Is there a blood-brain barrier at the optic nerve head?

Area of Science:

• Ophthalmology
• Neuroscience
• Cell Biology

Background:

• The retina and optic nerve possess a blood-brain barrier (BBB) restricting passive molecule passage.
• This barrier is crucial for protecting neural tissue from blood-borne substances.

Purpose of the Study:

• To investigate potential exceptions to the blood-brain barrier at the optic disc.
• To elucidate the pathway of tracer substance entry into the optic nerve head.

Main Methods:

• Utilized horseradish peroxidase (HRP) as an electron microscopy tracer.
• Employed normal rhesus monkeys as the experimental model.
• Examined the optic nerve head ultrastructure and tracer distribution.

Main Results:

• Horseradish peroxidase successfully traversed from the bloodstream into the optic nerve axons at specific optic disc regions.
• The tracer entered via the border tissue of Elschnig, originating from adjacent choroidal tissues.
• Glial cell junctions at the optic disc edge formed a barrier, preventing tracer spread into the subretinal space.

Conclusions:

• The optic disc represents a unique site where the blood-brain barrier is circumvented.
• The border tissue of Elschnig and specific glial cell junctions play critical roles in regulating substance passage at the optic nerve head.