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 sac-like structure that allows free movement of the eye while maintaining ocular surface health. 1

Basic Structure and Attachments

• The conjunctiva is a delicate, transparent epithelium with its own underlying stroma
• It overlies the white sclera of the eye
• Firm attachments occur at only two locations:
  • At the circular limbus (where sclera meets cornea)
  • At the eyelids
• Between these attachment points, the conjunctiva is loose, forming a blind sac (fornix) that is deepest under the upper and lower eyelids 1
• When removed intact, it appears as a flimsy sheet with an 11-mm circular defect in the center 1

Histological Layers

The conjunctiva consists of multiple layers that can be visualized using laser scanning confocal microscopy 2:

1. Epithelial Layer:

   • Superficial epithelium: appears as a mass of small cell nuclei (density: 2212 ± 782 cells/mm²)
   • Basal epithelium: cells with visible light grey borders but invisible nuclei (density: 2368 ± 741 cells/mm²)
   • Total epithelial thickness: approximately 32.9 ± 1.1 μm 2

2. Stromal Layer:

   • Dense meshwork of white fibers
   • Contains blood vessels with cellular elements 2

Specialized Cells and Structures

• Goblet Cells:

  • Specialized secretory cells that produce mucins
  • Density: approximately 111 ± 58 cells/mm²
  • Visible as orifices at the epithelial surface when they expel their contents
  • Produce both acidic and neutral mucins essential for tear film stability 3, 2

• Langerhans Cells:

  • Immune surveillance cells
  • Density: approximately 23 ± 25 cells/mm² in mature form 2

• Conjunctival Microcysts:

  • Present in deeper epithelial layers 2

• Junctional Proteins:

  • E-cadherin and zonula occludens 1 maintain epithelial integrity
  • Express cytokeratins including CK8 and CK19 3

Functional Aspects

1. Immunological Function:

   • Part of the Conjunctiva-associated Lymphoid Tissue (CALT)
   • Integrated into a dense neural and immunological network
   • Responds to various stimuli while minimizing collateral damage
   • Hosts resident microorganisms (conjunctival microbiome) 4

2. Transport Functions:

   • Moderately tight epithelium with various transport processes
   • Facilitates homeostasis of ions, solutes, and water in the conjunctival surface and tear film
   • Contains transport mechanisms for:
     • Amino acids
     • D-glucose
     • Monocarboxylates
     • Nucleosides
     • Dipeptides 5

3. Protective Functions:

   • Responds to oxidative stress by increasing production of reactive oxygen species
   • Produces inflammatory cytokines (IL-1β, IL-6, IL-8, IP-10) in response to stimuli like TNF-α 3
   • Contributes to tear film production and maintenance 5

Clinical Significance

• The conjunctiva serves as a potential drug delivery route to the posterior eye due to:

  • Larger surface area compared to the cornea
  • Expression of several key transport processes
  • Ability to maintain drug levels in the vitreous following subconjunctival injection 5

• Modulation of ion transport in the conjunctiva can alter transconjunctival fluid flow, which may be useful in treating dry eye conditions 5

• The normal conjunctival morphology described serves as a baseline against which changes due to disease, surgery, drug therapy, or contact lens wear can be assessed 2