In the beginning…very soon after conception, a human being is just a ball of cells called a Blastocyst. The cells are all the same, and each carries the potential to become any part of the developing human body – a blood cell, a bone cell, a nerve cell etc. They are called pluripotent stem cells.
The cells begin to differentiate, or develop into the specific type of cells they are going to be. At first the cells branch into three types, like the stem of a tree splitting into three branches. One of these branches is the developmental path for blood, bone, muscle and urogenital cells. On this branch, the cells permanently turn off the DNA needed to make other types of cells.
They are now Multipotent stem cells, because they can still become one of several types of cell, but only ones in their own branch of the tree. These cells could never go back and go down a different branch to become, say a nerve cell or a lung cell.
Embryonic stem cells are taken from inside a blastocyst and are pluripotent. Adult stem cells are multipotent. The word adult, here means from cells that have differentiated. Cells from babies and children (including cord blood cells) as well as cells from older people, are adult stem cells.
What’s the use of stem cells? Stem cells can be made to grow into brand new cells of different types. In theory they can be injected into a person’s body to replace damaged or diseased cells. For example, in the disease Alzheimers, cells in the brain die. Stem cells could grow back the missing parts of the brain. Or if someone has had a heart attack, they could grow new heart muscle using stem cells. There is even potential to re-grow severed nerves in people paralysed by accidents.
What’s the problem then? Embryonic stem cells are the easiest to work with – because they can become any type of cell, and because cells from a single embryo can grow and replicate in a laboratory to produce a lot of cells that last for a long time. The problem is ethical – many people believe that it’s wrong to use embryos in this way. Adult stem cells don’t raise such ethical dilemmas, but they are harder to work with. This is because they are difficult to remove from a person, they don’t survive or grow very well outside of the human body, and they can only grow into a limited number of cell types. One advantage of adult stem cells is that a person could be treated with cells taken from their own body which would get over problems with their body rejecting foreign cells.