Every human originates from a stem cell, capable of dividing and then differentiating into any cell type. Cells for several generations in the early embryo retain this capacity, hence identical twins.
Some cells present in the adult body may retain this ability, and might in the correct conditions be caused to develop into any replacement tissue, including with sufficient cleverness and effort whole formed organs. Since these would perfectly match the original individual, much of transplant surgery could be made significantly easier.
Stem cells possess the following unique characteristics:
- Self-renewal, i.e. one of the daughter cells retains the characteristics of the original mother cell.
- Potential to differentiate into more than one cell type (pluripotency).
- Potential for extensive proliferation.
Characteristics 1 & 3 and, to a lesser extent, 2 are features of neoplasms. It is now thought that some, if not most, cancers arise from cancer stem cells, i.e. normal stem cells that have accumulated genetic mutations that permit uncontrolled proliferation.