Blue and green orb in lower right corner, orange threads protrude from it

One hurdle to treating neurodegenerative diseases is the inability of neurons in the central nervous system to regenerate axons after damage. In glaucoma, retinal ganglion cell (RGC) axons, which make up the optic nerve and serve as cables to pass information from our eyes to our brains, are damaged and thus unable to regenerate. Shown here is a segment of a mouse retina that is sprouting new putative axons, termed neurites, from the RGCs after being cultured in a mixture of growth factors and hormones for one week. This segment is labeled with three different fluorescent probes. The new neurites are labeled with an antibody to a component of the RGC cytoskeleton (red), as well as a marker for cell nuclei (blue) and an antibody to a component of the astroglial cells, or nervous system cells, supporting the RGCs (green). The image was taken on a Ziess Axioplan 2 fluorescent microscope.

Courtesy Kimberly Toops, Ph.D. candidate, UW-Madison department of biomolecular chemistry

“Dogs have evolved to see well in both bright and dim light, whereas humans do best in bright light. No one is quite sure how much better a dog sees in dim light, but I would suspect that dogs are not quite as good as cats,” which can see in light that’s six times dimmer than our lower limit. Dogs, he says, “can probably see in light five times dimmer than a human can see in.”

Dogs have many adaptations for low-light vision, Miller says. A larger pupil lets in more light. The center of the retina has more of the light-sensitive cells (rods), which work better in dim light than the color-detecting cones. The light-sensitive compounds in the retina respond to lower light levels. And the lens is located closer to the retina, making the image on the retina brighter.

But the canine’s biggest advantage is called the tapetum. This mirror-like structure in the back of the eye reflects light, giving the retina a second chance to register light that has entered the eye. “Although the tapetum improves vision in dim light, it also scatters some light, degrading the dog’s vision from the 20:20 that you and I normally see to about 20:80,” Miller says.

The tapetum also causes dog eyes to glow at night.