Lecture 45 Flashcards
Neural retina and retinal pigmented epithelium (RPE)
The retinal part of the eye consists of the neural retina and the retinal pigment epithelium (RPE).
- Inner part of eye is sensory retina aka neural retina
- Surrounding outside of neural retina is non-sensory retinal pigmented epithelium (RPE)
Neural retina and RPE are both..
- part of the neural tube that is part of the CNS and derived from the neural ectoderm (along with the 2 layers of inner epithelium of the ciliary body and iris) and are therefore part of the central nervous system.
- continuous with the 2 ep layers that cover the ciliary body and inner portion of the iris.
1. RPE continues as a pigmented outer layer of ep cells.
2. Neural retina continues as non-pigmented inner layer of ep cells covering the ciliary body - BUT, both neural retina and RPE are pigmented at inner portion of iris
Optic disc
Optic disc: Where the optic nerve connects neural retina with the rest of the brain
- Where the optic nerve leaves the eye, and the optic nerve contains axons from the retinal ganglion cells (located in retina).
- optic nerve sends axons to optic disc, then axons leave the eye along the optic nerve
Retina layers
Dark layers are nuclear layer. Light layers are plexiform layer, bc very little cell bodies.
Retinal pigmented ep (RPE) is 1 cell thick. All non-RPE is the neural retina
Outer to Inner:
- Choroid (not part of retina)
- darkly-stained and has fenestrated capillaries to give nutrients to outer portion of retina
- red blobs in retina are blood capillaries. Inner portion of retina has capillaries. But outer part has no capillaries. So, outer part of retina receives nutrients from choroid
- Blood capillaries in brain are not fenestrated. The endothelial cells
that form capillaries are connected to each other by tight junctions, so the capillaries are separated from the fluid in the brain. These endothelial cells and no fenestration but
connected with tight junctions form the BBB. So, everything (glucose) that wants to go through brain needs to go through endothelial cells and astrocytes. - There is blood retina barrier, so material needs to go through endothelial cells and be actively transported across (unless it is
lipophilic) to get to the outer part of the retina. - Capillaries in choroid are fenestrated
- Pigmented epithelium (RPE)
- RPE is 1 cell layer, next to choroid, and connected to choroid via tight junctions.
- retina barrier is done by retinal pigmented ep cells and by the fact that capillaries in inner portion of retina are not fenestrated - Inner and outer segments of rods and cones
- contains, rod, cone, photoreceptor cells
- this is the photoreceptor layer - Outer limiting membrane
- formed by Mueller glia cells - Outer nuclear layer
- contains cell bodies of photoreceptors
- photoreceptor cell bodies (nuclei) - Outer plexiform layer
- no cell bodies
- has output structure (might be axon) of photoreceptors (are neurons but do not make AP)
- contains synapses
- has dendrites of 2 diff cell types: bipolar cell and horizontal cells
- only contains axons, synapses and dendrites
- synapses from rods and cones to horizontal cells and bipolar cells - Inner nuclear layer
- has cell bodies of horizontal cells (outer part), cell bodies of bipolar cells (middle part), and cell bodies of amacrine cells (inner part) - Inner plexiform layer
- has cell processes of amacrine cells and axons of bipolar cells
- synapses of bipolar cells to ganglion cells and amacrine cells - Ganglion cell layer
- contains retinal ganglion cell bodies
- contains ganglion cells (variable thickness)
- ganglion cells send axons through nerve fiber layer to optic disc to optic nerve to brain.
- ganglion cells fire APs. They have long thin axons that give neural signal to brain. APs are slow but good for long distance
- HOWEVER: all other neurons in retina layers do NOT rely on APs. Other neurons transmit signals by passive spread, which is at the speed of light. But passive spread attenuates/diminishes the signal, but this is not bad since the distance is short - Nerve fibers layer (axons of ganglion cells)
- axons of ganglion cells that course to optic disc - Inner limiting membrane
- basal lamina of Mueller cells
- limits the neuroretina to the vitreous body
- inner limiting membrane is formed by the glia cell of the retina called the MUELLER cell, which takes up all the space in bw neurons and synapses. It also
maintains homeostasis in extracellular fluid, it buffers potassium, it provides nutrients and nt to the cells. The cell’s end feet form the inner
limiting membrane to protect the retina
ALSO: amacrine, horizontal and bipolar cells do not fire APs
ALSO: the closer to the optic disc, the thicker the nerve fibers
Retinal pigment epithelium (RPE)
SUMMARY of RPE Functions
- Nurtures the outer layers of the retina and removes debris (shed disks)
- Is one layer of epithelial cells that are separated to choroid by Bruch’s membrane
- Connected by tight junctions (blood retina-barrier)
- Contain melanin granules and phagosomes
- Provide retinal (Vitamin A)
RPE Functions:
- Light absorption
- RPE is pigmented.
- Any light that goes through the retina and is not absorbed by photoreceptors is absorbed in RPE, so it does not harm other tissues - Epithelial transport
- Nutrient transport to outer layers of the sensory retina
- needs to transport ‘nutrients and metabolites that the retina needs to
get rid of’ across its cells.
- most important things to transport from
fenestrated capillaries in choroid are: glucose (fuel of the cells) and vitamin A (photopigment that photoreceptors need in order to absorb a photon)
- it also extracts h2o and Cl- from eye - Glia
- glia function is to buffer potassium and for metabolic support
- Potassium homeostasis - Visual cycle
- provides retinal to photoreceptors
- if retinal absorbs photon, it does a conformation change and is then recycled into 11-cis-retinal and given back to the photoreceptor cell by the RPE.
- they are important for the visual cycle to restore 11-cis-retinal that can absorb photons
- Synthesis of melanin and melanosome formation - Phagocytosis
- Phagocytosis of shed disks
- the rods constantly shed their outer portion of the outer segment (ROS).
- some membrane where the photopigments are located are constantly shed, so RPE picks up cell debris to degrade it and transport it away. They clean up by phagocytosis
- ROS (outer segments of rods) are sometimes broken, they are shed. they are included in phagosomes that fuse with liposomes and degrade the
membrane disks and get rid of them - Secretion
- they secrete growth factors, important for proper function and growth of retina - Synthesis of…
- Bruch’s membrane. Bruch’s membrane protects eye and is implicated in age related diseases in retina. It also thickens with age.
- Blood retina barrier
ALSO:
Transmission Electron microscopic Images of RPE
- dark spots are melanin that absorb light.
- there are photoreceptor outer segments anchored within the RPE to pick up the
sheded outer segments, degrade these segments, and then get rid of them
Rods
Summary: - Rods contain rhodopsin - They are very light sensitive - Discs are detached from outer membrane - Discs shed (turn over) - Rod spherules (synapses) contain synaptic ribbons, form triads with bipolar and horizontal cells
A type of photoreceptor (the other one is cones). Rods are very light sensitive.
- Photopigment of rod is called rhodopsin and the pigment that absorbs light is 11-cis-retinal (part of the rhodopsin)
- Photon absorption causes bleaching of rhodopsin, which is a process that is initiated by the photoconversion of 11-cis-retinal to all-trans-retinal
- Bleached rhodopsin = opsin and all-trans retinol
• Spherule forms synaptic contacts with bipolar and horizontal cells
Rods contain nucleus, inner segment and outer segment. Order: spherule, synapse, axon, nucleus, inner segment, outer segment, RPE, Bruch’s membrane, choroid.
- all the photopigment is in outer segment.
- Inner segment has all organelles that a cell contains, mostly mitochondria. Nucleus not here.
Rods constantly form new membrane
disks. Disks are newly synthesized, Outer disks shed.
- these membrane disks eventually detach from the outer membrane, becoming disks that are suspended inside the outer segment of the rod.
- at the top end of outer segment (border bw outer segment (cylindrically shaped) and inner segment), they form new disks.
- at the bottom end of outer segment, the disks are shed and picked up by RPE.
Also: - above nucleus is axon. - above axon is gigantic synapse. - the synapses from photoreceptor cells are giant pre-synaptic terminals. Each terminal makes contact with 3 different post-synaptic cells at the same time (bipolar cells and horizontal cells; this is called a TRIAD).
Cones
Summary:
- Cones contain iodopsins
- Humans are trichromats, they have red, green and blue cones
- Cones absorb light of different wavelengths – they see colours
- In cones, discs are not detached from outer membrane – comb structure
- Cone discs do not shed
- Cone pedicles contain synaptic ribbons, form triads to bipolar and horizontal cells
Order: Pedicle, synapse, axon, nucleus, inner segment, outer segment (conically shaped), RPE, Bruch’s membrane, choroid
Disks
- continuous with plasma membrane
- No shedding
- The disk membranes in cones are not detached from the outer membrane (unlike in rods). Cones form disks, but the disks are not detached and do not shed.
Photopigment is iodopsin
- Detailed vision
- Color
- 3 iodopsins (humans): Blue, green and red light
- Color blindness is lack of 1 or more iodopsins
Pedicle
- Thicker than spherule that is within rods
Outer segment section of cones is shorter than those of rods.
Pre-synaptic terminal is bigger in cone than in rod, and forms more triads.
Synaptic ribbon (synapse) has nt vesicles lined up on this band, which are constantly released.
Optical illusion - gray background with pink dots in circle (minus 1 dot) with + sign in middle
gray background contains all colours and excites all photoreceptors.
Pink dots excite red and blue cones.
Photoreceptors where the pink
dots are imaged on will adapt or bleach their photopigment, so that as soon as pink dot is gone, you would normally see gray. But red and blue cones are bleached, so you see green dots instead.
This is called after-image. Lilac chase.
Outer Plexiform and Inner Nuclear Layer
Cones and rods make synpases to bipolar cells, which convey info further to ganglion cells and horizontal cells.
Horizontal cells do lateral info processing
Inner Nuclear, Inner plexiform & Ganglion Cell Layer
Inner plexiform layer is where bipolar cells convey info to retinal ganglion cells.
- amacrine cells are located here too. They have cell processes that release nt and receive synaptic input. These amacrine cells also connect to neighbouring bipolar cells and neighbouring ganglion cells, they do contrast enhancement of edges
Ganglion cells
– Diffused (contact several bipolar cells)
– Midget (receive impulses from cones only)
– are the output cells of the retina. Their axons converge on the optic disk to form the optic nerve
Midget bipolar
– contracts only a single ganglion cell
– collects the visual input from cone and rod photoreceptors
Amacrine cells
– No axons or dendrites
– Has highly branched neuritic processes or neurites that conduct in both directions
– Function is to sample and modify the output of bipolar cells
Optical illusion - black background and white grid, looks like there are black dots at the corners or junctions of the white grid
junctions aka corners look darker
Called contrast enhancement due to amacrine cells in inner plexiform layer
Lateral Inhibition - Contrast enhancement process. Explains the black and white grid optical illusion
Contrast enhancement process. Lateral inhibition to the neighbouring cells, provided by amacrine cells
4 cells, Left to right: 100 start due to high intensity (-10 on left, -10 on right), 100 start due to high intensity (-10 on left, -2 on right), 20 start due to low intensity (-10 on left, -2 on right), 20 start due to low intensity (-2 on left, -2 on right)
Very left cell is 10% inhibition from each side, so 100 becomes 80.
At the edge of 100 and 20 (bw 2nd and 3rd cell), there is high inhibition from left cell and low inhibition from right cell. The left cell is 88 (100 minus 2 minus 10), which is a higher number and big contrast compared to the right cell at 8 (20 minus 2 minus 10).
- SO, at the edge of 88 and 80, the white at 88 is brighter (contrast enhancement)
- Also, bw the second (88) and third (8) cell, there is a very big contrast, so the white is very white here and the black is very black here.
- calculating the fourth cell: 20 minus 2 minus 2 is equal to 16
The Mueller Cell
- is a glial cell; glial cell of the retina
- fills all the gaps and spaces bw the cells in the retina
- wraps around each synapse and insulates them against each other
Fovea
Highest visual acuity
1: 1 connections
- 1 photoreceptor makes synapse to 1 bipolar cell to 1 ganglion cell
At periphery of fovea, many photoreceptors converge to 1 synapse, so ratio is bigger, and visual acuity or sharpness is less
Cones only
- in the fovea, we only have cones, so that is where we see colour
Bipolar and ganglion cells are squeezed to the side at the fovea.
- To have high visual acuity, we do not want the light to be scattered by going through the ganglion cell layer, the inner nuclear layer, and the outer nuclear layer until it eventually hits the outer segments of the retina; light travels through a lot of retina layers since our photoreceptors are located at the most outer layer of retina
- SO, we prevent light scattering by having inner nuclear layer and ganglion cell layer are squeezed to the side. So, the light can unobstructedly hit the outer segments of the photoreceptor cells
In central areas, signals are transmitted 1 photoreceptor to 1 bipolar cell to one ganglion cell. In periphery many photoreceptors converge on one bipolar cell and many bipolar cells converge on one ganglion cell. – High visual acuity in central retina, less visual acuity in periphery.
If something is faint, the cones are not as light sensitive as the rods. SO, to see a faint star at night, do not look directly at it (with
cones), look slightly away so it images on the rods (more light sensitive)
Fovea und Macula
Fovea
- dot in middle
- Cone-rich
- Devoid of inner retina layers
- Unobstructed light
- High quality vision
Macula
- area around fovea
- multiple layers of ganglion cells
- contains yellow pigment
- Inner retina layers have yellow pigment
- Absorbs short wavelength light
- macula absorbs things like blue light. Blue light is harmful to tissues so it is absorbed by macula
Optic Disk
- Site where axons of ganglion cells turn into optic nerve
- No photoreceptor cells
- Blind spot
- Blood vessels
No cell layers, just nerve fiber layer that becomes thick.
- All axons come, merge together and leave retina, then form optic nerve, which
becomes very thick.
- It becomes thick bc of myelination.
- As soon as axons leave eyeball, they become myelinated, which is the thick optic
disk
