CorneaSclera2

Question 1

What type of junctions are in the endothelium? What is the function of the tight junctions?

Answer 1

tight junctions; macula occludens
Function: barrier but not as good as epithelium
Do NOT encircle the whole cell
leaky barrier between stroma and aqueous

Question 2

What is the function of gap junctions in the endothelium? Where are they mostly located?

Answer 2

Intercelluar communication; do not form barrier

mainly located in the lateral membranes ; , although some are also present at the junctions of the apicolateral membranes

Question 3

What are two factors that contribute to prevention of corneal swelling? What percentage is water maintained at?

Answer 3

1. pumping function of endothelial
2. barrier function of endothelial

The barrier is incomplete compared with the epithelial barrier.

78%

Question 4

When the endothelium is disrupted, cornea swelling is at what amount?

Answer 4

127 µm/hr

Question 5

When the pump is disrupted cornea swelling is at what rate?

Answer 5

33 µm/hr

Represents movement of fluid and solutes from the aqueous humor into the stroma

Question 6

Why is the normal leakage of fluid to the cornea vital ?

Answer 6

This is how cornea gets nutrients (glucose and amino acids) for the cornea
(since cornea is avascular)

Question 7

Temperature Reversal of cornea and how it relates to swelling

Answer 7

Temperature has an effect on the corneal swelling; cooler temperature causes cornea to swell; normal temperature= normal corneal thickness

Early studies showed that maintenance of corneal thickness/water content is temperature dependent (a metabolic energy-dependent process)

Temperature reversal is clearly demonstrated by eye bank corneas, which swell during refrigeration and return to normal thickness and transparency after grafting.

Question 8

How much water is moved by endothelium from stroma to aqueous humor?

Answer 8

6 to 8 ml/hr of water is moved by the endothelium from stroma to aqueous humor.

Question 9

Endothelial Ion Transport

Answer 9

Driven by Na/K ATPase Pump located in basolateral membrane

present in normal humans at approximately1.5 x 106 pump sites per cell. The activity of Na+K+ATPase is vital to the maintenance of normal corneal hydration.

Question 10

What inhibits the pump in the endothelium?

Answer 10

Ouabain: inhibits the pump
stops sodium transport, causes corneal swelling, prevents temperature reversal, and eliminates the transendothelial potential difference

Question 11

Another reason pump responds?

Answer 11

responds to increases to endothelial permeability

human corneas with guttata have pump site densities of 6 x 106 sites per cell, which suggests a greater capacity for the endothelial pump to counteract the leak across the barrier.

Question 12

What is gutatta?

Answer 12

droplet-like accumulations of non-banded collagen on the posterior surface of Descemet’s membrane.

Question 13

Inflamed Edematous cornea

Answer 13

has decreased endothelial pump sites despite increased permeability.

Question 14

What is the sodium potassium ATPase and functions?

Answer 14

enzyme located in plasma membrane of cells

Functions:

1. Control of Hydration of cornea
2. Production of aqueous humor

Question 15

Na, K-ATPase and corneal hydration

Answer 15

Corneal stroma readily takes up water.
Excess water needs to be pumped out to maintain corneal deturgescence
Sodium ions is transported by Sodium Pottassium ATPase is pumped into channels between endothelial cells

Na ions being pushed generates osmotic pressure
To which water follows
The directionality is helped by density of sodium ions in Descemets membrane

Question 16

Endothelial wound healing

Answer 16

endothelial cells do not regenerate

Defects are covered by the spreading of cells from adjacent areas

When a large defect occurs, as a result of a keratoconus problem, more extensive cell migration occurs

Question 17

Ideal irrigating solution should contain?

Answer 17

buffer- bicarbonate
energy source- glucose
substrate- calcium, glutathione etc.

Question 18

T/F LASIK had an effect on endothelial cell density or percent of hexagonal cells

Answer 18

False, there was no significant change

Question 19

How many microns are needed for cornea to continue normalcy? adequately maintain and protect the corneal endothelial structure and barrier function.

Answer 19

200 microns

Question 20

Na ions in the pump and directionality of them

Answer 20

Na ions being pushed out generates osmotic pressure
Water follows the Na
Directionality is helped by density of sodium ions in Descemet’s membrane

Question 21

Irrigating system soultions
What do poorly buffered solutions expose?
Best soultion?,

Answer 21

they expose cornea to pH extremes and may cause edema

GBR

Question 22

Why is saline not a good buffer?

Answer 22

It lacks components of aqueous humor

Question 23

How do endothelial cells react to trauma? Large defects?

Answer 23

1. They don’t regenerate
2. cells spread out to cover wounded area
3. when larger defects, more migration of cells occurs

Question 24

Where do cells move after migration? What do they form?

Answer 24

move peripherally; form tight junctions to form endothelium barrier

Question 25

For proper corneal function there must be?

Answer 25

1. adequate endothelial cell density | 2. uniform size and shape of cells

Question 26

what is polymegathism and pleomorphism? How does age relate?

Answer 26

``` Polymegathism A change in cell size Pleomorphism A change in cell shape Both polymegathism and pleomorphism increase significantly with age ```

Question 27

What is CV and what is the normal CV?

Answer 27

Coefficient of Variation (CV): Standard deviation of mean cell area Mean cell area Normal endothelium CV ≈ 0.25

Question 28

how much does the cornea have hexagonal cells?

Answer 28

Healthy cornea has 70-80% hexagonal cells | A ⇩in hexagons with an ⇧in cells with < 6 sides shows endothelial stress = pleomorphism

Question 29

CV and LASIK

Answer 29

CV improves 3 years after LASIK but this is secondary to the patients that wear contacts

Question 30

What is cell count loss in those whove had PK

Answer 30

7.8% cell loss each year after 3-5 years of surgery. 13X higher than those with normal cornea

Question 31

Amount of endothelial cell loss after cataract sugery?

Answer 31

8.5% after 12 months ; lens adds 20 D

Question 32

How does cornea change in patients with KCN and Diabetes 2

Answer 32

morphology changes cell density does NOT change CV increases Hexagonal cells drop to 50% expect high astig or high myopia

Question 33

Cell density in Glaucoma Pts

Answer 33

cell density decreases IOP increases special care to these pts so that surgical stress to endothelial doesnt occur

Question 34

How does Polymegathism reported with long-term use of rigid polymethylmethacrylate lenses as well as with daily and extended-wear contact lenses change cornea?

Answer 34

morphologic abnormalities contact lens-induced hypoxia is responsible for the observed effects on the endothium CLUE syndrome, or contact lens-use endotheliopathy syndrome, to describe these effects.

Question 35

Nerves of cornea?

Answer 35

sensory nerves from trigeminal nerve But the Descemet’s membrane and the endothelium are NOT innervated in humans

Question 36

What are nociceptors?

Answer 36

corneal receptors stimulation results in perception of pain The receptors usually have the lowest threshold for mechanical stimulation This is why corneal abrasions, ulcers, and bullous keratopathy are all extremely painful conditions

Question 37

What leads to corneal denervation? and what does this cause?

Answer 37

Herpes simplex, stroke, diabetic neuropathy Sensory denervation causes epithelial erosions and neurotrophic ulcers The loss in foreign body sensations cause mechanical cornea damage

Question 38

What causes a decrease in cornea and conjunctival sensitivity?

Answer 38

LASIK up to 16 months | CL wearers

Question 39

Corneal metabolism depends on oxygen from

Answer 39

atmosphere (mostly) limbal vasculature aqueous humor

Question 40

O2 in aqueous humor vs tear film stroma vs epithelial

Answer 40

Amount of O2 present in the aqueous humor is low compared to the tears Epithelium consumes 10x more O2 compared to the stroma

Question 41

How is oxygen delivered to cornea while sleep?

Answer 41

through vascularized superior palpebral conjunctiva 21% eyes open 8% eyes closed even less if sleeping in CLs

Question 42

Oxygen after cataract surgery

Answer 42

After cataract removal (ie aphakia), oxygen tension may increase in the aqueous humor as a result of decreased oxygen metabolism by the crystalline lens. Thus,in the aphakic eye, oxygen in the aqueous humor may supplement oxygen dissolved in the tears, better meeting the corneal oxygen demands and allowing greater tolerance to hypoxic stress such as contact lens use.

Question 43

Metabolic requirements for cornea

Answer 43

supplied by aqueous humor via ciliary body | epithelial has glucose from glycogen stored

Question 44

Oxygen with CL wearers

Answer 44

contact lens wear causes a decrease in available oxygen to corneal epithelium Hard contact lens nearly 80% of reserve used up in 8 hours of wear Resulting in corneal edema (20% increase in corneal thickness)

Question 45

Embden-Meyerhof pathway

Answer 45

Glucose derived from the aqueous humor or from epithelial glycogen stores is converted to pyruvate by the anaerobic Embden-Meyerhof pathway (ie glycolysis), yielding two molecules of adenosine triphosphate (ATP) per glucose molecule.

Question 46

Under hypoxic conditions what causes corneal edema? What are symptoms of epithelial edema, stroma edema?

Answer 46

increase in lactate production Symptoms of epithelial edema: Halo and rainbow formation Increased glare sensitivity Decreased contrast sensitivity Stromal edema manifests in the posterior direction Buckling of the stroma and Descemet’s membrane cause vertical striae

Question 47

CL wearers and metabolism

Answer 47

Deturgescent cornea can be maintained with a sustained oxygen level as low as 25 mm Hg Small-diameter hard lenses- PMMA Impermeable to oxygen Good movement needed for tear exchange Large-diameter soft lenses Oxygen permeability needed for diffusion through the lens itself Tear pump exchange also helps to a lesser extent

Question 48

Underlying mechanism for polymegathism?

Answer 48

Underlying mechanism: Certain byproducts of long term hypoxia and stress that is12HETE (12-Hydroxyeicosatetraenoic acid) has the ability to inhibit the Na+K+ATPase of the endothelial metabolic pump Only occurs after long-term, volume-regulation stress (example CL wear) and in the corneas of diabetic patients

Question 49

How is metabolism altered in Extended Wearer CL

Answer 49

EW lenses alter metabolism and have been associated with: Decreased rate of mitosis Reduced oxygen uptake and glucose utilization Smaller numbers of intercellular desmosomes Because of these and reduced metabolic activity- the epithelial barrier function is compromised Increased likelihood of ulcerative microbial keratitis

Question 50

Lack of vitamin A causes what? and what is vitamin A required for?

Answer 50

Xerophthalmia Keratinization of the epithelium;with the corneal epithelium expressing keratins normally found in skin by terminally differentiated superficial epithelial cells. Primarily found in children of developing countries Required for mucin production

Question 51

Drug Penetration

Answer 51

The volume of the normal adult tear film is 7 to 9 µL, and the maximum amount of fluid that the cul-de-sac can maintain is 20-30 µL. This much of the 50 µL in the average drop of topical medication runs out of the eye immediately after instillation and the remainder is diluted in the preexisting tear film. Reflex tearing caused by irritating or hypertonic solutions results in more rapid tear dilution. Increased protein concentration in tears bathing inflamed or infected eyes may also decrease the bioavailability of drugs that bind to the protein.

Question 52

How does corneal epithelium affect drug penetration?

Answer 52

Initial barrier: Epithelium (tight juctions) Limit adsorption of hydrophilic, ionized substances Favoring penetration of lipid-soluble hydrophobic compounds Loss of corneal epithelium greatly enhances penetration of hydrophilic water-soluble pharmacologic agents

Question 53

Stroma and drug penetration

Answer 53

The hydrophilic nature of the stroma results in a barrier to lipid-based drugs.

Question 54

Drug penetration through endothelium

Answer 54

Drug penetration through the corneal endothelium is determined mostly be molecular size.

Question 55

How do you increase drug penetration?

Answer 55

by increasing duration of the contact of the drug with the ocular surface Press on the lacrimal sac Use viscous drops, suspensions, ointments Slow-release delivery systems Contact lenses Porcine collagen corneal shields

Question 56

Why are preservatives used in drugs and what are some examples

Answer 56

Preservatives are used to prolong shelf life of drugs and protect the eye from infection (prevent bacterial growth) Examples: benzalkonium chloride (BAK), chlorhexidine digluconate, polyquaternium-1, and thimerosal

Question 57

How does BAK work?

Answer 57

Antibacterial action of BAK is based on the detergent property of the compound Breaks down bacterial cell walls The detergent property also causes the epithelium and endothelium to be susceptible to damage

Question 58

BAK cont..

Answer 58

One drop of 0.01% BAK causes increase in permeability of the cornea to fluorescein BAK also inhibits epithelial wound healing Severe dry eye patients have increased fluorescein permeability secondary to the compromised epithelial barrier If they use tear solutions containing BAK they risk further damage

Question 59

What is EDTA?

Answer 59

used to stabilize BAK-containing formulations. E DTA chelates calcium, which is required for maintenance of tight junctions, and artificial tears containing EDTA may increase corneal permeability. Therefore, when possible, ophthalmic products should be formulated without EDTA, and formulations with EDTA should be avoided by patients with dry eyes. The preservatives BAK and chlorhexidine cause endothelial cell degeneration and corneal edema in vitro.

Question 60

Sclera? Water content? dry weight?

Answer 60

Connective tissue consists of collagen, proteoglycans, fibroblasts 70% water 30% dry weight The collagen fibers, 90% which are type I (resist tension), constitute 75% to 80% of the dry weight of the sclera.

Question 61

What overlies sclera?

Answer 61

Episclera

Question 62

Sclera mainly avascular with what exceptions?

Answer 62

Sclera is essentially avascular | Exceptions: superficial vessels of the episclera and the intrascleral vascular plexus

Question 63

Posterior Sclera contain?

Answer 63

sclera canal and lamina cribosa (passage of the optic nerve) Contains sites of perforation by the long and short posterior ciliary arteries, the short ciliary veins, the ciliary nerves, and the vortex veins Tendons of recti muscles insert into the superficial sclera collagen

Question 64

Size of sclera/ sclera thickness

Answer 64

95% of the total surface area of the globe Sclera thickness decreases towards the equator and increases near the optic nerve

Question 65

Diameter of scleral collagen fibrils

Answer 65

25-230 nm and are arranged in bundles

Question 66

Fiber Types in Sclera?

Answer 66

elastic, elaunin, and oxytalan.

Question 67

Myopia and Sclera

Answer 67

Form vision deprivation results in axial elongation of the ocular globe causing myopia Also associated with myopia: Changes in the rate of proteoglycan synthesis Accumulation within the sclera

Question 68

What happens to sclera as you increase in age?

Answer 68

Progressive degeneration of collagen and elastic fibers Loss of GAGs Scleral dehydration Accumulation of lipids and calcium salts

Question 69

Age in sclera causes?

Answer 69

Increase in tissue density Sclera thinning and yellowing Decreased sclera elasticity

Question 70

Scleral Innervation

Answer 70

Stroma of the sclera is devoid of innervation, but does allow the passage of nerves Sympathetic innervation present in monkeys Scleral spur axons of parasympathetic origin are present in humans Cholinergic innervations rare or absent in both monkeys and humans

Question 71

Transscleral Delivery of Drugs

Answer 71

New research investigating the transscleral delivery of drugs to treat posterior segment disease Primary mechanism: diffusion through an aqueous pathway

Question 72

What factors control diffusion rate?

Answer 72

Tissue hydration Tissue thickness Size and volume fraction of proteoglycans present The sclera swells least near a pH of 4

Question 73

What alters scleral permiability?

Answer 73

Age, cryotherapy, and diode laser treatment do NOT alter sclera permeability, whereas surgical thinning INCREASES permeability ange of IOP- 15 to 60 mm Hg can DECREASE sclera permeability to small molecules by half Scleral permeability is a WEAK function of transsceral pressure Strong function of molecular weight

Question 74

How is permeability affected by increased molecular weight and radius?

Answer 74

it decreases

Question 75

Prostglandins affect permeability?

Answer 75

When the sclera is exposed to various prostaglandins in organ culture there is INCREASED permeability This causes an INCREASED expression of matrix metalloproteinases Could be a mechanism by which prostaglandins decrease IOP when used topically

Question 76

How can transcleral delivery be improved for large molecules?

Answer 76

For large molecules, transscleral deliver could be improved significantly by: Taking advantage of thinner regions of tissue Increasing sclera hydration Transient modification of sclera ECM