Anatomy of the Cornea - Detailed Study Notes

Anatomy of the Cornea - Detailed Notes

Origin

  • Kerato: Derived from Greek meaning "horn" or "shield like". Ancient Greeks believed the cornea was made from material similar to the thinly sliced horn of animals.

Introduction

  • Characteristics:

    • Avascular and transparent.

    • Accounts for 70% of the total refractive power of the eye (+43D).

    • Principle optical surface of the eye.

  • Composition:

    • Cornea forms 1/6th and sclera forms 5/6th of the outer coat.

  • Refractive Indices:

    • Air: 1.00

    • Tear fluid: 1.33

    • Cornea: 1.37

    • Aqueous humor: 1.33

    • Lens: 1.42

    • Vitreous: 1.33

Dimensions

  • Anterior Surface: Elliptical

    • Horizontal Diameter: 11.75mm

    • Vertical Diameter: 11mm

  • Posterior Surface: Circular

    • Diameter: 11.5 mm

  • Corneal Diameter Classifications:

    • Microcornea: Horizontal Corneal Diameter (HCD) < 10mm

    • Macrocornea: Horizontal Corneal Diameter (HCD) > 13 mm

Radius of Curvature

  • Anterior Radius: 7.8mm

  • Posterior Radius: 6.5mm

  • Central Corneal Thickness: 0.52mm

  • Peripheral Corneal Thickness: 0.67mm

    • The central 5mm is the most critical for refracting power.

    • Central Thickness influences IOP measurement.

Histology - Six Layers of Cornea

  1. Epithelium:

    • Thickness: 50-90μm

    • Type: Stratified squamous (5-6 layers)

    • Cells shed into tear film; renewal by stem cells.

  2. Bowman's Membrane:

    • Binds stroma to epithelium, 0.5mm thick.

    • Not elastic; does not regenerate.

    • Provides resistance to infection.

  3. Stroma (Substantia Propria):

    • occupies 90% of corneal thickness.

    • Composed of collagen fibrils, keratocytes, and ground substances.

    • Lamellae are arranged in 200-250 layers.

  4. Dua's Layer:

    • Discovered in 2013, 15μm thick.

    • Strong and impermeable to air.

  5. Descemet's Membrane:

    • Strong and elastic layer; basement membrane of endothelium.

    • Regenerates, resistant to infection and chemicals.

    • Exhibits Hassel Henle bodies with age.

  6. Endothelium:

    • Layer of flat, polygonal cells; density declines with age.

    • Involves active secretion/synthesis; high mitochondria content.

Blood Supply

  • The cornea is avascular; nourished by small loops from anterior ciliary vessels that invade about 1mm at the periphery.

Nerve Supply

  • Highly sensitive tissue; supplied by long and short ciliary nerves from the trigeminal nerve.

  • Concentration is greatest in the anterior stroma but lacks innervation in Bowman's layer and endothelium.

Junction of Cornea and Sclera

  • At the limbus, the structures transition:

    • Corneal epithelium → Bulbar conjunctival epithelium.

    • Bowman's Membrane → Continuous with conjunctival lamina.

    • Stroma → Becomes sclera.

    • Descemet's Membrane → Becomes Schwalbe's line.

    • Endothelium → Lines trabecular meshwork.

Transparency Factors

  • Contributions:

    • Tight junctions in epithelial cells.

    • Endothelial pump mechanism.

    • Absence of blood vessels and pigments.

    • Minimal cell nuclei in stroma.

    • Uniform stroma structure.

Corneal Analysis Techniques

  • Techniques include:

    • Keratometer

    • Specular microscopy

    • AS-OCT

    • Orbscan

    • Aberrometer

    • Pentacam

    • Video keratoscope

    • Oculus topographer

    • Ocular response analyser (OCA)

References

  • Anatomy and Physiology of the Eye: Khurana

  • Clinical Anatomy of the Visual System: Remmington

Note: Ensure to understand each layer and function for complete knowledge on corneal anatomy.