OABP cornea

Three tunics of the eye

Fibrous (sclera and cornea)

Vascular (iris, ciliary body, choroid)

Sensory (retina and optic nerve)

Functions of cornea

Principle refracting surface (must be transparent, smooth, avascular, requires energy)

Protection (against external penetration and UV light)

Functions of sclera

Maintains the shape of the globe (stiff)

Protects against internal and external factors

Provides attachments for EOMs (won’t slip when muscles move)

Modified opening posteriorly so optic nerve can exit

The limbus

Junction between the clear cornea and white sclera

About 1-2 mm region

change in radius of curvature between cornea and sclera

Limbal transition epithelium

Squamous corneal epi —> columnar conj epi

Conjunctival stroma, episclera, and Tenon’s capsule begin

Limbal transition: deeper layers

Corneal stroma —> scleral stroma

Corneal endo stops to wrap around TM

Bowman’s and Descemet’s terminate

Superficial limbus

Limbal epithelium increases from 5 to up to 15 layers

Melanocytes may be present in basal layer

Langerhans cells are present

Palisades of Vogt

Palisades of Vogt

Series of radially oriented and bifurcating fibrovascular channels between ridges of thickened conjunctival epithelium

Linear projections average 0.36 mm in length and 0.04 mm in width, most obvious at superior and inferior limbal margins

Primarily serve limbal stem cells

Limbal epithelium stem cells

Eventually get to cornea for the continuous renewal and regeneration of the corneal epithelium

Transit amplifying cells

Pre basal cells

Can become epithelium cells that need to migrate

Active and well controlled

General dimensions of the cornea

12 mm horizontal

11 mm vertical

Microcornea

Corneas with horz. diameters from 7-10 mm

More common because of genetics

Megalocornea

Those with diameters of 12 mm or more in the neonate and 13 mm or more in the adult

Corneal slit lamp images

Help to detect anterior vs posterior problems

General Corneal Properties

Power

Curvature

Topography

Sphericity/shape-meridional disparity (astig), central vs peripheral asphericity

Thickness

Corneal biomechanical properties

Corneal refracting power

Anterior is 48.2D

Posterior is -6.2

Overall is +42D

Cornea provides ~70% of eye’s refracting power

Manual keratometry ranges

Normal eyes ~43D (40-47) usually symmetric

Corneal topography

“Maps”

Shows more of the cornea and the eye

Shows steepness

Oblate

Cornea is flattened

Lasik, PRK, ortho-k

Changes how light goes in

Prolate

More curved than normal (center is steeper)

Wavefront aberrometry

Many forms of optical aberrations

How is corneal thickness measured? Average ranges

Pachymetry

Average Central Corneal Thickness (CCT) 540-550 micrometers

Depends on ethnicity, corneal health, topical meds

Corneal thickness and glaucoma

Relationship between glaucoma risk and CCT

Thickness can affect measurements in eye pressure

Ocular response analyzer

Measures corneal biomechanical responses to an air-jet

Uses a rapid air-impulse and estimates IOP and corneal biomechanical behavior properties

Air jet flattens the cornea —> plane mirror

Cornea becomes concave then returns to normal

Corvis ST

Measures corneal biomechanical responses to an air-jet

Estimates IOP, corneal stiffness, and other corneal biomechanical responses

Layers of the cornea (superficial to deep)

1) epithelium (~50 micrometers)

2) Bowman’s layer (~10 micrometers)

3) Stroma (~500 micrometers, but highly variable)

4) Descemet’s membrane (~5-15 micrometers)

5) Endothelium (5 micrometers)

Dua’s Layer

10 micrometer thick layer between posterior stroma and Descemet’s layer

Consists of type I and VI collagen and elastin

High tensile strength

Fenestrated

Corneal epithelium

~50 micrometers, thickens in periphery where it becomes continuous with the conjunctival epi at the limbus

5-7 layers (single basal cell layer, wing cells, apical)

Occasionally there are melanocytes

Langerhans in peripheral cornea

Basal layer produces basal lamina (BM)

Basal Cells

Single layer of columnar cells (12 micrometers wide with a density of about 6000 cells/mm2

Rounded apical surface lies adjacent to wing cells, attachments by interdigitations and desmosomes (less numerous)

BM attaches via hemidesmosomes

Anchoring fibrils run from BM through Bowman’s and 1.5-2 microns into anchoring plaques into the stroma

Wing cells

2-3 layers

Wing-like lateral processes, polyhedral in shape with convex anterior surfaces and concave posterior surfaces to conform to basal cells around them

Joined to each other by desmosomes and gap junctions; join to superficial and basal cells

Surface layer (apical layer)

~2 cell layers thick

Non-keratinized stratified squamous cells (flattened)

Surface glycocalyx made and interacts with mucin layer of tear film

Microvilli and micro-plicae to increase surface area (stabilizes tear film)

Junctional complexes

Join the surface cells along the lateral walls near the apical surface

Provide a barrier to intercellular movement of substances from the tear layer

Prevent uptake of fluid from tears

Fluid and molecules can move though the cells but not between them

Held by hemidesmosomes

Epithelial replacement

As the superficial cells age, they are sloughed off and constantly replaced by cells from layers below

Renewal rate = X + Y = Z

X = basal cell mitosis

Y = centripetal movement of cells

Z = cell loss from surface

Bowman’s Layer composition

Acellular layer composed of randomly-oriented collagen fibrils

Transitional layer between epithelium and stroma—posterior surface of Bowman’s Layer merges with the highly organized collagen lamellae of the stroma

Acellular anterior portion of the stroma

8-14 microns thick, develops prenatally and thins with age

Mammals without Bowman’s Layer

Still have an epithelium to stromal transition

Bowman’s layer characteristics

Very resistant to damage

Does not regenerate, replaced by epithelial cells or stromal scar tissue if injured

What is the function of Bowman’s?

Not really sure

Could anchor to epithelium, structural integrity, prevent infectious invasion to stroma

Photorefractive keratectomy (PRK)

Epithelium of cornea is removed

Laser thins certain areas of the cornea

Corneal stroma

Thickest layer of the cornea made up mostly of highly organized collagen fibrils

~500 microns centrally, but highly variable

Water (78-80%), collagen fibrils, keratocytes, stromal stem cells, dendritic cells, non-collagen proteins, proteoglycans, electrolytes

Corneal stroma: Collagen fibrils

Uniform diameter (25-35 nm)

Predominately collagen type 1

Other collagen types play a role in reinforcement and assembly of fibrils

Fibrils organized into a lattice structure

More fibrils within peripheral cornea than central cornea

Corneal stroma: lamellae

Collagen fibrils are organized into bundles called lamellae

Fibrils organized into lattice with regular spacing within each lamella

Lamellar thickness varies from 0.2-2.5 microns

200-300 lamellae per cornea

Corneal stroma: Anterior

Anterior (1/3) stromal lamellae are thinner, have more interweaving, more branching, and are more densely organized

Many insert into Bowman’s layer

Corneal stroma: Posterior

Posterior (2/3) stromal lamellae are thicker, wider, have less interweaving, less branching, and are less densely organized

How do fibrils run to corneal surface

Fibrils are parallel and all lamella run in the same direction

Fibrils in adjacent lamellae are not parallel

Extension of the lamellae

Extend from limbus to limbus becoming circumferential at the limbus

Turn and anchor into the limbus, changes shapes

Structure is thought to help maintain shape of cornea

Cells-keratocytes (corneal fibroblasts)

Specialized fibroblasts, flattened spindle shape with stellate extensions

Lie between and occasionally within lamellae

Synthesize collagen and other ECM components

Distribution differs based on stromal depth

Cells-keratocyte function

Matrix turnover (more anterior than posterior)

Intracorneal communication

Reservoir for glycogen

Interlamellar tethering

Wound healing

Cell-keratocyte: wound healing

Keratocytes convert to myofibroblasts which contain actin-myosin bundles (muscle-like)

Scar formation

If severed, lamellar structure can not be reformed to pre-trauma strength and high-level organization

What happens first after stroma trauma?

Apoptosis of local keratocytes and then activation, proliferation, migration, and trans-differentiation of nearby keratocytes into myofibroblasts

Role of myofibroblasts in wound healing

Contract to close the wound and secrete ECM and proteinases to remodel stroma

Role of proteinases in wound healing

May degenerate the basement membrane allowing an influx of cytokines from overlying epithelium

Immune cells infiltrate the wound to clear cellular debris and prevent infection

What happens to the basement membrane

Regenerates (much slower than epithelium), myofibroblasts and immune cells disappear

What’s the ideal situation

Abnormal matrix is resorbed and transparency of the cornea is restored

What are keratocytes good at? What are they not good at?

Great at maintaining stromal organization but usually can not restore 100% pre-injury organization or transparency

Likely due to keratocyte differentiation into myofibroblasts

Local keratocyte apoptosis is likely a mechanism to prevent excessive inflammation but decreases population of keratocyte which produce regular, transparent ECM

Role of myofibroblasts is to repair not regenerate stroma, easier to repair with non-regular, nontransparent extracellular matrix

Cells-keratocytes transparency

Each keratocyte nucleus mildly scatters some light

How to decrease light scatter

Transparent intracellular cytoplasmic proteins with index of refraction that matches surrounding matrix

Corneal crystallin

Keratocyte thinness

Keratocyte even distribution within stroma

LASIK

Creates a flap

LASIK scar never 100% heals

Flap can open again

LASIK-epithelium ingrowth

Epithelium cells invade into the lasik flap

Other cells

Stromal stem cells-thought to reside in/near limbal stroma, unknown function

Dendritic cells, immune cells

Ground substances

Fills in the area between lamellae and cells

Composition-proteoglycans; core protein + GAGs

GAGs: hydrophillic, negatively-charged, attract and bind water, maintain the precise spatial rltshp between fibrils

Corneal Transparency-Lattice Theory

Equal diameter fibrils in a parallel arrangement within lattice

When light hits fibrils, secondary waves are created

Secondary waves do not cause light scatter due to destructive interference

Secondary waves allow light transmission into the eye due to constructive interference

Corneal transparency-Lattice Theory Defunct?

Fibril diameter varies

Not all corneal layers have regular fibril lattice

Even when lattice arrangement is present it isn’t perfectly regular

Non-fibril elements also exist in cornea

Not all transparent tissues have lattice patterns

What type of light scatterer are fibrils?

Ineffective

Based on large number of fibrils in the corneal stroma, at least some destructive interference of scattered light likely occurs

Descemet’s Membrane

Modified basement membrane of endothelium

Dense, thick, relatively transparent and cell-free matrix

Secreted by endothelial cells (begins prenatally, banded)

Thickens throughout life (doubles in thickness by 40; 5 microns in children 15 microns late in life, nonbanded)

Endothelium

Single layer of flattened cells

5 microns thick x 20 microns wide

Basal portion rests on Descemet

Apical portion into anterior chamber

Endothelial Cell Mosaic

Creates a protective barrier and regulate transport

Pump out fluid-corneal clarity

Endothelial Barrier

Barrier is leaky allowing large molecules to enter via the intercellular spaces

Glucose and amino acids from anterior chamber, water

Cornea must be deturgesced to maintain proper hydration —> endothelial pump

Endothelium vs epithelium

Epithelium acts as barrier to fluid, molecules, and microorganisms-allows oxygen to diffuse into cornea, perilimbal vessels provide small amounts of nutrients such as glucose

Endothelium is a poor barrier allowing water and molecules to enter cornea via the intercellular spaces-glucose and amino acids from anterior chamber, nutrition

Endo vs epi continued

Epi is highly miotic, but endo is not

Both layers are highly cellular

Both layers are highly metabolic

Both layers provide nutrition

Epi-oxygen

Endo-almost everything else via aqueous humor

Aging of Endothelium

Endothelial cells do not regenerate

Other cells change in shape and size to fill the space, become less regular with time

Descemet’s thickens with age

Descemet’s thickening peripheral

Hassall-Henle bodies bulge into AC

Descemet’s thickening centrally

Corneal guttata, indicative of endothelial dysfunction

Polymegathism

Variation in cell size

Polymorphism

Variation in cell shape

Average endothelial cell density by age

Starts at 3500 and drops to 1500

Less than 800 is considered theoretical corneal failure, not enough cells to keep cornea transparent

Anionic repulsion

From proteoglycans is thought to keep collagen fibrils at a defined distance

Endothelial Pump

Actively pumps water out of stroma

Stromal swelling pressure

Positive pressure provides resistance to water entering stroma

Product of anionic repulsion of GAGs and structural rigidity/elasticity of cornea

Creates resistance to water flow across stroma (+55mmHg)

Compression (increasing IOP, increases SP)

Expansion (corneal edema, decreases SP)

Imbibition pressure

Negative pressure pulling water into stroma

Product of cationic forces of GAGs

-40mmHg

Epithelial barrier structure

Doesn’t allow much water into stroma

Tear evaporation

Small loss of stromal water

Intraocular pressure

Forces water into stroma

~10-20mmHg

SP + IP = IOP

Corneal hydration

More than 5% increase in hydration corneal transparency starts to decrease

When lids are closed for prolonged periods of time (sleep) hydration increases and corneal thickness increase 3-9%-decrease oxygen levels —> decreased metabolism, decreased tear film evaporation, normalizes 1-2 hours after eyelid opening

Bicarbonate Secretion Model

Osmotic gradient formed by coupled anion secretion drives water into anterior chamber

Experimentally all the following were required to keep cornea from swelling; Na+/K+ ATPase, HCO₃ , Cl-, Na+/H+ exchanger

Carbonic anhydrase inhibitors slowed pump action by ~30%

Facilitated Lactate Transport

Movement of lactic acid from cornea to anterior chamber via transcellular monocarboxylate cotransporters could contribute to fluid movement into anterior chamber

Corneal blood supply

Normal cornea is avascular

Receives nutrients from diffusion from air, AC, limbal conjunctival and episcleral capillaries

Corneal neovascularization

Blood vessels in cornea

Corneal sensory innervation

Richly innervated with sensory nerves-V1, 70-80 large nerves enter peripheral stromal in a radial manner, lose myelin sheath about 1-2 mm in from limbus

What are the 3 sensory nerve networks formed

Stromal: below anterior 1/3 of stroma

Subepithelial: between Bowman’s and basal epithelium

Epithelial: naked nerve endings between cells (PAIN)