What are stem cells?
Your body contains ‘adult’ stem cells. Your bone marrow, for example, is teeming with stem cells, which turn into blood, cartilage and new bone as old cells die or are damaged. Embryonic stem cells found in four- to five-day-old human embryos are even more flexible, possessing the potential to turn into any kind of cell. Using fertilised embryos to extract embryonic stem cells is controversial. But in 2007 scientists found a way of reverting adult stem cells to become very similar to these prized embryonic cells. In future this may become a more accessible source of stem cells.