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HEALTHWATCH

An eye on stem cells

DR. MOHAN RAJAN, DR. SUJATHA MOHAN, DR. SRINIVAS K. RAO, DR. S. RADHIKA

Transplant of limbal stem cells offers hope to patients with corneal disorders.

Photo: K. Gopinathan

Unique capacity: A part of the eye constantly renews itself.

CAN you imagine a part of your eye that constantly renews itself for the whole of your life? Half the cells are replaced every two months under normal conditions. The cells replicate fast and move across the surface at an incredible speed of 60 to 80 mm per hour. These are the properties of the Corneal epithelium — the outermost layer, or "skin" of your cornea, composed of five to six layers of specialised cells that are critical for the normal functions of the cornea — vision and comfort.

STEM cells are a unique sub-population of cells — termed the corneal stem cells, located in the extreme periphery of the cornea. Since this region is termed the limbus, these cells are also called the Limbal stem cells

Constant renewal

Stem cells are unique in that they have the capacity for constant renewal and are capable of self-replication throughout life, they are undifferentiated cells and they constitute only 0.5 to 10 per cent of total cell population. The recognition of the importance of limbal stem cell function in the healing response of the cornea has paved the way for a newer treatment modality called Limbal stem cell transplantation.

Many diseases can result in damage to these important cells of the eye. Other causes include chemical or thermal injury, drug allergies, poorly fitted contact lenses, multiple surgeries, and some genetic disorders.

Limbal transplantation can be done either from the fellow eye of the same person (autograft) or from a cadaver (allograft). These limbal cells can be obtained from an eye that has been donated after death. However, the use of tissue from another human being will require use of powerful medicines called Immunosuppressives in the postoperative period. This is necessary to allow the donated limbal stem cells to survive in the donor's eye.

A recent development is a method that uses a very small two to four mm biopsy of the healthy limbus. The excised limbal tissue is then processed in the laboratory and using special culture media, the cells are encouraged to multiply and produce a large sheet about two cm in size. This is then transplanted onto the damaged eye — This has a good success rate. This procedure has been termed ex vivo Limbal stem cell replication.

Another common condition in our country — where ultraviolet light exposure is common — is the Pterygium, a growth of conjunctival tissue onto the cornea. This can result in poor cosmetic function and impairment of vision. This is another indication where the procedure of Limbal stem cell transplantation provides for the localised stem cell deficiency and prevents recurrence or regrowth of the tissue.

Harvesting membrane

The Amniotic membrane, or Amnion, comprises the innermost layer of the placenta. Its use in ocular reconstructive surgery has become important because of its ability to enhance wound healing and its antimicrobial potential in addition. The membrane is harvested during an elective Caesarian section and processed in a sterile manner and stored to be used at the time of transplantation. Common uses are in Chemical Injury; Limbal Stem cell deficiency

The more general usage of stem cell transplantation lies in Haematopoietic stem cells. They are used in certain specific conditions like Aplastic Anaemia, Leukemia, Lymphoma, Sickle Cell Anaemia and severe Immune Cell Deficiency.

The future research of stem cells is in exploring treatment options for Type I Diabetes mellitus (Pancreatic stem cells) and Parkinson's disease (Brain Stem cells).

In conclusion, the concepts of the corneal limbal stem cells and their transplantation are an exciting new advance in the understanding and management of a variety of hitherto poorly managed corneal disorders and offers new hope to many patients who are visually challenged due to these disorders.

E-mail: rajaneye@md2.vsnl.net.in

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