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Cards In This Set
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The cornea is part of the outer coat and is embryologically related to the skin with its pysiology being uniquely adapted to the functions.
________ and _________ are absent in the cornea.
It cover _________ of the circumference of the eye.
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Smooth Muscles
Blood Vessels
1/6
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What is the function of the cornea?
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1. Optical
Transparency - must be maintained.
Refraction - responsible for 2/3 of it
2. Protection
Somewhat rigid fibrous structure with a
renewable epithelial surface.
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What is the diameter of the cornea?
When does it reach adult size?
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11.7 X 10.6 (H X V)
At 2-3 years
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What is the central thickness of the cornea?
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Approx. 535um (Doughty)
or 532.1 (Li)
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What is the radius of curvature?
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Anterior 7.8 mm
Posterior 6.42 mm
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What is the front surface refractive index?
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40D
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What is the refractive index?
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1.376
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What % is water, collgen and other proteins?
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Water - 78%
Collagen - 15%
Other proteins - 5%
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What are the layers of the cornea?
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Epithelium (outermost layer)
Anterior Limiting Lamina (Bowman's Membrane)
Stroma
Posterior Limiting Lamina (Decemet's Membrane)
Endothelium (Innermost Layer)
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How thick is the epithelium?
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50.6 + 3.9 um
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What are three continuous layers of the epithelia?
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Corneal
Limbal
Conjunctival
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Describe the corneal epithelium.
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- It makes up 10% of the corneal thickness.
- Stratified layer with 5-7 layers of tightly packed cells .
- Embryologically derived from surface ectodermal cells.
- In the normal eye, the epi. is not keratinized b/c it does not express the cornified cytoskeleton typical of epidermal cells, although this may occur in severly dry eye or in case of Vitamin A deficiency.
- Contains three types of cells
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What are the three types and sheapes of cells in the epithelium?
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Basal Cells (tall columnar)
Wing Cells (umbrella or intermediate)
Squamous Cells (flattened)
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Describe the basal cells.
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- About 8 microns in height
- Attached to the basement membrane and the underlying anterior limiting lamina by means of hemidesmosomes.
- Along the lateral and anterior walls, basal cells form numerous desmosomal contacts with their neighbors. These desmosomes and interdigitations make the epithelium mechanically strong and resist mechanical forces.
- The weakest part of the epithelium is the basal cell layer, as the basal cells mature, they migrate towards the tear film.
- Once the basal cells have lost contact with the basement, it becomes postmitotic, losing ability to divide.
- Columnar shape of the cells change to flattened wing cells and eventually to squamous shape.
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Describe the squamoust epithelium, upon reaching the surface?
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- Upon reaching the surface, the squamous cells leave the corneal epithelium in an orderly manner as they are sloughed off in to the tear film.
- This process is known as exfoliation and may be driven by apoptotic mechanism.
- In rabbit cornea, it will take 3-4 days for some basal cells to reach the surface. Older reports suggest 7 days. Recent research indicates it may be far more complex, staying more than 14 days.
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