Serving as a repair system for the body, there are two main types: embryonic stem cells and adult stem cells.
Stem-cell treatments are procedures that introduce new cells into damaged tissue in order to treat a different disease or injury.
Many health experts believe they have the potential to drastically change medical science and provide benefits to patients with life-threatening or severely debilitating illnesses.
A number of stem-cell therapies already exist but most are still being trialled at centres around the world.
Experts believe adult and embryonic stem cells will one day be able to treat a range of illnesses, such as cancer, Parkinson’s disease, multiple sclerosis, cardiac failure and neurological disorders.
In many tissues within the human body they serve as a sort of internal repair system as they replenish cells as long as that person is alive.
When a stem cell divides, each new cell has the potential either to remain a stem cell or become another type of cell with a more specialised function, such as a muscle cell, a red blood cell or even a brain cell.
Stem cells are distinguished from other cell types by two important characteristics.
Firstly, they are unspecialised cells capable of renewing themselves through cell division, sometimes after long periods of inactivity. Secondly, they can be induced to become tissue or organ-specific cells with special functions.
In some parts of the body, such as in the gut and bone marrow, stem cells regularly divide to repair and replace worn out or damaged tissues. In other organs, however, such as the pancreas and the heart, stem cells only divide under special conditions.
Scientists discovered ways to derive embryonic stem cells from early mouse embryos 30 years ago.
Dolly the sheep later became the first mammal cloned from an adult cell, by experts at Edinburgh University’s Roslin Institute.