Highway for the Light

Anand Swaroop, Ph.D., a scientist at the Kellogg Eye Center, studies photoreceptor cells called rods and cones, which are found in the retina lining the back of the eye. These specialized neurons capture photons of light and transform them into electrical signals, which are processed by the brain to allow us to see.

According to Swaroop, the human retina contains about 120 million photoreceptor cells, and they are some of the hardest working cells in the body. Each photoreceptor produces about 6,000 light-grabbing molecules of rhodopsin or cone opsin every minute. That’s a total of 6 billion molecules synthesized every second in each human retina. All the molecular components required to make these proteins must travel through one slender cilium, which connects the photoreceptor’s inner and outer segments.

This photoreceptor cell has been stained to show the nucleus (blue), the outer segment (red) and the inner segment (green). Everything the cell needs to make rhodopsin or cone opsin — molecules vital to human sight — passes through one tiny cilium that connects the inner and outer segments of the cell. Photo: Sunil Parapuram, Ph.D. (Swaroop Laboratory)

“It’s a very vulnerable connection, and that’s why we have so many blinding diseases that involve ciliary or transport defects,” says Swaroop. “Any defect in the synthesis, regulation, transport or transduction of all these molecules can quickly lead to the degeneration of photoreceptors. These are non-dividing cells, so if you lose too many of them, you go blind.”

Because the cilia connection in photoreceptors is so delicate, Swaroop says even small losses in protein function can compromise vision, even if cilia on other types of cells are not affected.

—SP

Also:

Secrets of the Cilia

Picking Up the Scent