Mechanisms Shaping Foveola Cone Patterning Revealed

Johns Hopkins University scientists have determined that sharp human vision emerges during early fetal development through coordinated actions of a vitamin A derivative, retinoic acid, and thyroid hormones in the retina. Using lab-grown retinal organoids from fetal cells, the team monitored tissue over months to reveal how the foveola—the central region driving 50% of visual perception—develops its unique cone cell distribution. These light-sensitive cells specialize into red and green cones for daytime color vision, excluding blue cones that appear elsewhere in the retina.

The study challenges 30 years of theory by showing blue cones initially form in the foveola around weeks 10-12 but convert to red and green cones by week 14. Retinoic acid first limits blue cone creation by breaking down in the region, then thyroid hormones trigger the conversion of remaining blue cones. “First, retinoic acid helps set the pattern. Then, thyroid hormone plays a role in converting the leftover cells,” said Robert J. Johnston Jr., senior author of the study published in PNAS. “That’s very important because if you have those blue cones in there, you don’t see as well.” 

This cell fate specification and conversion process explains humans' rare three-cone system for broad color perception, absent in research models like mice or fish. Findings contradict the prevailing migration model, where blue cones were thought to relocate without changing type. “The main model in the field from about 30 years ago was that somehow the few blue cones you get in that region just move out of the way, that these cells decide what they’re going to be, and they remain this type of cell forever,” Johnston noted. “We can’t really rule that out yet, but our data supports a different model. These cells actually convert over time, which is really surprising.”

The work advances organoid models mimicking foveola function for potential vision restoration therapies. “By better understanding this region and developing organoids that mimic its function, we hope to one day grow and transplant these tissues to restore vision,” Johnston said. Lead author Katarzyna Hussey highlighted cell replacement potential: “The goal with using this organoid tech is to eventually make an almost made-to-order population of photoreceptors... These are very long-term experiments, and of course we’d need to do optimizations for safety and efficacy studies prior to moving into the clinic. But it’s a viable journey.”