A relatively undifferentiated cell in any tissue responsible for replenishing cells lost from a tissue through normal wear and tear, in the case of adult stem cells, or responsible for virtually all of a fetus's cells, in the case of embryonic stem cells; characterised by being able to replicate (through mitosis) seemingly for ever, either in the body or in tissue culture, without wearing out the ends of the chromosomes. Embryonic stem cells are controversial because they are derived from human embryos, sometimes by the process of somatic cell nuclear transfer, which involves creating embryos or embryo-like forms of a kind that could be used for reproductive cloning. The word stem comes from analogy with the stem of a plant giving rise to branches and further branches, representing the ever-increasing number of different paths a stem cell can differentiate into.

Embryonic stem cells.

Group of cells that differentiates within (but to one side of) the developing embryo when it is a blastocyst and which will, if all goes well after implantation, form the embryo-proper, or fetus. Cells from the inner cell mass can be used to produce embryonic stem cells.

A state in an embryo, fetus, animal or person in which a small number of cells occur that have a different genome. Arises naturally because cells (rarely, stem cells) can cross between a mother and her fetus, or experimentally by introducing foreign stem cells into an embryo, usually at the stage of blastocyst, when the introduced cells become part of the inner cell mass.

(SCNT) Transfer of the nucleus of a somatic cell into a secondary oocyte or zygote from which the egg or zygote's chromosomes have been removed, in such a way that cleavage still occurs, in effect causing the cell's nucleus to 'bud', or reproduce without genetic recombination, and hence a form of asexual reproduction or cloning now commonly used for reproductive cloning in animals and, it is hoped, for therapeutic cloning to create embryonic stem cells for treatment of serious degenerative disease and cancers.

The use of somatic cell nuclear transfer to produce embryonic stem cells suitable for differentiation into tissues that are a perfect match to treat disease in the person who provided the cell nucleus used. Ethically controversial.