Block 9 Flashcards
3 main functions of cornea
- refract light
- transmit light
- protect
Factors affecting refraction
- ant curvature
- ref index from air to TF
- corneal thickness
- post curvature
- ref index from cornea to aqueous
Total power of eye
60D
How much of the power of the eye comes from the cornea
40-48D
- 2/3 to 3/4 of power
How does the cornea transmit light
- little scattering
- little distortion
- max transmission
How much scatter is in the cornea
<1%
How does cornea protect long term vision
- TF protect from infection
- blocks UV light to protect retina
- hard to penetrate
- quick healing
How sensitive is the cornea
One of the most sensitive in the body
Innervation of cornea
Ophthalmic division of trigeminal nerve
- 60-80 long ciliary n.
The cornea is sensitive to
- touch
- cold
- chemicals
Normal CV of endothelium
.25
Shape of majority of endothelial cells
Hexagonal
Does the endothelium regenerate
No
What happens as you lose endothelial cells
Cell density decreases and cells change shape, becoming less hexagonal
How much of the cornea is the stroma
90%
How does cornea maintain transparency
Uniform size and spacing of collagen
What is the optimal water level in stroma
75-80% water
Which layer is resistant to deformation, trauma and FB
Bowman’s layer
Will Bowman’s layer scar?
Yes
Thickness of bowman’s layer
8-14 microns
(New) Layer between stroma and descemet’s membrane
Dua’s layer
How thick is dua’s layer
15 microns thick
How does the epithelium help maintain a proper hydration
Tight junctions
- keep out water
How does the stroma help maintain a proper hydration
Proteoglycans
- pull water in
How does the endothelium help maintain a proper hydration
Leaks fluid into cornea
Driving forces of water transport in cornea
Cl- extrusion and Na+ absorption
Two ways water moves through cornea
- stroma to endo to aqueous
- epi to TF
Bidirectional water transport channels
Aquaporins
What is lost during a minor corneal abrasion
Zonula occludens
2 things that happen with a minor abrasion
- edema
- haziness
How does edema affect VA
Decrease VA
What happens when there is a more extensive corneal abrasion
Fluid gets into the stroma
Fastest healing tissue in the body
Cornea
The healing of the cornea relies on
Peptide growth factors
How quickly does the cornea heal (minor abrasion)
24-48 hours
How quickly can the entire cornea be replaced
2 weeks
Which layers of the cornea consume the most oxygen
Endo and epi
Which layer of cornea can store glycogen
Epi
Compromises the integrity of the epi
Corneal ulcer
The epi is impermeable to
Ions
Epi and endo are…
Hydrophobic
What type of material penetrates the cornea easier
Lipid soluble
3 conditions from loss of stability and structure of cornea
- keratoconus
- ectasia
- high/low IOP
Corneal thickness at birth
Full thickness
Horizontal cornea diameter at birth
90% of full size
- 10mm
Axial length of eye at birth
2/3 of adult size
Why are kids typically more hyperopic
Bc the axial length is shorter at birth and must grow
At what age does the cornea bc adult size
3 years
Sclera is growing until what age
10-12
Most eyes are emmetropic by what age
6-8
When changes happen in central cornea, affecting transparency
Dec VA
Cornea aging changes: sensitivity
Decreased
Cornea aging changes: scatter
Increase
Cornea aging changes: curvature
Flattens
- becomes against the rule
Cornea aging changes: descemet’s thickness
Gets thicker
Most common aging change
Corneal arcus
White ring formed by cholesterol deposits in peripheral stroma
Corneal arcus
Corneal arcus is most common is what pt age range
60-80
What is suspected if a pt under 40 has corneal arcus
Hyperlipidemia
Corneal arcus: bi or unilateral
Bilateral
What layer is affected in corneal arcus
Bowman’s
What happens to the epi with age
Loss of intracellular junctions causing dec barrier function
Horizontal pigmented line at lower lid margin
Hudson-stahli line
What age does the hudson-stahli line appear
After 50
What causes the Hudson-stahli line
Iron deposits in epi cytoplasm (basal cells)
Degeneration of bowman’s
Limbal girdle of vogt
Yellowish/white opacity at 3 and 9 o’clock
Limbal girdle of vogt
What appear in the periphery due to changes in descemet’s membrane with age
Hassall-henle bodies
Which illumination will you see descemet’s warts
Retro
Endo cell density with age
Decreases
Avg endo cell density in kids
4000 cell/mm^2
Avg endo cell density in adults
2500 cell/mm^2
How does the cell density change over times (numerically)
Dec .6-1% per year
Greater than normal variation in size of endo cells
Polymegathism
Increased variability in size and shape of endo
Pleomorphism
Vertical line on posterior pole
Krukenberg’s spindle
Why is there a dec in corneal sensitivity with age
Loss of corneal nerves
Refractive error is due to what 2 factors
- axial length
- corneal curvature
Myopic eye length
Longer
Hyperopic eye length
Shorter
Development of myopia usually occurs around what age
8-14
What is myopia caused by
Poor image quality sends signal to sclera to move to get the retinal in the proper spot for a clear image
- lengthening of posterior eye
Increased outdoor activity does what to your chances of myopia
Reduced risk of myopia
Closer to the equator – more or less myopia
Less myopia
Scleral remodeling in myopia
Thinning
As the sclera thins, mainly at posterior pole, tissue can bulge out
Ectasia
Why can the sclera become yellow with age
Fatty deposits
Why does sclera become thin and lose elasticity with age
Degeneration of collagen and elastic fibers
3 biomolecules involved in corneal wound healing
- integrins
- cytokines
- growth factors
Sense changes in extracellular environment
Corneal integrins
Control growth of corneal cells
Cytokines
Maintain transparency and induce migration
Growth factors
How many cells thick is the epi
5 - 7
3 layers of epi
Surface
Wing
Basal
How many cells thick is the surface layer of the epi
2
How many cells thick is the wing layer of epi
2-3
How many cells thick is the basal layer of epi
1
Which layer of the corneal epi has cells that divide
Basal
How does the epi replace itself
- Stem cells from the periphery divide
- cells move to become basal
- then move to become wing, then surface
- surface cells shed into TF
**7 day turnover
Time frame for small corneal lesion to heal
24-48 hours
If the basement membrane is damaged, how long does it take for corneal to heal
Several months
Does the cornea scar
Usually not
Corneal epithelium sloughs off bc of poor adhesion (between epi and basement or between basement and underlying tissue)
Recurrent corneal erosion
Does bowman’s layer regenerate
No
Bowman’s layer is composed of
Collagen
90% of corneal thickness
Stroma
Stroma is composed of what 3 things
Collagen, keratocytes, ground substance
How long for stroma to reach full strength after wounded
Months
How are wounds healed in stroma
- Keratocytes become myofibroblasts
- regenerated collagen is larger and less precisely organized
- myofibroblasts back to keratocytes by apoptosis
Descemet’s mem healing is regenerated by
Keratocytes and endo
Single layer of flat cells
Endo
Cells of endo are known as
Polyhedral
- 70-80% hexagonal
- 20-30% heptagonal
Would healing of endo
Neighboring cells enlarge, cells remodel, inc number of ion pumps
–> doesn’t create new cells
Most glucose comes from
Aqueous humor
Minor glucose supply
TF and limbal capillaries
Glycolysis - 1 glucose gives how much ATP (net)
2
Krebs and ETC produce how much ATP
36
Which 2 layers will consume oxygen
Endo and epi
Where does the epi get oxygen
Limbal vasculature and TF (atmosphere - 155 mmHg)
Where does endo get oxygen
Aqueous (40mmHg)
Aerobic metabolism deals with which cycle
Krebs
Anaerobic metabolism deals with which cycle
Glycolysis and HMP shunt
Produces amino acids and intermediates for nucleotide prod (for DNA replication)
HMP shunt
Important for the production of components required for rapid regeneration of corneal epithelial cells
HMP shunt
In anaerobic conditions, what builds up
Pyruvate, which is converted to lactic acid
What does lactate buildup do
Induces epi and stroma edema
Situations that can cause too much time in anaerobic conditions
- CLs wearers
- lots of sleep
Painful condition caused by exposure to UV rays (natural or artificial sources)
Photokeratitis or UV Keratitis
Low exposure to UV
Inhibited cell mitosis
Medium to high exposure to UV
Swollen nuclei and cell death
Extreme UV exposure
Complete sloughing of epi cells
What also contributes to damage at the higher levels of UV damage
oxygen
UV light damage causes what dimers
Thymine (pyramids)
- cant replicate DNA
DNA repair enzyme
UV-specific endonuclease
exinuclease
Process of DNA repair from UV damage
- Exinucleause cuts DNA on both sides of damage
- remove damage
- gap filled by DNA Pol 1
- DNA ligase complete the bond
Deficiency in the excision endonuclease and a reduced ability to repair damage from UV light
Xeroderma pigmentosum
Symptoms of xeroderma pigmentosum
- skin damage and increased risk for cancer
- eyelid scarring
- corneal ulceration
Perseverance structural integrity of epithelial sheet
Hemidesmosomes
Cytoskeleton structure
Cell to matrix adhesion
Deposit iron in EPI
Hudson-stahli line
Degeneration of BOWMANS
Limbal girdle of Vogt
What layer does corneal arcus affect
Stroma
Warts appear in DESCEMETS
Hassall-henle bodies
Vertical line in ENDO
Krukenberg’s spindle
