Cornea Flashcards
Of which layer of the eye do the cornea and sclera belong to?
Outer fibrous layer
Describe the structure of the tear film, their function and the origin of each layer
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
What is the function of the tear film?
Provides nutrients and oxygen to ocular surface
Protects ocular surface from desiccation and bacteria
Lubricates ocular surface
Provides smooth and transparent ocular surface
How would you examine the tear film?
Corneal reflection and/or Schirmer tear test to measure tear production
Describe the gross structure of the cornea
Slightly larger horizontally than vertically
50-60um thick
Describe the microscopic anatomy of the cornea
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
Explain the mechanism responsible for maintaining corneal transparency
Smooth optical surface
Relatively dehydrated state
No keratin, blood vessels, or melanin
Low cell density
Regular arrangement of collagen fibrils
What is the function of the cornea?
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
What is the role of the corneal epithelium?
Physical barrier to injury and infection and also plays an important role in the ocular immune response system by producing inflammatory cytokines
What is the role of the corneal endothelium?
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.
Describe how you examine the cornea
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
What is keratoconjunctivitis sicca (KCS)? List the clinical signs
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
What is a corneal ulcer?
Break in continuity of corneal epithelium with exposure of underlying stroma
Aka ulcerative keratitis
List the causes of corneal ulcers?
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)
How does an ulcer affect the superficial (epithelial) layer?
Small amount of corneal oedema
How does an ulcer affect the deep (stromal) layer?
Corneal oedema around the ulcer but not in the centre of deep ulcer (see clear centre of the fluorescent staining)
How does an ulcer affect the descemetocele layer?
Explain the overall concept of corneal wound healing
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
How does the epithelial layer heal from a wound?
Cell proliferation, migration & adhesion
Faster if basement membrane remains intact
How does the stromal layer heal from a wound?
Starts once re-epithelialisation is complete
Fibroblasts migrate in & lay down new collagen
Requires vascularisation
Results in scar tissue
Remodelling over time
What is the role of vasculature in wound healing?
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
What are the different corneal opacities and what do they mean?
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
Describe the Sclera and its structure
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
What is the limbus?
Where cornea, sclera and bulbar conjunctiva merge
SOURCE OF STEM CELLS
What is the tenons capsule?
Connective tissue sheath
Connects bulbar conjunctiva to underlying sclera
What is the episclera?
Dense, highly vascular fibrous layer
Binds Tenon’s capsule to sclera
