Stem Cells Cure Blindness
Earlier this November, scientists from the University College London Institutes of Ophthalmology and Child Health and Moorfields Eye Hospital were able to restore vision to blind lab mice. This scientific breakthrough signifies that millions of people with optical conditions such as macular degeneration (loss of sight experienced by the elderly), diabetic retinopathy, and a variety of other forms of blindness could be able to regain sight through a remarkably simple procedure. However, the fact that the procedure requires stem cells from foetuses—currently viewed as a highly controversial method by many politicians—has prevented this procedure from becoming more publicized in the U.S. (1).
Researchers have identified certain cells on the margin of adult retinas that are similar to stem cells. Additionally, retinal cell replacement may be the most effective method of "cell transplant therapy because photoreceptor loss initially leaves the rest of the wiring to the brain intact” (1). In other words, major surgical reconstruction is not necessary. Any surgical procedure would only involve the superficial layer of the retina and not the particularly sensitive optic nerve wiring at the back of the eye. However, in order to attain human retinal cells at the necessary stage of development, stem cells would need to be extracted from a foetus during the second trimester of pregnancy (1). Because stem cells are able to proliferate and develop into many other types of cells within the human body, they can be extracted from any part of the foetus. However, the timing is imperative if the procedure is to work.
Three Blind Mice… See How the Procedure Works...
1. Early stage retinal stem cells were extracted from a 3 to 5 day old newborn mouse (1).
2. The retinal cells were transplanted onto the retinal surface of a blind mouse whose condition was genetically programmed to resemble the gradual loss of sight characteristic to the human disease retinitis pigmentosa or age-related macular degeneration (1).
3. The cells embed themselves and connected with other cells on the retina of blind mouse. Within 30 minutes the photoreceptors from the retinal stem cells implanted themselves and fused electrical connections with the animals' existing retinal nerve cells (3). As a result, the formerly blind mice's pupils began to respond to light and there was activity in the optic nerve (indicating that the eye was transmitting signals to the brain) (1).
Anatomy & Physiology of the Eye: Photoreceptors
The retina (around 0.5 mm thick) lines the back of the eye. It is lined with a network vascular blood vessels and neurons that gradually channel towards the optic nerve which contains the ganglion cell axons that connect the ganglion cells to the brain. The ganglion cells—the neurons of the retina that transmit images to the brain—are located in the innermost region of the retina and extend toward the lens, or anterior portion of the eye (2). The photoreceptors—the rod and cone shaped cells—are situated toward the outermost portion of the retina and are closer to the back of the eye (5). As a result, light must penetrate the nerve cells within the retina before reaching and activating the rods and cones. Once reached, the rods and cones absorb photons through their visual pigments and translate the photons into a biochemical message and then into an electrical message that stimulates all of the succeeding neurons of the retina. Consequently, “the retinal message concerning the photic input and some preliminary organization of the visual image into several forms of sensation are transmitted to the brain [by] the spiking discharge pattern of the ganglion cells” (2). From then on the brain is responsible for identifying, processing and interpreting the visual image (2).
Candidates for retinal cell replacement surgery must have some retinal cone and rod photoreceptors intact (1) . The retinal cell replacement surgery primarily serves to repair the nerve synapses in the retina, the macula lutea, and the fovea. The surgery cannot generate new photoreceptors.
The surgery mainly repairs the macula and the fovea. The macula functions as a short wavelength filter while the fovea, characterized by a dark circular area towards the back of the eye, is considered to be the most vital portion of the retina. Like the lens, it functions as “a protective mechanisms for avoiding bright light and especially ultraviolet irradiation damage” (2). The fovea is entirely composed of a mosaic cone photoreceptors that are arranged in a hexagonal structure. Outside of the foveal pit, the density of cone photoreceptors becomes increasingly more balanced with that of rod photoreceptors. There is a peak in the density of rod photoreceptors at about 4.5mm (or 18 degrees) from the foveal pit where the rod photoreceptors arrange themselves in a ring around the fovea (5). (Naturally, the optic nerve (the blindspot) is entirely free of photoreceptors) (5). If the macula or fovea cones are damaged (as happens gradually over many years), instant blindness results (2). However, macular degeneration could be easily remedied since stem cells take only about half an hour to develop into photoreceptors.
Repairing the Cornea
In August of 2003, Mike May, a Californian man who had been left blind for 40 years as the result of an accident that happened when he was three years old had his vision restored. Though the vision in his left eye was permanently lost, he could still sense light with his right eye. Researchers implanted corneal and limbal stem cells into his right eye. Five months after the surgery, May was able to sense movements and recognize simple shapes. After two years, he was able to see forms, color, and motion nearly accurately. His 3D perception and face and object recognition remained impaired, though his ability to sense motion was the best restored visual faculty (4).
Like photoreceptors, the cornea is responsible for channelling light through the eye's surface. The corneal surface refracts to provide 2/3 of the eye's focusing power. he corneal surface is entirely transparent and not lined with blood vessels, so the uniformity of cells may contribute to its ability to regenerate more rapidly than other cells in the human body. On the other hand, it is extremely sensitive. There are more nerve endings on the cornea than anywhere else on the human body (6). The cells that compose the layers of the cornea are found to regenerate at a rapid pace, though less rapid than photoreceptor cells. Again, a simple surgical procedure, most of which is processed by human mechanisms, could restore sight to millions of people if only the procedure were to be legalized.
courtesy of U.S. National Library of Medicine
(1) "Cell transplants 'restore sight.'" BBC International News Online. (http://news.bbc.co.uk/2/hi/health/6120664.stm)
(2) Simple Anatomy of the Retina. (http://webvision.med.utah.edu/sretina.html)
(3) "Cell Transplants Restore Vision in Mice." Live Science. (http://www.livescience.com/healthday/535968.html)
(4) "Cell Transplant Restores Vision." BBC International News Online. (http://news.bbc.co.uk/2/hi/health/3171993.stm)
(5) Photoreceptors. (http://webvision.med.utah.edu/photo1.html)
(6) "Cornea." Eye Anatomy. (http://www.stlukeseye.com/anatomy/Cornea.asp)