Stem cells build tissue when and where it's needed.
Without stem cells, wounds would never heal, your skin and blood could not continually renew themselves, fertilized eggs would not grow into babies, and babies would not grow into adults. Stem cells are quite unlike the specialized, or differentiated, cells in your body — such as the nerve cells, muscle cells and blood cells that enable you to function. In contrast, they are the body's silent reserves. At any given moment, many of the stem cells in your body won't be doing very much. They will only spring into action when you need either to produce more stem cells or make more of other, specialized types of cells. And they're not just found in people. All multicellular organisms, from plants to humans, need stem cells.
Usually, when a stem cell divides into two, one daughter cell goes on to make a more specialized type of cell, or even gives rise to several different cell types. The other daughter cell remains a stem cell, ready to produce more stem cells when they are needed. Only stem cells have this versatility, although some fully specialized cells, such as liver cells, can divide to give more cells exactly like themselves.
A fertilized egg is the ultimate stem cell, as it is the source of every type of cell in the body, from oxygen-carrying red blood cells to electricity-conducting nerve cells and throbbing heart muscle cells. But of course this doesn't happen all at once. As the fertilized egg divides to make an embryo, cells become specialized gradually.
Within three to six days after a human egg is fertilized, it has grown into a ball of a few hundred cells called a blastocyst. Within this ball lie a small number of cells that will go on to develop into the embryo. Scientists have learned to extract these stem cells from a thickening in the blastocyst called the inner cell mass and to grow them in the laboratory. These are known as embryonic stem cells or ES cells, and they have the potential to produce all the cell types in the human body. When a blastocyst implants in a woman's uterus, the cells of the inner cell mass will keep on dividing and differentiating into the earliest types of embryonic cells. Human ES cells in culture are not created from eggs that have been fertilized inside a woman's body. They come from the inner cell masses of 'spare' blastocysts that have been created in the laboratory as part of in vitro fertilization (IVF) programmes.
Less than three weeks after a human egg has been fertilized, these most flexible of stem cells have disappeared and embryonic cells become gradually more restricted in their potential. Instead of dividing to make one more specialized daughter cell and a back-up all-purpose stem cell, later embryonic cells are more likely to make two types of more differentiated cells when they divide. At this stage, the embryo's cells have 'committed' to become one of three general types of tissue that each has distinct types of stem cells. Many of these persist into adult life. As embryonic development continues, cells become even more specialized, forming recognizable tissues such as heart, muscle and blood.
In adults, dozens of stem-cell types have been described; more remain to be discovered. These stem cells are called tissue-specific as they will normally only replace one particular tissue. They are also sometimes called adult stem cells. The best understood are the stem cells that grow new blood cells, those that renew the skin, those that renew the gut lining, and those that can grow new skeletal muscles. Stem cells in the bone marrow make blood cells and have been used therapeutically for years. These are the cells that make it possible for a bone marrow transplant to renew a person's complete blood system.
Scientists across the world are trying to figure out exactly what these stem cells are capable of becoming in the lab and in the body; right now, however, tissue-specific stem cells appear to be specialists, quite good at making a few types of cells. Stem cells found in bone marrow naturally make new red blood cells and new white blood cells, for instance, but not new brain cells, at least, not robustly. Stem cells occurring in the brain make new neurons plus the cells that support them, but they don't seem to make muscle cells.