The Outer Coat

Question 1

How is the cornea adapted for its role as the major component of the eyes optical system?

Answer 1

• Its curved which increases refractive power, with the principal refractive surface of 42D
• Its also transparent and so transmits 90% of incident light

Question 2

Assuming a section was taken from the central cornea, what would be a typical distance in microns from the anterior to posterior surface?

Answer 2

540-550 microns

Question 3

What is the function of having a stratified squamous non keratinised epithelium on the anterior surface of the cornea?

Answer 3

For protection - similar to skin

Question 4

What features of the superficial squamos cells of the cornea make it suitable for its function?

Answer 4

• tight junctions between superficial cells
• microvilli with glycocalyx (for absorption and anchoring of tear film) he glycocalyx creates a hydrophillic surface to facilitate spreading of tear film

Question 5

What is the function of desosomes?

Answer 5

cell to cell adhesion

Question 6

What is the function of hemi desosomes?

Answer 6

cell to stroma adhesion

Question 7

What is the function of gap junctions?

Answer 7

Inter cellular communication

Question 8

What is the function of keratocytes in the normal cornea and what is their response following corneal stromal injury?

Answer 8

• Theyre dendritic cells that synthesise collagen and proteoglycans
• Upon injury to the cornea are stimulated to transition into repair

Question 9

What is the relationship between the organisation of the stroma and transparency?

Answer 9

• Collagen fibrils have a small diameter and are regularly spaced
• The intensity of the scattered light from a fibril depends on the amount of collagen with in the fibril
• So a small size would reduce the scatter

Question 10

How is the endothelium adapted for its role in maintaining corneal transparency?

Answer 10

• They’re tight junctions between adjacent cells that limit aqueous inflow
• Endothelial ion pumps create osmotic gradient such that flow of water occurs out of the cornea
• Contain numerous mitochondria to generate ATP for active transport

Question 11

How does the endothelium change with age?

Answer 11

• Progressive reduction in endothelial cell numbers with age
• Cells dont regenerate so existing cells have to be spread out to ensure a monolayer rather than have gaps
• There is also a variation in cell size (polymegathism) and shape (pleomorphism)

Question 12

Which nerves and blood vessels traverse the sclera?

Answer 12

• optic nerve
• short posterior ciliary arteries and nerves
• long ciliary nerves
• anterior ciliary arteries
• vortex veins

Question 13

What is the difference between the corneal and scleral collagen?

Answer 13

Cornea - small diameter, regularly arranged
Sclera - larger size and irregular arrangement of collagen means that sclera transmits v little visible light, so there is some transmission and this can be seen on transillumination

Question 14

How does this variation explain differences in transparency?

Answer 14

• Sclera has variability in fibril diameter and spacing

- Size and organisation of collagen fibrils in cornea allow forward transmission of light and eliminates scattered light

Question 15

What is the Tenons capsule?

Answer 15

Thin membrane, envelopes the eyeball from the optic nerve to limbus

Question 16

What is the relationship of the episclera with Tenon’s capsule?

Answer 16

-Tenons capsule found bet the conjunctiva and episclera (which forms the superficial vascular layer of the sclera)

Question 17

What are the 5 layers of the cornea?

Answer 17

Epithelium, bowman membrane, strong, descemets membrane and endothelium

Question 18

What is the epithelium of the cornea?

Answer 18

• stratified squamous and non keratinised
• consists of 6/8 cell layers
• consultant turnover of cells

Question 19

What are the 3 morphological types of epithelial cells in the cornea?

Answer 19

Basal cells, wing cells and squamous cells

Question 20

What do the squamous cells of the epithelium provide?

Answer 20

Provide a permeability barrier and also facilitate the spreading and attachment of the pre corneal tear film

Question 21

What is Bowmans membrane?

Answer 21

Appears as a cell free homogeneous layer which lies under the epithelium
-ultrastructurally it consists of randomly orientated fine collagen fibrils. It’s function has not been established

Question 22

What is the endothelium?

Answer 22

The corneal endothelium consists of a single layer of squamous cells that line the posterior surface of the cornea

Question 23

What is Descemets membrane?

Answer 23

The basement membrane of the endothelium, 3-4 microns at birth and continues to increase thickness with age to 10-15 microns

Question 24

What is the sclera?

Answer 24

Forms the bulk of the outer coat of the eye. It’s mechanical strength resists the intra ocular pressure and withstands the pull of the EOMs
Thickness varies from 1mm near optic nerve and thins anteriorly. It’s thinnest immediately behind the insertion of the EOMs (0.3mm)

Question 25

Histologically, what layers can the sclera be resolved into?

Answer 25

Episclera, sclera, storms and lamina fusca

Question 26

What is the episclera?

Answer 26

Represents the layer of loose vascular connective tissue which lies between the conjunctiva and sclera