Several eye diseases are considered excellent candidates for stem cell therapy. In particular, widespread ocular diseases such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP), as well as a genetic condition called Stargardt’s macular dystrophy (STGD) that afflicts young people, are all potential disease targets for therapeutic approaches.
The eye is an ideal system to test cellular therapies. The human eye is an immune-privileged site, and therefore less likely to reject transplanted cells than other areas of the body. It is also a relatively self-contained area, with barriers that keep cells from migrating to other parts of the body. The nature of the eye and the imaging tools used by ophthalmologists to examine it allow clinicians to watch what is happening in real time as they put the cells in, seeing how the cells interact within the eye.
Research has already led to successful stem cell therapy in the eye. Injuries or diseases affecting the cornea, the outer part of the eye, have been successfully treated by transplanting stem cells in the supportive tissue called the corneal limbus. Many patients have recovered clarity of vision in the cornea and the restoration of other vision benefits that have lasted more than ten years. However, treatments for diseases of the retina, which is in the back of the eye, are not as far along, with interventions just now being tested.
The retina is a layer of tissue in the back of the eye that senses light and sends images to the brain. The macula lies at the center of this nerve tissue and is responsible for central vision—it is essential for tasks such as reading, driving, and facial recognition. The photoreceptor cells in the macula (called “cones” and “rods”) react to light and send signals to the optic nerve and the brain. Supporting these cells is a layer of cells called the retinal pigment epithelium, or RPE, tissue that plays a critical supporting role in keeping the retina functioning well, including helping it get nutrients and balance fluids in the eye, facilitating the blood retina barrier, helping control bacteria in the eye, and even playing a role in immunity protection. Over the years, stem cell researchers have learned a lot about retinal cells and their roles in eye function.
While the RPE layer of tissue is a relatively simple structure, it is vitally important to the eye; if it dies, the retina dies, and vision is lost.
The retinal diseases mentioned above, AMD, RP, and STGD, result in the gradual deterioration of photoreceptors in the macula. As the rods and cones die off, eyesight deteriorates, eventually leading to blindness. There are currently no cures for these diseases.
Over the last decade, stem cell researchers have been studying ways to replace the RPE layer of the eye. Approaches using embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), have shown some success in generating RPE-like cells (read more about stem cell types here). These stem cell-derived RPE cells are good candidates for transplantation into diseased retinae for potential AMD, RP, and STGD treatments, and research and clinical trials are underway using these approaches.
In addition, researchers are finding that adult RPE cells have retained the potential to divide. By exploiting this potential in the laboratory, scientists may be able to successfully make the hundreds and thousands of RPE cells necessary to develop treatments for many retinal diseases.
Although clinical trials are ongoing, research into stem cell therapy for retinal disease continues. Scientists are analyzing the development of the retina in different species in hopes of gaining insight into how the retina forms and whether this knowledge can be applied therapeutically. A recent study in Stem Cell Reports focusing on the regenerative properties of glial cells in the retina of zebrafish will add to the body of research on how cells function within the retina.
If RPE replacement works, it may help pave the way for replacement of other retinal cells, and other central nervous system cells.
Learn more about research into Macular Degeneration at A Closer Look at Stem Cells.
Research timeline and additional resources: