Cornea Structure

Question 1

Overview x4

Answer 1

• cornea is transparent anterior bulbous part of the eye

• provides approx +42.00D of refracting power

• layered structure of fibrous connective tissue and epithelia

• cornea is avascular

Question 2

Location
Anterior
Peripheral
Posterior

Answer 2

Anterior
- covered by tears film
- in contact with atmosphere
- in contact with conjunctiva of eyelid

Peripheral
- limbus (stem cells) - where corneal tissue meets sclera, conjunctiva, ciliary body, iris and fascia of the globe

Posterior
- in contact with aqueous humour

Question 3

Function x4

Answer 3

• provides majority of eyes focal power

• gives mechanical protection via blink reflex

• helps maintain shape of the globe and maintain IOPs

• provides positive power due to the refractive index on each surface

Question 4

General dimensions

Vertical height
Horizontal diameter
Anterior surface radius
Posterior surface radius
Centre thickness
Edge thickness (limbus)
Refractive index

Answer 4

Vertical height = 10.6mm

Horizontal diameter = 11.6mm

Anterior surface radius = 7.8mm

Posterior surface radius = 6.7mm

Centre thickness = 0.5mm (550 nano)

Edge thickness = 0.7mm (750 nano)

Refractive index = 1.376

Question 5

Epithelium x3

Functions x4

Answer 5

• stratified squamous non keratised epithelium

• thickness of layer is approx 55nano

• 5-6 layers of 3 different cells - squamous, wing/umbrella, columnar

Functions
• damage protection through layers and sensitivity (blink reflex)

• protection from foreign organisms - cell renewal and lymphocytes

• pathway for metabolites from tears

• provides smooth surface for refraction

Question 6

Bowman’s layer (A.L.L) x4

Function x1

Answer 6

• fine collagen fibrils with random arrangement

• nerve fibres leaving cornea aquire Schwann cells but NO myelin sheath until 0.5mm from limbus

• may form scar tissue if damaged

• terminates at limbus marking anterior corneo-limbal junction

Function
• mechanical support and protection against infection

Question 7

Stroma x6

Answer 7

• approx 450nano thick in central cornea

• consists of mainly lamellae of collagen in mucopollysacharide matrix (approx 2nano thick of variable width)

• fibroblast cells found between lamellae (wound repair)

• collagen fibrils evenly spaced in parallel bands

• lamellae cross each other obliquely but stay parallel to surface

• regular arrangement of collagen is key factor in maintaining corneal transparency

Question 8

Descements membrane (P.L.L) x4

Function x1

Answer 8

• fine collagen fibrils in layers

• 8-10 nano thick in adults - increases with age

• can be reformed if damaged in minor trauma

• terminates at edge of limbus and marks posterior corneo-limbal junction

Function
• basement membrane for endothelium

Question 9

Endothelium x4

Functions x3

Answer 9

• single cell layer - simple epithelium

• cell membranes interdigitate to limit passage of water between cells

• cells packed with mitochondria

• cells cannot regenerate

Functions
• regulates corneal hydration

• active transport of metabolites from aqueous to stroma

• produces material for poster limiting lamina and reforms it

Question 10

Metabolite supply

Answer 10

Nearest blood vessels are found in capillary loops at limbus, and in tarsal / palpebral conjunctiva of eyelids

Question 11

Metabolite supply
Anterior surface

Answer 11

• tears film when lids open / daytime

• tarsal / palpebral conjunctiva when lids closed / nighttime

Question 12

Metabolite supply
Posterior surface

Answer 12

• bathed in aqueous humour

• primary glucose supply

Question 13

Metabolite supply
Periphery

Answer 13

• limbal blood vessels

• minor role compared to aqueous

Question 14

Nerve supply x5

Answer 14

• cornea only contains sensory neurones, no need for motor innervation

• free nerve endings pass through columnar & wing cells, terminating in a varicosity

• pass back into A.L.L covered in Schwann cells, travel to the peripheral cornea to exit at limbus in 70-80 trunks

• corneal nerves join ciliary nerves, derived from ophthalmic division of trigeminal nerve

• nerves of cornea are unmyelinated until 0.5mm from limbus

Question 15

Factors affecting transparency x5

Answer 15

• avascular - blood vessels are opaque

• corneal nerves are unmyelinated

• epithelium and endothelium are non-keratinised

• regular arrangement of stroma collagen fibres

• too much fluid = separation of collagen fibrils in stroma means loss of transparency