Stem cells are a very special type of cell; they’re like a blank piece of paper. A normal cell, say a liver cell, will stay as a liver cell wherever you put it; if you put it with a heart cell for example, it would keep being a liver cell. Take a stem cell however, and put that with a heart cell—or a liver cell, a kidney cell, a skin cell, or more-or-less any other kind of cell you can think of—it will become that kind of cell.
During pregnancy, the foetus will have a lot of stem cells. This is because the baby is growing, and building its body parts, and all body parts are made up of cells; arms, legs, ears, eyes, hearts, kidneys, etc. The cells which make up these parts all start out as stem cells. As we grow, we have fewer stem cells; this is because we have less need to grow new body parts. Some scientists think that stem cells are what starfish use to grow new limbs if one is severed.
Human embryonic stem cells, and below, neurons produced from stem cells. (Image: Wikimedia)
There are two main types of stem cells:
- Embryonic stem cells are found in the embryos which go on to form the foetus, and eventually the baby.
- Adult stem cells are found in much lower numbers than embryonic stem cells, but are found in a number of tissues, including umbilical chord blood. Bone marrow is also rich in this type of stem cell, which is why bone marrow transplants are often used to treat conditions such as leukemia, and spinal chord injuries.
Most research done using stem cells takes them from either mice or humans. When a woman is struggling to have a baby, she might undergo a treatment where embryos are taken out, and fertilised outside the body, before being put back in for the foetus to grow. Once this treatment is over, and the woman has had her baby, there are often embryos left over which may be used to obtain stem cells. The removal of the stem cells however, destroys the embryo. Some people see the unfertilised embryo as a potential human life, and so say this procedure is similar to an abortion, which is from where some controversy around stem cell research comes.