By Norbert Herzog and David Niesel
Blind mice don’t only exist in nursery rhymes, but in scientific laboratories. Scientists have been able to restore the vision of some such mice that were impaired by the same family of eye diseases that cause blindness in millions of Americans.
The landmark study successfully transplanted immature photoreceptor cells, which are nerve cells in the retina responsible for sight.
Photoreceptor cells gradually die off in people with certain degenerative eye diseases. Once enough of the cells are gone, a person goes blind and no treatment can reverse the damage.
Members of the research team from University College London Institute of Ophthalmology proved they can reverse the damage in adult mice with nonfunctioning rods. Rods and cones are the two types of photoreceptor nerve cells that make up the retina. These nerve cells convert light energy into signals that travel the optic nerve to the brain. Cones detect such things as color and detail and provide the center field of vision. Rods detect black and white and enable peripheral and low-light vision.
In the study, researchers took mice without functioning rods and injected their retinas with immature rod receptor cells from young, healthy mice. In four to six weeks, one in six of the mice with transplanted rod precursor cells seemed to be functioning nearly as well as mice with normal rod photoreceptors. These transplanted cells formed nerve connections resembling normal rod cells and generated signals that were transmitted to the brain for visual processing. This is the first time that transplanted photoreceptor cells have been shown to integrate into the circuitry of the retina and improve vision.
To test their results, researchers placed treated mice and diseased mice into a dimly lit water maze. Recall the diseased mice do not have functioning rods, which means they can’t see in low light. If the transplant worked, the mice with new rods would be able to spot a visual cue for a hidden platform and get out of the water. The mice with implants had no difficulty finding the platform and climbing out. The untreated, diseased mice took much longer and did a thorough search of the maze before finding the platform.
Researchers say the study’s success was due to the large number of photoreceptor cells they implanted — 200,000 compared to less than 1,000 cells in previous studies. Next, researchers plan to test whether the procedure is as successful in transplanting precursor cone cells in mice.
A similar but separate study in humans is already undergoing clinical trials involving 12 patients with Stargardt disease, the most common form of inherited juvenile macular degeneration. By age 50, half are legally blind. The participants have been injected with 50,000 to 200,000 embryonic stem cells. The aim of these early clinical trials is to determine if the implant of embryonic stem cells is safe and well tolerated. So far, the trials are promising.
Professors Norbert Herzog and David Niesel are biomedical scientists at UTMB. Learn more at www.medicaldiscoverynews.com.