Ocular Phys: Cornea Part 1

Functions of the cornea

transparency - major refractive surface of eye

structural support - protect intraocular contents

biodefense

These functions require

smooth anterior surface

instance epithelium and endothelium

normal corneal contour

clarity of tissue

normal thickness

the anterior corneal barrier

zonula occludens - tight junctions

located in the *superficial squamous cells

Anterior surface of the cornea contains

glycocalyx

glycocalyx is includes

glycoprotein, glycolipid, carbohydrate molecules and membrane anchored mucins

*membrane anchored mucins produced by

superficial epithelial cells and goblet cells of conjunctiva

Mostly richly innervated tissue of the body is the

cornea

Nerves directly maintain the health of the epithelium through

trophic factors

Cell Replacement: daughter cells of the stem cells migrate onto the cornea and become

transient amplifying cells

transient amplifying cells differentiate to become

basal cells

basal cells divide once

wing cells --> superficial squamous cells

Which are the terminally differentiated cells of the corneal

superficial squamous

Which are the post mitotic cells

wing cells

Basal cells of epithelium anchor to bowmans layer by

Type VII collagen anchoring fibrils which anchors basal lamina to anchoring plaques in bowmans layer

This stabilizes association between

surface epithelium, BM and underlying stroma

bowmans membrane is _____ attatched to stroma

strongly

Bowman's layer function

protects against shearing damage

Stroma functions

transparency

mechanical strength

maintains corneal curvature

------ stroma is more resistant to change in stromal hydration compared to the _____ stroma

anterior

posterior

water accumulates easily in posterior!!

Stroma contents

water (relatively dehydrated)

collagen

non collagenous proteins

cells

Stroma collagen is transparent due to

- fibril in parallel orientation

- fibrils in small and highly uniform diameter

- close packing and uniform distance between fibrils

each fibril is surrounded by

proteoglycans

proteoglycans are responsible for

uniform distance between fibrils

Anterior 1/3 of cornea

thin

less regular

obliquely arranged lamellae

SHEAR RESISTANCE

Posterior 2/3

thicker

approx right angles arranged lamellae

TENSILE STRENGTH

what gives the cornea its shear restiance

anterior stroma

what gives the cornea its tensile strength

posterior stroma

Proteoglycans are

core protein with GAG side chains

Proteoglycans are highly ______ charaged

negatively

This negative charge pulls in sodium and therefore

water

important for hydration of the cornea

Proteoglycans functinos

- pressire exerting polyelectrolyte gel

- create uniform spacing between fibrils

- Resist compressive forces - Prevent collapse

- Give viscoelastic properties to cornea

- GAGs contribute to the tendency of the stroma to swell

Stromal cells

keratocytes

Keratocytes

normally quiescent (modified fibrocytes)

Keratoctyes function

synthesize - collagen ECM and crystalins

helps with transparancy and wound healing

Stroma crytallins functions

- contribute to cellular transparency

- act as antioxidants to reduce UV induced damage

Endothelium cells are

metabolically active!!

Endothelial cells have

focal or leaky tight junctions

allows paracellular transport

What are functions of the endothelial cells

- leaky tight junctions permit passage of nutrients from aqueous into cornea

- help maintains relatively dehydrated state (transparant)

- Synthesize descemets membrane

review slide is slide 36

Sources of nutrients to the cornea

tear film

aqueous

tear film provides

oxygen (front)

aqueous provides

glucose

oxygen (back)

_________ has the second highest metabolic rate in the eye

endothelium

it has to keep pumps working to keep cornea dehydrated

Glycolysis leads to accumulation of...

lactate in the stroma

This leads to

stromal acidosis

osmotic imbalance

stromal edema

----- increases lactate and causes corneal swelling

corneal hypoxia

(contact lens wear)

Cornea transmits _______ of light in the visible spectrum

80-90%

light scattering (loss of transparancy) due more to

cells than collagen

transparancy is maintained by minimizing the

fluctuations in refractive index

Transparency is due to

- smoothness of the anterior surface of the cornea

- lattice like arrangement of collagen fibirls in the stroma

- homogeneity of refractive indices of corneal tissue

- thinness of cornea - hydration level

- avascularity

Transparency is maintained

1. barrier function of epithelium and endothelium

2. Size and arrangement of collagen fibrils

3. uniform index if refraction

4. relative dehydration of the stroma

we have uniform index of refraction by

1. keeping keratocytes close together

2. crystalins

how do crystallins help with transparancy

match the overall refractive index if the cytoplasms of the cells with the surrounding ground substance

Factors contributing to stromal hydration

- GAGs surrounding collagen fibrils

- endothelial permeability

- Swelling pressire

-imbibition pressure

- intraocular pressure

GAGs account for swelling tendency

highly negatively charged

repel each other

attract Na

absorb water

can cause the cornea to swell beyond normal 78% water content

endothelial permeability

allow nutrients in but must provide barrier so water does not flow in

leaky tight junctions

What causes the swelling pressure in the stroma

GAGs have negative charge

negative chrages repel

creates reuplusive and expansive force in the stroma

PUSHES OUT

Imbibition pressure

a negative pressure drawing fluid into cornea

PULLS IN

Na pulled into stroma fixed anions of GAGS attract water

Intra ocualr pressure

pressure pushing on back of endothelial cells

water wanting to enter!

intraocular pressure

There is no significant effect on corneal hydration

if IOP is within normal range and if the endothelial cell layer is normal ***

damage to the endothelial cells will result in

more swelling than the damage to the epithelial cells

due to IOP

(tear film is not under pressure)

Tear film hypertonicity

occurs with normal evaporation of tears

draws fluid out of cornea

necessaey prerequisites to maintain relatively dehrysared state

- intact cellular barriers

- active metabolic pumps

Epithelial pumps

pumps Cl out of epithelium into tears and pulls some water with it

Endothelial pump

pumps Na and HCO3 out of cornea

establishes osmotic gradients - pulls water out of stroma

swelling og more than ____% we start to lose transparancy

5%

As IOP increases

it exerts compressive forces on stromal tissue which transmit stromal edema to the epithelium

When we have fluid accumulating

the water does not change the diameter of the colalgen fibrils !!!!!

the fibrisl does not swell

the distance between the fibers is different!

Almost all stromal swelling is in

posterior 2/3!!!!

this pushes the back surface of the cornea

backward toward the anterior chamber. - folds in descemnts membranes

not the anterior !