Cornea

Question 1

Of which layer of the eye do the cornea and sclera belong to?

Answer 1

Outer fibrous layer

Question 2

Describe the structure of the tear film, their function and the origin of each layer

Answer 2

Lipid - meibomian glands
Limits evaporation
Aqueous -Orbital gland (70%) Nictitans gland (30%)
Most of the volume
Mucin - Conjunctival cells
Aids spread & adherence of tear film

Question 3

What is the function of the tear film?

Answer 3

Provides nutrients and oxygen to ocular surface

Protects ocular surface from desiccation and bacteria

Lubricates ocular surface

Provides smooth and transparent ocular surface

Question 4

How would you examine the tear film?

Answer 4

Corneal reflection and/or Schirmer tear test to measure tear production

Question 5

Describe the gross structure of the cornea

Answer 5

Slightly larger horizontally than vertically

50-60um thick

Question 6

Describe the microscopic anatomy of the cornea

Answer 6

Layered
Epithelium
Stratified squamous (non keratinised)
Contains tight junctions & has abasement membrane
Continual cell turnover
Stroma
90% corneal thickness
Collagen fibrils in regularly spaced layers
Relatively dehydrated
Descemet’s Membrane

    Endothelium
             One cell layer thick
	 Na/K ATPase pump : pumps ions from stroma into aqueous 
             humour
         Numbers decrease with age

Question 7

Explain the mechanism responsible for maintaining corneal transparency

Answer 7

Smooth optical surface
Relatively dehydrated state
No keratin, blood vessels, or melanin
Low cell density
Regular arrangement of collagen fibrils

Question 8

What is the function of the cornea?

Answer 8

Refract and transmit light - major refractive component of eye

Tensile strength – lends rigidity to globe

Protection
Chemical barrier

Rich subepithelial nerve plexus (trigeminal nerve V)

Igs & other antimicrobials in tear film

Epithelial cell desquamation

Migrating Langerhans cells and macrophages from limbus

Question 9

What is the role of the corneal epithelium?

Answer 9

Physical barrier to injury and infection and also plays an important role in the ocular immune response system by producing inflammatory cytokines

Question 10

What is the role of the corneal endothelium?

Answer 10

Transports water from the stroma into the anterior chamber. This movement of water counters a natural tendency for the stroma to swell and is necessary to maintain a transparent cornea.

Question 11

Describe how you examine the cornea

Answer 11

Light source +/- magnification

Darkened room

Looking for:
Irregularities
Opacification
Vascularisation
Pigmentation

Fluorescein staining
Orange dye that turns green in alkaline tears

                 Adheres to and stains hydrophilic tissues such as exposed 
                 corneal stroma

    Extremely useful for diagnosis of corneal ulcers

        Easily seen by owners to help understanding

Question 12

What is keratoconjunctivitis sicca (KCS)? List the clinical signs

Answer 12

Inadequate tear production caused by local immune-mediated destruction of orbital & Nictitans lacrimal glands

Clinical signs:
Conjunctivitis
Thick mucoid discharge
Blepharospasm

Corneal ulceration

Cornea; vascularisation & pigmentation

     Reduced vision

Commonly seen in toy breeds & cocker spaniels

Question 13

What is a corneal ulcer?

Answer 13

Break in continuity of corneal epithelium with exposure of underlying stroma

Aka ulcerative keratitis

Question 14

List the causes of corneal ulcers?

Answer 14

Trauma
Lacerations, abrasions, foreign body

Eyelid abnormalities
e.g. Entropion

Eyelash lesions
e.g. ectopic cilium

Dystrophies/Degenerations

Infections
e.g. IBK in cattle

Keratoconjunctivitis sicca (KCS)

Question 15

How does an ulcer affect the superficial (epithelial) layer?

Answer 15

Small amount of corneal oedema

Question 16

How does an ulcer affect the deep (stromal) layer?

Answer 16

Corneal oedema around the ulcer but not in the centre of deep ulcer (see clear centre of the fluorescent staining)

Question 17

How does an ulcer affect the descemetocele layer?

Question 18

Explain the overall concept of corneal wound healing

Answer 18

Corneal mebrane is self renewing :
Proliferation of basal epithelial cells at limbus –>
Movement of peripheral cells towards centre of cornea –>
Epithelial cells lost from corneal surface into tear film

Complex interaction which involves
Proteases
Growth Factors
Cytokines
Epithelial cells
Stromal Keratocytes
Inflammatory cells
Lacrimal glands

Question 19

How does the epithelial layer heal from a wound?

Answer 19

Cell proliferation, migration & adhesion

Faster if basement membrane remains intact

Question 20

How does the stromal layer heal from a wound?

Answer 20

Starts once re-epithelialisation is complete

Fibroblasts migrate in & lay down new collagen

Requires vascularisation

Results in scar tissue
Remodelling over time

Question 21

What is the role of vasculature in wound healing?

Answer 21

Conjunctival hyperemia (with or without episcleral hyperemia)

Endothelial budding

Superficial corneal blood vessels (with or without deep corneal blood vessels)

Lesion resolves and blood become hypoperfused

Corneal ghost vessels

Question 22

What are the different corneal opacities and what do they mean?

Answer 22

Blue - oedema
(fluid enters stroma disrupting collagen fibre arrangement)
Red - vascularisation
(infectious KSC ‘pink eye’)
White - scarring, lipid, calcium, cellular infiltration
(corneal lipidosis, cellular infiltration eg infected ulcer, corneal
scarring)
Dark - pigment

Question 23

Describe the Sclera and its structure

Answer 23

Opaque

Largest portion of the outer coat of the eye

Composed of collagen & fibroblasts

3 layers:
Lamina fusca
Sclera proper/stroma
Episclera

Anterior portion covered by bulbar conjunctiva

Question 24

What is the limbus?

Answer 24

Where cornea, sclera and bulbar conjunctiva merge
SOURCE OF STEM CELLS

Question 25

What is the tenons capsule?

Answer 25

Connective tissue sheath

Connects bulbar conjunctiva to underlying sclera

Question 26

What is the episclera?

Answer 26

Dense, highly vascular fibrous layer

Binds Tenon’s capsule to sclera