Stem cells are undifferentiated cells. There are two types of stem cells:
Each of us began our lives when our father's sperm fertilised our mother's egg. At this point we were one embryonic stem cell. At this stage this cell could turn into any of the two hundred or so cells types that make up a human. For this reason, embryonic stem cells are essential for new life, but also extremely valuable in medical research.
Embryonic stem cells can divide to produce new cells, which can then divide into different cell types. They therefore have the potential to be transplanted into patients to treat medical conditions and disease. They could be used to replace cells that have been damaged or destroyed, eg:
Embryonic stem cells can differentiate into a wider range of cell types, but are difficult to obtain. The best source is the five-day-old embryo. These are often left over embryos from in vitro fertilisation. This is an ethical issue which people may have strong feelings or opinions on due to religious or moral reasons.
Adult stem cells are present in developed organisms which no longer have embryonic stem cells. They are found in bone marrow, brain, muscle, skin and heart. However, they are not as useful for medical research as they will only differentiate into a narrower range of cell types. Bone marrow transplants are an example of adult stem cell transplant. Bone marrow cells will differentiate into different types of blood cell, but not any other type of cell. Bone marrow transplants are carried out: