Q. Where do stem cells come from?

A. All human beings start their lives from a single cell, called the zygote, which is formed after fertilisation. The zygote divides and forms two cells; each of those cells divides again, and so on. Pretty soon, about five days after conception, there is hollow ball of about 150 cells called the blastocyst. The blastocyst is smaller than a grain of sand and contains two types of cells, the trophoblast and the inner cell mass. Embryonic stem cells are the cells that make up the inner cell mass. As embryonic stem cells can form all cell types in an adult, they are referred to as pluripotent stem cells.

Stem cells can also be found in very small numbers in various tissues in the adult body. For example, bone marrow stem cells are found in the marrow of the bone and they give rise to all specialised blood cell types. Adult stem cells are typically programmed to form different cell types of their own tissue; they are called multipotent stem cells. Adult stem cells have not yet been identified in all vital organs. In some tissues like the brain, although stem cells exist, they are not very active, and thus do not readily respond to cell injury or damage. Scientists are now also exploring ways in which they can induce the stem cells already present to grow and make the right cell types to replace the damaged ones.

Stem cells can also be obtained from sources like the umbilical cord of a newborn baby. This is an accessible source of stem cells, compared to adult tissues like the brain and bone marrow. Although scientists can grow these cells in culture dishes, they can do so only for a limited time. Recently, scientists have also discovered the existence of stem cells in baby teeth and in amniotic fluid-the "water bath" that surrounds an unborn baby-may also have the potential to form multiple cell types. Research on these cells is very promising but at a very early stage. These cells will have to be characterised and studied further.

Q. What are the obstacles that must be overcome before the potential uses of stem cells will be realised?

A. One of the first obstacles that must be overcome is the difficulty in identifying stem cells from adult tissues, which contain numerous mixtures of various cells. The process of identifying and growing the right kind of stem cell, usually a very rare cell in the adult tissue, involves painstaking research.

Second, once stem cells are identified and isolated, the right conditions must be developed to cause these cells to differentiate into the specialised cells. This too will require a great deal of experimentation.

In general, embryonic and foetal stem cells are believed to be more versatile than adult stem cells. However, scientists are still working on developing proper conditions to differentiate embryonic stem cells into specialised cells. As embryonic stem cells grow very fast, scientists must be very careful in fully differentiating them into specialised cells. Otherwise, any remaining embryonic stem cells can grow uncontrolled and form tumours.

Assuming that the above obstacles can be overcome, new issues arise when the specialised cells (grown from stem cells) are implanted into a person. The cells must be integrated into the patient's own tissues and organs and "learn" to function in concert with the body's natural cells. Cardiac cells that beat in a cell culture, for example, may not beat in rhythm with a patient's own heart cells. And neurons injected into a damaged brain must become "wired into" the brain's intricate network of cells and their connections in order to work properly.

Yet another challenge is the phenomenon of tissue rejection. Just as in organ transplants, the body's immune cells will recognise transplanted cells as "foreign", setting off an immune reaction that could cause the transplant to fail and possibly endanger the patient. Cell recipients would have to take drugs to temporarily suppress their immune systems, which in itself could be dangerous.

Thus, research on stem cells and their applications to treat various diseases is still at a preliminary stage. However, results from animal models are very promising and many researchers believe that it is only a matter of time before the same results can be achieved with human stem cells.

Q. What is an embryonic stem cell?

A. Embryonic stem cells are derived from the cells that make up the inner cell mass of the blastocyst. Both mouse and human embryonic stem cell lines exist. Mouse embryonic stem cells are capable of generating any and all cells in the body, under the right conditions. Therefore, they are said to be pluripotent and have unlimited potential as far as growth and differentiation. The cells divide continuously in tissue culture dishes in an incubator, but at the same time maintain the ability to generate any cell type when placed into the correct environment to cause their differentiation.

Human embryonic stem cell lines are currently being studied and several research teams are working to determine whether or not they possess the same properties as mouse embryonic stem cells. Because human embryonic stem cells were isolated relatively recently, and therefore we know less about them, it is currently more difficult to work with human systems than mouse. However, scientists are making remarkable progress that could ultimately lead to therapies to replace or restore damaged tissues using these human cells.