cornea

Question 1

functions of the cornea

Answer 1

protection
lens to refract light
to let light penetrate - transparent

Question 2

properties if the cornea

Answer 2

no blood supply hence avascular which makes it transparent

Question 3

how much refraction does the cornea do? what is its refractive index?

Answer 3

two thirds of it, 1.376

Question 4

what are the layers if the cornea?

Answer 4

epithelium
bowmans layer
stroma
descements layer
endothelium

Question 5

give three features of the corneal epithelium

Answer 5

-most regular squamous stratified epithelium in the body
-most anterior layer
5-6 cells thick

Question 6

what is the cycle of the epithelial squamous cells and how long does it take?

Answer 6

10 day cycle,
1. basal
2. wing
3. squamous
4. apoptosis

Question 7

what are the features of the bowmans layer?

Answer 7

-no cells
-narrow collagen fibrils

Question 8

what does the corneal stroma do?

Answer 8

provides strength, elasticity and and corneal form

Question 9

what is the stroma made of?

Answer 9

collagen - the fibrils withstand radial tension

Question 10

how is corneal lamellae structured?

Answer 10

-via collagen fibrils that run at various angles from each limbus
-posterior lamellae don’t interweave while anterior lamellae do

Question 11

what and where are keratocytes?
whats their function?

Answer 11

flattened cells with long pseudopodia joined by gapjunctions mostly between the lamellae and are in highest conc in the anterior stroma. responsible for slow turnover of collagen. this helps maintain fibril order important in wound healing. Also, become active when there’s injury to corneal tissue

Question 12

what are proteoglycans?

Answer 12

long repeating sulphated disaccharides and are highly negatively charged so they are hydrophilic to maintain corneal hydration which keeps it transparent

Question 13

where are proteoglycans found and what are they do?

Answer 13

they are attached to the protein core and they act as collagen spacers
-essential in maintaining corneal transparency

Question 14

what is the descements membrane?

Answer 14

a structureless band that gives mechanical strength and a base for endothelium

Question 15

give three features of the endothelium

Answer 15

-most prosterior layer
-one cell thick
-source of endothelial pump

Question 16

what happens to the number of endothelial cells as you get older?

Answer 16

they decrease and so the cells cover a larger area by spreading during growth or post injury

Question 17

Give 6 features of the limbus

Answer 17

• nourishment of the peripheral cornea
  -corneal wound healing
  -contains pathways of aqueous humour outflow
  -immunosurveillance
  -contains circumferential collagen fibrils
  -Site of surgical incisions

Question 18

Give three features of the sclera

Answer 18

-outer coat of the eye
-contains collagen, proteoglycans and elastic fibres
-relatively a vascular

Question 19

Where are corneal nerves?

Answer 19

Derived from ophthalmic division of the trigeminal nerve and branches radiate to anterior cornea from the limbus

Question 20

What is the corneal touch threshold and how can this be used to calculate sensitivity

Answer 20

The minimum pressure applied to the cornea to be felt. Sensitivity is the reciprocal of this

Question 21

What is the donnan effect?

Answer 21

Alters distribution of diffusible cations and anions across any membrane where there is a non diffusable charged species present on one side of the membrane

Question 22

Where can you see more info on the donnan effect?

Answer 22

YouTube video check history

Question 23

Give the basic steps of LASIK laser eye surgery

Answer 23

1. Corneal flap is cut and hinged backwards
2. Central lamellae flattened using excimer laser
3. Corneal flap replaced

Question 24

What are the advantages and disadvantages of LASIK over PRK

Answer 24

-advantages: is not removed so there’s less wound healing response
-disadvantage: lack of sufficient wound healing means flap never heals which causing the cornea to be mechanically weaker

Question 25

How does PRK work?

Answer 25

The surface is either
-flattened to reduce the power of the eye
-made more curved to increase the power of the eye

Question 26

What makes keratocytes transparent?

Answer 26

Their cytoplasm contains corneal crystallins that give them a uniform refractive index close to that of the rest of the stroma and hence makes them transparent