what is the refractive index of the cornea?
1.376
what are the main cornea structures?
anterior to posterior
- epithelium (top layer is tear film )
- bowman’s membrane
- stroma
- Descemet’s membrane
- endothelium
describe the epithelium
- it is 10% of cornea thickness, 5-7 layers
- CAN regenerate
- anterior-most layer of the cornea
- acts as a barrier to prevent the entry of tear film into cornea. this is maintained by the tight junctions (zonula occludens)
- comprises of 3 cells
1a. superficial/squamous cell
1b. wing cells
1c. basal cells
describe the superficial/squamous cells
- surface epithelial cells
- polygonal-flattened in shape
- non-keratinised (the reason y its transparent)
- has tight junctions (zonula occludens) to prevent water entry into the cornea and desmosomes and gap junctions in OTHER layers
describe the wing cells
- irregular in shape
- has gap junctions and desmosomes (to hold the cells tgt)
describe the basal cells
- single layer of columnar epithelial cells that is elongated in shape
- has gap junctions and desmosomes
- has hemidesmosomes
(to connect them to the Bowman’s membrane
-> capable of mitosis (to produce more cells)
-> it is able to regenerate and help during epithelial
wound healing
describe the bowmans’s layer
- it is connected by the hemidesmosomes and it is the basement layer of the epithelium
- is NOT able to regenerate
describe the stroma
- 90% of the corneal thickness, about 500 microns thick
- has few cells, flattened keratocytes
- can NOT regenerate in case of injury
- made up of collagen fibres
- **-> according to Maurice’s theory, the stroma is made up of collagen fibres that are equidistant and of equal diameter, they are parallel within the lamellae and are perpendicular to adjacent lamellae. this orderly arrangement of the fibrils minimises light scattering by creating destructive interference and thus increasing the light transmission. this leads to a transparent cornea that allows almost 98-99% of light transmission.
describe the descemet’s membrane
- basemen layer of the endothelium
- can regenerate partially (can heal in minor trauma)
describe the endothelium
- posterior/inner-most layer of the cornea, 500 microns thick
- single layer of hexagonal cells with tight junctions (macula occludens) and gap junctions
- can NOT regenerate
- hight metabolic activity
- impt function : to maintain cornea water content by 78% (by 2 ways)
1. acts as a barrier to prevent aqueous humour from entering the cornea. This is maintained by tight junctions (macula occludens)
2. endothelial pump function. the pump function is maintained through ions exchange through the gap junctions.
what are the functions of the cornea?
- maintain corneal transparency
2. transmit and focus light onto the retina
what are the factors for cornea transparency?
- lattice (regular arrangement)
- absence of blood vessels and acellularity
- corneal deturgescence
describe how lattice arrangement contribute to corneal transparency
***** according to maurice’s theory, the stroma is made up of collagen fibres that are equidistant and of equal diameter. they are parallel within lamellae and are perpendicular to adjacent lamallae.
this orderly arrangement of fibrils minimises light scattering by creating a destructive interference and thus increasing light transmission.
this leads to a transparent cornea which allows light transmission of 98-99%.
how does the cornea being avascular and acellular lead to it being transparent?
the absence of blood vessels and cells allows for efficient transmission of light through the cornea as there is no blockage in the pathway of light
what are the factors that preserve corneal deturgescence?
3a. anatomical integrity/barrier function of epithelial and endothelial
3b. metabolic pump
3c. changes in osmolarity / loss in equilibrium
3d. IOP (intraocular pressure)
describe anatomical integrity/barrier function of epithelium and endothelium
i. the presence of intact epithelium and endothelium with all cell layers and connections
ii. presence of tight junctions in both epithelium and endothelium
epithelium ‘s tight junctions = zonula occludens
endothelium’s tight junctions = macula occludens (leaky barrier)
describe the metabollic pump
[it helps with the exchange of ions (when ions move, water will follow) so the pump does not actually pump water)]
- to maintain water content within stroma of 78%
leak= movement of water from AH into stroma pump= movement of water from stroma into AH
- when leak > pump, water freely enters the stroma, lattice arrangement is lost and cornea will be swollen
both epi and endo pumps are essential as it facilitates the movement of fluid out of the cornea, maintaining deturgescence.
-> 3 endothelial pump situations: normal, stressed, decompensated
describe the 3 types of endothelium pump situations
- > normal endothelial cell density
- cell density of 2000-3000 cells
- leak = pump
- water content is maintained
- > stressed endothelium
- cell density of 800-1500 cells
- leak = pump
- water content is maintained but endothelium is under alot of stress
- > decompensated endothelium
- cell density is less than 500 cells
- leak > pump
- water content cannot be maintained as water flows freely into cornea
describe changes in osmolarity/ loss in equilibrium
- it is seen in contact wearers with either tight, ill-fitted, overwear or sleeping in contact lenses
- hypoxia occurs
(look at next slide for hypoxia description
what happens during hypoxia?
- when there is a lack of oxygen in the cornea, there is an increase in lactate ions and anaerobic metabolism.
- pH level is reduced which affects the functioning of the endothelial pump function
- the pump is then unable to function properly which results in the leak being more than the pump.
- this allows water to freely enter the cornea which causes the cornea to swell/corneal oedema
- the lattice arrangement is disrupted and the destructive inference is lessen/gone
this results in increased light scattering hence causing blur vision
describe IOP and what happens when it is too high
- is seen in glaucoma
- the sudden increase in IOP causes the endothelial pump function to be affected and function abnormally
- this causes the leak to be more than pump, allowing water to freely enter the cornea which results in cornea swelling/corneal oedema
- the lattice arrangement is disrupted and hence the destructive interference is lessen/ lost
- this causes there to be more light scattering, causing blur vision
why is the corneal metabolism needed?
- transport processes/ pump functions
- wound healing
- maintains ocular temperature
what are the 3 corneal metabolic pathways?
- anaerobic glycolytic pathway / Embden- Meyerhof pathway
- aerobic Krebs cycle (TCA cycle)
- hexose monophosphate shunt/pentose phosphate pathway
- both anaerobic and aerobic
describe corneal healing
- > epithelial wound healing (for most cases)
1. the adjacent epithelial cells, mainly wing cells and squamous cells will migrate to cover the abraded area
- then, the basal cells will undergo mitosis to produce new and fresh cells to help restore the normal cell layers
- this process usually takes up to a week
- > endothelial wound healing
1. the cells are not able to regenerate hence during injury, the cells enlarge and spread to cover the area of loss resulting in pleomorphism ( loss in cell shape), causing the endothelium to be thinner.
