Corneal Disorders Treated with Limbal Stem Cells
By Dr. Mohan Rajan, The Hindu, December 3, 2006
During the course of a personís life, there is a certain part of the individualís eye that continually renews itself. Under normal conditions, half the cells are replaced every two months. At unbelievable speeds of 60 to 80 mm per hour, the cells replicate and move across the surface of the eye. The cells are critical for the normal function of the cornea, vision, and comfort. These special cells are part of the corneal epithelium; the outermost layer, or "skin" of your cornea, which is composed of five to six layers of specialized cells.
Stem cells are a unique sub-population of cells. Corneal stem cells specifically, are located in the extreme periphery of the cornea. These cells are also called the limbal stem cells, since this region is called the limbus.
Stem cells are undifferentiated cells and they constitute only 0.5 to 10 per cent of total cell population. They are unique in that they have the capacity for constant renewal and are capable of self-replication throughout life.
The way to limbal stem cell transplantation is paved by the recognition of the importance of limbal stem cell function in the healing response of the cornea.
These important cells of the eye can be damaged by a multitude of diseases. Other causes include drug allergies, poorly fitted contact lenses, chemical or thermal injury, multiple surgeries, and some genetic disorders.
Using cells from the fellow eye of the same person (autograft) or from another individual (allograft), the procedure of limbal transplantation can be performed. These limbal cells can be obtained from an eye that has been donated after death as well. However, powerful immunosuppressive medication would need to be administered during the post operative period if the cells are harvested from another human being. This is to avoid immune rejection so that the limbal stem cells can survive in the patientís eye.
A method using a very small two to four mm biopsy of healthy limbus is a recent development. The cells are encouraged to multiply and produce a large sheet about two cm in size while they are processed in a laboratory using special culture media. The damaged eye is then given a transplant with this newly grown tissue. The success rate to date has been good. The procedure is called ex vivo limbal stem cell replication.
Pterygium, which is the growth of conjunctive tissue onto the cornea, is another common condition in India given the high ultraviolet light exposure. This condition results in impairment of vision and poor cosmetic function. Limbal stem cells transplantation prevents the re-growth of the tissue and also provides for the localized stem cell deficiency.
Covering the innermost layer of the placenta is the amniotic membrane. Given its antimicrobial potential and ability to enhance wound healing, the amniotic membrane has been important in ocular reconstructive surgery. The membrane is stored after it is harvested from a willing donor during elective caesarian section birth. It is most commonly used in the cases of limbal stem cell deficiency and chemical injury.
Hematopoietic stem cells are part of more general stem cell transplantations. Specific conditions such as Lymphoma, Sickle Cell Anemia, Aplastic Anemia, Leukemia, and severe Immune Cell Deficiency.
Type I diabetes mellitus (pancreatic stem cells) and Parkinsonís disease (brain stem cells) are conditions that the future of stem cells research will focus on.
In conclusion, the transplantation of corneal limbal stem cells is an exciting advance in adult stem cell treatment. Corneal disorders have been poorly managed to date, and these new advances offers new hope to many patients who are visually challenged due to these disorders.