the formation of different cell types at different times during retinal differentiation which will lead to the different neuronal types that we can see in the retina.

Question 38

How was the competence model shown?

Answer 38

* Kaede is a green fluorescent protein, that glows red under UV light. * The researchers expressed Kaede in cells of the retina and following the cells to see what they were giving rise to * They found that different cell types derive from different progenitors * One cell give rise to lots more! * Each labelling event was giving rise to different results – lots of variability. This does not fit with one progenitor one cell type.

Question 39

The stochastic model for retinal cell fate differentiation

Answer 39

• They proposed that progenitors in the retina make stochastic choices on how to divide and what to differentiate into, however these choices happen with a certain probability. This probability varies and changes over time so that early stages retinal development there will be a high probability it will give rise to two progenitors but as development progresses there will be a higher probability for the progenitors to divide to give rise to specific cell types