Cornea Flashcards
Which embryologically tissues does the cornea develop from?
Surface ectoderm = epithelium, secretes thick matrix - the primary stroma which consists of collagen fibrils and glycosaminoglycans.
Mesenchymal neural crest cells migrate between surface ectoderm and optic cup to give rise to corneal endothelium and stroma, anterior iris stroma, ciliary muscle and most structures of iridocorneal angle.
Endothelium forms first and secretes descemet’s membrane around days 30-35 in the dog
In growth of mesoderm between epithelium and endothelium which is continuous with sclera and forms stroma.
When does the cornea achieve transparency and how does it change over the first few weeks of life?
Towards end of gestation = transparency
Following eyelid opening at 14 days in dog initial decrease in corneal thickness over 4 weeks as endothelium becomes functional. Then gradual increase in thickness over next 6 months.
How much of the outer tunic of the eye does the cornea make up?
1/6th (in continuity with surrounding sclera)
What are the functions of the cornea?
- Refract and transmit light through to the lens and hence the retina
- Protection from chemical and physical insults
Which structure is the major refractive component of the eye?
How much dioptre of power towards convergence of an image on the retina is there from this structure?
Anterior cornea = major refractive component of the eye
48 dioptres of plus power towards convergence of an image on the retina.
What is essential for optical clarity of the cornea? What does this mean for how the cornea receives nutrition?
Cornea needs to be transparent hence why cornea = avascular
Lack of blood vessels so has to source nutrition elsewhere
Tear film = oxygen provision/hydration
Aqueous humour = other nutrition.
How is a combination of transparency and tensile strength achieved in the cornea?
Collagen fibrils - uniformly sized and maintained at close regular periodicity
(Highly dependent on state of corneal hydration)
How is a chemical barrier formed by the cornea?
Tight junctions between superficial corneal epithelial cells
How does the cornea have sensitivity without compromising clarity?
Rich subepithelial nerve plexus with extensive fine endings interdigitating between corneal epithelial cells
Superficial cornea = very sensitive due to the abundance of these nerve endings.
How is the cornea protected from pathogens despite the lack of blood vessels?
Tear film = immunoglobulins and antimicrobial factors, physical blinking to remove debris
Epithelial cell desquamation
Migrating Langerhans cells and macrophages from limbus.
Into which 3 layers is the cornea divided anatomically?
Epithelium, stroma, endothelium
What type of epithelium are present on the cornea? How are these epithelial cells arranged.
Stratified squamous (non keratinised, non secretory)
Basal layer = columnar cells adhered to 50nm basement membrane
2-3 layers interdigitating, wing or polygonal cells make up intermediate layer, irregularly shaped cells with oval nuclei. These cells are in an intermediate state of differentiation.
3-4 layers flattened, nucleated squamous cells called squames
Outermost cells = most differentiated - possess tight junctions, zona occludens and form permeability barrier to the cornea.
Anterior plasma membrane of most superficial layer of cells = express microvilli and micropliae whose glycocalyx coat interacts and helps stabilise the pre corneal tear film.
How thick is the corneal epithelial layer?
50-60 um
5-7 layers total
How thick is the whole cornea on average?
0.6mm
How do epithelial cells adhere to one another on the cornea?
To maintain stable cornea need appropriate cell-cell adhesion and cell-substrate adhesion.
Superficial cells = desmosomes + tight junctions. Tight junctions only in the superficial corneal epithelial cells and ensure barrier function. Tight junctions or zona occludens completely encicle to cell and represent anastomosis of lipid bilayer of adjoining membranes. Enable superficial cells to provide effect semi-permeable barrier on surface of cornea.
Wing cells - desmosomes (both to superficial cells above and other wing cells). Gap junctions present within wing cell layers allow high degree of intercellular communication.
Basal columnar cells - also desmosomes and gap junctions but smaller and fewer in number than wing cells. Also hemisdesmosomes via which the basal cells and thereby the whole epithelium attaches to the basement membrane and stroma. (SCCED - failure of hemidesmosomes and recurrent erosions)
What are the roles of the corneal basement epithelial membrane?
Basement membrane = specialised extracellular matrix
40-60nm thick
- Structure - maintain tissue architecture
- Anchorage for adjacent cells
- Selective barrier to migrating/invading cells
- Facilitating filtration/temporary storage of macromolecules
- Intimately involved in embryonic development and cellular differentiation.
How thick is the stroma?
500um thick - 90% of corneal thickness
How is the stroma unique amongst connective tissue within the body?
Stroma = most highly organised and transparent connective tissue in the body
What is the stroma composed of? How is it excreted and maintained?
Lamellae - flattened bundles of collagen fibrils orientated in parallel manner equidistant apart
Secreted and maintained by stromal fibroblasts known as keratocytes which reside between layers of the lamellae within the collagen matrix.
Majority of stroma = collagen there are also numerous proteoglycan molecules cross linking the stromal lamellae - keratin, sulphate and chondroitin.
What is the predominant type of collagen within the stroma?
Type 1 collagen
Lesser amounts of type 3, 5 and 6
How is the stroma remodelled?
What happens if there is overproduction of these remodelling enzymes?
Stroma = constant state of remodelling
Maintained by keratocytes
Collegen molecules broken down and reformed as these cells move through stroma
Keratocytes produced proteases which enable remodelling - overproduction by these fibroblasts, inflammatory cells or bacteria = corneal melting.
What is Descemet’s membrane? How thick is it?
Inner thickened basement membrane of the stroma
12um thick
How thick is the endothelium? Describe its anatomy.
Endothelium = 1 cell thick
Monolayer = regular arrangement and mainly hexagonal in shape
Lateral membrane = interdigtiation with hemidesmosomes
Tight junctions and gap junctions. (tight junctions do not completely encircle like with epithelium)
Endothelium = “leaky barrier” between AH and stroma due to not full surrounding of tight junctions
Does still impede free flow of water and solutes.
How does the endothelium regulate stromal hydration? (i.e prevent stroma from having excess water)
Na K+ activated adenosine triphosphatase (ATPase) pump
Can the corneal endothelium regenerate? What happens to the endothelium with age?
No corneal endothelium is not self renewing and number of cells decreases with age
What response do corneal endothelium cells have when damaged?
Cells around defect enlarge and slide in to fill the defect - consequence = oedema due to influx of water into stroma
How is the cornea innervated?
Trigeminal nerve = corneal sensitivity
Myelinated fibres pass from trigeminal through anterior stroma in groups termed leashes.
As individual nerves leave leashes they maintain a Schwann cell sheath.
Penetrate through epithelial basement layer, sheath is lost and naked nerve endings send its terminus up between the cell layers, terminating amongst outer most squamous cells.
How many more nerve endings does the corneal epithelium have compared to the dermis?
300-400 more nerve endings per unit
Where does the cornea obtain it’s nutrition from?
Oxygen - tear film (+hydration)
Limbal blood vessels/aqueous humour = glucose and amino acids
Describe the anatomy of the limbus - how does it differ from the surrounding cornea/conjunctiva?
Limbus = junction between corneal and conjunctival epithelia
(Anatomically zone also includes Schlemm’s cana; and trabecular meshwork but more commonly it is the superficial portion referred to as the limbus.
Transitional zone 10-12 layers thick
Unlike conjunctival epithelium lacks goblet cells
Similar cell-cell and cell-substrate junctions as cornea
Basal cells smaller and less columnar than corneal epithelium
More mitochondria in basal cells of limbus
Smaller area of basal cells with hemidesmosomes - undulation = additional adhesive strength and possibly increased surface area for absorption of nutrients from limbal vasculature.
Some of these basal cells hypothesised to be stem cells of the corneal epithelium.
Connective tissue below limbal epithelium = more loosely and irregularly arranged than cornea
Similar collagen, proteoglycans, soluble glycoproteins as cornea
Stroma of limbus = fibroblasts, melanocytes, macrophages, langerhans cells, mast cells, lymphocytes, plasma cells.
Blood vessels and lymphatics loop into this region along with bundles of unmyelinated nerves.
Connective tissue - large radial folds/ridges (palisades of vogt) house small blood vessels, lymphatics and nerves
Crypts of limbal epitheliu, reach down into valley between palisades of vogt and hypothesised that deep housing of basal cells protects stem cell population.
Blood supply - anterior ciliary arteries extending from rectus muscles. Drained by venules reversing over same direction.
Is the corneal epithelium self renewing? What is the normal resting state of the cornea?
Yes - stratified squamous epithelium = self renewing
Constant state of “healing”
Squamous cells shed into tear pool whilst simultaneously being replaced by cells moving centrally from the limbus and anteriorly from the basal layers of the epithelium
How is the corneal epithelial mass maintained during normal physiological state? How does it change with wound healing.
Exaggerated version of normal physiological maintenance phase when wound healing.
Cellular and subcellular events occurring under influence of extracellular matrix proteins and growth factors.
Under normal circumstances epithelial mass does not change
X+Y = Z (corneal epithelial cell mass maintenance equation)
X = proliferation of basal epithelial cells
Y = contribution to cell mass by centripetal movement of peripheral cells
z = epithelial cell loss from surface
Cell multiplication occurs predominantly at the limbus - limbal stem cells source of multiplying and migrating cells.
What 3 components are required for corneal epithelial wound healing?
Cell elongation + migration, cell proliferation, cell adhesion
Describe the process of corneal epithelial defect healing?
3 phases of healing:
1. Cell elongation followed by migration
2. Cellular proliferation
3. Cellular differentiation and adhesion
1st 4-6 hours no appreciable decrease in wound size occurs (may become slightly larger due to necrotic cell removal and rounding off of cells at wound edge)
Basal and squamous cells in vicinity show thickening and separation.
Within 2hrs all hemidesmosomal attachments between basal cells and basement membrane disappear
Demsmosomes between cells remain intact allowing to move as a sheet.
Epithelial cells flatten and move across the defect until completely covered - active energy consuming process independent of cell proliferation.
Migration = increased synthesis of proteins and glycoproteins with glycogen metabolism being main energy source.
Both basal and suprabasal cells participate in migration
Flattened epithelial cells filling defect show finger like (filopodia) and coral like (lamellipodia) processes extending onto wound surface - formation of these marks beginning of cell migration.
In order to migrate basal cells must release their adhesive junctions and other means of adhesion to allow spreading over the wound. Temporary contacts known as focal contacts formed - actin filament bundles inserting into cell membrane.
Fibrin and fibronectin stimulate epithelial cells to release plasminogen activating factor - plasminogen to plasmicn which lyses cell to substrate adhesions allowing cells to advance and form new adhesions.
Cycle repeated until migration ceases at wound closure.
Normal thickness restored by epithelial proliferation - restore cell numbers and mass.
Wave of mitosis generated from periphery - stem cells at limbus produce transient amplifying cells and move from periphery to the wound and continues until wound has healed and normal thickness restored.
Wound healing not complete until newly regenerated epithelium has anchored itself to underlying connective tissue.
Rapidity with which new hemidesmosimal attachements occur depends on whether basement membrane was intact or not at time of wounding.
If was intact new epithelial cells can utilise it and migrate rapidly within 7 days to fill defect forming new strong adhesions.
Basement membrane not intact e.g stromal ulcer, normal adhesion may not be for several weeks as a new basement membrane must be constructed by the epithelium and stroma before new adhesions can form.
What happens with stromal wound healing (lack of basement membrane)
Stroma cannot heal/remodel until re-epithelialisation complete
Fibronectin from tear film deposited onto ulcer bed and basal epithelial cells use fibronectin to make focal adhesions and migrate across the ulcer bed.
Once re-epithelialised fibroblasts (keratocytes) from surrounding stroma migrate to wounded area and start to lay down new collagen matrix (type 3)
Collagen is not as regularly arranged as the unwounded cornea and this results in opaque scar formation.
With time the fibroblasts can remodel the scar tissue and the opacity may reduce in size.
What are the main responses of the cornea to insult?
Oedema
Pigmentation
Vascularisation
Fibrosis
Cellular infiltration
Lipid deposition
Calcium deposition
Most corneal disease will incorporate a number of these signs.
What colour changes can we see with the cornea?
Blue/grey - corneal oedema
Red - neovascularisation
White - lipid/calicum/cellular infiltrate
Black/Brown - pigmentation
Describe location and distribution of this colour change.
What is corneal oedema?
Fluid within cornea - due to epithelial or endothelial dysfunction
Disruption to regular arrangement of collagen fibrils = opacity
List the causes of corneal oedema
Epithelial dysfunction - corneal ulceration (usually deep = stroma)
Endothelial dysfunction - primary (age/breed related) or secondary (intraocular inflammation/uveitis, anterior lens luxation, increased IOP, intraocular mass, intraocular surgery/trauma)
What tests should be performed in all cases of corneal oedema?
Full ophthalmic exam
Fluorescein
IOP - tonometry important
Consider ocular ultrasound if unable to visualise eye with diffuse severe corneal oedema.
Why does vascularisation occur in the cornea?
Can get superficial and/or deep vascularisation of the cornea in response to release of angiogenic factors by the injured (anoxic) cornea, infiltrating cells or neoplasms.
Try to establish if the blood vessels are superficial or deep.
What is pigmentation of the cornea generally associated with?
Chronic corneal disease
Melanin - superifical or endothelial
Corneal sequestrum in cats = their version of pigmentation.
What types of white opacities can we see in the cornea?
Fibrosis - from stromal healing - irregular collagen deposition therefore these areas are opaque. Overtime remodelling can occur and regular lamellae arrangement restored reducing opacity.
Cellular infiltrate (white/pink) - usually superficial and proliferative. Identification of cell type with corneal cytology (scraping lesion with topical anaesthesia). May consider keratectomy for diagnostic/therapeutic purposes.
Lipid deposition (white and SPARKLY, think fat = glistening) - various casues, superficial or deep deposits
Cholesterol or triglycerides.
Corneal lipid dystrophy
Lipid Keratopathy
Arcus lipoides corneae
Steroid keratopathy
Calcium deposition - (white and GRITTY, think mineral) - degenerative condition usually seen in older dogs. Often may have concurrent diseases e.g renal/cardiac disease
How can non ulcerative corneal disease be categorised?
Infiltrative e.g cellular, neoplastic, non cellular
VS
Non infiltrative - congenital/acquired
Vs
Infectious
Describe Chronic Superficial Keratitis (Pannus)
Which breeds are predisposed?
Predisposed = GSD, Collies, Greyhounds
Usually 3-5 years old at presentation
“Red eyes”
Corneal lesions start at ventro-lateral limbus and spread out across cornea
Vascularisation, infiltration and pigmentation characteristic
Conjunctival involvement including TEL common and in some cases may be affected without corneal lesions (plasma cell infiltrate of TEL - plasmoma, loss of pigmentation of TEL)
Lesions bilateral generally and may be complicated by lipid deposition
Rare to be ulcerated
How is CSK diagnosed and treated?
CSK - often suspected based on presentation/breed however could confirm with biopsy - plasma cells dominate
Treatment = topical steroids or ciclosporin (Optimmune = licensed)
Most cases respond rapidly to treatment but often prone to relapse once therapy withdrawn - affected dogs require repeat courses of treatment/lifelong therapy
Flares ups often associated with high UV
Vascularisation and infiltration will decrease with treatment but lipid deposition/pigment often persists.
?Superficial keratectomy if vision severely impaired by pigmentation.
Describe eosinophilic keratoconjunctivitis in the cat.
Immune mediated disease
White/pink deposits (resembling cottage cheese) in superficial cornea and stroma
Can also involve conjunctiva (occasionally just conjunctiva affected - loss of pigmentation of eyelids also)
Young - middle aged cats
?possible link to FHV-1
How would you diagnose eosinophilic keratoconjunctivitis in the cat?
Diagnosis = cytology of corneal scrapings or biopsies
Mast cells and eosinophils predominate with occasional mixed inflammatory cells.
What is the treatment for eosinophilic keratoconjunctivitis?
Topical steroids or ciclosporin - clinical remission
Many cases will have seasonal reoccurence and require repeat treatments.
In past megoestrol acetate was used for treatment but s/e (increased risk of mammary neoplasia, appetite increase, weight gain, increased risk diabetes etc) mean it is reserved only for refractory cases.
Which breeds are predisposed to punctate keratitis? How does it appear? What is the treatment?
Breeds = Shetland Sheepdog, Minature Dachshunds
Multiple pin point fluorescein +ve punctate corneal opacities
Treatment = topical steroids (only time steroids are indicated when fluorescein +ve)
What types of neoplasia may we see of the cornea itself?
Corneal neoplasia alone very rare
Squamous cell carcinomas - can occur at limbus and reported in cats, also dogs associated with chronic keratitis
Limbal/epibulbar melanoma - slow growing benign tumour in dogs, darkly pigmented breeds e.g Labradors/GSD
Surgical resection tx of choice and can be followed with Strontium 90 radiation therapy. Larger lesions may need reconstructive techniques e.g corneo-scleral transposition.
Cryotherapy/laser photoablation also options.
Haemagioma/haemangiosarcoma - reported at limbus and on cornea.
What is the difference between corneal lipid dystophy and corneal lipidosis?
Dystrophy = implies inherited
Lipidosis = inheritance unproven
Describe corneal lipid dystrophy (crystalline stromal dystrophy)
Which breeds are predisposed?
Lipid = sparkly appearance
Breeds = Cavalier King Charles, Rough Collies, Shetland Sheepdogs, Afghan Hounds, Beagle, Boxer, Husky, GSD, Samoyed
Lipid deposition in corneal stroma of both eyes
Not a systemic disease although systemic factors can influence
NOT ASSOCIATED WITH VASCULARISATION
Condition usually seen in young adults (occasionally as puppies)
Bilateral and reasonably symmetrical (one eye may be affected in advance of the other)
Opacity usually central/paracentral and typically crystalline sparkling appearance.
Integrity of epithelium confirmed with lack of fluorescein uptake.
No associated inflammation and opacity once formed remains static. Occasionally will regress and occasionally progress in which case low grade inflammatory response may follow on.
In bitches sometimes associated with oestrus, pregnancy and lactation.
What is the predominant lipid type in corneal lipid dystrophy (crystalline stromal dystrophy)
Cholesterol = main type of lipid - free and esterified form with lesser quantities of free fatty acids and phospholipids
What is the main defect in corneal lipid dystrophy (crystalline stromal dystrophy)
Main defect = with keratocytes of stroma
Accumulates large amounts of lipid before dying in situ - crystalline cholesterol associated with death and necrosis of the keratocytes.
Changes restricted to anterior third of the corneal stroma
