Molecular physiology of transparency - The Cornea. Lecture 3 Flashcards
What is the function of the cornea?
1) Major physical barrier
2) Major refractive (optical) element (40diop)
What are the layers of the cornea?
1) Epithelium (Ant) Bowmans layer 2) Stroma (90%) Descements layer 3) Endothelium (Post)
What covers the cornea and what is its role?
A tear film covers the cornea forming a smooth optical element. However the epithelium secretes a mucin layer to prevent tear (water) lipid interaction.
Describe the cellular structure of the epithelium?
Stratified, non-keratanized, squamous epithelium 5-7 layers thick.
What are the cellular layers within the epithelium?
1) Superficial
2) Wing cell
3) Basal
What allows the epithelium to act as a physical barrier>
The high expression of high resistance tight junctions.
Describe the Basal cellular layer;
Single layer of cuboidal cells.
Capable of dividing into wing cells.
Sits on the bowman layer.
Describe the Wing cellular layer
1-3 cells thick
Intermediate stage of differentiation.
Gives rise to superficial cells
Describe the superficial cellular layer.
Terminally differentiated squamous cells that are sloughed off into the tears.
Whats the function of the epithelium?
Act as a physical barrier (high resistance tight junctions)
To secrete fluids into the tear film to maintain steady state stroma volume.
What does damage to the corneal epithelium result in?
Rapid wound healing (as cell turnover is every 7 days and damage accelerates this process)
How is the corneal epithelium maintained?
- The corneal epithelium maintains a constant thickness.
- Epithelium turned over every 7 days.
- XYZ hypothesis for cell turnover
Whats the XYZ hypothesis?
X = Proliferation basal cells Y = Migration of stem cells Z = Cells lost from the surface.
What percentage of the cornea is the stroma?
90%
What cells are found in the stroma?
- Keratocytes (fibroblasts of the eye)
- ECM (PG’s, water, Collagen fibrils)
What collagen types are found in the stroma?
- Collagen fibrils are made up of type 1
- lesser of type 5 and 6
What is the function of type 5 collagen?
Required to initiate the assembly of collagen type one into fibrils.
Describe the histological appearance of keratocytes:
Long, thing, stellate cells which run PARALLEL to the corneal surface.
Whats the function of keratocytes?
They secrete collagen and its associated ECM.
What structures does collagen form in the cornea?
Collagen fibrils form 200-250 lamellae bundles.
What are the properties of collagen?
Add to corneal strength, elasticity and shape.
What is the arrangement of collagen fibrils/ lamellae?
Uniform spacing <200nm to create transparency.
How are PG’s arranged in the stroma?
PG’s form ladder like attachments between collagen fibrils and along them.
What determines the arrangement of collagen fibrils?
Their size and charge
What modulates the diameter of collagen fibril?
Modulated and maintained by the PG core protein.
What are the core proteins of PG’s?
Decorin
Lumican
Keratocan
Biglycan
What are the types of gag side chains in the retina?
1) Chondroitin + Dermatan sulphate
2) Keratin Sulphate
3) Herapan Sulphate
What determines the spacing between collagen fibrils?
The hydration properties of the PG sulphate side changes determines the fibril spacing.
i.e the side chains charges draw in external ions, which in turn cause osmosis.
Where does water flow into the cornea from?
The tear film and the aqueous humor.
What problem does the hydration properties of PG side chains present?
The flow of water into the stroma can disrupt the charges of the PG side chains, therefore this process must be managed.
Whats the function of the stroma?
To be transparent and be a optical element.
How much light does the stroma scatter?
Less than 10%
How does the stroma scatter less than 10% of light?
The fibril/lamellae lattice structure acts as a diffraction grating, allowing light scattering fibrils to be cancelled by destructive interference (as the lamellae are arrange at 1/2 the wavelength of light)
What specific PG’s control collagen fibril diameter?
Decorin and lumican
What happens if osmosis of the stroma is not controlled?
The stroma would swell interfering with lamellae spacing therefore light would be scattered. Decreasing transparency.
How is excess osmosis in the stroma countered?
Active transport of ions by the epithelium and endothelium
Describe the structure of the endothelium:
Single layer of Hexagonal cuboidal cells attached to the desemonts membrane
Whats the function of the endothelium?
Maintenance of stroma hydration
Whats unique about the corneal endothelium?
High density of mitochondria to pump ions.
What removes majority of fluid from the stroma?
The endothelium removes the majority of fluid from the stroma.
How does the endothelium balance fluid into and from the stroma?
Active outward directed transport of HCO3- and Na+ drive fluid secretion that balances passive ion and water leakage into the stroma.
Where does the HCO3 come from?
CO2 water interaction in the stroma. CO2 from aerobic respiration.
What does the endothelium require to maintain stroma hydration?
O2 and energy. (to perform aerobic respiration)
Where does the cornea obtain nutrients from?
Aqueous humour - Glucose, AA, Vitamins
Tears - Glucose, O2
These are carried into the stroma by the the passive diffusion of water.
What happens to the ability of the cornea to obtain O2 when eyes are open, closed or have contacts on.
Eyes open. Large diffusion gradient for O2. Lesser for eyes shut and significantly impaired by contacts. Tennis profiles reduced.
O2 obtained from the tear layer.
