Lecture 5 - Corneal & Scleral Physiology

What does kerato mean?

Greek word meaning horn or shield. Reflects the resilience and tough nature of the cornea.

What are the 5 layers of the cornea and their thicknesses?

1. Epithelium (~50 um)

2. Bowman’s layer (~10-15 um)

3. Stroma (~470 um; 90% of thickness)

4. Descemets’s membrane (~10 um)

5. Endothelium (~5 um)

What are the three cell types found in the corneal epithelium?

Superficial squamous cells, wing cells, and basal columnar cells

How do basal columnar cells in the epithelium attach to the underlying layers?

They secrete a basement membrane (BM) that attaches to the cells below via hemidesmosomes.

Which cells are the only mitotic cells in the cornea?

Basal epithelial cells (which arise from corneal stem cells in the palisades of vogt)

What are the dimensions and shape of the cornea?

It is ellipsoid in shape, with a horizontal diameter of ~12mm and a vertical diameter of ~11mm.

How does corneal curvature and thickness change from the center to the periphery?

Curvature is steeper at the apex and flatter at the periphery. The average central thickness is ~550-555 microns, while the periphery is thicker at ~670-700 microns.

What are the average refractive power and refractive index of the cornea?

Refractive power is ~43-44D; refractive index is 1.376.

What is the ideal tear osmolarity, and what does "osmolarity" define?

Ideal osmolarity is 308 mOsm/L; it defines the concentration of solute particles in a solution

What are the clinical consequences of hyperosmolarity (>308 mOsm/L) in tears?

It indicates reduced aqueous content (common in dry eye), stimulates epithelial damage/apoptosis/inflammation, and damages corneal nerves, affecting tearing and reflexes.

What is the water composition of the cornea?

78% total water (66% cellular matrix, 12% intracellular).

How many types of collagen are in the human cornea, and which are most prevalent?

13 types are present (out of 28 total types). The most prevalent are Type I (58%) and Type IV (24%).

What are the percentages of keratocytes and proteoglycans in the cornea?

Each makes up 1% of the cornea.

Which glycosaminoglycans (GAGs) do corneal proteoglycans bind?

Keratan sulfate (60%) and dermatan sulfate (40%)

Why is collagen imporant in the cornea?

Corneal collagen is important for transparency and tensile strength / resistance. It combines to form collagen fibrils that run parallel with each other forming lamellae with keratocytes in between.

What are the only two transparent structures in humans?

The cornea and the lens.

What percentage of incident light is scattered by a healthy cornea?

Less than 1%

List the corneal components contributing to light scatter in decreasing order.

• Endothelial cells

• Epithelial cells

• Nerve cells

• Keratocytes

• Collagen fibrils and extracellular matrix

What does Ultraviolet radiation (UVR) lead to at a molecular level in the cornea?

An increase in reactive oxygen species (ROS).

What is the result of increased ROS in the cornea?

Increased oxidative stress and damage.

Which three substances work to decrease ROS in the cornea?

Ascorbate (vitamin C), glutathione, and corneal crystallins

At what wavelength threshold does the cornea absorb light?

Wavelengths below 320nm (UVR)

What percentage of UVR rays does the cornea absorb?

100% of UV-C , ~90% of UV-B , and ~35% of UV-A.

What is the range of light wavelengths that the cornea transmits

From 320 to 2500nm.

What type of UVR is absorbed by the ozone layer of the earth’s stratosphere?

UVC. This is why the eye enounters very little UVC.

What structural factors maintain corneal transparency?

Thinness, smooth surface, tear film, ineffective collagen scatter, crystallins, regular lattice spacing.

What is optimal corneal hydration and what happens when it changes?

78% (deturgescence); ↑ hydration = edema, ↓ hydration = opacification.

What causes photokeratitis and what are its signs?

UV overexposure; punctate keratitis, conjunctival injection.

What is the term for optimal corneal hydration and its percentage?

Corneal deturgescence = 78% water.

Why must excess water constantly be removed from the cornea?

Because the endothelial barrier is leaky.

What are the two directional barriers for corneal hydration?

Epithelium ↔ tear film; Endothelium ↔ anterior chamber.

What do zonula occludens in the epithelium prevent?

Movement of ions/fluid from tears into the cornea; low permeability; pathogen protection; reduced evaporation.

How do endothelial junctions differ from epithelial junctions?

They are “leaky,” allowing fluid from anterior chamber to stroma; they have high permeability aiding nutrition and transparency

How do stromal proteoglycans affect corneal hydration?

Anionic proteoglycans attract water.

What role do osmotic gradients and ion transport play in corneal hydration?

Water follows ion concentration gradients (e.g., Cl⁻ expulsion, Na⁺ absorption).

What is the role of the glycocalyx and mucin layer?

Allow tear film to adhere to hydrophobic epithelium.

Why can’t water pass directly between epithelial cells?

Tight junctions force molecules to move through cells, not between them.

What are aquaporins and where are they found?

Bidirectional osmotic water channels in epithelium and endothelium; integral membrane proteins

What is the role of Aquaporin-1?

Water-selective; epithelium & endothelium; moves water from stroma → endothelium → aqueous.

What is the role of Aquaporin-3?

Transports glycerol and small solutes; present in epithelium.

What is the role of Aquaporin-5?

Water-selective; epithelium; moves water from epithelium → tears.

How do ions move across epithelial membranes?

Ion channels, co-transporters, Na⁺/K⁺ ATPase pumps create gradients that water follows.

Describe Na⁺ movement in epithelial water regulation.

Tears → epithelium via apical channels; epithelium → stroma via Na⁺/K⁺ ATPase.

What is the role of the Na⁺/K⁺/2Cl⁻ cotransporter?

Moves Na⁺, K⁺, Cl⁻ from stroma into epithelium through the basolateral membrane.

What causes Cl⁻ to move from epithelium to tears?

High intracellular K⁺ leads to K⁺ leak to stroma, forcing Cl⁻ out apically.

What ion movement is the main driving force for water into tears?

Cl⁻ movement.

What is the main mechanism regulating endothelial hydration control?

Ion transport and osmotic gradients involving Na⁺, Cl⁻, HCO₃⁻.

What enzyme is critical in endothelial hydration regulation?

Carbonic anhydrase.

How many Na⁺/K⁺ ATPase pump sites are present per endothelial cell?

~3 million in basolateral membrane.

What reaction does carbonic anhydrase catalyze?

H₂O + CO₂ ⇌ HCO₃⁻ + H⁺.

How does Na⁺ movement influence HCO₃⁻ transport in endothelium?

Low intracellular Na⁺ pulls Na⁺ in from stroma with HCO₃⁻ via Na⁺/HCO₃⁻ cotransporter.

How does HCO₃⁻ exit endothelial cells?

Via HCO₃⁻/Cl⁻ transporter back to stroma and into anterior chamber.

How does Cl⁻ exit endothelial cells?

Down gradient into anterior chamber.

What ions drive water into the anterior chamber from the stroma?

HCO₃⁻ and Cl⁻.

What factors reduce endothelial pump function?

Na⁺/K⁺ ATPase inhibition, low temp, low bicarbonate, carbonic anhydrase inhibitors, low endothelial density.

How does the cornea compensate for endothelial cell loss?

Increased activity at existing pumps, more pump sites, polymorphism, polymegathism.

What is normal endothelial cell density through life?

Birth 5000 → 5yo 3500 → 14-20yo 3000 → late adult 2500 cells/mm².

Edema <500.

What defines corneal edema?

Water >78%; thickened cornea; light scatter; reduced transparency.

How do epithelial vs endothelial damage cause edema differently?

Epithelial: loss of tight junctions → localized edema. Endothelial: pump failure → diffuse stromal edema.

Why is posterior stroma more prone to fluid retention?

Anterior stroma and Bowman’s are more rigid.

What conditions can cause corneal edema?

Fuch’s dystrophy, hypoxia, high IOP (fluid moves AC → stroma).

What is corneal hysteresis?

Ability to absorb/dissipate energy; viscoelastic shock absorption.

What structures give cornea elasticity vs viscosity?

Collagen = elasticity; ground substance = viscosity.

How is corneal hysteresis measured?

Ocular Response Analyzer; air puff; P1 inward, P2 rebound; CH = P1-P2.

What is Goldmann-correlated IOP calculation from ORA?

(P1 + P2)/2.

What are normal corneal hysteresis values and trends?

10–11 mmHg; decreases with age and high IOP; relevant for glaucoma.

What fuels corneal metabolism?

Oxygen and glucose.

Where does the cornea obtain oxygen in the open-eye environment?

Mainly from tear film/atmosphere; secondarily from aqueous and limbal capillaries.

Where does the cornea obtain oxygen in the closed-eye environment?

Mainly from palpebral capillaries and aqueous humor.

Where does the cornea obtain glucose?

Mainly from aqueous humor; secondarily from limbal capillaries.

Where is glycogen stored in the cornea?

Basal cell layer of corneal epithelium.

What percentage of corneal glucose metabolism is anaerobic and what are its byproducts?

85%; produces lactate and hydrogen.

Where do lactate and hydrogen byproducts go after anaerobic metabolism?

Accumulate in stroma and move to anterior chamber.

What percentage of corneal glucose metabolism is aerobic and which layer relies on it?

15%; corneal endothelium relies highly on it.

(Has a very high metabolic demand for O2 to make ATP)

What is the hexose monophosphate shunt and where does it occur?

Oxygen-independent pathway in corneal epithelium; 35% of epithelial glucose uses it; produces NADP.

How does hypoxia affect corneal metabolism?

Only anaerobic respiration occurs; lactate and hydrogen build up.

What does lactate buildup during hypoxia cause?

It causes edema by shifting osmotic balance, drawing water into the cornea.

How does hydrogen buildup during hypoxia damage cells?

Lowers pH → K⁺ loss → cell shrinkage/apoptosis.

When is some hypoxia considered normal?

In closed-eye environment; resolves after awakening.

What can cause abnormal hypoxia?

Contact lens wear, especially poor compliance/hygiene.

Why is the cornea avascular?

Necessary for transparency.

How do nutrients and immune cells reach the cornea?

By diffusion.

What is angiogenic privilege of the cornea?

Prevents neovascularization via endostatin and thrombospondin-1 inhibiting VEGF binding to the corneal endothelium.

VEGF = vascular endothelial growth factor

What is immune privilege of the cornea?

Reduced immune response due to low MHC I expression; reduces graft rejection.

What causes corneal neovascularization?

Hypoxia, infection, inflammation, contact lens wear; mediated by VEGF.

How densely innervated is the cornea?

Most densely innervated tissue; 400× skin; 7000 nociceptors/mm².

What functions do corneal sensory nerves provide?

Touch, temperature, pain, blinking, tearing, nourishment, trophic factors.

What is the corneal innervation pathway?

Trigeminal nerve → ophthalmic nerve → nasociliary nerve → long/short ciliary nerves → stromal plexus → subepithelial plexus → subbasal plexus → intraepithelial plexis/nerve terminalis.

Where is the stromal plexus located and what happens to myelin there?

Anterior stroma; myelin sheath is lost; enclosed by basal lamina & Schwann cells.

Where is the subepithelial plexus located and what is lost?

Within Bowman’s layer; Schwann covering lost.

Where is the subbasal plexus located?

Between basal epithelial cells and Bowman’s layer.

What are intraepithelial nerve terminals?

Naked nerve endings that reorganize as epithelium turns over.

What is the clinical significance of prominent visible corneal nerves?

Normal peripherally; central prominence may indicate disease.

How is corneal sensitivity tested and what does it assess?

Cotton wisp or Cochet-Bonnet; tests V1 function.

What decreases corneal sensitivity?

Long-term CL wear, aging, disease.

What happens when corneal nerves are damaged?

impaired corneal health, ↑ permeability, ↓ mitosis, ↓ adhesion, poor healing, ↓ tear signaling.

Which corneal layers regenerate and which do not?

Epithelium, stroma, Descemet’s regenerate; Bowman’s and endothelium do not.

How does stromal regeneration differ from original structure?

Injuries cause keratocyte proliferation and myofibril formation. Regenerated collagen is larger and less precisely organized.

What cells assist in Descemet’s membrane regeneration?

Stromal keratocytes and endothelial cells (secrete membrane)

What happens to the endothelium when it is damaged?

Pleomorphism, polymegathism, increased number of ion pumps