*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)