Stem Cells can be used to treat some human diseases- there are two main sources or stem cells.
They can be collected from an early embryo before the cells differentiate- these embryonic stem cells can form any type of human cell, organ or tissue. There are ethical concerns with this operation.
Stem cells can also be collected from adult bone marrow. It is a simple operation but can be painful. These stem cells mainly differentiate into blood cells, and they have been used for many years to treat leukaemia.