Page 1: Introduction to the Cornea

Anatomy and Physiology

• Cornea Overview: Complex structure with dual functions.
  • Protective role.
  • Contributes about 75% to the optical power of the eye.
• Hydration: Normal hydration level of the cornea is approximately 78%.
• Nutrient Supply and Waste Removal:
  • Nutrients supplied via aqueous humour (posterior).
  • Waste products removed by tears (anterior).
• Innervation:
  • Most densely innervated tissue in the body.
  • Conditions such as abrasion and bullous keratopathy cause significant pain and reflex lacrimation.
  • Innervation provided by the first division of the trigeminal nerve.

Dimensions of the Cornea

• Average Diameter:
  • Vertical: 11.5 mm
  • Horizontal: 12 mm
• Thickness:
  • Centrally averages 540 μm and increases peripherally.
• Variations:
  • Thickness does not differ between males and females but varies among individuals and races.
  • Central corneal thickness is critical for determining intraocular pressure (IOP).

Structure of the Cornea

• The cornea is organized into several layers:

  1. Epithelium

  • Type: Stratified squamous and non-keratinized.
  • Components:
  • Single layer of columnar basal cells attached to the basement membrane via hemidesmosomes.
  • 2-3 strata of ‘wing’ cells.
  • 2 layers of squamous surface cells.
  • Outer cell surface area increased by microplicae and microvilli (aids in tear film attachment).
  • Lifespan: Superficial cells shed into the tear film after a few days.
  • Corneal Stem Cells:
  • Located at the corneoscleral limbus (palisades of Vogt).
  • Deficiency can cause chronic epithelial defects.
  • Significant for maintaining a physiological barrier, preventing conjunctival tissue growth onto cornea.
  • Deficiencies may be treated with stem cell transplantation (autologous or allogeneic).

  2. Bowman Layer

  • Acellular layer made of collagen fibers.

  3. Stroma

  • Comprises approximately 90% of corneal thickness.
  • Structure: Regularly oriented layers of collagen fibrils maintained by proteoglycan ground substance (chondroitin sulfate and keratan sulfate) and interspersed keratocytes.
  • Importance: The regular arrangement and spacing of collagen is essential for optical clarity.
  • Scarring: Possible but cannot regenerate after damage.

  4. Descemet Membrane

  • Composed of fine latticework of collagen fibrils, different from stroma collagen.
  • Structure consists of a banded zone (from utero development) and a posterior non-banded zone produced throughout life by the endothelium.
  • Regeneration: Possesses regenerative potential.

  5. Endothelium

  • Monolayer of polygonal cells which maintain corneal deturgescence by pumping excess fluid from the stroma.
  • Normal density in young adults: approximately 3000 cells/mm².
  • Density decreases by about 0.6% per year; neighboring cells enlarge to occupy space.
  • At around 500 cells/mm², corneal edema develops leading to impaired transparency.
  • Sixth Layer: Existence of the Dua layer (between stroma and Descemet membrane) proposed, subject to debate.

Clinical Tip

• Corneal abrasion is associated with intense pain, photophobia, and reflex lacrimation due to high innervation density.

Diagram: Anatomy of the Cornea (Fig. 7.1)

• Components:
  • Tear film.
  • Surface cells, wing cells, basal cells, and basement membrane contributing to the epithelium.
  • Bowman layer, stroma, Descemet membrane, and endothelium in structural layout.