L6 Cornea and Sclera 2 Flashcards
What % of fluid is removed from stroma by epithelium into tears?
5%
How does fluid removal from stroma by epithelium work?
Na/K ATPase antiporter sets up high K+ and low Na+ inside –> primary transport
Na/K/Cl symporter transports Cl from stroma into cell –> secondary transport
Cl- builds up inside cell and then is transported out of epithelium via cAMP dependent Cl- channel
With Cl-, water leaves via paracellular transport out of epithelium
What leads to increased chloride and water secretion?
increased cAMP and sympathetic secretion
How is pH maintained?
Lactate/H+ co-transporter and Na/H+ exchanger regulate pH in the basolateral membrane
pH must be maintained between 6.8-8.4 to prevent corneal swelling
Where is majority of fluid removed from stroma via?
Endothelium
What transport system drives the secretion that balances passive water leakage into stroma caused by osmotic pressure imposed by ECM? (endothelium)
Outward direction transport of HCO3- and Na+
What nutrients does cornea get from aqueous humor?
Glucose, amino acids, vitamins
What nutrients does the cornea get from tears?
Small bits of glucose, mostly oxygen
What vessels does the cornea get nutrients from
Small amounts from limbal vessels
Why doesn’t oxygen exist in aqueous humor?
Lens exists in low O2 environment, oxygen can cause cataract to lens
How does oxygen get into tears?
Oxygen diffuses through tears
How does a lack of nutrients or O2 affect corneal transparency?
Reduces ATP supplied to endothelial pumps, inhibits fluid transport and causes the stroma to swell, causing loss of transparency
How do oxygen levels in tears change during blinking?
155mmHg when open, 55mmHg when shut
What is polymegethism
Neighbouring cells fill up gaps of dead cells by enlarging, resulting in shaped cells
What are the consequences if the cornea doesn’t get enough oxygen?
Bloodshot eyes, polymegethism, new blood vessels grow from the limbus, eye becomes more vulnerable to infections
Loss of epithelium by trauma is generally healed within…
24-48hrs
What is the process of epithelium wound healing?
Basal cells lose attachment to basal membrane, cells enlarge and migrate to cover the wound by sending out filopodia towards the center of the wound. Limbal stem cells differentiate into basal cells, and migrate into the cornea. The wound is covered by a multilatered sheet, basal cells increase in height and mitosis resumes to restore the epithelium to its normal configuration.
How soon after does the epithelium re-establish healing after the stroma
wound has been sealed off?
Rapidly after
Why does the stroma swell when damaged?
Due to influx of neutrophils from tears
What is the process of stromal wound healing?
Keratocytes activate and move across the wound, producing collagen and fibronectin. Collagen forms over 4-6 days, then tensile strength increases over 3-6 months
Why is the cornea transparency initially compromised?
Direction of keratinocytes and collagen is not parallel to cornea fibrils within lamallae, but it will later remodel.
How is the loss of endothelial cells compensated?
Migration and enlargement of cells to cover the wounded cells, little evidence of cell division
When will corneal transparency return after endothelial cell damage?
Once wound is healed, HCO3- pump continues, the cornea thins
What is the structure of the sclera?
CT consisting of fibroblasts embedded in ECM.
What is the function of the sclera?
Provide structural integity and defines the shape of eye
Why does the eye length change throughout life
Due to growth of the eye and sclera
The sclera is an avascular tissue except for the…
Episclera and intrasclera vascular plexus.
What is the scleral collagen composed off?
Type 1 and 3
What is the structure of scleral collagen?
Collagen fibrils form entangled bundles that branch and intertwine with each other. Different to corneal fibrils which are arranged in parallal lamallae
Why is the sclera opaque?
Due to arrangement of collagen fibrils, spacing, and size
What is the difference between scleral and corneal collagen?
Scleral collagen is irregularly arranged and has a varied thickness and is thicker. Corneal thickness is uniform –> 30nm and spaced 250nm
What is the limbus?
Transition from regular cornea lamallae to irregular organisation of sclera
What is the function of the limbus?
- Serve as the barrier to infiltration of blood in the cornea
- Home to stem cells
What are the palisades of Vogt?
Radial projections within the limbus that extend into cornea, home of stem cells.
Describe the movement of stem cells from the limbus to the basal cell layer of epithelium
Centripetal movement, transcient amplifying (TA) cells are progenitor cells that differentiate from limbal stem cells and then migrate into the central stroma.
