Structure and permeability to ferritin of the choriocapillary endothelium of the rat eye.

The choriocapillaris is the fenestrated capillary network that supplies a large portion of the nutrients required by the retinal pigment epithelium, photoreceptor cells, and other cells of the outer neural retina. The permeability of these capillaries was investigated in the rat by the use of ferritin (mol. wt. approx. 480,000; mol. diam. 110A) as a tracer. Ninety minutes after intravascular ferritin administration, a high concentration of tracer particles was distributed uniformly in the capillary lumina but few particles were present in Bruch's membrane, the multilayered basement membrane that separates the choriocapillary endothelium from the retinal pigment epithelium. The bulk of the tracer remained in the capillary lumina with a definate blockage seen at fenestral, channel, and vesicle diaphragms. These results indicate that the ratchoriocapillary endothelium, unlike the fenestrated endothelia lining other capillary beds, constitutes an important barrier to the passage of ferritin and presumably of circulating native molecules of similar size.