Eyes are an unusual body part: they are a part of the nervous system, yet are non-vital, and further are easily accessible. The internals of the eye can be easily inspected without the need for invasive procedures. Eyes are prone to a number of well-studied progressive disorders, such as the varieties of macular degeneration, that seem very amenable to treatment through stem cell therapies.
So given all of this, and especially the point about accessibility, we might expect regeneration in the eye to be a proving ground for a class of techniques and therapies aimed at structures in the nervous system. It should be easier and less costly to make progress in therapies aimed at rebuilding damaged vision than for repair of other portions of the nervous system. Over the years this sort of difference in cost tends to add up to incrementally faster research and development.
A recent publicity release makes this point:
"The eye is a transparent and accessible part of the central nervous system, and that's a big advantage. We can put cells into the eye and monitor them every day with routine non-invasive clinical exams," Tsang says. "And in the event of serious complications, removing the eye is not a life-threatening event."
The work in question involved the creation of induced pluripotent stem cells (iPS cells), which were then used to generate a population of retinal cells, photoreceptors that respond to light by sending signals back to the optic nerve. These newly created photoreceptor cells can then be deposited into a damaged retina to take over the function of local cells that are dead or genetically damaged:
In their study, the researchers injected the iPS-derived retina cells into the right eyes of 34 mice that had a genetic mutation that caused their retina cells to degenerate. In many animals, the human cells assimilated into mouse retina without disruption and functioned as normal retina cells well into the animals' old age. Control mice that got injections of saline or inactive cells showed no improvement in retina tests.
"Our findings provide the first evidence of life-long neuronal recovery in a preclinical model of retinal degeneration, using stem cell transplant, with vision improvement persisting through the lifespan," Tsang says. "And importantly, we saw no tumors in any of the mice, which should allay one of the biggest fears people have about stem cell transplants: that they will generate tumors."
Tsang hopes to begin a clinical trial for macular degeneration patients in the next three years, after more preclinical testing in animal models.
As you can see, matters are progressing rapidly in this field. Ten years ago, people were getting terribly excited about simple advances in transplanting stem cells laboriously purified from donor tissue. Yet now no-one bats an eye when researchers create perfectly matched stem cells as needed from a patient skin sample, manipulate them to create arbitrary types and amounts of cell to order, and then put them to use in ways that may successfully repair a broad range of serious medical conditions.
These are interesting times that we live in. Ten years from now, we can be sure that the presently sophisticated manipulation of stem cells will seem pretty crude and backward in comparison - perhaps because researchers will have figured out how to direct existing stem cell populations in the body to take the required actions without the need for any sort of processing and cell production outside the body.
Still, there is a lot of back-filling that has yet to happen. It remains the case that there are hundreds of varied types of cell in the body, the bulk of which seem to be varieties of nerve cell. The research community is far from possessing the recipes needed to produce each and every type of cell on demand given a pluripotent cell to start with. Developing that set of protocols will be a fairly laborious process, even distributed between the world's laboratories as it is, and that is but one of the tasks which will likely differ greatly for each of the types of cell.