Stem cells:  Single cells that have the capacity 1) to self-renew as well as 2) to differentiate to more mature cells.

Totipotent stem cells:  Stem cells that include in their progeny all cell types that can be found in an embryo, fetus, or developed organism, as well as the cells that form the embryonic components of the trophoblast and placenta.

Pluripotent stem cells:  Stem cells that include in their progeny all cell types that can be found in a postimplantation embryo, fetus, or developed organism, but not embryonic components of the trophoblast and placenta (these are usually called extra-embryonic).

Multipotent stem cells:  Stem cells whose progeny are of multiple differentiated cell types, but all within a particular tissue, organ, or physiological system.  For example, blood-forming (hematopoietic) stem cells (HSC) are single multipotent cells that can produce progeny that include HSC, blood cell-restricted oligopotent progenitors, and all cell types and elements (e.g., platelets) that are normal components of the blood.

Embryonic stem (ES) cells:  ES cells are pluripotent cell lines established, usually, from the inner cell mass of the blastocyst stage of development.  Within the population of cultured ES cells are cells that can produce more ES cells, or under conditions of differentiation, give rise to collections of cells that include most, if not all cell types that can be found in a postimplantation embryo, fetus, or developed organism, but not trophoblast or placenta.  To date no ES cells cultured in vitro can give rise to developed organisms or even developed organs.  A test of pluripotency of mouse ES cells is to inject them into blastocysts prior to implantation; the progeny of these cells can participate in all germline and somatic tissues along with host blastocyst-derived cells.

Embryonic germline (EG) cells:  EG cells are pluripotent stem cell lines derived from the primitive germline cells that exist between the blastocyst stage of development until their conversion within gonads to egg or sperm stem cells.  Their properties are similar to those of ES cells.

Unipotent stem cells:  Stem cells that self-renew as well as give rise to a single mature cell type; e.g., spermatogenic stem cells.

Adult stem cells:  Stem cells in the developed organism; these include the various types of multipotent and unipotent stem cells, but to date do not include known pluripotent stem cells.

Transdifferentiation:  The ability of a particular cell of one tissue, organ or system, including stem or progenitor cells, to be caused to differentiate into a cell type characteristic of another tissue, organ, or system; e.g., HSC to liver hepatocytes.

Pre-implantation embryos:  Fertilized eggs (zygotes) and all of the developmental stages up to, but not beyond, the blastocyst stage.

Post-implantation embryo:  Implanted embryos in all early stages of development until the establishment of the body plan of a developed organism with identifiable tissues and organs.

Fetus:  The postembryonic stages of development from embryo to developed organism, usually ending at parturition.

Autologous transplantation:  Cell, tissue, or organ transplants from one person back to the same person.  Such transplants from self do not induce an immune response and are not rejected.  In one example, a cancer patient may have her HSC or bone marrow removed and stored during treatment with sufficient radiation or chemical therapy to kill all blood-forming cells (and, perhaps, all cancer cells), and then her blood-forming capacity is rescued with autologous HSC or bone marrow.

Allogeneic transplantation:  Cell, tissue, or organ transplants from one member of a species to a genetically other member of the same species.

Graft versus host disease:  A transplantation syndrome wherein mature T lymphocytes from an allogeneic donor undergo an immune response against tissues and organs of the host.  These immune responses can vary in degree, from mild skin, gut, and liver damage to lethal disease.  The usual circumstance is an allogeneic bone marrow (BM) or mobilized peripheral blood (MPB) transplant from healthy donors to hosts who have received substantial pre-transplant conditioning, which prevents host immune rejection of donor cells.  In that circumstance, donor mature T lymphocytes are present as contaminants of HSC and blood progenitors.  Pure allogeneic HSC transplants do not cause graft versus host disease.

Hematopoietic cell transplantation (HCT):  Transplantation of cells with blood-forming potential, usually bone marrow (BM).  HSC are the only cells in HCT that provide rapid and sustained reconstitution of blood formation.  HSC are found in adult BM, umbilical cord blood, mobilized peripheral blood (MPB, the nucleated cell fraction of blood after treatment of the donor with agents that increase the passage of HSC from BM to blood), and in fetal liver.

Progenitor cells:  Cells that can differentiate into mature cells, but which lack the capacity to self-renew.  Progenitor cells are derived from stem cells and are intermediate to the production of mature cells.

Oligopotent progenitor cells:  Progenitor cells that can produce more than one type of mature cells.  For example, the clonal common myeloid progenitor is a progenitor cell which can give rise to blood granulocytes, monocytes, red blood cells, platelets, basophiles, eosinophiles and dendritic cells, but not T lymphocytes, B lymphocytes, or natural killer (NK) cells.

Zygote:  The fertilized egg, the result of sperm/egg interactions leading to the fusion of sperm and egg nuclei.  The zygote usually begins cell division to give rise to the stages of the pre-implantation embryo, from the 2 cell stage to the blastocyst; but only the fertilized egg is the zygote.

Germline cells:  Cells that arise from the inner cell mass and are irreversibly committed to give rise to eggs or sperm.  Primitive germline cells (which are germline stem cells) migrate from the posterior of the early postimplantation embryo to the developing gonads, known as genital ridges, where they commit to spermatogenesis (in testes) and oogenesis (in ovaries).

Somatic cells:  All cells within the developing or developed organism with the exception of germline cells.