The early human embryo contains many
stem cells. Stem cells are undifferentiated themselves, but have
the potential to develop into many different types of specialised
cells. The very earliest cells in an embryo are known as totipotent – they
have the potential to develop into any one of the different types
of cells needed to make a complete human being.
An early human embryo |
By the time the embryo is four or five days old and has formed a
hollow ball of cells (blastocyst) the stem cells are slightly
less flexible. They have become pluripotent – they
can form almost all of the cell types needed in future, but not tissue
such as the placenta.
Thompson and Gearhart (see what
is stem cell research about?) realised that embryonic stem cells
have the potential to be harvested and cultured in the laboratory
to produce huge numbers of undifferentiated cells. Once they had found
the conditions which made it possible for embryonic stem cells to
survive, they managed to do just that, persuading the cells to reproduce
successfully in cultures in the laboratory, without mutating or changing,
for up to eight months. This time is likely to get longer and longer.
This may seem a strange idea, but long lived cell cultures are widely
used by scientists – HeLa cells are commonly cultured in laboratories
all over the world and they all come originally from cancer cells
from the cervix of a woman who died around fifty years ago!
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The next big step is to turn undifferentiated
stem cells into useful cells, tissues or organs needed in
the body of a patient. By changing the culture conditions
of the embryonic stem cells, scientists have persuaded them
to differentiate into different types of adult cells, forming
clusters of cartilage, bone, nerve and intestinal cells. So
far no-one knows how to control exactly which adult cells
develop – but that will surely come.
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There is hope that developments of this technique will make it
possible to grow nerve cells, heart cells, brain cells to repair
damaged tissue in Alzheimer’s and Parkinson’s disease,
islet cells to produce insulin – the possibilities are almost
endless. Because embryonic cells do not trigger the normal immune
response in their mother, transplants of tissues from embryonic
stem cells may not cause the rejection which has previously beset
tissue and organ transplantations. In the future it may be possible
to collect stem cells for each of us during embryonic development
or from the umbilical cord at birth, stem cells which could then
be stored and saved until we needed them in later life.
Another, more immediate, benefit is that scientists can use cells
and tissues grown from embryonic stem cells to carry out tests on
new medicines. These tests could potentially replace some of the
animal tests which are currently required when new medicines are
being developedfor human treatment.
However there are some difficulties – scientists don’t
know how to control fully the development of embryonic stem cells
into all the tissues they want whilst avoiding uncontrolled growth.
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