Chapter 8: Cornea

Question 1

What is the innervation of the cornea?

Answer 1

Long Posterior Ciliary Nerves (branch of CN V1) penetrate the cornea to in 3 planes: the scleral, episcleral, and conjunctival
Peripherally, 70-80 branches enter the cornea and lose their myelin sheaths 1-2 mm from the limbus
a plexus posterior to bowmans layer sends branches anteriorly into the epithelium

Question 2

Describe the 02 supply of the cornea?

Answer 2

02 is supplied via the pre corneal tear film, eyelid vasculature, and aqueous humor

Question 3

What is the source of glucose for the cornea?

Answer 3

Glucose is received primarily from the aqueous via carrier-mediated transport through the endothelium
Epithelium receives it via passive diffusion from the stroma
10% of glucose is supplied by the preocular tear film and limbal vessels

Question 4

How is glucose metabolized by the cornea?

Answer 4

TCA cycle (much more active in the endothelium than the epithelium), anaerobic glycolysis, and HMP shunt
In epithelium, 35-65% go glucose metabolized via HMP pathway

Question 5

How do keratocytes metabolize glucose? Why?

Answer 5

Keratocytes metabolize glucose via the HMP shunt, because they lack the enzyme 6-phosphogluconate dehydrogenase

Question 6

What is the thickness of the corneal epithelium?

Answer 6

The corneal epithelium is 50 micrometers thick, and constitutes 5-10% of total corneal thickness

Question 7

What is the composition of the corneal epithelium?

Answer 7

composed of 4-6 layers.
1-2 layers of superficial squamous cells
2-3 layers of broad wing cells
inner layer of columnar basal cells

Question 8

What is the composition of bowman’s layer of the cornea? What is the thickness?

Answer 8

8-12 micrometers thick

composed of randomly packed type I and V collagen

Question 9

What cells produce the bowman’s layer? When does the Bowmans layer develop?

Answer 9

Anterior stromal keratocytes and epithelium secrete bowmans layer during embryogenesis

Question 10

What happens to Bowmans layer when it is inured?

Answer 10

As it is acellular, the cells do not regenerate. Therefore, a scar will form.

Question 11

What is the function of Bowmans Layer of the cornea?

Answer 11

Bowmans layer seves to prevent expire of the stromal corneal keratocytes to GFs secreted by the epithelial cells

Question 12

What is the composition of the corneal stroma? What percentage of total corneal volume does it occupy?

Answer 12

The stroma is composed of 200 layers of lamellae, which are collagen fibrils enmeshed in a matrix consisting of proteoglycans, proteins, and glycoproteins.
Makes up 90% of total corneal volume

Question 13

What percentage of corneal volume is made up by keratocytes?

Answer 13

30-45 %, depending on age

Question 14

What is the orientation of collagen fibrils in the corneal stroma?

Answer 14

The collagen fibrils run parallel to each other from limbus to limbus, with regular spacing of 55-60 nm from center to center
Adjacent lamellae are positioned at 90 degrees to each other (< 90 in anterior stroma, >90 in posterior stroma)

Question 15

What type of collagen is the major component of the corneal stroma? What other types of collagen are also found in the stroma?

Answer 15

Type I collagen is the major component of the corneal stroma–constitutes 70% of total stromal dry weight
Types V, VI, VII, XII, XIV
Type III is also produced and is associated with wound healing

Question 16

What is the second most abundant biological constituent of the corneal stroma?

Answer 16

Proteoglycans, which constitute 10% of dry weight of corneal stroma and confer hydrophilic properties to the stronga

Question 17

What glycosaminoglycans are found in the corneal stroma?

Answer 17

Keratan sulfate
Chondroitan sulfate
Dermatan sulfate

Question 18

What is the only MMP found in a healthy cornea? What types are pro ducted in response to injury?

Answer 18

MMP-2

S/P injury, MMP types 1, 3, and 9 are synthesized

Question 19

What is the function of proteinase inhibitors in the cornea? Where do they come from?

Answer 19

• They restrict damage during corneal inflammation, ulceration, and wound healing.
• Some are synthesized in the cornea, others come from the tears, aqueous humor, and limbal blood cells

Question 20

What is Descemets Membrane?

Answer 20

a specialized basement membrane composed predominately of type IV collagen that is 10-12 micrometers thick.
Has an anterior banded portion and a posterior non-banded portion

Question 21

Where does Descemets membrane come from?

Answer 21

It is secreted by the corneal endothelium

Question 22

What is the corneal endothelium? What is the cell density in young people? When do you begin to see guttata?

Answer 22

A single layer of polygonal cells that are 20 micrometers in diameter.
3000 cells/mm^2
guttata form at < 2000 cells/mm^2

Question 23

What is the function of the corneal endothelium?

Answer 23

it is a permeability barrier b/w the stroma and aqueous humor, and a pump to maintain the corneal stroke in a dehydrated state (generates a negative hydrostatic pressure)

Question 24

Where does the endothelium receive 02 from?

Answer 24

The endothelium receives enough 02 from the aqueous to maintain its pumping function

Question 25

Does healing/regeneration occur in the corneal endothelium? If so, how?

Answer 25

Healing occurs via cell migration, rearrangement, and enlargement of the residual cells.
Regenerative capabilities are limited

Question 26

What is a Retrocorneal fibrous membrane? How and when is it produced? What affect does this have on the eye?

Answer 26

After an injury, it forms between the corneal endothelium and descemets membrane.

S/P injury, PMNs infiltate and endothelial cells become fibroblastic, synthesizing RCFM.

Causes a decrease in VA