Developmental changes in the blood-ocular barriers in chicken embryos

Changes in the permeability characteristics of the developing chicken eye were studied using light and electron microscopy. Chicken embryos from 6–19 days of gestation (E9–E19) and within 1 day of hatching (P1) were injected intravascularly with horseradish peroxidase (HRP) and examined 1 and 5 min after injection. Within 1 min of injection there was focal accumulation of horseradish peroxidase reaction product (HRP-RP) at the angle of the anterior chamber in embryos up to E15. By 5 min post-injection the HRP diffused into the anterior and posterior chambers and vitreous body; and extended posteriorly into the developing uveal tract. No leakage was detected in the E19 or older animals. In the posterior segment of the eye prior to E12, HRP from the adjacent connective tissues diffused into zone III of the optic nerve. After E12 the developing meninges prevented the influx of the HRP. At the level of the lamina cribosa HRP permeated zone II up to E19 after which only 25% of the animals examined showed HRP-RP in this area. In zone I in all ages examined no HRP-RP was detected 5 min after injection. Developing blood vessels in the deep iris stroma, optic nerve and pecten remained impermeable even as they grew, while the choroidal vessels were consistently leaky. This study suggests: (1) that proteins from the vascular system reach the intraocular chambers before E19; (2) that the leakage occurs from vessels located in the uveal tract adjacent to the angle; (3) that the permeability of the optic nerve depends on the development of the meninges and the border tissues associated with the lamina cribosa; and (4) that the growing blood vessels in the developing eye have permeability characteristics similar to those found in mature vessels.