Cornea 2 - Week 2

Question 1

What is the corneal limbus?

Answer 1

The border of the cornea and sclera

Question 2

The limbus contains radially oriented fibrovascular ridges that may harbour a stem cell population. What is the name fro these ridges?

Answer 2

Palisades of Vogt

Question 3

How does central corneal thickness (CCT) vary throughout the day?

Answer 3

Thickest during sleep; thinnest at end of the day

Question 4

Which controls most of the corneal thickness? Epithelium or Endothelium?

Answer 4

Corneal endothelium

Question 5

How does chemical damage to the endothelium affect corneal thickness? How does this compare to if epithelium is scraped off?

Answer 5

Massively increases corneal thickness

Epithelium scraped off has less of an effect

Question 6

Describe sympathetic control of corneal hydration

Answer 6

Release of catecholamines (e.g. noradrenaline) may modulate this pathway (cAMP secondary messenger controls apical chloride transport)

i.e. noradrenaline acts on beta-adrenergic receptors, main goal is to increase amount of chloride flowing out of the cell

Question 7

Describe the steps for stromal water control by the epithelium

Answer 7

1. tight junctions are high resistance
2. Na+/K+ATPase pump (on basolateral membrane) pumps Na+ towards stroma
3. This reduces net -ve charge, maintaining gradient for function of Na/Cl co-transporter
4. co-transporter brings Cl and Na from stroma to cell
5. Cl- ions diffuse into tears; create osmotic gradient drawing water out of stroma

Question 8

Which maintenance of corneal hydration is more important? Epithelial or Endothelial?

Answer 8

Endothelial

Question 9

Where does the majority of epithelial glucose come from?

Answer 9

Aqueous

Question 10

Where does the majority of epithelial oxygen come from?

Answer 10

is atmospheric and diffuses from tear film

Question 11

Where does particularly high levels of peithelial glucoes methabolism occur?

Answer 11

Pentose Phosphate Pathway

Question 12

How do keratocytes of the stroma metabolise high amounts of glucose?

Answer 12

Pentose Phosphate Pathway

Question 13

Why do we need to actively remove fluid from the stroma?

Answer 13

Because fluid is continuously and passively entering the stroma

Question 14

Describe the cells of the corneal endothelium

Answer 14

single layer of flattened hexagonal cells (that are critical to control of corneal hydration)

Question 15

What is the effect of the leaky tight junctions in the corneal endothelium?

Answer 15

Allows entrance of nutrients from aqueous

Question 16

How much of the cornea is water (normal cornea)?

Answer 16

78% water

Question 17

How does the corneal endothelium control stromal water flow? List the steps

Answer 17

1. Na+/2HCO3- load endo cells with bicarbonate and Cl
2. These ions move passively from cell into aqueuos
3. Carbonic anhydrase converts bicarbonate to CO2
4. Co2 diffuses into aqueous and then membrane bound CA-IV converts back into bicarbonate
5. Intracellular HCO3- can also diffuse exit via CFTR
6. Basolateral Na+/K+/2Cl- cotransport loads cells with Cl-; Cl- negative offset by ion channels
7. Sodium pumped out by basolateral pump in lateral space
8. Sodium moves to aqueous
9. Transendothelial osmotic gradient brings water back into aqueouous
10. ATP diffuses out of cell and autoregulates adenosine receptors which mediate function of apical pumps

Question 18

Factors that affect epithelial wound healing?

Answer 18

1. Anatomic location: limbal vs central (central faster)
2. size of wound
3. layers of cornea involved
4. infection
5. topical drugs administered

Question 19

List the 4 main stages of epithelial wound healing

Answer 19

1. Latency: apoptosis, increased cell metabolism, re-organisation of cell structure, fibronectin band-aid
2. Migration: filopodia at leading edge, celular ‘avalanche’
3. Proliferation: limbal stem cells divide rapidly to replace destroyed cells
4. Attachment (part that takes longest). Re-establishment of hemidesmosomes to anchoring filaments in BM

Question 20

Where are most of the stem cells located?

Answer 20

In the limbus of the cornea