Embryonic stem cells are pluripotent, meaning that they have the ability to differentiate, or change, into many directions.
Development starts with a fertilized egg cell, which divides and produces an embryo. Part of the cells in the embryo become the new individual whereas the other cells produce the placenta and other tissues outside of the fetus. The cells in the early embryo, which give rise to the new individual, are called pluripotent embryonic stem cells. These cells can develop into all the approximately 200 different kinds of cell types that are needed in the body.
When these cells differentiate to make specialized cells and tissues, they gradually lose their plasticity, or ability to change into different kinds of cells. A one-week old embryo only contains multipotent stem cells, which can develop only in a certain direction. The tissues are finally made by these kinds of multipotent stem cells. Almost all adult tissues contain multipotent stem cells, which take care of renewing the tissue in different ways depending on the tissue.
The embryonic stem cells, that develop into the new individual, reside in the so-called inner cell mass in a few-days old embryo, called a blastocyst. During the last 25 years or so, researchers have been able to culture embryonic stem cells extracted from embryos, that have been left over from fertility treatments. Embryonic stem cells can produce all the cells of the body.
In the laboratory, it’s theoretically possible to maintain embryonic stem cells indefinitely, and it’s also possible to differentiate them into specialized cells, such as nerve cells or gut cells. This is based on gradually guiding the cells by mimicking the natural events of embryonic development. When the internal programs of the cells are turned on, the result is cells, that closely resemble real functional tissue cells.
A fertilized egg cell is totipotent and can produce both all the cells of the new individual as well as the tissues outside of the embryo, such as the placenta. The cells that reside in the inner cell mass of the early embryo, called the blastocyst, are pluripotent, and can develop into all cell types in the body. During embryonic development, the pluripotent stem cells of the early embryo produce more specialized, multipotent stem cells, which can only make for instance the cells of a specific tissue. Blood stem cells, which are found in the bone marrow, are an example of multipotent stem cells. The plasticity of stem cells, or their ability to change into other cells, declines as the degree of specialization increases. A unipotent stem cell can develop into only one type of cell.
During the last 15 years or so, researchers have been able to reprogram specialized cells back to a pluripotent state. These cells are called induced pluripotent stem cells, or iPS cells.