Among humans, a tiny area in the center of the retina, called the fovea, is critically important to viewing fine details.  Densely packed with cone photoreceptor cells, it is typically used while reading, driving and gazing at objects of interest.  Certain animals have a similar feature in their eyes, but researchers had previously believed that the fovea was unique to primates.  According to vision scientists at the University of Pennsylvania, dogs also have an area of their retina that strongly resembles the human fovea.  In addition, this region is susceptible to genetic blinding diseases in dogs, much as it is in humans.

It’s really amazing to believe that humans and dogs have been interacting with each other for some 20,000 years, and we’re now learning something about them that’s not only eluded us for all of this time, but is also clinically relevant.  It is known that dogs have what is called an “area centralis”, a region around the center of the retina with a relative increase in cone photoreceptor cell density.  However, dogs lack the pit formation that humans have and before this study was done, it was believed that the increase in cone photoreceptor cell density was nowhere near what is seen in primates.  The highest reported density in dogs was previously 29,000 cones per square millimeter, compared to more than 100,000 cones per square millimeter seen in the foveas of humans and macaques.

However, it turns out that previous studies in dogs had missed a miniscule region of increased cell density.  In this study, while examining the retina of a dog with a mutation that causes a disease akin to a form of X-linked retinal degeneration in humans, the Penn researchers noticed a thinning of the retinal layer that contains photoreceptor cells.  After focusing in on this region, researchers examined the retinas of normal dogs using advanced imaging techniques, such as confocal scanning laser ophthalmoscopy, optical coherence tomography and two-photon microscopy.  By enabling the scientists to visualize different retina layers, they were able to identify a small area of peak cone density and estimate cone numbers by counting the cells in this unique area.  Researchers found that cone densities reached more than 120,000 cells per square millimeter in a never-before-described fovea-like region within the area centralis, a density of par with a primate’s fovea.  They also recognized that the “output side” of this cone-dense region corresponded with an area of dense retinal ganglion cells, which transmit signals to the brain.

Human patients with macular degeneration experience a loss of photoreceptor cells at or near the fovea, which results in a loss of central vision.  To see whether the fovea-like region was similarly affected in dogs, the Penn researchers used the same techniques to examine animals that had mutations in two genes (BEST1 and RPGR) that can lead to macular degeneration in humans.  In both cases, the onset of disease affected the fovea-like region in dogs in a very similar way that the disease affected humans, with central retinal lesions appearing earlier than lesions in the peripheral retina.  Why the fovea is so susceptible to early disease expression for certain hereditary disorders and why it’s spared under other conditions is as of yet unclear.  However, these findings could allow for translational research by allowing researchers to test treatments for human foveal and macular degenerative diseases in dogs.

In addition, this discovery offers unique insight into a rare human condition, fovea plana, in which people have normal visual acuity but no “pit” in their fovea, so that their fovea resemble that of dogs.  The fact that dogs have a fovea-like area of dense photoreceptor cells could indicate that dogs are seeing more acutely than people had previously assumed.  Looking ahead, researchers may focus on this fovea-like area in studies of therapies not only for X-linked retinal degeneration and Best disease, but also other sight-related problems that affect the macula and fovea.