The retinal pigment epithelium is the unique site of transthyretin synthesis in the rat eye.

Transthyretin (TTR), or prealbumin, is a 55-kD tetrameric protein which plays an important role in the plasma transport of thyroxine, and through its interaction with retinol-binding protein, of retinol. Four major sources of TTR synthesis have been identified in the mammal: liver hepatocytes, visceral yolk sac endoderm, choroid plexus epithelium, and the eye. We now report in situ hybridization studies demonstrating that in the rat eye, the retinal pigment epithelium is the unique source of TTR synthesis. This finding underscores the developmental, structural, and functional homology between the choroid plexus epithelium and the retinal pigment epithelium. Although the functional significance of ocular TTR synthesis is unclear, it is likely that it serves to transport thyroxine or retinol across the blood-retina barrier, thereby facilitating their effects on differentiation and morphogenesis. Considering the importance of retinol in the biochemistry of the visual process, we propose that TTR may play a role in the intraocular cycling of retinol.