Week 4: the cornea and limbus Flashcards
Describe the size of the Cornea
Convex and elliptical shape
11.7mm horizontally and 10.6mm vertically
anterior radius of curvature (7.8mm anteriorly and 6.5mm posteriorly)
corneal thickness - centre 0.5mm and 0.7mm periphery
Function of the cornea
Transparency - must be transparent to let light reach the retina
- refraction - responsible for 2/3rds of the refraction taking place in the eye
Protection
- physical barrier to protect the contents of eyeball from injury (due to rigid fibrous structure with renewable outer epithelial surface)
- rich innervation of corneal nerves make it sensitive to detect foreign bodies and the need to blink and reflex tearing to maintain the health of the external surface of the eye
List the layers of the cornea
Epithelium Bowmans' Layer Stroma Descemet's membrane Endothelium
Describe the regeneration process of all the layers of the cornea
Epithelium - epithelium damage = chemical messengers = neighbouring epithelium to detach form basement membrane and migrate to close the defect
Cornea stroma - slow and collagen must be replaced causing irregularity
endothelium - cant’t regenerate, just change shape to fill the void
nerves - after injury a rapid nerve degeneration occurs and normal innervation is restored in 4 weeks
Discuss the degenerative process leading to Keratoconus
- Focal disruption to Bowmans’ Layer
- Adhesion between collagen fibrils is lost causing slippage and displacement of lamellae
- The stroma degenerates and thins and the affected area projects outward in a cone shape due to intraocular pressure
- folds in the descemet’s membrane and posterior stroma occur
Describe corneal transparency
Epithelium + Collagen Matrix + Glycosminoglycans (GAGs) + Endothelium pump
GAG = maintain lateral interfibrillar distance
level of hydration determinded by the the endothelium pump
90% pump out of the endothelium pump, 10% pump out of epithelium
Describe the process of corneal hydration
This relative corneal deturgescence (78% water content) requires precise control of stromal extracellular water content and is dependent upon: (1) the barrier functions of the epithelium and endothelium, (2) the anionic characteristics of molecules within the stromal matrix that account for the tendency of the stroma to imbibe water, and (3) water and ion transport through the epithelial and endothelial cell membranes (including ion channels, ion cotransporters, and energy-utilizing ion pumps). Fluid is continually entering the cornea through the leaky barrier formed by the junctions joining the endothelial cells. Ion transporters in both the epithelium and the endothelium help to maintain the concentration gradient change that can facilitate water movement from the stroma into the tear film through the epithelium and into the anterior chamber through the endothelium. Net transport of solute into the anterior chamber exceeds that into the tears and corneal deturgescence is primarily reliant on endothelium and minimally on epithelium.
Describe oxygen requirements of the eye
Minimum oxygen transmissibilty to avoid corneal oedema for daily wear was: 24.1 +/- 2.7 X 10^-9 Dk/t
Minimum oxygen transmissibilty to avoid corneal oedema for overnight wear was: 87.0 +/- 3.3 X 10^-9 Dk/t
Describe the effect of contact lens wear on the cornea
- epithalial and stroma thinning
- Decreased number of keratocytes
- changes to endothial mosaic pattern
- pleomorphism and polymegathism have been found in long term extended wear of c/l
Describe the limbus
The transition zone between cornea and conjunctiva, 1.5mm - 2mm the encircles the peripheral cornea
Describe the function of the epithelium?
- 50nm
- 5 to 6 layers of tightly packed cells
- continous with the conjuctival epithelium
- non - kerantised - relatively fragile compared to skin
- where we find epithelial cells
Describe the function of the epithelium?
- 50nm
- 5 to 6 layers of tightly packed cells
- continous with the conjuctival epithelium
- non - kerantised - relatively fragile compared to skin
- where we find epithelial cells
- physical protection - renewable layer of the ocular surface
- optical - maintain a clear, transparent smooth surface for light refraction
- tear stabilisation - aided by microvilli
- Barrier to fluid movement
- microorganism shield, prevents invasion of organisms from the conjuctiva
Describe the function of the Bowmans’ Layer?
- 10nm
- composed of interwoven collagen fibres
- allows nerves to perforate
- ends at the limbus
- not replaced following injury
- function to adhere epithelium to stroma
Describe the function of the Stroma?
- 500nm
- 90% of the cornea’s thickness
- Stromal cells do not regenerate
- composed of sheets of collagen fibrils called lamallae and keratocytes that lie flat and parallel to the surface
- lamellae lie parallel to corneal surface and are much thinner than the wavelength of visible light
- arranged in various angles in a uniform fashion this allows for transparency
- keratocytes hold lamellae in place, store glycogen and produce new collagen after stromal damage
- posterior stroma is more easily separated than anterior stroma (one of the signs of oedema, vertical striae in the posterior stroma)
How do you get a corneal oedema?
- Oxygen reduced - hypoxic cornea
- Anaerobic glycolysis
- build up of lactate
- osmotic balance disrupts
- pulls water into cornea (stroma)
- corneal oedema
