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Human Stem Cell Research: Opportunities for Health and Ethical Perspectives A Discussion Paper GLOSSARY Blastocyst: a very early stage in mammalian development (about 1 week post-fertilization, approximately 100 cells in total). A blastocyst consists of an inner cell mass (which develops into the embryo and from which embryonic stem cells may be derived), covered by a thin layer of cells known as the trophoblast (from which the placenta develops). Cloning: transplantation of a nucleus from a somatic cell to an ovum (egg), which then develops into a zygote following an appropriate stimulus. “Therapeutic cloning”: sometimes used to refer to cloning of an embryo for the purpose of deriving pluripotent stem cells for therapeutic application. “Reproductive cloning”: sometimes used to refer to cloning of an embryo for transplantation into a uterus with the intent of producing offspring genetically identical to the nuclear donor. Differentiation: the process by which cells acquire new characteristics and form more specialized cell types. Embryo: an organism in the early stages of development; in humans, from the 2-cell stage until approximately 6 weeks, after which it is termed a ‘fetus’. 1 Embryonic germ (EG) cell: pluripotent stem cells derived from fetal gonads. Embryonic stem (ES) cell: pluripotent stem cells derived from the inner cell mass of a blastocyst stage embryo. Gamete: the sex cell (sperm or egg). In animals (and humans) the functional, mature, male gamete is called a sperm while the female gamete is called the ovum, or egg. Gonad: an organ that produces sex cells (a testis or an ovary). Hematopoietic: related to the formation of blood cells. Hybrid: an organism in which the chromosomes contain DNA from two different species or subspecies. In vitro fertilization (IVF): fertilization of an egg by a sperm in an artificial environment. Oocyte: an immature ovum (egg). Pluripotent stem cells: the most primitive stem cells, with the capacity to differentiate into the complete range of specialized cell types. Somatic cells: the cells of an organism, other than gametes. Somatic cell nuclear transfer (nuclear transfer, nuclear replacement: see ‘cloning’): inserting the nucleus of a cell from one of the body’s organs or tissues into an unfertilized egg (oocyte) which has had its nucleus removed (i.e. has been ‘enucleated’). Stem cells: any cells that can renew tissue; these cells have the property of being able to either reproduce themselves (self-renewal) or differentiate into a variety of more specialized cell types. Zygote: a fertilized egg formed as a result of the union of the male (sperm) and female (egg) sex cells. 2