Corneal Anatomy and Histology

Introduction

• The eye's outer connective tissue resembles two joined spheres.

• The cornea is a smaller, transparent sphere located anteriorly.

• The radius of the cornea is 8 mm.

• The sclera is larger, opaque sphere located posteriorly.

• The radius of the sclera is 12 mm.

• The globe's dimensions are:

  • Anteroposterior: 24 mm
  • Vertical: 23 mm
  • Horizontal: 23.5 mm

Corneal Histologic Features: Overview

• The cornea is the eye's primary refractive component.
• It is transparent and avascular, which is crucial for optimal light transmission.
• The anterior surface is covered by the tear film.
• The posterior surface borders the aqueous-filled anterior chamber.

Corneal Histologic Features: Overview (cont.)

• The cornea is continuous with the conjunctiva and sclera at its periphery.
• It has five layers:
  • Epithelium
  • Bowman's Layer
  • Stroma
  • Descemet's Membrane
  • Endothelium

Corneal Epithelium - Structure

• The epithelium is the cornea's outermost layer.
• Stratified corneal epithelium is typically 5-7 cells thick (approximately 50 µm).
• It thickens at the periphery and is connected to the conjunctival epithelium.

Corneal Epithelium - Surface Layer

• This layer is two cells thick and displays a smooth anterior surface.
• It's composed of non-keratinized squamous cells.
• Microvilli and microplicae enhance tear film stability by increasing surface area.

Corneal Epithelium - Tight Junctions

• Tight junctions (zonula occludens) connect surface cells along their lateral walls.
• They act as a barrier, preventing excessive fluid uptake from the tear layer and regulating substance movement.
• This creates a highly effective semipermeable membrane.

Corneal Epithelium - Middle Layer

• This is comprised of 2-3 layers of wing cells.
• Each wing cell has a 20 µm diameter.
• Wing cells have wing-like lateral processes.
• These cells are polyhedral with convex anterior and concave posterior surfaces, fitting over basal cells.

Corneal Epithelium - Basal Layer

• This is the innermost corneal epithelial layer, composed of a single layer of columnar cells.
• Cell diameters are 8-10 µm, and nuclei are oval-shaped.
• The rounded apical surface of each basal cell is adjacent to the wing cells.
• The basal surface is attached to the basement membrane (basal lamina).
• Basal cells secrete and form the basement membrane, anchoring the cells to underlying tissue.
• This layer is the germinal layer, where mitosis occurs.

Corneal Epithelium - Epithelial Replacement

• Corneal surface smoothness is maintained through continuous cell replacement.

• Surface cells are shed into the tear film.

• Cell proliferation occurs in the basal layer.

• Basal cells become wing cells, then surface cells.

• Only cells in contact with the basement membrane can divide.

• Stem cells located in a band around the corneal periphery renew the basal cell layer.

• Continual migration of basal cells from the periphery to the center of the cornea occurs.

• The complete turnover time for the entire corneal epithelium is roughly 7 days, which is faster than other epithelial tissues.

• Minor abrasions heal within hours, while larger ones within overnight.

• If the basement membrane is damaged, complete healing with basement membrane replacement can take months.

Bowman's Layer

• The second layer of the cornea, approximately 8-14 µm thick.
• It is a dense, fibrous sheet made of interwoven collagen fibrils randomly arranged within a mucoprotein ground substance.
• Fibrils have a 20-25 nm diameter and run in various directions.
• It's a transition layer, not a true membrane, between the epithelium and stroma, differing from the stroma in its lack of cells and smaller collagen fibril diameter.
• Bowman's layer may provide biomechanical rigidity and shape to the cornea in addition to contributing to its biomechanical characteristics.
• The structure of the anterior surface is irregular and mirrors the bases of basal cells in the epithelium; the posterior transition to the stroma is notably gradual.

Stroma

• The middle layer of the cornea, 500 µm thick (about 90% of total corneal thickness).
• The stroma is composed of collagen fibrils, keratocytes, and extracellular ground substance.
• Collagen fibrils have a uniform 25-35 nm diameter, run parallel, and form lamellae (flat bundles).

Stroma - Collagen Fibrils

• The 200-300 lamellae run parallel to the corneal surface.
• Adjacent lamellae intersect at angles, maintaining continuity with regular spacing.
• Each lamella and fibril spans the entire cornea, extending from the limbus.
• Fibril arrangement varies slightly across the cornea; the anterior region featuring thinner, highly interweaving lamellae and the posterior characterized by more regular, thicker lamellae.

Stroma - Keratocytes

• Keratocytes (corneal fibroblasts) are flattened cells intermingled between collagen lamellae.
• They're not distributed haphazardly but form a discernible corkscrew pattern from anterior to posterior.
• Keratocytes exhibit extensive branching processes, interconnected by gap junctions (lateral and anteroposterior branches).

Stroma - Ground substance

• The ground substance fills spaces between fibrils and cells within lamellae.
• It contains proteoglycans (PGs) with attached glycosaminoglycan (GAG) side chains.
• PGs significantly contribute to corneal tensile strength.

Stroma - Transparency

• Regular arrangement of stromal components.
• Small diameters of collagen fibrils (1.411 refractive index).
• Extracellular matrix with 1.365 refractive index.
• Short distances between refractive index variations. These help minimize light scattering.
• Specific spacing between fibrils induces destructive interference of reflected light, substantially reducing light scattering.