Stem cells are undifferentiated cells: they have not yet become specialised, they can divide many times, and they can develop into different types of specialised cell. Embryonic stem cells come from human embryos and can become almost any type of cell. Adult stem cells, such as those in bone marrow, can only form a limited range of cell types.
Stem cells can be used to replace damaged tissue. Bone marrow stem cells are transplanted to treat blood diseases such as leukaemia. Research aims to grow nerve cells to repair spinal injuries, and insulin-making cells to treat diabetes. The benefit is that diseases that cannot be cured now might be treated, and tissue could be grown without waiting for a donor.
There are risks. Transplanted stem cells can divide out of control and form a tumour, and a virus in the donor cells can infect the patient. Transplanted tissue also carries antigens on its cells. If these are different from the patient's own, the immune system sees them as foreign and attacks, so the tissue is rejected. Rejection is reduced by matching the tissue to the patient, using drugs that suppress the immune system, or using the patient's own stem cells.
There are ethical issues too. Obtaining embryonic stem cells destroys embryos. Some people think this is wrong because an embryo is a potential human life, while others think the benefit to patients is more important.
