Glial Cells in the Fish Retinal Nerve Fiber Layer Form Tight Junctions, Separating and Surrounding Axons

Area of Science:

• Neuroscience
• Cell Biology
• Ophthalmology

Background:

• Teleost fish retinas exhibit continuous cell addition throughout life.
• Axonal growth and proliferation occur in a functioning retina.
• The extracellular environment's regulation is crucial for retinal development and function.

Purpose of the Study:

• Investigate components regulating the extracellular environment in the fish retinal nerve fiber layer (NFL).
• Determine if growing axons are surrounded by different structures than signal-conducting axons.
• Characterize novel tight junctions found in the fish retinal NFL.

Main Methods:

• Immunohistochemistry to identify cellular components and protein expression.
• Freeze fracture electron microscopy to visualize membrane structures.
• Polymerase Chain Reaction (PCR) to analyze gene expression.
• Functional assays using Evans Blue dye to assess barrier properties.

Main Results:

• Müller cell endfeet express aquaporin-4, suggesting involvement in water homeostasis.
• Conspicuous tight junctions were discovered in the NFL, forming branching strands and meshworks around axons.
• Expression of various claudins (e.g., claudin-1, -3, -19) was confirmed via PCR.
• Newly growing axons are associated with Müller cell endfeet, not the observed myelin-like wrappings.
• Evans Blue dye retention indicated the tight junctions function as a diffusion barrier.

Conclusions:

• The fish retinal NFL possesses unique tight junctions with potential roles beyond nerve conductance.
• These tight junctions may separate extracellular spaces, facilitating both axonal conductance and growth.
• Müller cells play a role in water homeostasis and support developing axons.