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What are the 3 functions of the cornea?
Supports intraocular contents
Refracts light (40-42 diopters)
What does the aqueous humor deliver to the cornea?
What is a major source of oxygen for the cornea?
With sleeping or eye closure, what provides the oxygen to the cornea?
The palpebral conjunctiva
What are the cornea's 6 mechanisms to maintain clarity?
Highly organized arrangement of small diameter collagen fibrils
How does the highly organized arrangement of small diameter collagen fibrils act as a mechanism of clarity for the cornea?
These fibrils extend the entire diameter of the cornea
This arrangement allows 99% of the light to pass through the cornea without scatter
What is the job of the Na-K-ATPase pumps in the endothelium?
To remove fluid influx;
maintain relative dehydration
What is the job of the NA-K-ATPase pumps in the intercellular junctions between endothelial cells and epithelial cells?
To prevent fluid influx;
Maintain relative dehydration
What percent of the cornea is water, as compared to most other tissues?
most other tissues are 95%
Why is maintaining the relative dehydration of the cornea an active process?
The cornea is bathed by fluid on both sides eager to enter; aqueous humor on the inside, and tears on the outside
In a normal cornea, what is the process of maintaining clarity through relative dehydration?
The endothelial cells constantly pump Na+ out of the corneal stroma back into the aqueous humor and aqueous humor that was imbibed into the deeper stroma follows the Na+ back into the anterior chamber, and the epithelial barriers keep tears out
How does the tear film help maintain the clarity of the cornea?
Prevents keratinization and washes away resident and transient bacteria so their numbers stay in check
What are the 7 ways that the cornea may respond to disease?
What are the causes of corneal edema?
Occurs with damage to endothelium or loss of epithelium
Why is corneal edema present during corneal ulceration?
Because the missing epithelium cannot keep tears out, so they enter the stroma in the area of the ulcer causing a mild, faint edema
What is dense, widespread corneal edema usually caused by?
How does cellular infiltration of the cornea occur?
Via tears and limbus;
With complicated corneal ulcers
What cells infiltrate the cornea during complicated corneal ulcers?
Immune mediated disease (pannus)
What is scar formation caused by?
Abnormal arrangement of collagen fibers
What may lead to faster scar resolution?
Why does repair of a corneal stromal defect result in decreased opacity?
When the collagen is secreted to repair the defect, it is laid down haphazardly and does not attain the regular periodicity necessary for corneal clarity
How might pigmentation occur?
With any chronic irritation, active or inactive
How quickly do corneal vessels grow?
1 - 2 mm per day, but they take about a week to get started
What does vascularization indicate?
Active chronic disease
Why does vascularization occur?
When the cornea is injured or inflamed, the metabolic needs change. The cornea then recruits vessels from an arcade of limbal vessels to meet these changing needs
What are ghost vessels?
As keratitis resolves, the cornea's need for a blood supply diminishes. The endothelial channel that had formed within the cornea collapses, but the "track" that it laid down remains and appears as a translucent line within the cornea
What 2 types of vessels develop during vascularization?
Superficial and deep
Why do superficial vessels develop?
In response to superficial keratitis
Why do deep vessels develop?
In response to either deep corneal disease or intraocular diseases (like uveitis and glaucoma)
What is the formation pattern of superficial vessels?
They are usually solitary vessels that branch dichotomously and often extend far into the cornea
What is the formation pattern of deep vessels?
They form more of a "brush border", with multiple short straight vessels densely packed together; they usually do not make it to the center of the cornea
What is keratinization most common with?
Why/how does keratinization occur?
When the epithelium lacks moisture from the tears, it becomes more skin-like and keratinizes even forming rete pegs
What does secondary uveitis result from?
An axonal nerve reflex in the trigeminal nerve and is referred to as axon reflex uveitis
What does anterior uveitis refer to?
Inflammation of the iris and ciliary body
What are signs of anterior uveitis (caused by the axon reflex mechanism)
Protein and cellular leakage from uveal blood vessels (aqueous flare)
Ciliary spasm (resulting in pain)
How long does it take an entire healthy cornea to reepthelialize?
4 - 7 days
What are the two mechanisms of healing for stromal healing?
What triggers the mechanism of epithelial defect healing to begin?
When corneal epithelial cells sense a loss a "contact inhibition"
What are the 3 stages of the mechanism of epithelial defect healing?
1. Basal epithelial cells slide across the bare corneal stroma
2. Fibronectin is laid down on stromal surface, fibronectin receptors on the basal aspect of the basal epithelial cells attach to them, then "rachet" across the stroma using actin and myosin filaments
3. Once the stroma is covered, the epithelial cells undergo mitosis to thicken back to the normal 7-8 cell thickness
What is the process of the stromal defect avascular mechanism of healing?
1. Neutrophils come in via chemotaxis
2. Local keratocytes die
3. Surrounding keratocytes "transform" into fibrocytes, migrate, synthesize collagen and mucopolysaccharides
4. Matrix metalloproteinase (MMP) enzymes produced by both the cornea and invading PMNs degrade necrotic tissue in a relatively controlled fashion
5. Epithelium migrates to cover the defect
How long does it take maximum wound strength to be achieved after avascular healing of a stromal defect?
6 - 12 months
What is the process of the stromal defect vascular mechanism of healing?
Cellular infiltrate is greater
1. Vessels invade the stroma and granulation tissue is created
2. Eventually the vessels collapse and become ghost vessels
Is scar formation more dense with avascular or vascular healing?
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