After fertilisation, cell division
takes place and a hollow ball of
cells called an embryo is formed.
The cells on the inside of
an embryo are called
embryonic stem cells.
They are unspecialised (not designed
for a particular job). As the embryo
develops the cells begin to differentiate
and change into different types of cell.
As the cells mature, they can no
longer differentiate but some of our
stem cells remain into adulthood.
For example, there are adult
stem cells in our bone marrow
which give rise to new blood cells.
Small numbers of stem
cells remain in other
body tissues as well.
If stem cells can be made to
multiply and differentiate, we
would have an unlimited
supply of different types of
cells, which could be
transplanted into people
whose tissues are damaged.