What is the anatomy of the conjunctiva?

Anatomy of the Conjunctiva

The conjunctiva is a transparent mucous membrane that covers the anterior surface of the eye from the corneal limbus to the inner surfaces of the eyelids, forming a protective barrier and contributing to ocular surface homeostasis through its specialized epithelium and associated lymphoid tissue. 1, 2

Structural Organization

Anatomical Boundaries and Attachments

• The conjunctiva extends from the corneal limbus (the border between cornea and sclera) to the inner surfaces of the eyelids 2
• It forms a blind sac (fornix) that is deepest under the upper and lower eyelids 1
• The conjunctiva is firmly attached only at two points:
  • At the circular limbus (cornea-sclera junction)
  • At the eyelids 1
• Between these attachment points, the conjunctiva is loose, allowing free eye movement 1

Gross Anatomy

• When removed intact, the conjunctiva appears as a delicate, transparent sheet with an 11mm circular defect in the center 1
• It typically has adherent fascial tissue called Tenon's capsule 1
• The loose structure between attachment points forms folds that create the conjunctival fornices 1

Microscopic Anatomy

Epithelial Layer

• The conjunctival epithelium consists of multiple layers:
  • Superficial epithelial layer: composed of small cell nuclei with sometimes visible cell borders 3
  • Basal epithelial layer: cells with clearly visible light grey borders but invisible nuclei 3
• Mean thickness of the bulbar conjunctiva: 32.9 ± 1.1 μm 3
• Cell density measurements:
  • Superficial epithelial cell density: 2212 ± 782 cells/mm² 3
  • Basal epithelial cell density: 2368 ± 741 cells/mm² 3

Specialized Cells

• Goblet cells:
  • Present throughout the epithelial layers 3
  • Density: approximately 111 ± 58 cells/mm² 3
  • Produce mucins (both acidic and neutral) essential for tear film stability 2, 3
  • Surface orifices may represent goblet cells that have released their contents 3
• Langerhans cells:
  • Mature forms observed in the epithelium 3
  • Density: approximately 23 ± 25 cells/mm² 3
  • Part of the conjunctiva's immune defense system 4

Stromal Layer

• Composed of a dense meshwork of white fibers 3
• Contains blood vessels with cellular elements traversing through the stroma 3
• Houses components of the conjunctiva-associated lymphoid tissue (CALT) 4

Functional Aspects

Protective Functions

• Forms part of the ocular mucosal system 4
• Contains conjunctiva-associated lymphoid tissue (CALT) that perceives stimuli and provides defense while minimizing collateral damage 4
• Maintains a balance with resident microorganisms (commensal flora/microbiome) 4

Cellular Junctions and Barrier Function

• Features junctional proteins including:
  • E-cadherin
  • Zonula occludens 1 2
• Expresses cytokeratins including CK8 and CK19 2
• Functions as a moderately tight epithelium 5

Transport Functions

• Serves as a conduit for drug clearance into systemic circulation or for transport to deeper eye tissues 5
• Contains various transport processes for homeostasis of ions, solutes, and water 5
• Expresses several ion-coupled solute transport processes for:
  • Amino acids
  • D-glucose
  • Monocarboxylate
  • Nucleosides
  • Dipeptides 5

Clinical Significance

• The conjunctiva's larger surface area compared to the cornea and its rich transport processes make it an attractive route for drug delivery to posterior eye segments 5
• Subconjunctival injection can maintain reasonable drug levels in the vitreous 5
• Modulation of ion transport in the conjunctiva can alter transconjunctival fluid flow, potentially useful in treating dry eye conditions 5