Stem cells are found in a number of areas of the body and occur at the earliest stages of human development through to adulthood. Whether they come from an early embryo, a fetus or an adult, stem cells have two key properties. Firstly, they have the ability to reproduce themselves almost indefinitely through cell division. Secondly, they can be directed to generate cells with special functions that make up the tissues and organs of the body, such as the beating cells of the heart or the insulin-producing cells of the pancreas.
Scientists all over the world are studying stem cells to learn what makes them different from specialised cells. They are also trying to understand how stem cells are able to renew themselves and what triggers them to specialise into other types of cell. As the investigators learn more about stem cells, it may become possible to collect them and direct them to form particular types of cell from which to grow healthy supplies of tissue for treating patients who are ill or injured. It may also be possible to use stem cells for screening new drugs and toxins and understanding birth defects and the progression of certain diseases.
Embryonic stem cells
When a sperm cell fertilises an egg, a one-cell embryo is formed. This very early embryo has the capacity to develop into every cell needed for full human development as well as tissues such as the placenta and umbilical cord.
After about five days of development, the embryo consists of a ball of 50 to 100 cells called a blastocyst. Embryonic stem cells are derived from an inner cell mass within the blastocyst. At this stage the blastocyst is no bigger than a grain of sand and its cells have no fixed destiny. There is no trace of any structure such as a nervous system which would, for example, produce sensations of pain. Embryonic stem cells taken from the blastocyst can be manipulated to generate all the cell types of our bodies although they cannot develop into a complete embryo on their own.
Adult stem cells
Adult (or somatic) stem cells are found, for example, in bone marrow, blood, the eye, the brain and skeletal muscle. Their purpose is to replace and replenish cells that are continually lost by depletion and damage, such as blood cells.
Because they are already partially specialised, adult stem cells do not appear to have the same capabilities as embryonic stem cells. Under laboratory conditions, they have been manipulated to form other cell types. It may be possible, eventually, to direct these cells to function in other areas of the body to replenish damaged or diseased body tissue. However, adult stem cells are rare within the body and it is not known whether they are present in some organs, such as the heart. They are also difficult to extract and grow using the techniques that are currently available.
Bone marrow transplants are a type of adult stem cell therapy. For more than 20 years, patients with blood disorders such as leukaemia have been treated by introducing haematopoietic (blood-forming) stem cells into their bodies through bone marrow transplants. This has been possible because haematopoietic stem cells are readily accessible, unlike many other adult stem cell types found in our bodies, and they are able to replenish blood cells continuously at high rates.
There are advantages and disadvantages of both adult and embryonic stem cells. Embryonic stem cells are able to develop into many more different cell types, it is easier to control their growth and specialisation, and they are more plentiful and easier to isolate than many adult stem cells. On the other hand, a patient’s own adult stem cells or stem cells from a matched donor could be used to treat a patient, minimising the chance of their body rejecting the new cells as it might reject embryonic cells. Scientists therefore continue to research both types of stem cell in the quest for medical advances and new treatments.