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. But dogs lack the pit formation that humans have, and, before this study, it was believed that the increase in cone photoreceptor cell density didn't come close to matching what is seen in primates. Prior to this study, the highest reported density in dogs was 29,000 cones per square millimeter compared to more than 100,000 cones per square millimeter seen in the human and macaque foveas.
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.
Zeroing in on this region, they examined retinas of normal dogs using advanced imaging techniques, including confocal scanning laser ophthalmoscopy, optical coherence tomography and two-photon microscopy. By enabling the scientists to visualize different layers of the retina, these techniques allowed them to identify a small area of peak cone density and then estimate cone numbers by counting the cells in this unique area.
Based on their observations, the researchers found that cone densities reached more than 120,000 cells per square millimeter in a never-before-described fovea-like region of the area centralis a density on par with that of primate foveas.
"There's no real landmark for this area like there is in humans," Aguirre said, "so to discover such a density was unexpected."
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 e
|Contact: Katherine Unger Baillie|
University of Pennsylvania