In the normal adult body, different classes of stem cells are responsible for the renewal of different types of tissue. Some tissues, however, seem incapable of repair by the genesis of new cells because no competent stem cells are present. Recent discoveries have opened up new possibilities for manipulating stem-cell behaviour artificially so as to repair tissues that previously seemed unrepairable. Epidermal stem cells taken from undamaged skin of a badly burned patient can be rapidly grown in large numbers in culture and grafted back to reconstruct an epidermis to cover the burns. Neural stem cells persist in a few regions of the adult mammalian brain, and when grafted into a developing or damaged brain can generate new neurons and glia appropriate to the site of grafting.
Embryonic stem cells (ES cells) are able to differentiate into any cell type in the body, and they can be induced to differentiate into many cell types in culture. Stem cells of some adult tissues, such as bone marrow, when placed in a suitable environment, seem able to generate a much wider range of differentiated cell types than they produce normally.
When cells are removed from the body and maintained in culture, they generally maintain their original character. Each type of specialized cell has a memory of its developmental history and seems fixed in its specialized fate, although some limited transformations can occur. Stem cells in culture, as in tissues, may continue to divide, or they may differentiate into one or more cell types, but the cell types they can generate are restricted. Each type of stem cell serves for the renewal of one particular type of tissue. For some tissues, such as the brain, regeneration is impossible in adult life because no stem cells remain. There seemed to be little hope, therefore, of replacing lost nerve cells in the mammalian brain through the genesis of new ones, or of regenerating any other cell type whose normal progenitors are no longer present.
Recent discoveries have overturned this gloomy judgement and have led to a more optimistic perception of what stem cells can do and how we may be able to use them. The change has come from several findings that demonstrate exceptional forms of stem-cell versatility that could scarcely have been suspected from knowledge of the normal life histories of cells in tissues.