ANATOMY - CORNEA

Question 1

How thick is the corneal epithelium in terms of cell layers?

Answer 1

six layers thick

Question 2

The anterior surface of the cornea is ….. shaped whereas posterior surface of the cornea is ….. shaped

Answer 2

1. eliptical
2. spherical

Question 3

What is the horizontal diameter of cornea?
What is the vertical diameter of the cornea?
What is the average diameter of cornea?

Answer 3

11.7mm
11mm
11.5mm

Question 4

What is the radius of the curvature of cornea?
What is the radius of curvature of the sclera?

Answer 4

1. cornea – 7.8mm
2. sclera – 12mm.

Question 5

What is the average central corneal thickness?
What is the average peripheral corneal thickness?

Answer 5

520 micrometers
670 micrometers

Question 6

What is the refractive power of the cornea?
What is the refractive power of the lens?

Answer 6

+45 D
+15 D

Question 7

What is the refractive index for these structures?
1. Air
2. Cornea
3. Anterior Chamber
4. Lens
5. Vitreous

Answer 7

Air - 1
Cornea - 1.37
AC + vitreous -1.33
Lens - 1.42

Question 8

Where are the corneal layers derived from?
(Embryologically)

Answer 8

surface ectoderm - epithelium

mesoderm - stroma, descemet’s endothelium

Question 9

What type of epithelium is the cornea?

Answer 9

Stratified (5-6 layers)
Squamous (superficial cells flattened)
Non keratinised (nucleated, no keratin)
Epithelium

Question 10

How long does it take for entire cornea epithelium to be replaced?

Answer 10

7 days

Question 11

What is the function of the microvilli in the cornea?

Answer 11

Are present with glycocalyx and help in adhesion and stability of the tear film to prevent tear film evaporation and subsequent dry eye.

Question 12

Where are the corneal epithelium stem cells found?

Answer 12

In the limbal basal epithelium at the palisades of Vogt

Question 13

What is the thickness of Bowman’s membrane?

Answer 13

8-16 micrometers

Question 14

What is Bowman’s membrane composed of?

Answer 14

randomly arranged collagen fibres generally resistant to trauma - cannot be regenerated

Type I
Type VII

Question 15

What are the main components of the corneal stroma (3)

Answer 15

1.Uniform Collagen fibrils (arranged in flat bundles called laminae - 300 laminae present)
2. keratocytes which secrete collagen
3. Extracellular ground substances

Question 16

What is responsible for the transparency of the cornea?

Answer 16

The highly uniform collagen fibres in the stroma (25-35micrometers) and distance between two corneal fibres is uniform (41.5nm)

Question 17

What are the reasons for corneal transparency? (5)

Answer 17

1.Optically smooth tear film
2.Role of corneal epithelium
3.Arrangement of stromal fibres
4.Avascularity of the cornea
5.Absence of myelination in corneal nerves

Question 18

What is the thickness of descemets membrane at birth and in adulthood?

Answer 18

3 micrometers
10 micrometers

Question 19

What is descemets membrane composed of?

Answer 19

Type IV collagen
Laminin
Fibronectin
Heparan sulfate

Question 20

Descemet’s membrane is ….. attached to the stroma and is …… to enzyme degradation by phagocytes and toxins

Answer 20

weakly

resistant

Question 21

What is the main shape of the corneal endothelium?
How are they connected together? (3)

Answer 21

hexagonal forming continuous mosaic pattern

Various junctional complexes
(1) zonula occludens (tight junctions)
(2) macula occludens
(3) macula adherens

Question 22

What do the corneal endothelial pumps do?

Answer 22

Sets osmotic gradient that causes fluid to move from stroma to the aqueous to maintain 70% corneal hydration to keep cornea transparent

Question 23

What are the 3 different endothelial pumps?

Answer 23

Na/K –> 3 Na exit to AH, 2 K+ enter
Na/H –> 1 Na enters, 1 H+ exits
Na/HCO3 –> Na exits to AH, bicarb exits

Question 24

What endothelial cell density leads to corneal decompensation?

Answer 24

800 cells / mm2

Question 25

What is the blood supply of the cornea?

Answer 25

No blood supply

Anterior ciliary artery (ophthalmic artery) forms arcade in limbal region

Question 26

Is the corneal epithelium hydrophilic or lipophilic?
Is the corneal stroma hydrophilic or lipophilic?

Answer 26

epithelium - lipophilic
stroma - hydrophilic

Question 27

Corneal endothelium is held to its basal lamina (descemets membrane) with what strucuture?

Each endothelial cell is held together by what structure?

Answer 27

Endothelium to basal lamina - hemidesmosomes
Endothelium to endothelium -desmosomes.

Question 28

What is the structureBowman’s layer?

Answer 28

structure posterior to the corneal epithelium basement membrane - it is the condensation of stroma made up of acellular type 1 collagen