What is the evolution of the human eye?
- Began with a light spot (light sensitive proteins) in single called organisms
- Light cups grew deeper; the opening at the front grew smaller, creating a pinhole effect increasing the resolution by reducing distortion by only allowing a thin beam of light into the eye
- Cornea evolved through transparent cells covering the opening, reducing infection, allowing the inside of the eye to fill with fluid (transparent humor) that optimises light sensitivity and processing
- Crystalline proteins (lens) form at the surface which is important at focusing light at a single point on the retina —> this is key for eye’s adaptability, changing its curvature to adapt to near and far vision
- Iris formed which controls the amount of light entering the eye
- Sclera is the tough outer layer which maintains the structure
- Tear glands produce a protective film
- Expansion of the visual cortex of brain to produce sharper images
- We have a blind spot as human retina is inverted, with the light detecting cells facing away from the eye opening
What is the prototypic eye?
- Photoreceptors (cells responsible for detecting light)
- Cup of pigment cells (provide support to photoreceptors and prevent damage and light dispersion)
- They are the common origin of all cells
-Despite the huge diversity in morphology and shapes in eye organisation of animal kingdom, all of them are composed by photoreceptors and ligament cells
What is the most simple eye?
Ocelloid of dinoflagellates
- Unicellular
- Single cell surrounded by membrane with organelles and flagellum (moves by following visual stimuli)
- Membrane is folded which contains rhodopsin in a crystalline body/hyalosome (follows visual stimuli)
- Rhodopsin detects light and is present in photoreceptors of multicellular organisms
What is Pax6?
- Pax6 is really important for eye formation
- Pax6 is master-regulator eye formation
- When you remove Pax6 in Drosophila, eye did not form
- Detected by Walter Gehring’s labwork in Drosophila
- Involved in mutagenesis screening
- They found an eyeless Drosophila and called it ‘eyeless’
- When they went on to find which gene was affected, they found that it was Pax6 (called ‘eyeless’) which is a transcription factor
- Pax6 was already known for eye formation
- Walter Gehring and team took out Pax6 gene from Drosophila from its original position and moved it to different locations
- He used GAL4 UAS system: transcriptional activator called GAL4 which comes from yeast that combine to enhancer sequences called UAS
- GAL4 and UAS don’t exist in Drosophila genome but we can engineer flies that will drive whatever gene we put under the control of UAS (under the expression of GAL4).
- we can get GAL4 to be expresses and we have the UAS enhancer (which we can use to control another gene)
- When they combined GAL4UAS with Pax6 embryonic cDNA in different parts of the Drosophila, they generated ectopic eyes, wherever they were driving the generation of Pax6 (forced expression in different parts of Drosophila). This led to the idea that Pax6 was enough for eye formation
- They also found that the mouse Pax6 gene also makes eyes in Drosophila
- RX is not needed in Drosophila in eye development
- It’s important to address that Pax6 is not the only eye-master gene. There is complex genetic interaction, many transcription factors are involved.
What happens in loss of Pax6?
-Leads to defects across phyla
In humans:
- cornea opaque
- iris absent
- retina degenerate
- lens opaque
- increase pressure of aqueous humour of eyeball
In mouse:
- decreases eye size
- lens fused to form cornea
- iris morphology
What are the important eye development genes in vertebrae?
For eye field:
-Pax6, TII, RX, T/Tbx3, Six3/6
For optic vesicle:
-Otx2, Mitf, Pax6, Vsx2
RX is very important for eye formation in vertebrate because when we remove or mutate RX, embryos won’t develop eye
-Mutant for RX leads to absence of eye formation which is anophthalmia
What is the morphogenesis of the vertebrae eye?
- The cells that 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 rearrangement and inductive events, these group of neuroepithelial cells transform into your optic cup (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
- Retina and pigmented epithelium arise from the central nervous system primordium (neuroectoderm)
- Axons of neurones are collected into optic nerve. Optic nerve exits through the optic stalk in the embryo to innervate parts of the brain involved in visualising
-Retinal Pigmented epithelium covers the retina. It is heavily pigmented. It has trophics and protective functions for the eye
What is the eye derived from?
- Ectoderm = neuroepithelial cells, lens and cornea
- Endoderm = e.g forms blood vessels
Retinal pigmented epithelium covers retina
What are the key points of eye morphogenesis?
- Specification of eye field
- Optic vesicle evagination
- Patterning of optic premordiun
- Optic cup folding
- Lens specification and morphogenesis
- Differentiation of retina, retinal pigment epithelium and optic nerve
What is the eye field specification?
- Promotes by a group of transcription factors known as Eye Field Specification transcription factors (EFSTF) —> Pax6, TII, RX, T/Tbx3, Six3/6
- They are expressed in overlapping patterns in an anterior portion of neural plate
- The region where they are expressed at the same time will become the neural plate
- The EFSTF regulate Lhx2 which is transcription factor that determines eye fate
- Eye field specification is tightly linked to anterior-posterior patterning of the neural tube
- Neuroectoderm was specified from dorsal region of ectoderm in gastrulating embryo
- As gastrulation progresses, these neuroectoderm cells become rough patterned along the anterior-posterior axis due to posteriorising signals such as Fgfs, retinoic acid, Wnt and antagonists of the posteriorising signals in the anterior portion of the neuroectoderm and progresses.
- This leads to activation of different transcription factors in the neuroectoderm and establishment of anterior and posterior regions in the neuroectoderm
- One of the first subdivisions between anterior and posterior is the establishment of midbrain and hindbrain boundary
- Anterior to the mid-hindbrain boundary, there is an expression of a group of transcription factors called Otx (midbrain fate)
- Posterior to it are another group of transcription factors called Gbx (hindbrain fate)
- The eyefield becomes specified in the anterior most portion of the anterior neural plate (which is the OTX positing region of the neural plate)
- Eye field can only be specified in a OTX-positive region of the neural plate
- EFTFs are expressed (combinational expression) in the OTX-positive region of the neural plate
- They demarcate the eye field and lead to its specification and its differentiation into 2 vesicles
- The eyefield is continuous along the midline of the embryo
- Once the eye field is specified, its split into 2 domains by signals released at the underlying midline by activity of Shh
- Shh is expressed along the ventral midline of developing embryo
- It’s important in activating a number of genes that are important in midline formation
- Shh represses Pax6. By repressing Pax6 in the midline, it leads to separation of single eyefield into 2 fields on both sides of midline and allows specification of optic stalk later.
-Optic vesicle = presumptive retina
What other mechanisms are involved in splitting the eyefield in lower vertebrates?
- Physical separation by anterior movement of midline tissue
- Active movement of eye field cells away from the midline
Optic vesicle evagination on zebrafish:
- anterior neural plate will fold over itself go give rise to neural tube
- tissues at side of eyefield will be brought together towards the dorsal midline as the folding occurs
- the tissue underlying the eyefield will squash through the eyefield; cells of the eyefield will simultaneously evaginate towards the lateral tissue, forming the optic vesicles
What are the similarities and differences in optic vesicle organisation in mouse and fish?
Similarities:
- both have neuroepithelium
- basal part of neuroepithelium is on the outside and apical side is facing the lumen
Differences:
- mouse has a bug lumen
- in zebrafish, lumen is oblurated (the apical sides of dorsal and ventral halves of optic vesicle are touching each other)
What is optic vesicle patterning?
- As the optic vesicle becomes fully enveloped, it will become patterned to give rise to 3 main neural derivatives
- These 3 derivatives are the optic stalk, neural retina and retinal pigment epithelium
- This organisation follows proximal to distal disposition
- Proximal: optic stalk
- Middle: neural retina
- Distal: retinal pigment epithelium (most dorsal; the optic vesicle turns around to become optic stalk again)
1) Most proximal part of optic vesicle
- Shh from midline promotes optic stalk fate by inducing expression of Pax2 and Vax1/2
- Shh also represses Pax6 expression to more distal parts of the optic vesicle
- Subsequently, Pax2 and Pax6 repress each other and maintain the optic stalk-neural retina boundary
2) Middle portion of the optic vesicle
- Overlying ectoderm is in close contact with her optic vesicle
- Fgfs are released by the overlying ectoderm which induce transcription factor Vsx2 in the middle region of optic vesicle.
- Vsx2 and Pax6a induce neural retina fate
3) Distal portion of the optic vesicle
- At the same time, in the distal part of the vesicle, Wnts and BMPs are secreted by the extraocular mesenchyme that surrounds the optic vesicle
- They promote retinal pigment epithelium by inducing Otx2 and Mitf (transcription factors)
- Mitf and Vsx2 repress each other and maintain the neural retina-retina pigment epithelium
-This leads to fully patterned optic vesicle
- Many of the retina and retina pigment epithelium genes are initially expressed throughout the optic vesicle
- Neural retina and retina pigment epithelium genes are initially expressed throughout the optic vesicle.
- Neural retina and retinal pigmented epithelium specification involves gradual restriction of fate
- Gradual restriction of fate is affected by inductive events and by cell intrinsic events
What is optic cup folding?
- Optic vesicle folds to give rise to optic cup
- Patterning is important for correct folding
-Tissues that are specified within optic vesicle during patterning (e.g optic stalk, neural retina and pigmented retinal epithelium) will undergo different morphological rearrangement (structure and shape) —> this drives optic cup folding
- Retinal pigmented epithelium cells become pigmented and spread at the back of the retina
- TheI function is to maintain homeostasis of retina which protects the eye from photo-damage
- Folding of the optic cup is not symmetrical
- Opening along ventral portion of optic cup is called ‘optic fissure’ or ‘choroid fissure’ which is important for eye formation as blood vessels enter the eye here
- Axons of neurones of retina forming the optic nerve will exit the eye to go to the brain
- This fissure fuses at the end of the eye formation so that retina and pigmented retinal epithelium become continuous along ventral portion of optic cup
- As the optic fissure closes, the optic stalk narrows down (ventral sides come closer together and fuse)
- The interface between retina and optic stalk become a small opening called the ‘optic disc’ (where the optic nerve enters)
- Axons from the retinal ganglion cells (that innervate the brain) will exit the eye through the optic disc and migrate towards the midline along the optic stalk
- Signals released by the growing axons (Shh signals) promote the differentiation of the optic stalk cells into astrocytes that ensheath the optic nerve
-This completes the differentiation of neural derivatives of optic cup of eye
What are the retinal pigmented epithelium function?
- Formation of outer blood-retinal barrier
- Transepithelial transport of nutrients and waste in and out of eye
- Phagocytosis and degradation of outer photoreceptor segments
- Protection against excessive light and prevents scattering
What is the shaping of the optic cup?
- Lens become invaginated by overlying ectoderm and forms lens vesicle. The rest of the overlying ectoderm will differentiate to give rise to cornea (protective layer)
- The optic cup, lens and cornea develop in parallel and interact with each other. This morphogenesis is tightly coordinated by the release of signals from retina and lens that influence neighbouring tissue.
-Once the lens differentiate, it releases factors (signals that promote cornea differentiate)
What are the sequences of events leading to lens induction?
- Lens ectoderm/lens primoridum becomes specified very early on at the same stage as eyefield becomes specified within neural plate
- The lens ectoderm becomes specified at the edge between the neural plate and ectoderm from a region called preplacodal ectoderm
- Lens ectoderm becomes specified in a small portion of the preplacodal ectoderm - at this stage, eye field becomes located in the anterior portion of the neural plate and the Lens ectoderm is adjacent to neural plate
- As neurulation progresses and neural plate invaginates and forms neural tube, the eyefield invaginates to give rise to optic vesicles
- These vesicles become very intimately oppose to Lens ectoderm
- This intimate contact between lens placode and optic vesicle is necessary for further morphogenesis of eye primordial.
- This is affected by signals released by Lens ectoderm and optic vesicles. This triggers the morphogenesis and folding of the optic cup and invagination of Lens ectoderm to form lens vesicle
What is the organisation of the retina?
1) Most external layer
- photoreceptors (perceive light stimuli) make synaptic connections within cells in the next layer
2) Innernuclear layer
- consists of amacrine, bipolar and horizontal cells
- they integrate the visual stimuli received from the photoreceptors
- they send this partially processed information to retinal ganglion cells by synaptic connections (located in the most inner layer of retina)
3) Retinal ganglion cells
- extend their axons along the optic nerve and innervate different regions of the brain for processing visual information
-In between the layers, we have plexiforn layers in which axons become organised and synaptic connections are present
What are the 6 types of neuronal types?
- Retinal ganglion cells
- Amacrine cells
- Bipolar cells
- Horizontal cells
- Cone cells
- Rod cells
- They all differentiate over a prolonged period of time
- They arise at different stages of retinal differentiation
What happens in neurogenesis?
- Neurogenesis proceeds in waves
- Several waves of neurogenesis sweep through the retina (where the progenitor cells exit cell cycle and differentiate)
- Each one generates a different type of neurone
- Wave progression are controlled by a feedback loop between a signalling molecule and transcription factor
- The best understood wave is the one that generates retinal ganglion cells (RGCs)
How are retinal ganglion cells generated?
- Ath5/Tg is present in RGC as they start to differentiate
- This differentiation sweeps all the way across the retina
- Waves of differentiation starts at the anterior ventral portion of retina (ventronasal region of retina)
Molecular POV:
- it started by signal from the midline tissues
- these signals are the hedgehog pathway which initiates neurogenesis (first wave signals = Shh, AthS, RCCs)
- Shh induces cell cycle exit and triggers retinal ganglion cell differentiation
- FGF is involved and produced in ventral portion
- Retinal ganglion cell progenitors express Ath5 which is induced by Shh which drives differentiation
-Differentiated retinal ganglion cells express Shh. This influences the retinal ganglion cell progenitors nearby and start differentiation.
It’s not clear whether all waves of neurogenesis follow the same pattern and respond to the same signalling molecules. However, all of your neurones derived from retinal progenitors seem to appear in a similar wave to retinal ganglion cells
What is the competence model?
-How neurogenesis is occurring in the retina
- Retinal progenitor cells within retina are all equivalent
- They can undergo assymetric or symmetric division
- Over time, their competence to giverise to different types of differentiated cells would change
- This change in competence would lead to different cell types are different times during retinal differentiation which leads to differentiation of all the different neurone types
- However, there was some overlap in waves of differentiation so this theory was contradicted
- Different cells follow different temporal waves of differentiation
What is an alternative theory (one-progenitor-one cell type)?
- There are different progenitor types, one for each type of retinal cell
- We can test this hypothesis by labelling one single progenitor and its progeny in the retina
- If this hypothesis is true, it should result in only one cell type being labelled
- This expire was done in zebrafish using a fluorescent protein and UV light
- They found that different cell types were derived from each progenitor (e.g bipolar cells, horizontal cells, photoreceptors etc)
- This experiment showed that this doesn’t fit with one progenitor-one cell type
What is the stochastic model for retinal cell fate differentiation?
- Progenitors make stochastic choices on how to divide and what to differentiate into
- These choices will happen with a certain probability
- The probability will vary over time so that at early stages of retinal development, the high probability would be that when a progenitor is dividing, it gives rise to 2 progenitors
- But as development progresses, there will be higher probability for progenitors to divide to give rise to specific cell types
-This explains the variability seen in the previous experiment
