Adult stem cell — An undifferentiated cell found in a differentiated tissue that can renew itself and (with limitations) differentiate to yield the specialized cell types of the tissue from which it originated.
Blastocyst — A preimplantation embryo of 150 cells. The blastocyst consists of a 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).
Chimera — An organism composed of cells derived from at least two genetically different cell types. The cells could be from the same or separate species.
Differentiation — The process whereby an unspecialized early embryonic cell acquires the features of a specialized cell, such as a heart, liver, or muscle cell.
Embryo — 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.
Embryologist — A specialist who performs all of the laboratory aspects of assisted human reproductive technologies, endocrinology and andrology in an IVF clinic setting. This includes egg preparation, egg identification, sperm preparation, fertilization, micro-manipulation, embryo incubation, and embryo preparation for transfer.
Embryonic stem (ES) cells — Primitive (undifferentiated) cells derived from the early embryo that have the potential to become a wide variety of specialized cell types.
Fertilization — The process whereby male and female gametes unite to form a zygote (fertilized egg).
Gamete — A mature male or female germ cell, that is, sperm or oocyte, respectively.
hES cell — Human embryonic stem cell; a type of pluripotent stem cell.
In vitro — Literally, "in glass," in a laboratory dish or test tube; in an artificial environment.
In vitro fertilization (IVF) — An assisted reproductive technique in which fertilization is accomplished outside the body.
In vivo — In the living subject; in a natural environment.
Nuclear transfer (NT) — Replacing the nucleus of one cell with the nucleus of another cell.
Oocyte — Developing egg; usually a large and immobile cell.
Primitive streak — The initial band of cells from which the embryo begins to develop. The primitive streak establishes and reveals the embryo's head-tail and left-right orientations.
Somatic cells — Any cell of a plant or animal other than a germ cell or germ cell precursor.
Somatic cell nuclear transfer (SCNT) — The transfer of a cell nucleus from a somatic cell into an egg (oocyte) whose nucleus has been removed.
Stem cell — A cell that has the ability to divide for indefinite periods in vivo or in culture and to give rise to specialized cells.