L5 Structure of Cornea Flashcards
What are the roles of the cornea?
- Major refractive surface of the eye
- Interacts with the mucin later of the tear film to produce a smooth optical surface
- Barrier between outside environment (pathogen) and inside
- Corneal epithelium secretes fluid into tear film - contributes to the removal of fluid from stroma
Describe the structure of the cornea? (ABCDE)
Anterior epithelium
Bowman’s membrane
Central stroma
Descement’s membrane
Endothelium
What are the 3 layers of corneal epithelium?
Superficial cells, intermediate/wing cells, basal cells
What do the different layers of corneal epithelium represent?
Different stages of epithelial terminal differentation
What is the structure of basal cell layer?
Single layer of cuboidal cells that sit on bowman’s membrane, capable of division into wing cells
What is the structure of wing cell layer?
1-3 cells thick in an intermediate stage of differentation and gives rise to superficial cells.
What is the structure of superficial cells?
Terminally differentiated squamous cell that are sloughed off into tears
What is the rate of turnover of corneal epith?
7 days
What features of corneal epithelium allows it to carry out its function?
Has high resistance tight junctions between superficial cells
What is the X + Y = Z hypothesis
Process of differentiation which allows epithelium to turnover every 7 days. Corneal thickness is maintained by a balance between:
- Cell loss from surface (z)
- Proliferation of basal cells (x)
- Central migration of stem cells from limbus (Y)
What does the cornea consist of?
Primarily water, ECM, collagen fibrils (mainly 1 and some 5,6), keratocytes
What is the structure of ECM?
Made up of proteoglycans which are ladder like attachments along collagen fibrils.
What are the two classifications of proteoglycans in the ECM?
Chondroitin/dermatan sulphate and Keratin sulphate
What is the structure of chondroitin sulphate and where is it found?
Decorin which binds to type 6 collagen and is abundant in the posterior stroma
What is the structure of keratin sulphate and where is it found?
Lumican which is found in the central stroma
How does a high PG content cause high colloid osmotic pressure?
It attracts water, which results in the swelling of the stroma and disrupts the spacing of collagen fibrils, causing light to scatter
How is the swelling of the stroma counteracted?
By active transport of ions by epithelium and endothelium, driving paracellular transport of water out of the stroma.
What is the structure of corneal collagen?
Lamellar arrangements of collagen fibrils, 200 layers
What are keratocytes?
Long thin stellate cells which run parallel to corneal surface
What do keratocytes do in the cornea?
Secrete collagen and is associated with ECM
What is the function of the stroma?
Transmit light which enters the eyes and scatters less than 10% of light
What is the structure of stroma which allows it to scatter less light?
Due to arrangement and size of collagen fibrils, it has a lattice structure.
Which model explains the non-scattering property of stroma?
Light scattering by individual fibrils is cancelled out by destructive interference with scattered light from adjacent fibrils.
What does the distance between fibrils and fibril diameter need to be to maintain transparency?
Less than one half of the wavelength of light
What is the structure of endothelium?
Single layer of haxagonal, cuboidal cells attached to the descement’s membrane
What is the function of endothelium?
Maintain stroma hydration
Why does the endothelium contain a high amount of ATP?
Contains lots of mitochondria which produce ATP for ion pumps to maintain hydrations
Where does the endothelium receive nutrients and oxygen from?
Nutrients from aqueous humor, oxygen from tears
How does the thickness of endothelium and descement’s membrane change with age?
Endothelium layer becomes thinner, descement’s membrane thickens
What happens when the endothelium layer gets damaged?
Swells up to cover injured areas
What is the number of corneal endothelium cells?
500,000 non-mitotic cells with limited replacement.
Is the cornea supplied with nerves?
Yes, it is the most sensitive part of the body. But, deep stroma has no nerves
Which ciliary nerves supply the cornea and where do they originate from?
Long ciliary nerve originates from opthalmic branch of trigeminal nerve
Short ciliary nerve formed by branches of ciliary ganglion
