6. Cornea - KMK Ocular Anatomy

What are the three functions of the cornea?

- transmit light

- refract light

- serve as a barrier against pathogens and edema

What is the total refractive index of the cornea?

- 44D

What is the index of refraction of the tear film?

- 1.336

What is the index of refraction of the cornea?

- 1.376

The tear cornea interface contributes ___D and the cornea/aqueous humor interface contributes ___D to the total refractive power of the cornea.

- (+5D)

- (-6D)

Is the cornea thicker in the periphery or in the center?

- thicker in the periphery (0.67 versus 0.55mm)

Okay homies, what is WTR versus ATR astigmatism....

- WTR: the cornea is steeper in the vertical meridian

- ATR: the cornea is steeper in the horizontal meridian

As the lens ages, what type of astigmatism shift might you see? What else besides age could lead to this shift?

- you might see it shift towards ATR

- a loss of lid tension may also result in the flattening of the vertical meridian of the cornea = increase in ATR

What is the central radius of curvature of the anterior corneal surface?

- 7.8mm

What is the central radius of curvature of the posterior corneal surface?

- 6.5mm

What is the average anterior horizontal diameter of the cornea?

- 11.7mm

What is the average anterior vertical diameter of the cornea?

- 10.6mm

What is the average posterior horizontal diameter of the cornea?

- 11.7mm

- same as the vertical = that's why it lowkey looks spherical from behind

What is the average posterior vertical diameter of the cornea?

- 11.7mm

- same as the horizontal = that's why it lowkey looks spherical from behind

Okay money moves, time to lock in: what are the corneal layers and their thicknesses, if applicable? ***i know some of these numbers don't match our notes but know the general idea k

- epithelium = 52um

(basement membrane = basal lamina & reticularis lamina)

- Bowman's layer = 8 - 14um (livs bday)

- Stroma = 450um

(Dua's Layer = 15um = pre descement's)

- Descemet's membrane = 5 - 15um

- Endothelium = 5um

***average corneal thickness is 550um

What is the average corneal thickness?

- 550um

What produces descement's membrane?

- corneal endothelium

what are the four layers of the corneal epithelium?

- surface layer

- wing cells

- basal layer

- stem cells

What type of cell tissue is found in the epithelium?

- stratified squamous non-keratinized epithelium

What makes up the surface layer of the epithelium?

What does the plasma membrane of these cells secrete?

What happens to the cells as people age?

- composed of two layers of non-keratinized squamous cells

- the plasma membrane of these cells secretes a glycocalyx and contains microvilli and microplicae to increase the SA and enhance the stability of the tear film

- as cells age, they are sloughed off into the tear film #ew

What two cell type junctions form a tight barrier between cells to impede the intercellular movement of particles in the cornea?

- zonula occludens

- desmosomes

What makes up the wing cells of the corneal epithelial layer?

- consists of 2 - 3 cell layers interconnected by desmosomes which link them to each other and adjacent layers

What type of cell junction exists to keep wing cells linked to each other?

- desmosomes

What is the only mitotic layer of the corneal epithelium?

- basal layer

Describe the basal layer of the corneal epithelium. #bigdaddy

- it's the only mitotic layer int he corneal epithelium

- composed of one layer of columnar cells

- secretes its own basement membrane (aka the basal lamina)

- attaches to its basement membrane via hemidesmosomes

How does the basal lamina/basement membrane stay attached to the basal layer? to the underlying bowman's layer?

- it attaches to the BM via hemidesmosomes

- it attachs to bowman's via hemidesmosomes too that penetrate bowman's and attach to the EC matrix of the corneal stroma

What happens to the basement membrane of the basal layer in the corneal epithelium?

- it reduplicates with age

- by age 60 it's doubled in thickness #okaypopoff

Basement membranes in the body consist of two layers. What are they?

- basal lamina (secreted by epithelial cells)

- reticular lamina (secreted by underlying stromal cells in the cornea)

What are the three factors that could increase the risk of recurrent corneal erosions (RCE)?

- poor hemidesmosome attachments

- epitehlial basement membrane dystrophy / EBMD

- age related thickening of the basement membrane / BM

Where do the stem cells of the corneal epithelium originate?

- Palisades of Vogt (aka the band around the limbus)

The palisades of vogt are at the same level as what other layer in the cornea? Why's this important? #killmeplz

- they are on the same level as the basal layer

- important because it allows for an easy transition as stem cells migrate circumferentially to become basal cells

Describe the flow of cell development within the epithelium of the cornea. aka this cell becomes this becomes this etc #youknowwhatimean

- stem cells become basal cells

- basal cells become wing cells (then travel anteriorly)

- wing cells become surface layer cells of the epithelium

What triggers stem cells to become basal cells?

- signals via V1

Limbal stem cell deficiency has been shown to contribute poor corneal epithelial maintenance in individuals with what three conditions?

- aniridia

- SJS

- alkali corneal burns

What type of collagen makes up bowman's layer?

- type 1 #uno

What type of collagen makes up the stroma of the cornea?

- type 1

What type of collagen makes up descement's membrane?

- type 4 #okaytobiasokay

Describe the composition of bowman's layer.

- an acellular layer primarily composed of random type 1 (majority) and type 5 collagen fibrils (type 3 and 7 are also found but to a lesser extend)

- tough layer that's resistant to damage BUT it can't regenerate so if it's hurt, it'll scar :(

What type of layer is Bowman's? What is it not?

- it's a transition layer from the epithelium into the stroma

- it is NOT a basement membrane

How thick is bowman's?

- 8 - 14um #opium

Can bowman's regenerate?

- NO JUST NO IT SCARS IF HURT - FOR THE LOVE OF ALL THINGS HOLY DO NOT GET THIS WRONG

This layer is thought to maybe play a role in maintaining the correct curvature of the cornea.

- bowmans

Off the bat, what are the six clinical conditions related to bowman's layer?

- Band keratopathy

- pterygia

- croc shagreen

- reis buckler's dystrophy

- kerataconus / KC

- refractive surgery

The stroma makes what percentage of the cornea?

- 90%

What makes up the stroma of the cornea?

- made up of dense regular connective tissue

- composed of:

- keratocytes (fibroblasts)

- collagen fibrils

- ground substance

- water

How much of the corneal stroma is water?

- 78 - 80%

What are keratocytes in the cornea?

- they're fibroblasts of the cornea that produce collagen fibrils and the EC matrix

Describe how collagen fibrils are composed within the stroma overall, anteriorly, and posteriorly.

- the stroma alone has 200 - 300 layers of uniform spaced 30um lamellae that run PARALLEL to the corneal surface

- anterior 1/3 = higher incidence of cross linking btw collagen fibers aka creating more rigidity and helping to maintain corneal curvature

- posterior 2/3 = more organized and has uniformly spaced lamellae that are larger and have fewer branches and less cross linking aka higher incidence of corneal edema can occur

What section of the corneal stroma is more susceptible to corneal edema?

- posterior cornea bc of lamellar organization

What is the ground substance of the corneal stroma? How does it relate to GAGs?

- basically it fills the space btw collagen fibrils and keratocytes

- it contains GAGS that attract water, contributing to the precise spacing btw collagen lamellae needed for transparency

What is the predominant GAG found in the ground substance of the corneal stroma?

- keratin sulfate (66%)

How does the thickness of descemet's membrane change with age?

- it increases

Can descemet's regenerate?

- YES by using the endothelium

Where does descemet's terminate? What's that called?

- terminates at the limbus

- becomes Schwalbe's line

What are Hydrops?

- occur in keratoconus / KC as a result of ruptures within descemet's

What are Haab's striae?

- they're folds in descemet's that occur in congenital glaucoma

What are hassall - henle bodies? Do they increase or decrease with age? How do they affect vision?

- small areas of thickened descemet's in corneal periphery that protrude toward AC

-increase in # with age

-no visual significance

Where is dua's layer?

- located between the posterior stroma and descemet's

What happens to the corneal endothelium with age? (2)

- it thins

- amount of organelles and mitochondria decrease

How do endothelial cells maintain corneal hydration and transparency within the cornea? (2)

- use Na+/K+ ATP pumps

- regulate water and ion flow btw the aqueous humor and corneal stroma

How are corneal endothelial cells linked/attached? Is this weak or strong?

- linked together at their apical borders (facing the anterior chamber) by maculae occludens junctions

- this creates a weak barrier that allows amino acids, glucose, and nutrients form the aqueous humor to enter into the cornea

Can the corneal endothelium regenerate?

- NO

Okay so the corneal endothelium can't regenerate. BooHoo. What happens then?

- instead, as the cells decrease in number with age, neighboring cells change shape (PLEOMORPHISM) and size (POLYMEGATHISM) to compensate for the decrease in cell density

The loss of endothelial cells results in a decrease in Na+/K+ ATP pumps and may lead to what?

- stromal edema

The cornea is avascular #duh. So like where does it get it's nutrients (2).

- diffusion from aqueous humor

- limbal conjunctival and episcleral capillary networks

- palpebral conjunctival networks

The main source of oxygen for the cornea under OPEN eye conditions is what?

- tear film that contains oxygen that has diffused from the atmosphere

The main source of oxygen for the cornea under CLOSED eye conditions is what?

- palpebral conjunctival blood vessels

Why does corneal neovascularization happen? Where do these new vessels arise from?

- happens as a defense response to oxygen deprivation

- new vessels from from endothelial cells of the limbal capillary network in response to cytokines and growth factors (#VEGF)

Corneal innervation is necessary for what two things?

- pain sensation

- proper wound healing

What nerve directly innervates the cornea?

- V1

- the ophthalmic division of the trigeminal nerve divides into nasociliary which then divides into long and short posterior ciliary nerves that innervate the cornea

How do the LPCNs and SPCNs enter the eye in order to innervate the cornea?

- they enter mid stroma

When do the LPCNs and SPCNs lose their myelination?

- after traveling 2 - 4mm inside the stroma, they lose their wrap as they penetrate thru bowman's to enter the epithelium

- once they're naked they are highly sensitive due to the nocireceptors that mediate pain

How does neurotrophic keratitis deal with corneal sensitivity? #GOTTAKNOWITGOTTAHAVEIT

- NK is characterized by poor corneal sensitivity and wound healing and is secondary to damage to V1 of the trigeminal nerve (ex. herpes simplex, herpes zoster, CVA, DM)

What is the nerve network found in the corneal epithelium referred to as?

- referred to as the intraepithelial plexus

What is the nerve network found in the anterior stroma/bowman's layer referred to as?

- referred to as the subepithelial plexus

What is the nerve network found in the mid stroma referred to as?

- referred to as the stromal plexus

What nerves are found in the posterior corneal stroma? Descemet's nerves? Corneal endothelial nerves?

- NONE ARE FOUND IN THOSE LAYERS