Stem cells are defined as undifferentiated biological cells that can differentiate into specialized cells and can divide (through mitosis) to produce more stem cells. They are found in multicellular organisms.
In mammals, there are two broad types of stem cells:
Because a person's own (autologous) cord blood stem cells can be safely infused back into that individual without being rejected by the body's immune system, and because they have unique characteristics compared to other sources of stem cells, they are an increasing focus of regenerative medicine research.
"A number of studies have reported that both embryonic stem cells and iPS cells may be turned into different types of retinal cells in the laboratory."
Sight is perhaps one of a person's most important senses. People rely on vision to navigate through their surroundings. A loss of vision may have a large impact on a person's life, yet a number of the disorders that cause blindness are hard or impossible to treat. Researchers are now using stem cell technology to explore potential new approaches to treatment for vision loss.
The Human Eye
The eye is the organ responsible for a person's ability to see the world around them. It is able to detect light from the surrounding environment and transfer information about what it has detected to a person's brain. The eye is a very complex organ made up of multiple specialized components. The components, or tissues, are made up of a number of types of cells, each with a specific job to perform to enable the tissues to perform specialized roles.
The main parts of the human eye include the following.
Lens: The lens performs just like the lens in a camera by focusing light that enters the eye.
Cornea: The cornea is the transparent, 'window,' on the front of the eye that allows light to enter.
Retinal Pigment Epithelium: The retinal pigment epithelium is a sheet of black cells positioned beneath the retina. The sheet supports the retina and has several important roles, to include processing nutrients.
Retina: The retina is the most complex component of the eye and is comprised of a number of different types of cells with specific roles. The cells include photoreceptor cells which detect light entering the eye and produce an electrical signal.
Optic Nerve: The optic nerve is a biological wire that connects a person's eye to their brain. It is responsible for transferring the electrical signal produced in the retina to the person's brain. The person's brain then interprets the signal to provide the person with an image of their environment. The optic nerve is closely associated with the retina.
Diseases or disorders of the eye happen when one or more of the components above is damaged and/or stops working as they should. Different disorders develop depending on which components are not working. The difficulty in treating these issues is that new biological components for the eye are not easy to come by. Stem cell technology may be of use and provide a way to replace damaged cells in a person's eye. There are a number of types of stem cells that could be used in various ways, depending on the particular disorder the person experiences.
Limbal Stem Cells and Cornea Repair
Cells that make up the cornea are constantly being damaged by blinking and exposure to the world outside. To repair the damage, people have a small number of stem cells at the edge of their corneas known as, 'limbal,' stem cells. These cells are responsible for making new corneal cells to replace ones that are damaged. If these stem cells are lost because of disease or injury, the cornea can no longer be repaired. The loss of these cells affects the ability of light to enter the person's eye, resulting in a notable loss of vision.
Transplantation of limbal stem cells from a healthy eye might repair the cornea and give the person their sight back. The procedure; however, presents some risks to both the healthy donor eye and to the person receiving the transplant, as well as other issues which mean the approach is not ideal. If a person; for example, has damage to both of their eyes, it might not be possible to obtain any limbal stem cells. Cells from a donor may be used, but donors are in short supply. Success rates are lower and donor cells are usually only effective in the short and medium terms.
At the moment, this is the only available stem cell treatment in the eye that has been proven to work. It is not yet available widely. Additional clinical studies with larger numbers of people needs to be carried out before this type of therapy can be approved by regulatory authorities for greater use.
Recent research has led to improvements in methods for growing limbal stem cells in the laboratory and to improve transplantation techniques. There are still limitations; however, in the amount of new cells that may be obtained from a sample taken from the eye. Researchers are investigating the potential of using a different approach, starting from embryonic stem cells or, 'induced pluripotent stem cells (iPS) to make new limbal stem cells in the laboratory. The approach would remove the need for complex surgery for donors, as well as provide a theoretically endless source of large quantities of limbal stem cells for people who need new ones. It is hoped this approach will be available for people in the future.
Replacement of Retinal Pigment Epithelial Cells
Retinal pigment epithelial (RPE) cells have several important jobs to perform, to include looking after the adjacent retina. If the cells stop working appropriately due to disease or injury, certain portions of the retina perish. As the retinal is the component of the eye responsible for detecting light, this leads to the onset of blindness. RPE cells may be damaged in a number of diseases such as retinitis pigmentosa, age-related macular degeneration (AMD) and Leber's congenital aneurosis.
One means of treating these diseases would be to replace the damaged RPE cells with transplanted healthy ones. At this time it is not possible to take healthy RPE cells from donors, so it is necessary to find another source of cells for transplantation. Scientists have produced new RPE cells from both iPS cells and embryonic stem cells in the laboratory. The safety of embryonic stem cell-derived RPE cells is being tested at this time in clinical trials for people with Stargardt's macular dystrophy and it is hoped that a similar trial will follow for people who experience age-related macular degeneration (AMD).
Replacement of damaged RPE cells will only be effective in people who still have at least a portion of working retina and therefore, some level of vision. The reason why is because the RPE cells are not themselves responsible for seeing, but are actually responsible for supporting the seeing retina. Sight is lost in these types of diseases when the retina starts to degenerate because the RPE cells are not working appropriately. So the RPE cells need to be replaced in time for them to support a retina that is still working. It is hoped that transplantation of new RPE cells will permanently stop additional vision loss and in some instances, might even improve a person's vision to some degree.
Replacement of Retinal Cells
In a number of instances where vision is lost, it is found the problem lies with malfunctioning retinal circuitry. Different disorders happen when particular, specialized cells in the circuit either stop working appropriately or perish. Despite the retina being more complicated than other components of the eye, it is hoped that if a new source of retinal cells can be found, it might be possible to replace the damaged or dying cells and repair the retina. The approach might also help to repair damage caused to the optic nerve.
Scientists have turned to stem cell technology to provide a source of replacement cells. A number of studies have reported that both embryonic stem cells and iPS cells may be turned into different types of retinal cells in the laboratory. Within the eye, a type of cell called the Muller cell is found in the retina and is known to act as a stem cell in some species such as zebra fish. It has been suggested that this type of cell might also have the ability to act as a stem cell in human beings, in which case it may provide another source of retinal cells for the repair of the retina.
Unlike RPE cell transplantation, direct repair of the retina might allow people who have already lost their vision to have it restored to a certain level. What this does is give hope to people with disorders such as late-stage age-related macular degeneration, where the light-sensitive photoreceptor cells in the retina have already been lost. Research might also provide new treatments for those who experience retinal diseases like retinitis pigmentosa and glaucoma. Despite encouraging evidence; however, such research is still in its infancy. There are currently no patient clinical trials planned using this type of approach, significant research is still needed beforehand.
Stem cell technology has great potential for improving the lives of people who experience visual disorders. Several studies are currently being pursued in order to develop new therapies to treat and/or prevent vision loss. Central to the research is the development of our understanding of how different types of stem cells behave and how to best harness their potential in the eye. A specific approach is needed, dependent on the particular issue a person is experiencing. Stem cells are not an all-in-one cure, yet they do hold exciting potential for the production of new biological components that may be used to repair a person's eye.
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