CORNEA & SCLERA (PART 1) Flashcards
How many diopters of power does the cornea have?
48 diopters plus power in normal cornea
there is 60 diopters of power total in normal eye.
what is the measurement of the cornea peripherally and centrally? which is thickest part of cornea.
cornea is thickest peripherally.
centrally= 0.52mm
peripherally= 0.65mm
> 99% of incident light above 400nm is transmitted through the cornea. True or False.
True.
How much radiation does the cornea transmit from ultraviolet and infrared light?
UV= 310 nm infrared= 2500 nm
Name the 5 layers of the cornea from anterior to posterior.
epithelial layer bowman's layer stroma descemete's membrane endothelium
Describe epithelium of cornea. How many layers does it have? what kinds of cells make it up?
stratified, non-keratinized, non-secretory, squamous epithelium.
has 5-7 layers and 3 layers and types of cells that make it up (surface cells, wing cells and basal cells).
describe surface cells?
3-4 cells thick. differentiated squamous cells grow and then are sloughed off surface.
describe wing cells?
(Middle Layer) 1-3 cells thick. intermediate differentiation stage.
describe basal cells?
(basement layer) 1 cell where mitosis occur or cell division and adhere to basement membrane. daughter cells move toward the surface of the cornea and start to differentiate.
where does cell division occur in cornea? surface, wing, or basal layer
basal layer
how many days does it take for the cornea epithelial cells to turnover?
7 days
describe the characteristics of basal cells.
derived from stem cells of limbal epithelium, high metabolic rate, prominent mitochondria, and Golgi apparatus and ER, high glycogen storage.
The mitotic rate of epithelium is 10-15% per day. True or False.
True.
what do young and mature epithelial cells look like?
young= small, light mature= large dark, have breaks or holes in epithelium. will be sloughed.
what are exfoliation holes
breaks in epithelium. cells in process of peeling off the surface of cornea.
basal cell adhere to basement membrane and stroma via what?
hemidesmosomes (linked to anchoring fibrils that pass through bowman’s layer into stroma).
what are anchoring fibers function in epithelium of cornea? what is it made out of?
they are made of type VII collagen and provide firm edition to epithelial layer and goes into stroma 2um). they end in structures known as anchoring plaques.
during surgery adhesion of epithelium is destroyed and takes at least a month to grow back. True or False.
True
what layers are zonula occludens found and what is its function?
Serve as a barrier. ONLY found between superficial cells of epithelium. completely encircle cells.
what layers of gap junctions found?
found in ALL LAYERS. more numerous in basal cells than superficial cell layer.
Na+ is pumped from tears to stroma. Cl- is pumped from stroma to tears. Net Influx= 0
TRUE OR FALSE
TRUE
What is EBMD? what happens as a result of this.
Epithelial Basement Membrane Dystrophy. Painful recurrent epithelial erosions. Basal cells have decreased amount of hemidesmosomes produced.
onset of edema.
Describe Reduplication of Basement membrane.
thickening of basement membrane. Causes epithelial erosions. Happens as result of aging and in diabetic patients.
anchoring fibrils cannot penetrate as deep into the thickened basement membrane into the stroma.
what happens when epithelial layer of cornea is wounded?
Quick response. Must cover wounded area rapidly. Mitosis stops. Cells enlarge, epithelial cells move by ameboid movement to cover wounded area. When wound is closed mitosis starts again. The process is rapid 48 hours. Protein synthesis increases at time of cell migration healing.
During cell migration, in the healing process of the epithelium in cornea when wounded, is it aerobic or anaerobic process? What happens to the glycogen levels, do they increase or decrease?
During the healing process of the cornea epithelium, glycogen levels decrease and glycolytic activity increase, stored glycogen is used to supply energy to cells. An anaerobic process is used.
There is an decrease in protein synthesis and growth factors during healing of cornea epithelium. True or False.
False. there is an increase.
What is EGF growth factor? where is it found?
Epithelium Growth Factor. Can be expressed by corneal epithileum in tears fluid we produce.
what is KGF and HGF? where can they be found?
Kerotocyte and hepatocyte growth factors. synthesized by stromal kerotocytes.
Healing in Diabetic patients of the corneal epithelium is delayed because normal adhesion to cannot occur. They cannot use glucose properly. True or False.
True.
During corneal healing, there is a increase in the length of the cell cycle and decrease in cell production. True or False.
False. There is a decrease in cell length cycle and increase in cell production.
The adhesion of epithelium is established by new hemidesmosomes. If the basement membrane has been damaged it takes longer. True or False.
True.
In a keratectomy procedure, what part of the cornea is removed? How long does it take to replace during healing process after LASIK surgery? Do we want the healing to happen fast or slow? What is prescribed to slow the process of healing?
In a keratectomy the basement membrane of the epithelium is removed from the cornea. Therefore it has to grow back first after surgery before normal adhesion complexes can be established again. The process of healing is approx. 1 year or 12 months. Steroids are prescribed so that healing doesn’t happen too fast. that would not be good.
Where is Bowman’s layer located? Describe is structure and composition.
Bowman’s layer is under the basement membrane of the epithelial layer of cornea. It is the anterior most part of the stroma layer. Acelluar. Has Type 1 collagen. Not in a structured order.
“modified superficial layer of stroma”
Describe the Stroma of the cornea.
An extracellular matrix (ECM) comprised of a lamellar arrangement of collagen fibrils
Parallel to corneal surface
Individual collagen fibrils separated by a matrix of proteoglycans
Keratocytes and fibroblasts – produce and maintain the ECM
Run limbus to limbus
Collagen fibers are made of:
Mostly type I collagen
Also type V and VI collagen
Refractive index
n= 1.411 for the collagen fibers
n= 1.365 for the extrafibrillar matrix
Despite the disparity, minimal light scattering occurs (only 10%): because of the highly uniform size and spacing of the collagen fibers!
anterior stroma run obliquely or orthogonally?
obliquely.
posterior stroma run obliquely or orthogonally?
orthogonally.
Describe Maurice Proposal.
Maurice proposed that corneal transparency is a consequence of a crystalline lattice arrangement of collagen fibrils within stromal lamellas and that light scattered by individual fibrils of uniform diameter is canceled by destructive interference with scattered light from adjacent fibers; therefore light is scattered only in the forward direction.
Such an arrangement requires that all collagen fibrils be of equal diameter and that all fibrils be equidistant from each other. Subsequent studies showed that these conditions are not satisfied in the cornea.
why is the sclera not transparent like the cornea?
Fibers are large with greatly varying diameters
Not orderly or closely spaced
This is why the sclera has a great deal of light scatter and is nontransparent.
why is the cornea transparent?
The distance between the fibrils must be less than1/2 the wavelength of visible light
This maintains the transparency. collagen fiber sizes vary but very slightly.
what happens when there is damage to the corneal epithelial or endothelial wall?
With damage to epithelial or endothelial barrier, the cornea swells = loss of corneal transparency
What does “Lakes of water” in the stroma where there is no collagen fibers cause?
“Lakes” of water in the stroma where there aren’t collagen fibers – these cause:
increased divergence of refractive index
increase in distance between collagen fibrils
All resulting in a loss of light transmittance
Fibril diameter in posterior cornea is greater than anterior. True or False.
False. greater in anterior cornea.
Density of fibrils is greater in anterior or posterior cornea?
posterior cornea.
There is a two fold increase of light scatter in the human in the anterior vs. posterior cornea. True or False.
True
what is Fuch’s dystrefy?
Fuch’s dystrefy- Disease of endothelial cells. Endothelium not functioning very well. Abnormal Descemete membrane. Cornea begins to decompensate because of decrease in endothelial cells.
(water is going in but no water pushing out which cause edema). No good. Edema is even worse in the morning.
what is the rate of evaporation of the cornea?
2.5 µL/cm2/hr
There is 5% more thinning of cornea in the morning than at night when sleep. True or False.
True.
Describe Dellen. What can persistence of this cause?
Dellen is localized corneal drying. If this persist can cause:
a decrease in stromal fluid flow when stroma hydration is abnormal or
minimal lateral flow of water in the cornea
What happens when IOP is more than 50mm Hg or abnormal in endothelial function?
epithelial edema and increase in in stromal thickness.
Stromal swelling pressure decreases with increased corneal thickness.
Thus mild corneal edema with elevated pressure can lead to high imbibition pressure and subsequent epithelial edema and bullae. True or False
True
incisions made far from the limbus instead of the peripheral part of the cornea heal faster. True or False.
False. incisions made in the peripheral (near limbus) heal faster.
what effect can steroids have on the healing process after surgery? What is the advantage and disadvantage?
Steroids can slow healing down. This is good because it gives the epithelium time to heal correctly, better epithelial cells. However, it is bad because now more prone to infection.
Where is Descemet’s membrane located?
It is the basement membrane that the corneal endothelium rest on.
Composed of collagen IV, fibronectin, and laminin.
Does Descemet’s membrane increase or decrease with age?
increase. it thickens with age.
Desmentele means what?
Desmentele= out pouching of descmete’s membrane. This happens when severe ulceration has happened to corneal epithelium and stroma.
Descemete’s membrane is highly resistant to proteolytic enzymes. True or False.
True.
Describe Guttata.
Collegenous material formed at the posterior surface of membrane.
thinning and enlargement of endothelial cells.
endothelium is a single layer or multi-layer structure.
single polygonal layer of cells.
The endothelium increases with age and can regenerate, like the epithelium of cornea.
True or False
False. No endothelium decreases and thins with age. CANNOT REGENERATE.
Newborn cell density: > 5500 cells/mm2
Adult cell density: 2500-3000 cells/mm2
Minimum cell density = 400-700 cells/mm2 ( for cornea to remain clear)
True or False
True
Cells are metabolically active in transport, synthesis, and secretory functions SO … endothelial cells contain: Large nucleus Numerous mitochondria Prominent endoplasmic reticulum Golgi apparatus True or False
True
Gap junctions are present in endothelial layer of cornea. True or False.
False. Only in epithelium. Mainly in lateral membranes.
Endothelium has tight junctions. What are they called? are they as good as the tight junctions (zonular occludens in the epithelium of cornea?
The tight junctions are called macular occludens. No they are not as good as the zonular occludens in epithelium. They are made to leak, for tears to get through and bring nutrients to cornea.
Macular occludens in endothelium completely encircle cells like zonular occludens in the epithelium of cornea. True or False
False. does not completely encircle cells but acts as a barrier.
