L6 Cornea and Sclera 2

Question 1

What % of fluid is removed from stroma by epithelium into tears?

Answer 1

5%

Question 2

How does fluid removal from stroma by epithelium work?

Answer 2

Na/K ATPase antiporter sets up high K+ and low Na+ inside –> primary transport
Na/K/Cl symporter transports Cl from stroma into cell –> secondary transport
Cl- builds up inside cell and then is transported out of epithelium via cAMP dependent Cl- channel
With Cl-, water leaves via paracellular transport out of epithelium

Question 3

What leads to increased chloride and water secretion?

Answer 3

increased cAMP and sympathetic secretion

Question 4

How is pH maintained?

Answer 4

Lactate/H+ co-transporter and Na/H+ exchanger regulate pH in the basolateral membrane
pH must be maintained between 6.8-8.4 to prevent corneal swelling

Question 5

Where is majority of fluid removed from stroma via?

Answer 5

Endothelium

Question 6

What transport system drives the secretion that balances passive water leakage into stroma caused by osmotic pressure imposed by ECM? (endothelium)

Answer 6

Outward direction transport of HCO3- and Na+

Question 7

What nutrients does cornea get from aqueous humor?

Answer 7

Glucose, amino acids, vitamins

Question 8

What nutrients does the cornea get from tears?

Answer 8

Small bits of glucose, mostly oxygen

Question 9

What vessels does the cornea get nutrients from

Answer 9

Small amounts from limbal vessels

Question 10

Why doesn’t oxygen exist in aqueous humor?

Answer 10

Lens exists in low O2 environment, oxygen can cause cataract to lens

Question 11

How does oxygen get into tears?

Answer 11

Oxygen diffuses through tears

Question 12

How does a lack of nutrients or O2 affect corneal transparency?

Answer 12

Reduces ATP supplied to endothelial pumps, inhibits fluid transport and causes the stroma to swell, causing loss of transparency

Question 13

How do oxygen levels in tears change during blinking?

Answer 13

155mmHg when open, 55mmHg when shut

Question 14

What is polymegethism

Answer 14

Neighbouring cells fill up gaps of dead cells by enlarging, resulting in shaped cells

Question 15

What are the consequences if the cornea doesn’t get enough oxygen?

Answer 15

Bloodshot eyes, polymegethism, new blood vessels grow from the limbus, eye becomes more vulnerable to infections

Question 16

Loss of epithelium by trauma is generally healed within…

Answer 16

24-48hrs

Question 17

What is the process of epithelium wound healing?

Answer 17

Basal cells lose attachment to basal membrane, cells enlarge and migrate to cover the wound by sending out filopodia towards the center of the wound. Limbal stem cells differentiate into basal cells, and migrate into the cornea. The wound is covered by a multilatered sheet, basal cells increase in height and mitosis resumes to restore the epithelium to its normal configuration.

Question 18

How soon after does the epithelium re-establish healing after the stroma
wound has been sealed off?

Answer 18

Rapidly after

Question 19

Why does the stroma swell when damaged?

Answer 19

Due to influx of neutrophils from tears

Question 20

What is the process of stromal wound healing?

Answer 20

Keratocytes activate and move across the wound, producing collagen and fibronectin. Collagen forms over 4-6 days, then tensile strength increases over 3-6 months

Question 21

Why is the cornea transparency initially compromised?

Answer 21

Direction of keratinocytes and collagen is not parallel to cornea fibrils within lamallae, but it will later remodel.

Question 22

How is the loss of endothelial cells compensated?

Answer 22

Migration and enlargement of cells to cover the wounded cells, little evidence of cell division

Question 23

When will corneal transparency return after endothelial cell damage?

Answer 23

Once wound is healed, HCO3- pump continues, the cornea thins

Question 24

What is the structure of the sclera?

Answer 24

CT consisting of fibroblasts embedded in ECM.

Question 25

What is the function of the sclera?

Answer 25

Provide structural integity and defines the shape of eye

Question 26

Why does the eye length change throughout life

Answer 26

Due to growth of the eye and sclera

Question 27

The sclera is an avascular tissue except for the…

Answer 27

Episclera and intrasclera vascular plexus.

Question 28

What is the scleral collagen composed off?

Answer 28

Type 1 and 3

Question 29

What is the structure of scleral collagen?

Answer 29

Collagen fibrils form entangled bundles that branch and intertwine with each other. Different to corneal fibrils which are arranged in parallal lamallae

Question 30

Why is the sclera opaque?

Answer 30

Due to arrangement of collagen fibrils, spacing, and size

Question 31

What is the difference between scleral and corneal collagen?

Answer 31

Scleral collagen is irregularly arranged and has a varied thickness and is thicker. Corneal thickness is uniform –> 30nm and spaced 250nm

Question 32

What is the limbus?

Answer 32

Transition from regular cornea lamallae to irregular organisation of sclera

Question 33

What is the function of the limbus?

Answer 33

• Serve as the barrier to infiltration of blood in the cornea
• Home to stem cells

Question 34

What are the palisades of Vogt?

Answer 34

Radial projections within the limbus that extend into cornea, home of stem cells.

Question 35

Describe the movement of stem cells from the limbus to the basal cell layer of epithelium

Answer 35

Centripetal movement, transcient amplifying (TA) cells are progenitor cells that differentiate from limbal stem cells and then migrate into the central stroma.