Investigation of barrier characteristics in the hyaloid-retinal vessel of zebrafish.

The blood-retinal barrier (BRB) is essential for the physiological integrity of the retinal vessels. In particular, ocular pathologies of retinal neovascularization could be causally related to the BRB breakdown. Zebrafish have emerged as an advantageous model for studying vascular development and characteristics. Here we investigated for the first time the barrier characteristics of the hyaloid-retinal vessel using fli1-EGFP transgenic zebrafish. By 7 dpf, the hyaloid-retinal vessel was formed between lens and retina, where intercellular junctional complexes were already present between endothelial cells. Interestingly, NG-2 expression, but not GFAP, was colocalized with EGFP-positive cells of the hyaloid-retinal vessel. Among endothelial tight junction proteins, claudin-5 was expressed on EGFP-positive cells of the hyaloid-retinal vessel, whereas occludin and ZO-1 were not observed on the vessel. In addition, the hyaloid-retinal vessel was so leaky that a mixture of fluorescein tracers (2,000-kDa FITC-dextran, 10-kDa rhodamine-dextran, and 350-Da DAPI) diffusely infiltrated into all retinal layers. Our results suggest that, unlike retinal vessels of higher vertebrates, the hyaloid-retinal vessel of zebrafish shows insufficient characteristics to meet a functional endothelium-based CNS barrier. Therefore, it might be not suitable to use the hyaloid-retinal vessel of zebrafish for studying BRB biogenesis.