Tight junction complexity in the retinal pigment epithelium of the chicken during development.

In the avascular retina of birds, the pigment epithelium (RPE) is the main site of the blood-retina barrier. Tight junctions (TJs) connect the pigment epithelial cells and represent the structural substrate of the barrier function. We investigated, by means of the quantitative freeze-fracturing technique, the TJs of the chicken RPE during development and compared them with the TJs of choroid capillary endothelial cells which are known to be fenestrated. The association of TJs with the protoplasmic membrane leaflet (P-face) is more pronounced in the RPE than in the choroid vessels. Between embryonic day 15 (E15) and E19, we observed a significant increase in the TJ complexity in the RPE, but not in the choroid vessels. The increase coincides with the morphological and functional maturation of the chicken retina suggesting that complex P-face-associated TJs in the RPE are necessary for the formation of an effective blood-retina barrier.