abortion An event in which a pregnancy is terminated. Some abortions occur naturally, while others are conducted for health, personal, or other reasons.
adult stem cell An undifferentiated cell found in some specific types of tissue (such as muscle or nerve tissue) with the ability to renew itself and develop into the type of tissue cell in which it is found. Adult stem cells are also called somatic stem cells. Evidence suggests that some types of adult stem cells may be able to differentiate into tissue cells different from themselves, nerve cells from blood stem cells, and muscle cells from nerve blood cells, for example.
allogeneic transplantation The process by which cells, tissues, or organs from one individual are transplanted into a second individual from the same species.
astrocyte A large cell found in nerve tissue.
autologous transplantation The process by which cells, tissues, or organs from an individual are transplanted back into the same person.
blastema A mass of undifferentiated cells from which an organ or body part develops.
blastocoel A cavity in the blastula of the developing embryo.
blastocyst An early stage of an embryo prior to its implantation into the uterine wall, usually made up of about 150 cells consisting of an inner cell wall and inner cavity, and an outer layer of cells, the trophoblast.
blastomere A cell formed in the first stages of embryonic development, after a fertilized egg has undergone division but before a blastocyst has formed.
blastula A hollow ball of cells one cell thick that appears in the early development of an embryo.
bone marrow stromal cell A stem cell that occurs in bone marrow that may develop into a bone, cartilage, fat, or fibrous connective tissue cell.
cell culture The process of growing cells in an artificial medium for the purpose of scientific research.
cell-based therapy A medical procedure in which stem cells are transplanted into a body with the expectation that they will develop into some specific type of cell that will repair damaged cells or augment the number of cells in some specific tissue.
cell division The process by which a single cell divides to produce two new cells.
cell line A collection of cells kept alive in an artificial environment that continues to reproduce itself essentially forever until its fate is changed by some external factor, such as through an experiment.
chimera An organism whose cells are derived from at least two different organisms, such as a mouse and a human. The term comes from an animal in Greek mythology with the head of a lion, the body of a goat, and the tail of a serpent.
clone An organism that is genetically identical to some original cell from which it was originally derived.
co-culture A group of two or more different kinds of cells that have been grown together.
conceptus A term used to describe an organism in its earliest stages of life, that is, as a zygote, an embryo, or a fetus. The term is sometimes used in an attempt to keep discussions “value-free,” and avoid talking about the organism as an “unborn child,” a “baby,” a “human being,” or some other term with philosophical or religious context.
cord blood Blood found in the umbilical cord and placenta.
cryopreservation The process of preserving an organic material by lowering its temperature to a very low point. In most in vitro fertilization facilities, fertilized eggs are cryopreserved by being suspended in liquid nitrogen at a temperature of 196°C.
dedifferentiation A process by which a mature cell with specialized structures and functions reverts to a simpler, more primitive state, as when a unipotent adult somatic cell reverts to a simpler pluripotent or totipotent stemlike cell.
differentiation The process by which a primitive unspecialized cell develops into a specialized cell, such as a muscle, heart, or nerve cell.
diploid cell A cell that has two sets of chromosomes, one set from the father and one from the mother.
directed differentiation The process by which a researcher establishes conditions so as to encourage a stem cell to develop into some specific type of cell.
DNA An acronym for deoxyribonucleic acid, a chemical compound found in the nucleus of all cells that carries instructions for making the proteins of which all cells are made.
ectoderm The outermost layer of the three layers of cells present in an embryo. The ectoderm eventually gives rise to the cells that make up the skin and the nervous system.
embryo A very early stage in the development of an organism. In humans, the term is used to describe the structure that exists from the time of fertilization until the end of the eighth week of gestation.
embryoid body A clump of cells that develops when stem cells aggregate with each other during the process of cell culturing.
embryonal carcinoma cell (EC cell) A cell derived from a teratoma.
embryonic germ cell (EG cell) A cell that occurs in the gonadal ridge portion of an embryo or fetus. Its properties are similar to those of an embryonic stem cell.
embryonic stem cell (ES cell) A primitive undifferentiated cell that occurs in an embryo with the potential for developing into any one of many kinds of tissue cells, such as heart, muscle, liver, nerve, or brain cells.
embryonic stem cell line A group of embryonic stem cells that have been cultured under in vitro conditions and that have been maintained without differentiation for long periods of time, ranging from a few months to many years.
endoderm The innermost layer of the three layers of cells present in an embryo. The endoderm eventually gives rise to the cells that make up the digestive and respiratory systems.
ex utero fertilization Fertilization that takes place outside of the body. Similar to and, in most cases, identical with in vitro fertilization.
feeder layer A group of cells used in a co-culture to maintain pluripotent stem cells.
fertilization The process by which male and female cells are joined to each other.
fetus A term used to describe an unborn young organism. In humans, the term is used to describe the unborn child from about two months after conception to birth.
gene A unit of heredity that consists of a specific segment of DNA that directs the formation of a protein.
germ cell An egg or sperm cell. Germ cells originate in the inner cell mass of the embryo before migrating outward and beginning to differentiate.
gonad An organ that produces germ cells. For example, the testis produces sperm cells and the ovary produces oocytes, or egg cells.
haploid cell A cell containing only a single set of chromosomes, half the number normally found in a somatic cell. Haploid cells are usually germ cells.
hematopoietic cell transplantation (HCT) Transplantation of cells with blood-forming potential. Such cells are most commonly removed from human bone marrow, but they may also be obtained from umbilical cord blood, the fetal liver, and a few other sources.
hematopoietic stem cell A stem cell found in bone marrow from which all kinds of red and white blood cells eventually develop.
human embryonic stem cell A pluripotent stem cell found in the inner cell mass of the blastocyst of the human embryo.
implantation The process by which the blastocyst is embedded into the endometrium, the lining of the uterine wall.
in vitro A Latin phrase that literally means “in glass,” referring to some type of procedure carried out in a test tube, on a laboratory dish, or in some other artificial environment.
in vitro fertilization (IVF) An artificial method of reproduction in which male and female cells are joined to each other outside the human body.
in vivo A Latin phrase that means “in life,” referring to a procedure that occurs within a living organism, as within a laboratory animal or a human.
informed consent Permission granted by a person to participate in a research decision, based on that person’s understanding of the potential risks and benefits associated with his or her participation.
inner cell mass (ICM) A cluster of cells within the blastocyst that eventually gives rise to the embryonic disk of the embryo and, ultimately, to the fetus.
long-term self-renewal The process by which stem cells replicate themselves without differentiating (that is, they remain stem cells) over long periods of times, ranging from a few months to many years.
mesenchymal stem cell According to one definition, “a multipotent cell found in embryonic connective tissue and, much more rarely, in adult bone marrow and connective tissue; capable of differentiating into bone, cartilage, and fat cells” (Dirckx, at http://www.hpisum.com/perspectives/ issue50/update.pdf). However, one authority in the field of stem cell research has reviewed the literature and concluded that “there is no accepted definition of a mesenchymal stem cell, not even an operational one”.
mesoderm The middle layer of the three layers of cells that make up the embryo. The mesoderm eventually gives rise to cells that make up connective tissue, muscles, bones, blood, the genital system, and some glands.
morula A ball of cells with the appearance of a mulberry (hence, the name) that forms three to four days after fertilization. The morula consists of 16, 32, or 64 cells.
multipotency The ability of a stem cell to differentiate into more than one type of tissue cell, although all of the cells into which it differentiates are of the same tissue type. For example, a blood stem cell may differentiate into any one of a variety of white blood cells or red blood cells, but does not typically develop into a nerve, muscle, skin, or nonblood type of cell.
murine Pertaining to mice or rats.
neural stem cell A type of stem cell found in human nerve (neural) tissue that develops into various kinds of nerve cells.
neuron A nerve cell.
niche A matrix of tissue cells and molecules in which one or more stem cells is embedded and which controls self-renewal and prevents the differentiation of those stem cells.
oligopotent progenitor cell A progenitor cell with the capability of differentiating into more than one, but only a limited number, of different kinds of cells.
oocyte (oocyte) A female gamete, or egg cell.
parthenogenesis The development of an unfertilized egg into a mature individual. The process occurs naturally in some animals and has been produced artificially in other animals. The process has been proposed as a method for producing embryonic stem cells without the intermediary step of fertilizing an egg.
plasticity The ability of a stem cell to grow and differentiate into a different kind of cell.
pluripotency The ability of a stem cell to develop into many different kinds of cells. The term often refers to the ability of a stem cell to differentiate into all kinds of cells found in the postimplantation embryo, fetus, or developed organism, but not in the trophoblast or placenta (so-called extra-embryonic entities).
preembryo A term sometimes used to describe the earliest stages of life, ranging from the fertilized egg to any larger entity in which cells have not yet begun to differentiate. The term is the subject of a great deal of dispute, with many experts in the field of embryology suggesting that it has no scientific meaning and is used only for political purposes.
preimplantation genetic diagnosis and screening (PGD) A set of procedures in which embryos that have been created by in vitro fertilization are tested for certain genetic traits so as to determine which of those embryos is to be implanted.
primitive streak A band of cells that develops about 14 days after fertilization along the longitudinal axis of the body that later becomes the fetal spinal cord.
progenitor cell A cell present in fetal or adult tissue that, like a stem cell, can differentiate into another kind of specialized cell. Unlike a stem cell, however, it is unable to continually renew itself by repeated cell division.
proliferation The multiplication of a single cell or small group of cells into a large population of identical cells through the process of cell division.
regeneration In medicine, the process by which an organism regrows tissue, organs, or some other body part.
regenerative medicine A field of medicine in which stem cells are introduced into a person’s body and induced to differentiate into some specific type of cell tissue in order to repair or replace damaged tissue.
reproductive cloning A process by which a complete new organism is created by somatic cell nuclear transfer (SCNT) beginning with a single body cell of another organism, to which the new organism is genetically identical; cloning of an embryo for transplantation into a uterus in order to produce a mature organism that is genetically identical to the nuclear donor.
somatic cell Any cell that is not a germ cell, that is, a sperm or egg cell.
somatic cell nuclear transfer (SCNT) The process by which the diploid nucleus of a somatic cell is transplanted into an unfertilized oocyte from which the nucleus has been removed. When this chimeric cell begins to divide, it produces totipotent stem cells that are genetically identical to the donor of the diploid nucleus.
stem cell A cell that is capable of dividing over some indefinite period of time and differentiating to produce one or more kinds of specialized cells.
“stemness” A term that is often applied to a stage in a cell’s life during which it has the properties of a stem cell (ability to divide over many generations and to eventually differentiate into a specialized cell) that can be characterized by certain biological and chemical characteristics of the cell. teratogeny The experimental study of teratomas.
teratoma A tumor that contains tissues from all three embryonic germ layers— endoderm, ectoderm, and mesoderm—most commonly found in the gonads (ovaries and testes).
therapeutic cloning The use of somatic cell nuclear transfer (SCNT) to produce an embryo that is allowed to develop to the blastocyst stage, at which point it is sacrificed for the purpose of harvesting embryonic stem cells contained within it.
totipotency The capacity of a stem cell to differentiate into any one of the 210 different types of cells normally found in the human body, or into all types of the cells found in some other organism.
transdifferentiation The process by which an adult stem cell from one kind of tissue differentiates into a cell of a different type of tissue.
trophoblast The outer layer of cells in the blastocyst that attaches to the inner wall of the uterus during implantation of the embryo, eventually becoming the placenta through which food passes to the embryo and, later, the fetus.
unipotency The ability of a stem cell to differentiate into a single type of mature somatic cell.
zygote Technically, a diploid cell formed by the fusion of two haploid cells during sexual reproduction. More commonly, a fertilized egg that has been formed by the fusion of a sperm cell and an egg cell.
