Cornea Flashcards
Anterior surface of cornea -shape, horizontal diameter, vertical diameter
Cornea makes up the anterior 1/6th of the eyeball.
It’s anterior surface is elliptical with horizontal diameter 11.7 mm and vertical diameter 11 mm.
Posterior surface of cornea
Circular in shape and an average diameter of 11.5 mm.
Thickness of cornea
Centre— 0.5-0.6 mm
Periphery—1-1.2 mm
Thickness of the cornea is measured using
Corneal pachometer
Indications- Performed prior to refractive surgeries, keratoconus surgery, screening of patients with high risk factors for glaucoma.
Refractive INDEX of cornea
1.376
Refractive POWER of cornea
45D
Total refractive power of the eye is 60 D. 3/4th of which is contributed y the cornea.
Anterior surface of cornea has a refractive power of +48D but a negative power of the posterior surface of -5.8D results in final refractive power of about 43-44D.
Histology of the cornea
Anterior to posterior
1. Epithelium
2. Bowman’s membrane
3. Stroma
4. Pre Descemet’s or dua’s layer
5. Descemet’s membrane
6. Endothelium
Epithelium of Cornea
Anterior most later of cornea
6 layer of cells.
1 layer of Basal columnar cells
3 layers of umbrella or wing shaped cells
2 layers of flat cells with superficial micro villi
Flat cells have tight junctions high prevent entry of tear film into cornea
Microvilli adsorb tear film onto corneal Surface.
Hydrophobic corneal surface to hydrophilic surface
Limbal epithelium
Junction of cornea and sclera
Basal epithelial cells at limbus- limbal stem cells
Proliferate and differentiate into corneal epithelium
Damage to limbal epithelium leads to proliferation of conjunctiva as corneal epithelium.
Bowman’s membrane of cornea
Acellular layer
Consists of condensed collagen fibrils
Condensed superficial part of stroma
Strong and resistant to infections but once damaged, can not regenerate and heals by scarring.
Stroma of cornea
Hydrated matrix of proteoglycans (chondroitin sulphate and keratan sulphate)
Collagen fibrils (type 1 and type 5 fibrillae with type 6 collagen)
Lamellae have peculiar arrangement explained by lattice theory of Maurice
Between lamellae- keratocytes, histiocytes, wandering macrophages, leukocytes.
Lamellae have peculiar regular refractive index- Goldmann and Bendek theory
Dua’s layer
Pre- Descemet’s membrane
Acellular, strong and impervious to air
Descemet’s membrane
Basement membrane of the endothelium
Resistant to. Pathogens, chemicals etc and develops a descemetocoele.
Made of collagen and glycoproteins
Can regenerate
Normally it remains in a state of tension and when tears, curls into itself.
Periphery- Schwalbe’s line- angle of anterior chamber
Endothelium of cornea
Made of hexagonal epithelial cells
At Young age, they are many cells which gradually decrease with age, and to compensate cells increase in size-polymegathism.
Active-pump mechanism to maintain dehydrated state of cornea for corneal transparency.
Nerve supply of cornea
Long ciliary nerves which are branches of nasociliary nerve of ophthalmic division of trigeminal nerve.
Dichotomous division- Stroma, subepithelial and intrepithelial plexus
