cornea Flashcards
functions of the cornea
protection
lens to refract light
to let light penetrate - transparent
properties if the cornea
no blood supply hence avascular which makes it transparent
how much refraction does the cornea do? what is its refractive index?
two thirds of it, 1.376
what are the layers if the cornea?
epithelium
bowmans layer
stroma
descements layer
endothelium
give three features of the corneal epithelium
-most regular squamous stratified epithelium in the body
-most anterior layer
5-6 cells thick
what is the cycle of the epithelial squamous cells and how long does it take?
10 day cycle,
1. basal
2. wing
3. squamous
4. apoptosis
what are the features of the bowmans layer?
-no cells
-narrow collagen fibrils
what does the corneal stroma do?
provides strength, elasticity and and corneal form
what is the stroma made of?
collagen - the fibrils withstand radial tension
how is corneal lamellae structured?
-via collagen fibrils that run at various angles from each limbus
-posterior lamellae don’t interweave while anterior lamellae do
what and where are keratocytes?
whats their function?
flattened cells with long pseudopodia joined by gapjunctions mostly between the lamellae and are in highest conc in the anterior stroma. responsible for slow turnover of collagen. this helps maintain fibril order important in wound healing. Also, become active when there’s injury to corneal tissue
what are proteoglycans?
long repeating sulphated disaccharides and are highly negatively charged so they are hydrophilic to maintain corneal hydration which keeps it transparent
where are proteoglycans found and what are they do?
they are attached to the protein core and they act as collagen spacers
-essential in maintaining corneal transparency
what is the descements membrane?
a structureless band that gives mechanical strength and a base for endothelium
give three features of the endothelium
-most prosterior layer
-one cell thick
-source of endothelial pump
what happens to the number of endothelial cells as you get older?
they decrease and so the cells cover a larger area by spreading during growth or post injury
Give 6 features of the limbus
- nourishment of the peripheral cornea
-corneal wound healing
-contains pathways of aqueous humour outflow
-immunosurveillance
-contains circumferential collagen fibrils
-Site of surgical incisions
Give three features of the sclera
-outer coat of the eye
-contains collagen, proteoglycans and elastic fibres
-relatively a vascular
Where are corneal nerves?
Derived from ophthalmic division of the trigeminal nerve and branches radiate to anterior cornea from the limbus
What is the corneal touch threshold and how can this be used to calculate sensitivity
The minimum pressure applied to the cornea to be felt. Sensitivity is the reciprocal of this
What is the donnan effect?
Alters distribution of diffusible cations and anions across any membrane where there is a non diffusable charged species present on one side of the membrane
Where can you see more info on the donnan effect?
YouTube video check history
Give the basic steps of LASIK laser eye surgery
- Corneal flap is cut and hinged backwards
- Central lamellae flattened using excimer laser
- Corneal flap replaced
What are the advantages and disadvantages of LASIK over PRK
-advantages: is not removed so there’s less wound healing response
-disadvantage: lack of sufficient wound healing means flap never heals which causing the cornea to be mechanically weaker
How does PRK work?
The surface is either
-flattened to reduce the power of the eye
-made more curved to increase the power of the eye
What makes keratocytes transparent?
Their cytoplasm contains corneal crystallins that give them a uniform refractive index close to that of the rest of the stroma and hence makes them transparent
