Eye development Flashcards
(39 cards)
What do photoreceptors do?
cells that are responsible for detecting light
What do pigment receptors do?
provide trophic support to the photoreceptors to protect from damage and to avoid dispersion
Is there diversity in the the morphology of the eye?
Yes
What did Darwin suggest the eye consisted off?
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….
What does the planarian eye consist of?
It consists of photoreceptors protected by a cap of pigment cells
What are dinoflagellates?
• This is a unicellular organism (one cell surrounded by a membrane with organelles inside and a flagella).
How does the dinoflagellates move?
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.
Describe the eye structure in dinoflagellates
- 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
What TF is needed in eye formation
Pax6
What does mutations in Pax6 cause?
Eyeless - no eyes formed
Are there lots of homologues of eyes
Yes
How did researchers induce the formation of an ectopic eye?
- 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
Is Pax6 the only gene that drives eye formation
No
refer to image
What is Rx and is it found in drosophila?
- 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
What are the steps in eye formation?
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
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?
- 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
In mice, when does the optic vesicle start evaginating?
• The optic vesicles start evaginating prior to the closing of the neural tube
What does the optic vesicles form?
What is the lumen of the vesicles in contact with?
- 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
What specifies the eye field?
Where are these expressed?
- 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
Where does the eye field become specified?
• 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
What demarcates the eye field?
- 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
What happens when the eye field is specified?
Does it specify the optic stalk?
• 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
What other mechanisms are involved in splitting of the eye?
• 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
Following optic vesicle evagination what structures can be be seen in zebrafish and mouse?
The zebrafish and mouse eye both look very different, however there are a number of similarities:
- Neural epithelium is found in both the zebrafish and mouse on the apical side and basial side
- 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
