Salamanders don't get glaucoma because they can readily regenerate retinal cells. The same is true of newts, frogs, and some types of fish. "We're trying to understand the remarkable regenerative powers of these lower vertebrates, and through this understanding, develop strategies to stimulate regeneration in the human retina," Reh said.
While salamanders can regenerate retinal cells through their life, many other species lose this ability as they age. "At some point in each species life cycle, the stem cells in the retina make a transition from a regenerative cell to a cell that will make a scar in response to injury, like the cells that cause scars in the spinal cord," Reh said. "Chickens make the transition a few weeks after hatching in most of their retina, though they retain some limited capacity to regenerate retinal cells throughout life. In rats, it's only a matter of a few days after the cells are generated that they lose their ability to regenerate other retinal cells."
Human retinas seemingly can't repair themselves, yet in recent studies human retinal cells have grown new neurons when cultured in the laboratory. "The hope is that many of the molecular and cellular mechanisms necessary for regeneration, that serve amphibians so well, are still in place in humans," Reh said. "Future studies from the nervous system, as well as other organ systems, should enable us to define the roadblocks in the regenerative process, and develop strategies to go around them."
Behold the power of Salamanders!
[LOL!]
As to why fish and amphibians can regenerate retinal cells, but people and rats can't, and chickens have only limited abilities to do so, my hypothesis is based on two suppositions: first, endothermic vertebrates (birds and mammals) either semi- or fully lost their regenerative abilities when they gained the advantage of warm-blooddedness (the same reason why we can't regenerate limbs the way reptiles and amphibians can). Secondly, mammals lost their ability to see in color because as nocturnal animals, it wasn't necessary; whereas birds, being primarily diurnal animals, kept color vision (primates, including mammals, regained their color cones when they returned to an active day life). Similarly, because mammal eyes didn't need the continual protection from intense visible and UV light, they may have completely lost the capacity for retinal regeneration.