Chapter 13: RPE Flashcards
What is the Retinal Pigment Epithelium (RPE)?
a single layer of cuboidal epithelial cells that constitutes the outermost layer of the retina, located b/w the choriocapillaris and the photoreceptor outer segments
How many cells are located in the RPE layer? What is the ratio of RPE cells to photoreceptors?
4-6 x 10^6 RPE cells/eye
45:1 ratio of photoreceptors: RPE cells
Embyrologically, what is the RPE derived from?
Derived from the optic cup (same as neural retina), but differentiates into secretory epithelium
Describe the anatomy of the RPE cell
- RPE are polarized epithelial cells, with long microvillous processes on their apical surface that interdigitate with outer segments of photoreceptors.
- On their basal surface there are many infoldings, which serve to increase surface area for solute exchange
- Joined near their apical sides via tight junctions, which constitutes the outer blood-retina barrier
- In addition to organelles, contain phagosomes, peroxisomes and melanin (suggest active in detoxifying radicals and oxidized lipids)
How does the RPE produce energy?
Produces energy via the TCA cycle, glycolysis, and pentose phosphate pathway
What is the primary carbon source for the RPE? Secondary sources?
Primary source= glucose
Other sources: glucosamine, fucose, galactose, mannose
What component of the RPE accounts for > 80% of its wet weight?
Water
What is RPE Na+K+ATPase? why is it important?
located on the apical surface rather than the basal surface (different than orientation in most cells) and functions to maintain the balance of Na+ and K+ in the subretinal space
What is RPE65?
a protein unique to the RPE, is an obligate component of the isomerization and hydrolysis of vitamin A
What percentage of the wet weight of the RPE is derived from lipids?
3%; 50% of which are phospholipids, >80% of which are phosphatidylcholine and phosphatidylethanolamine
What is the function of saturated fatty acids in the RPE?
used for retinol esterification and enegy metabolism by the RPE mitochondria
What are the functions of the RPE?
- Visual Pigment Regeneration
- Phagocytosis of shed photoreceptor outer segment discs
- Transport
- Pigmentation
- Retinal Adhesion
How does the concentration of vitamin A in the RPE compare to other tissues in the body?
Has the second highest vitamin A concentration in the body (second to the Liver)
What is the basic function of the RPE in the visual process?
generation of 11-cis-retinaldehyde (used in formation of rhodopsin)
What molecules are used to create rhodopsin?
Opsin (synthesized by photorecepter) + 11-cis-retinaldehyde
What happens to rhodopsin when it becomes photolyzed?
Isomerizes to 11-trans-retinaldehyde, which is released, converted to all-trans-retinol via retinal dehydrogenase, returned to the RPE via interphotoreceptor retinoid binding protein (IRBP), where it is converted into retinol ester for storage.
When needed for rhodopsin regeneration, is converted to 11-cis-retinol–>11-cis-retinal and returned to the photoreceptor via the IRBP
How does the RPE acquire vitamin A?
1- through release during bleaching of rhodopsin and return via regeneration of visual cycle
2- from circulation
3- via phagocytosis of shed photoreceptor outer disc segments
Why does the RPE store retinoids as esters?
Stores retinoids as esters b/c the alcohol and aldehyde formations are membranolytic
What happens to photoreceptor outer disc segments?
They are encapsulated in phagosomes, which fuse with lysozymes and are digested.
Building blocks are then recycled into photoreceptors for use in synthesis and assembly of new discs
Where does lipofuscin in the RPE come from?
derived from digested photoreceptor disc membranes
is responsible for generating the signal during fundus autofluorescence imaging
How many discs does each photoreceptor outer segment shed each day? How many discs does each RPE digest per day?
Outer segments shed approx. 100 discs/day
RPE digests >4000 discs per day
What kind of a pattern for disc shedding do the photoreceptors follow?
They follow a circadian rhythm
Cones- shed most vigorously at onset of darkness
Rods- shed most vigorously within 2 hrs of light onset
How does dopamine interact with photoreceptors?
Dopamine acts on the photoreceptor-RPE complex to control disc shedding
What direction does Na+ flow through the RPE? K+?
Na+ is actively transported from the choriocapillaris to the subretinal space, while K is transported out of the RPE
Where does active HCO3 transport occur on the RPE?
On the apical surface, similar to Na+K+ATPase
What is the basis for the Electro-Oculogram?
the Trans-RPE potential is the basis for the EOG, which is the most common electrophysiologic test for evaluating the RPE
Where is the melanin in the RPE located?
Melanin is found predominantly in the apical and mid-portions
Why does the fundus in older people appear less pigmented than in younger people?
B/c as humans age, melanin granules fuse with lysozymes
What is the role of melanin in the RPE?
Exact mechanism is unknown, but there is speculation
May act as a neural-density filter to decrease light scatter
Though to play a role in development (when melanin is below a critical level, aberrant neuronal migration in the visual pathway, lack of foveal development, low vision, nystagmus, and strabismus have been shown to occur)
How does the retina adhere to the RPE?
- passive hydrostatic forces
- interdigitation of RPE and photoreceptor outer segments
- active transport of SRF
- complex structure and binding properties of interphotorecepter matrix
