*Topic 2: cornea Flashcards
what is the tear film?
Anterior most refractive layer of the eye, separating the corneal epithelium and the atmosphere
what are the 3 types of cells that make up the epithelium?
superficial squamous cells, wing cells and basal cells
what are the 4 characteristics of superficial squamous cells?
- flattened
- non-keratinised
- polygonal
- Have tight junctions (Zonula Occludens) to prevent entry of water from tear film
what are the 2 characteristics of wing cells?
- Irregular cells
2. have gap junctions and desmosomes.
what are the 3 characteristics of basal cells?
- single layer of columnar cells
- have gap junctions, desmosomes and hemi desmosomes (attachment to the basement membrane-Bowman’s layer).
- Ability for MITOSIS, hence high metabolic demand
what are the 5 layers of the cornea?
- epithelium
- Bowman’s membrane
- stroma
- Descemet’s membrane
- endothelium
what is the thickness of epithelium? Can it heal?
10% of total corneal thickness, 50 microns thick. 5-7 cell layers
yes. in case of injury, can regenerate due to the Basal cells which have the ability for mitosis. Usually takes up to 1 week for epithelial healing.
what is the Bowman’s membrane? can it heal?
Basement membrane for the epithelial cells. Cannot regenerate if injured.
what is the thickness of stroma? Can it heal?
90% of the corneal thickness, about 500 microns thick.
Cannot regenerate in case of injury.
what is the stroma made up of and how is it arranged? how does its arrangement allow for corneal transparency?
Has few cells, flattened Keratocytes. Mostly made up of collagen fibers which are equidistant and of uniform diameter and arranged in regular lattice arrangement.
Results in destructive interference (Maurice’s theory) leading to a transparent cornea (allows almost 98-99% light transmission).
what is the Descemet’s membrane? can it heal?
Basement membrane for the endothelium.
Can regenerate partially.
what is the thickness of endothelium? Can it heal? what is its main function?
5 microns thick, single layer of hexagonal cells with tight junctions (macula occludens).
Cannot regenerate.
Important function is to maintain the water content of the corneal stroma (78%) by preventing entry of water from aqueous humour. !! high metabolic activity.
what are the 4 factors responsible for Corneal deturgescence ? (BMCI)
- Barrier function of epithelium and endothelium
- Metabolic pump
- Changes in osmolarity
- Intra-ocular pressure (IOP)
how does the barrier function of epithelium and endothelium maintain cornea deturgescence?
Epithelium - tight junctions - zonula occludens = prevent entry of water
Endothelium - tight junctions - macula occludens (leaky barrier) = only allows water to leave and not enter
how does the metabolic pump maintain cornea deturgescence?
the pump mainly transport ions and water follows the movement of the ions. (Do NOT pump water).
leak is the movement of water from aqueous humour into stroma
pump is the movement of water from stroma into aqueous humour
For normal endothelial cell density, leak = pump (water content maintained)
Stressed endothelium (cell density reduced to 800-1500 cells), leak = pump (water content is maintained but endothelium under lot of stress)
Decompensated endothelium (<500 cells), Leak > pump (water content cannot be maintained, water freely moves into the stroma)
how does changes in osmolarity affect cornea deturgescence? (relate to hypoxia!)
seen in CL wearers (either tight, ill fitted CLs or overwear of CLs or sleeping with CLs)
Hypoxia (lack of oxygen) - increased anaerobic metabolism - increased lactate ions - reduced pH (osmotic balance is disrupted) - endothelial pump function affected
leak > pump (water enters the corneal stroma) - lattice arrangement is lost - results in light scattering - causing blurred vision
how does IOP affect cornea deturgescence?
sudden increase in IOP (seen in acute angle closure glaucoma)
endothelial pump function is affected - leak > pump - water enters the cornea
corneal edema disrupts the lattice arrangement - increases light scattering - causing blurred vision
why is there a need for corneal metabolism? (4 reasons) (TPWM)
- transport processes
- pump functions
- wound healing
- maintain ocular temp
what are the 3 corneal metabolic pathways?
- Anaerobic glycolytic pathway
- Aerobic Kreb’s cycle
- Hexose monophosphate shunt (anaerobic)
what happens in epithelial wound healing? how long does it take?
mitosis then migration
The basal cells carry out mitosis to prodice epithelial cells before migrating upwards, transforming into wing cells and supercifial epithelial cells
1 week
what happens in endothelial wound healing?
cells cannot regenerate
Hexagonal cell shape is lost (pleomorphism)
cell enlargement occurs (polymegathism)
endothelium becomes thinner due to cell loss and cell enlargement
what are the 5 factors affecting rate of corneal wound healing? (SDLIM)
- size (smaller sound=faster healing)
- depth (Superficial wounds=faster healing)
* Epithelium can regenerate but deeper injuries to stroma can result in scar (since it cannot regenerate)
3.location (peripheral wounds heal faster than central wounds.)
In the corneal periphery, there is presence of blood vessels where as central cornea is avascular hence takes longer to heal.
- infection (no infection=faster healing)
if infection is present then the injury will heal slower since the repair mechanism will need to control the infection first.
5.medication
medication like steroids speed up the recovery process, whereas anaesthetics (numbing drops) will slow down the wound healing
3 Sources of nutrition of the cornea?
- Diffusion through the tear film - anteriorly
- Limbal blood vessels (anterior ciliary arteries) - peripherally
- Aqueous humour - posteriorly
what is the blood supply of the cornea?
cornea is avascular but one of the sources of it’s nutrition are the limbal blood vessels which are branches of the ANTERIOR CILIARY ARTERY
what is the nerve supply of the cornea? Where is it located
TRIGEMINAL NERVE (CRANIAL NERVE 5) the nerve endings lie in the sub-epithelial region and extend to the mid-stroma
