Cell types in the eye have ancient evolutionary origins  

Karthik Shekhar and his colleagues raised a few eyebrows as they collected cow and pig eyes from Boston butchers, but those eyes — from 17 species — are providing insights into the evolution of the vertebrate retina and could lead to better animal models for human eye diseases. 

The retina is a miniature computer containing diverse types of cells that collectively process visual information before transmitting it to the rest of the brain. In a comparative analysis across animals of the many cell types in the retina — mice alone have 130 types of cells in the retina, as Shekhar’s previous studies have shown — the researchers concluded that most cell types have an ancient evolutionary history. These cell types, distinguished by their differences at the molecular level, give clues to their functions and how they participate in building our visual world. Their remarkable conservation across species suggests that the retina of the last common ancestor of all mammals, which roamed the earth some 200 million year ago, must have had a complexity rivaling the retina of modern mammals. In fact, there are clear hints that some of these cell types can be traced back more than 400 million years ago to the common ancestors of all vertebrates — that is, mammals, reptiles, birds and jawed fish.

The findings, published in Nature, were a surprise, since vertebrate vision varies so widely from species to species. For example, fish need to see underwater, mice and cats require good night vision, and monkeys and humans evolved very sharp daytime eyesight for hunting and foraging. Yet, numerous cell types are shared across a range of vertebrate species, suggesting that the gene expression programs (which genes are turned “on” or “off” in a given cell) that define these types likely trace back to the common ancestor of jawed vertebrates, the researchers concluded. “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,” Shekhar said.

The team found, for example, that one cell type — the “midget” retinal ganglion cell — that is responsible for our ability to see fine detail, is not unique to primates, as it was thought to be. The researchers discovered evolutionary counterparts of midget cells in all other mammals, though these counterparts occurred in much smaller proportions. This is important for research on glaucoma, the leading cause of irreversible blindness in the world, as “the midget cells are probably what we should care about the most in human glaucoma,” Shekhar said. While mice are a favorite model animal for studying glaucoma, they have very few of the midget retinal ganglion cell counterparts.  “Knowing their counterparts in the mouse will hopefully help us design and interpret these glaucoma mouse models a little better,” Sekhar added.

Edited by Suditi Kedambadi, high school volunteer, and Miriam Kaplan, PhD

Source: University of California – Berkeley, ScienceDaily, December 13, 2023; see source article