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Sources of neuronal material for implantation

The adult brain is an organ that does not have the natural ability to replace cells that have been lost through damage. Possible human interventions to rectify this situation include transplanting either developing neural tissue into the damaged host brain or transplantation of neural stem cells (cells that have the capacity to proliferate into neural cells and self‐replicate) into the damaged area. Fetal or embryonic stem cells can be extracted and differentiated in vitro into the specific desired progeny (e.g. neurons). The neuronal stem cells themselves can be extracted from fetuses and multiplied in culture and then transplanted into the damaged brain. There is the possibility of de‐differentiation, in which cells of one type can be converted into a different cell type; for example, a differentiated blood cell could be de‐differentiated back to its own hemopoietic stem cell and that stem cell could be converted into a neuronal stem cell which could then be differentiated into a neuron. It is probable that methods of generating large numbers of committed stem cells to treat conditions such as Alzheimer's disease will soon be increasingly common.