Endocytosis in the retinal and choroidal microcirculation.

Area of Science:

• Ophthalmology
• Cell Biology
• Vascular Biology

Background:

• Endocytosis is a key cellular process for internalizing substances.
• Retinal and choroidal vasculature have unique properties influencing transport.
• Horseradish peroxidase (HRP) is a commonly used tracer for studying endocytosis.

Purpose of the Study:

• To compare the mechanisms of HRP endocytosis in retinal and choroidal vascular cells.
• To elucidate the specific pathways and vesicle types involved in HRP uptake.
• To investigate the role of different membrane structures in endocytosis.

Main Methods:

• Comparison of HRP endocytosis in immersion and perfusion-fixed rat eyes.
• Analysis of tracer localization within vascular cells.
• Identification of vesicle sizes and membrane structures involved in HRP uptake.
• Ruthenium red-osmium tetroxide staining to visualize the glycocalyx.

Main Results:

• Identical endocytosis mechanisms observed in both retinal and choroidal vessels.
• Bulk of internalized HRP found in macropinosomes (300-400 nm).
• HRP also internalized in smaller vesicles (100-130 nm), potentially from clathrin-coated regions.
• Tracer localized to a layer coincident with the glycocalyx.
• Smooth plasmalemmal caveolae (80 nm) did not participate in endocytosis.

Conclusions:

• HRP endocytosis in retinal and choroidal vascular endothelium likely involves plasma membrane recycling.
• HRP may be internalized as a 'contaminant' of the glycocalyx in coated pits.
• Caveolae are not involved in absorptive endocytosis or vesicular transport in these cells.