Blood-eye barriers in the rat: correlation of ultrastructure with function

The function of different vascular beds in the rat eye and brain was evaluated by measuring the transfer of a vascular tracer, 14C-alpha-amino-isobutyric acid, from blood to tissue. The density of vascular pores was measured in electron micrographs of perfusion-fixed, age-matched tissue to determine whether the differences in tracer transfer were paralleled by differences in ultrastructure. Tracer transfer in retina was approximately four times that in brain of the same animal. The transfer constant was not changed by the inclusion of cold alpha-amino-isobutyric acid, showing that transport across retinal vessels is not saturable, and indicating that, as in brain, transport is due to passive diffusion. Ultrastructurally, retinal vessels have a higher density of interendothelial junctions and of endothelial vesicles, both of which suggest higher vascular permeability. However, pericytes, which contribute to a second line of defence in the blood-brain barrier, are approximately four times as numerous in retina as in brain, and we suggest that in the retina, they act to compensate for a more permeable endothelial barrier. Ciliary body vessels had a high transfer of tracer, probably as a consequence of the fenestrations in their walls. Iridial vessels had a relatively low transfer of tracer, similar to that in retina even though a proportion of the interendothelial junctions in iridial vessels had expanded junctional clefts suggestive of open paracellular channels. However, both iris and ciliary body may lose tracer to the anterior chamber fluid, leading us to underestimate the vascular permeability in these sites.