Stem Cell Glossary

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.

Blastocyst: After the nucleus of a somatic cell is inserted into the place of the evacuated egg cell nucleus, the resulting cell begins to grow and divide. In three to five days, the resulting ball of cells is called a blastocyst. The sphere is made up of an outer layer of cells, a fluid-filled cavity, and a cluster of cells on the interior. This inner-cell mass of malleable stem cells are then extracted and grown in a petri dish for research or therapy.

Clone: A line of cells that is genetically identical to the originating cell -- in this case, a stem cell.

Cytoplasm: The liquid, organelles, proteins and enzymes within a cell wall but outside the cell’s nucleus. Cytoplasm has structures for the manipulation, manufacture and processing of proteins, enzymes and sugars, as well as structures that give the cell shape, regulate its water content and eliminate wastes from energy consumption and cell division.

DNA: Deoxyribonucleic acid, a chemical found primarily in the nucleus of cells. DNA carries the instructions for making all the structures and materials in the body.

Embryonic stem cells: Primitive (undifferentiated) cells from the embryo that have the potential to become a wide variety of specialized cell types.

Embryonic stem cell line: Embryonic stem cells that have been cultured under in vitro conditions that allow proliferation without differentiation for months to years.

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.

Genome: All the genetic information or hereditary material possessed by an organism; the entire genetic complement of an organism.

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.

Long-term self-renewal: The ability of stem cells to renew themselves by dividing into the same nonspecialized cell type over long periods (many months to years) depending on the specific type of stem cell.

Multipotent: Stem cells that can reproduce only limited cells and tissues in the body. In limited numbers, these stem cells are found, with pluripotent stem cells, in the marrow of adult humans.

Neurons: Nerve cells, the structural and functional unit of the nervous system. A neuron consists of a cell body and its processes, an axon, and one or more dendrites. Neurons function by the initiation and conduction of impulses and transmit impulses to other neurons or cells by releasing neurotransmitters at synapses.

Pluripotent: Stem cells that can reproduce every cell and tissue in the body, except the placenta. The inner cell mass of the blastocyst resulting from somatic cell nuclear transfer. These are the cells of highest interest to stem cell researchers.

Somatic cell nuclear transfer: Process in which the nucleus containing half a genome is removed from a human egg cell and replaced with a nucleus containing a full genome from an adult cell of any kind from a donor. The cytoplasm within the egg awakens, or turns on, the full DNA strand in the nucleus, allowing it to begin to direct cell division. The result is a set of totipotent stem cells that soon begin to specialize into pluripotent, and then, within two weeks, multipotent stem cells.

Stem cell: Unspecialized cells that renew themselves for long periods through cell division. Under certain physiologic or experimental conditions, they can be induced to become cells with special functions such as the beating cells of the heart muscle or the insulin-producing cells of the pancreas. Stem cells are the foundation of every cell and tissue in the body.

Totipotent: Stem cells that can reproduce every cell and tissue in the body, including the placenta. The first four of five divisions of the cell after somatic cell nuclear transfer result in totipotent stem cells. It is at this stage in natural fertilization that identical twinning can occur.

National Catholic Reporter, June 18, 2004