Eye cells in humans, mice and 400-million-year-old ancestor remarkably similar

The retina of species such as mice and humans have changed very little since the last common ancestor of all mammals 200 million years ago, and even back to the last common ancestor of all vertebrates 400 million years ago according to new research.

Understanding the evolutionary links between eye cells in vertebrates could lead to better animal models for human eye diseases.

Think of the retina like a tiny computer in your eye. The diverse cells within it work together to take in and process visual information before sending that information to the brain.

Vertebrate vision is quite varied.

Fish need to see underwater. Mice, cats and owls need good night vision. Humans and our primate relatives have developed sharp daytime eyesight to find and hunt for food.

Despite differences in colour perception and visual acuity, the cell types across species have been shown to be linked by a long evolutionary history. This suggests that the genetic expression of different retinal cells dates back to the common ancestor of jawed vertebrates.

The new research published in Nature looked at the eye cells of 17 species including humans.

For example, the “midget” retinal ganglion cell is responsible for our ability to see fine detail. It was previously believed to be unique to primates. But large-scale genetic analysis revealed evolutionary counterparts in all other mammals.

“What we are seeing is that something thought to be unique to primates is clearly not unique. It’s a remodelled version of a cell type that is probably very ancient,” says research leader Karthik Shekhar, an assistant professor at the University of California, Berkeley. “The early vertebrate retina was probably extremely sophisticated, but the parts list has been used, expanded, repurposed or refurbished in all the species that have descended since then.”

The number of cell types in vertebrates vary considerably. Mice have 130 kinds of retinal cell, while humans have 70. The origin of these diverse cells has been a mystery.

Primate evolution appears to have reduced the need for signal processing in the eye itself as the brain became more complex.

“Our study is saying that the human retina may have evolved to trade cell types that perform sophisticated visual computations for cell types that basically just transmit a relatively unprocessed image of the visual world to the brain so that we can do a lot more sophisticated things with that,” Shekhar explains. “We are giving up speed for finesse.”

Diagram showing retinal cells of different animals including human and mouse pictured on left
The retina of vertebrate species, such as mice and humans, are remarkably conserved since the origin of jawed vertebrates more than 400 million years ago. This diagram shows the similarities between the retinal cells of humans and mice, including the ON and OFF “midget” retinal ganglion cells (MGCs). Credit: Illustration by Hugo Salais, Metazoa Studio, Spain.

The team believe their research will have clinical benefits.

Sekhar’s group is also studying the molecular signs of glaucoma, the leading cause of irreversible blindness in the world.

Mice are the preferred model animal for studying glaucoma. But the midget retinal ganglion cells crucial in understanding glaucoma make up only 2–4% of ganglion cells in mice, compared to 90% in humans.

“This work is clinically important because, ultimately, the midget cells are probably what we should care about the most in human glaucoma,” Shekhar says. “Knowing their counterparts in the mouse will hopefully help us design and interpret these glaucoma mouse models a little better.”

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