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WHITE PAPER: Alternative Sources of Pluripotent Stem CellS

Table of Contents

The President's Council on Bioethics
Washington, D.C.
May 2005


Adult stem cell: An undifferentiated cell found in a differentiated tissue that can renew itself and (with certain limitations) differentiate to yield all the specialized cell types of the tissue from which it originated. (NIH)

Altered Nuclear Transfer (ANT): A proposed method, using a modified form of somatic cell nuclear transfer (SCNT), of producing a biological artifact from which human pluripotent stem cells could be derived.

Anencephalic fetus: A fetus with a congenital defect related to development of the brain, with absence of the bones of the cranial vault and absent or rudimentary cerebral and cerebellar hemispheres, brainstem, and basal ganglia. (SMD)

Aneuploid: Having an abnormal number of chromosomes. (SMD)

Autologous: Derived or transferred from the same individual's body.

Biological artifact: As employed here, this phrase denotes an artificially created non-embryonic but embryo-like cellular system, engineered to lack the essential elements of embryogenesis but still capable of some cell division and growth.

Biopsy: Process of removing tissue from patients for diagnostic examination. (SMD)

Blastocyst:In mammals, an early stage of embryonic development at which the embryo (roughly 100-200 cells) is a hollow sphere made up of an outer layer of cells (the trophectoderm), a fluid-filled cavity (the blastocoel), and a cluster of cells on the interior (the inner cell mass).

Blastomere: A cell contained within an early embryo (up to two days after conception, at which point the embryo comprises about 8 blastomeres).

Blastomere biopsy: Removal of one or two blastomeres from the embryo in vitro at about the 8-cell stage, usually in order to perform preimplantation genetic diagnosis and screening.

Blastula: An early stage of embryonic development (roughly 100-200 cells) at which the cells of the morula are rearranged to form a hollow sphere; at this stage of embryonic development in humans and other mammals, the embryo is generally called a blastocyst

Bone marrow: The soft, fatty, vascular tissue that fills most bone cavities and is the source of red blood cells and many white blood cells.

Chimera:In experimental embryology, the individual produced by grafting an embryonic part of one animal on to the embryo of another, either of the same or of another species. (SMD)

Chromosomes:Structures inside the nucleus of a cell, made up of long pieces of DNA coated with specialized cell proteins, which are duplicated at each mitotic cell division. Chromosomes thus transmit the genes of the organism from one generation to the next. (CR)

Cleavage arrest: Spontaneous cessation of cell division in an early embryo.

Cloned embryo: An embryo arising from the somatic cell nuclear transfer process as contrasted with an embryo arising from the union of an egg and sperm. (CR)


Cloning-to-produce-children-Production of a cloned human embryo, formed for the (proximate) purpose of initiating a pregnancy, with the (ultimate) goal of producing a child who will be genetically virtually identical to a currently existing or previously existing individual.

Cloning-for-biomedical-research-Production of a cloned human embryo, formed for the (proximate) purpose of using it in research or for extracting its stem cells, with the (ultimate) goals of gaining scientific knowledge of normal and abnormal development and of developing cures for human diseases.

Human cloning-The asexual reproduction of a new human organism that is, at all stages of development, genetically virtually identical to a currently existing, or previously existing, human being. (CR)

Cord blood: Blood in the umbilical cord and placenta.

Cryopreservation and Cryostorage: Freezing of IVF embryos for later use.

Cytoplasmic: Of or pertaining to the substance of a cell, exclusive of the nucleus. (SMD)

Dedifferentiation: A procedure whereby differentiated, somatic cells are restored to a more undifferentiated, multipotent condition.

Diploid:Refers to the full complement of chromosomes in a somatic cell, distinct for each species (forty-six in human beings). (CR)

Embryo: (a) In humans, the developing organism from the time of fertilization until the end of the eighth week of gestation, when it becomes known as a fetus. (NIH) (b) The developing organism from the time of fertilization until significant differentiation has occurred, when the organism becomes known as a fetus. An organism in the early stages of development. (CR)

Embryogenesis: That phase of prenatal development involved in establishment of the characteristic configuration of the body of the embryo; in humans, embryogenesis is usually regarded as extending from the end of the second week to the end of the eighth week, after which the product of conception is usually spoken of as a fetus. (Based on SMD)

Embryonic germ layers: The three initial tissue layers arising in the embryo-endoderm, mesoderm, and ectoderm-from which all other somatic tissue-types develop. (NRC)

Embryonic stem cells (ESCs): Primitive (undifferentiated) cells, derived from the inner cell mass of the embryo, that have the potential to become a wide variety of specialized cell types. (Based on NIH)

Enucleated oocyte: An egg cell from which the nucleus has been surgically removed.

Ex vivo: Outside the body, frequently the equivalent of "in vitro"; the opposite of "in vivo."

Fertilization: The process whereby male and female gametes unite. (NIH)

Fetus: A developing human from usually two months after conception to birth. (NIH)

Gamete:A reproductive cell (egg or sperm). (CR)

Gene: A functional unit of heredity that is a segment of DNA located in a specific site on a chromosome. A gene directs the formation of an enzyme or other protein. (NIH)

Genome: The total gene complement of a set of chromosomes. (SMD)

Genotype: The genetic constitution of an organism or a group of organisms. (SMD)

Hydatidiform mole: An abnormality during pregnancy; a tissue mass or growth that forms within the uterus as the result of a genetic error during the fertilization process.

Implantation: The attachment of the blastocyst to the lining of the uterus, and its subsequent embedding there. (Based on SMD)

In vitro fertilization (IVF): The union of an egg and sperm, where the event takes place outside the body and in an artificial environment (the literal meaning of "in vitro" is "in glass"; for example, in a test tube). (CR)

Inner cell mass: The cluster of cells inside the blastocyst. These cells give rise to the embryonic disk of the later embryo and, ultimately, the fetus. (NIH)

IVF embryo: An embryo produced by in vitro fertilization.

Karyotype: The chromosome characteristics (number, shape, etc.) of an individual cell or cell line, usually presented as a systematized array in pairs. (SMD)

Lineage: The descendants of a common ancestor.

Mesenchymal stem cells: Cells from the immature embryonic connective tissue. A number of cell types come from mesenchymal stem cells, including chondrocytes, which produce cartilage. (NIH)

Morphology: Configuration or structure, shape.

Morula: An early stage of embryonic development (roughly 16-64 cells) at which the embryo is a solid spherical mass of cells, resulting from the early cleavage divisions of the zygote; so called because of its resemblance to a "little mulberry" (in Latin, morula).

Mosaic: Possessing two or more genetically different cell types; an early embryo is said to be mosaic when some of its cells exhibit chromosomal abnormalities while others appear chromosomally normal.

Multipotent adult progenitor cells (MAPCs): Cells isolated from bone marrow that can be differentiated into cells with characteristics of cartilage, fat, and bone.

Multipotent cell: A cell that can produce two or more different types of differentiated cells; adult stem cells are multipotent.

Oocyte: Unfertilized egg cell.

Organismic death (of an embryo)-concept andcriterion: As proposed by Landry and Zucker, the concept of organismic death for an early-stage human embryo is defined by irreversible loss of "the capacity for continued and integrated cellular division, growth, and differentiation"; their proposed criterion for determining organismic death is "irreversible cessation of cell division in the embryo observed in vitro."

Parthenogenesis: A form of reproduction in which an unfertilized egg develops into a new individual (SMD); the process of inducing an unfertilized egg to initiate cell division.

Parthenote: The primary product of parthenogenesis; more precisely, an unfertilized egg that has been activated to initiate cell division.

Placenta: The oval or discoid spongy structure in the uterus from which the fetus derives it nourishment and oxygen. (NRC)

Pluripotent cell: A cell that can produce all the cell types of the developing body; embryonic stem cells, as well as the inner cell mass cells of the blastocyst, are pluripotent.

Pluripotent stem cell: Any stem cell that has the same functional capacity-that is, stable pluripotency-as an embryonic stem cell, though not necessarily the same origin.

Preimplantation genetic diagnosis (PGD): A method of testing IVF embryos for chromosomal or genetic disorders before they are transferred to the uterus; typically one or two blastomeres are removed for genetic testing at about the 8-cell stage of embryonic development.

Somatic cell: Any cell of an organism other than the gametes. (Based on SMD)

Somatic cell nuclear transfer (SCNT): A method of cloning: transfer of the nucleus from a donor somatic cell into an enucleated oocyte to produce a cloned embryo.

Stem cells: Stem cells are undifferentiated multipotent precursor cells that are capable both of perpetuating themselves as stem cells and of undergoing differentiation into one or more specialized types of cells. (CR)

Stem cell line: Stem cells which have been cultured under in vitro conditions that allow proliferation without differentiation for months to years. (NIH)

Superovulation: Drug-induced stimulation of a woman's ovaries to produce many mature oocytes in a single menstrual cycle.

Teratoma: A tumor consisting of different types of tissue, as of skin, hair, and muscle, caused by the development of independent germ cells. (SMD)

Totipotent cell: A cell that can give rise to the entire organism, including the extra-embryonic membranes; the fertilized egg or zygote is totipotent.

Trophectoderm: In early embryos at the blastocyst stage, the outer layer of cells that will give rise to the placenta.

Uterine transfer: Transfer of an IVF embryo to a woman's uterus with a view to implantation and gestation.

Xenotransplantation: A transplant of tissue from an animal of one species to an animal of another species.

Zygote: The diploid cell that results from the fertilization of an egg cell by a sperm cell. (CR)

Definitions marked "(CR)" are from the Council's report on human cloning (Human Cloning and Human Dignity: An Ethical Inquiry, Washington, D.C.: Government Printing Office, 2002). Definitions marked "(NIH)" are from the National Institutes of Health online stem cell glossary at (accessed April 1, 2005). Definitions marked "(NRC)" are from the National Research Council report, Stem Cell Research and the Future of Regenerative Medicine (Washington, D.C.: National Research Council, 2001). Definitions marked "(SMD)" are from Stedman's Medical Dictionary.


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