Anatomy of the Cornea

The Cornea

• The cornea is the anterior 1/6 of the outer coat, acting as a covering lens, and is avascular and transparent.

Biochemistry

• Epithelium:
  • High activities of enzymes of glycolysis, Kreb's cycle, and ATPase pump.
  • High concentration of acetyl choline and choline esterases.
• Stroma:
  • Low enzymatic activity.
  • Collagen fibrils and glycosaminoglycans occupy the interfibrillar space.
  • Fibrils prevent scattering of light, and swelling of fibrils increases light scatter with a hazy appearance of the cornea.

Glucose

• Main substrate.
• Stored glycogen in epithelium.
• Glucose metabolism:
  • Anaerobic oxidation (glycolysis): 1 molecule of glucose gives 2 molecules of lactic acid and 2 ATP.
  • Aerobic oxidation (Kreb's cycle): 1 mole of glucose is burnt into CO2, water, and 38 ATP.

Pentose (HMP) Shunt

• Extramitochondrial oxidation of glucose.
• Energy produced is not stored in ATP, but in NADPH2.
• Energy is used in biosynthesis.
• 35% of glucose is oxidized in this shunt.

Corneal Permeability

• Influences diffusion of oxygen, water, and nutrients from bathing fluids.
• Regulates corneal hydration.
• Determines transport of drugs across the cornea.

Passive Permeability

• Diffusion: molecules move across semi-permeable membranes (epithelium or endothelium).
• Molecules achieve equal concentration on the two sides of the membrane.
• They pass through intercellular pores.
• Solubility is more important than molecular size.

Phase Solubility

• Fatty phase in the epithelium and endothelium: permeable to fat-soluble substances.
• Watery phase in the stroma: permeable to water-soluble substances.
• Biphasic substances (both fat and water soluble) are more permeable.

Active Mechanism

• Endothelium sodium pump:
  • (Na-K)ATP-ase enzyme.

Corneal Vascularisation

• Normal cornea is avascular.
• Compactness of stroma prevents vascularization.
• Stromal edema precedes vascularization.

Corneal Sensitivity

• Touch, cold, and pain receptors.
• Pain receptors are very sensitive, increasing from periphery to center.
• More sensitive in the horizontal meridian than the vertical.
• More sensitive in the temporal half than the nasal.