Glossary     

Adult stem cell — A partially differentiated cell present in tissue that can renew itself and form the specialized cell types of the tissue from which it originated.

Blastocyst — A four to five day old pre-implanted embryo of about 150 cells resulting from cell division that occurs after the egg is fertilized by the sperm. The blastocyst is a sphere made up of an outer layer of cells that will form the placenta, a fluid-filled cavity and a cluster of cells in the interior (the inner cell mass).

Bone Marrow Transplant (BMT) — Procedure to replace bone marrow that has been destroyed by high doses of anticancer drugs or radiation. Bone marrow, which is the spongy tissue found inside large bones, is responsible for making blood cells and is home to the immune system. For a BMT, bone marrow cells are collected from either the patient or a donor and then expanded in culture before being transplanted.

Cell-based therapies — Treatment in which stem cells are triggered (induced) to differentiate into the specific cell type required to repair damaged or destroyed cells or tissues.

Cell culture — Growth of cells in vitro (in a dish; Latin for “in glass”) in an artificial medium for experimental research.

Cell division — Method by which a single cell divides to create two cells.

Cell line — Stem cells which have been cultured in vitro (in a dish) in laboratory conditions that allow them to proliferate (duplicate) without differentiating (changing) for months or years. A line of cells comes from a single blastocyst.

Clone — Generate identical copies of a molecule, cell or organism. When it is used to refer to cells grown in a tissue culture dish, a clone is a line of cells that is genetically identical to the originating cell. Generating the clone cells is referred to as therapeutic cloning. The term clone may also be used to refer to an animal produced by nuclear transfer and then transplanted into a uterus. This is reproductive cloning.

Cord blood stem cells — See Umbilical cord blood stem cells.

Culture medium — The liquid that covers cells in a culture dish and contains nutrients to feed the cells. Medium may also include other growth factors added to produce desired changes in the cells.

Differentiation — The process whereby an unspecialized embryonic cell acquires the features of a specialized cell such as a heart, liver, or muscle cell. Differentiation is controlled by the interaction of a cell's genes with physical and chemical conditions outside the cell, usually through signaling pathways involving proteins embedded in the cell surface.

Embryo — In humans, the developing organism from the time of fertilization until the end of the eighth week of gestation, when it is called a fetus.

Embryonic stem cells — Primitive (undifferentiated) cells derived from a four to five day old pre-implanted embryo that have the potential to become every cell type in the body.

Feeder layer —Cells used in co-culture to maintain pluripotent stem cells. For human embryonic stem cell culture, typical feeder layers include mouse embryonic fibroblasts (MEFs) or human embryonic fibroblasts that have been treated to prevent them from dividing.

Fertilization — The joining of the male gamete (sperm) and the female gamete (egg).

Fetus — A developing human from approximately eight weeks after conception until the time of its birth.

Gamete — An egg (in the female) or sperm (in the male) cell. See also Somatic cell.

Gene — A functional unit of heredity that is a segment of DNA found on chromosomes in the nucleus of a cell. Genes direct the enzymes and other proteins.

Glia — Tissue that surrounds and supports neurons in the central nervous system; glial and neural cells together compose the tissue of the central nervous system

Hematopoietic stem cell — A stem cell that gives rise to all red and white blood cells.

Human embryonic stem cell (hESC) — A pluripotent stem cell derived from the inner cell mass (ICM) of the blastocyst that can form every cell type in the body.

In vitro — Latin for "in glass"; in a laboratory dish or test tube; an artificial environment.

In vitro fertilization — A technique that unites the egg and sperm in the clinic in a dish, instead of inside the female body.

Inner cell mass (ICM) — The cluster of cells inside the blastocyst which give rise to the embryo and ultimately the fetus. When grown in vitro, ICM cells can form embryonic stem cells.

Induced Pluripotent Stem (iPS) cells — A type of pluripotent stem cell, similar to an embryonic stem cell, formed by introducing certain embryonic genes to a somatic cell.

Mouse embryonic fibroblast (MEF) — Often used as "feeder cells" in human embryonic stem cell research. Many researchers are phasing out MEFs in favor of culture media with precisely defined ingredients of human derivation with synthetic supplements to eliminate the chance of contamination.

Multipotent — Ability of a stem cell to develop into more than one cell type of the body. See also pluripotent. After embryonic stem cells specialize into a specific type of cell like a blood cell or skin cell, they are referred to as adult or ‘tissue-specific’ stem cells. They can only form a limited number of cell types corresponding with their tissue of origin and are called multipotent cells.

Neural stem cells — A stem cell found in adult neural tissues that can give rise to neurons and glial (supporting) cells.

Neurons — Nerve cells, the principal functional units of the nervous system. A neuron consists of a cell body and its processes — an axon and one or more dendrites. Neurons transmit information to other neurons or cells.

Pluripotent — Ability of a stem cell to give rise to all of the various cell types of the body. Pluripotent cells cannot make so-called "extra-embryonic" tissues such as the amnion, chorion, and other components of the placenta. Because Embryonic stem cells, which only exist during the first four through 14 days of embryonic development, are unspecialized but have the potential to form all cell types of the body, they are referred to as ‘pluripotent’ stem cells.

Proliferation — Expansion of the number of cells by the continuous division of single cells into two identical daughter cells.

Regenerative medicine — A field of medicine devoted to treatments in which stem cells are induced to differentiate into the specific cell type required to repair damaged or destroyed cells or tissues. (See also cell-based therapies).

Replating — After embryonic stem cells in a plastic laboratory culture dish containing nutrients divide and spread over the surface of the dish the for several days, they crowd the dish. They are then removed and distributed between several fresh culture dishes.

Reproductive cloning — The process of using somatic cell nuclear transfer (SCNT) to produce a normal organism (e.g., animal) genetically identical to the donor of the somatic cell nucleus. The embryo is implanted in a uterus and develops into a live being. The first animal to be created by reproductive cloning was Dolly the sheep, born at the Roslin Institute in Scotland in 1996. See also somatic cell nuclear transfer (SCNT).

Self renewal — The process by which a cell divides and replaces itself with another cell that has the same potential.

Somatic cell — Any body cell other than gametes (egg or sperm).

Somatic cell nuclear transfer (SCNT) — A technique that combines an unfertilized enucleated egg (nucleus removed) and the nucleus of a somatic cell with the goal of making a blastocyst. SCNT can be used for therapeutic or reproductive purposes, but the initial stage that combines an enucleated egg and a somatic cell nucleus is the same. See also therapeutic cloning and reproductive cloning.

Stem cells — Cells with the ability to divide for long periods in culture and to give rise to specialized cells.

Therapeutic cloning — The goal of therapeutic cloning is to create pluripotent cells that exactly match a patient. By combining a patient's somatic cell nucleus and an enucleated egg, a scientist may harvest embryonic stem cells that can be used to generate tissues that match the patient's body. This means the tissues created are unlikely to be rejected by the patient's immune system. See also Somatic cell nuclear transfer (SCNT).

Umbilical cord blood stem cells — stem cells collected from the umbilical cord at birth that can produce all of the blood cells in the body (hematopoietic). Cord blood is currently used to treat patients who have undergone chemotherapy to destroy their bone marrow due to cancer or other blood-related disorders.

Undifferentiated — A cell that has not yet developed into specialized cell type.

Virus — A microscopic living matter that relies on a host to reproduce and survive. When it comes in contact with the host cell, a virus can insert its genetic material into the host, taking over its function. While we commonly think of viruses as infecting people with disease or computers with problems, because viruses can transfer genetic material between different hosts, they are extensively used in genetic engineering, including converting adult cells to iPS cells. See also Induced pluripotent stem cells.

Most of the definitions in this glossary are from the NIH at <http://stemcells.nih.gov/info/glossary.asp>.