Table 1.

Glossary and definition of terminology

Potency: Sum of developmental options available to cell.

Totipotent: Ability of a single cell to divide and produce all the differentiated cells in an organism, including extraembryonic tissues, and thus to (re)generate an organism in total. In mammals only the zygote and the first cleavage blastomeres are totipotent.

Pluripotent: Ability of a single cell to produce differentiated cell types representing all three embryonic germ layers and thus to form all lineages of a mature organism. Example: embryonic stem cells.

Multipotent: Ability of adult stem cells to form multiple cell types of one lineage. Example: hematopoietic stem cells.

Unipotent: Cells form one cell type. Example: spermatogonial stem cells (can only generate sperm)

Reprogramming: Change in epigenetics that can lead to an increase in potency, dedifferentiation. Can be induced by nuclear transfer, cell fusion, genetic manipulation.

Transdifferentiation: The capacity of a differentiated somatic cell to acquire the phenotype of a differentiated cell of the same or different lineage. An example is epithelial–mesenchymal transition, a process whereby fully differentiated epithelial cells undergo transition to a mesenchymal phenotype giving rise to fibroblasts and myofibroblasts.

Plasticity: Hypothesis that somatic stem cells have broadened potency and can generate cells of other lineages, a concept that is controversial in mammals.

Embryonic Stem Cell (ESC): Cell lines developed from the inner cell mass of early developing blastocysts. ESCs have the capacity for self-renewal and are pluripotent, having the ability to differentiate into cells of all embryologic lineages and all adult cell types. However, ESCs cannot form extraembryonic tissue such as trophectoderm.

Adult Stem Cell: Cells isolated from adult tissues including bone marrow, adipose tissue, nervous tissue, skin, umbilical cord blood, and placenta that have the capacity for self-renewal. In general, adult stem cells are multipotent, having the capacity to differentiate into mature cell types of the parent tissue. Some populations of adult stem cells, such as MSCs, exhibit a range of lineage differentiation that is not limited to a single tissue type. Whether adult stem cells exhibit plasticity and can differentiate into a wider variety of differentiated cells and tissues remains controversial.

Adult Tissue–Specific Stem Cell: Same as adult stem cells but with defined tissue specificity. A relatively undifferentiated cell within a given tissue that has the capacity for self-renewal through stable maintenance within a stem cell niche. Adult tissue-specific (endogenous) stem cells have a differentiation potential equivalent to the cellular diversity of the tissue in which they reside. The hematopoietic stem cell is a prototypical adult tissue stem cell.

Induced Pluripotent Stem Cell (iPSC): Reprogrammed adult somatic cells that have undergone dedifferentiation after the expression of reprogrammingtranscription factors such as Oct 3/4, Sox2, c-Myc, and Klf4. iPSCs are similar to ESCs in morphology, proliferation, gene expression, and in the ability to form teratomas. In vivo implantation of iPSCs results in formation of tissues from all three embryonic germ layers. iPSCs have been generated from both mouse and human cells.

Progenitor Cell: A collective term used to describe any proliferative cell that has the capacity to differentiate into different cell lineages within a given tissue. Unlike stem cells, progenitor cells have limited or no self-renewal capacity. The term “progenitor cell” is commonly used to indicate a cell can expand rapidly but undergoes senescence after multiple cell doublings. Terminology that takes into account the functional distinctions among progenitor cells is suggested below.

Transit-Amplifying Cell: The progeny of a endogenous tissue stem cell that retain relatively undifferentiated character, although more differentiated than the parent stem cell, and have a finite capacity for proliferation. The sole function of transit-amplifying cells is generation of a sufficient number of specialized progeny for tissue maintenance.

Obligate Progenitor Cell: A cell that loses its ability to proliferate once it commits to a differentiation pathway. Intestinal transit-amplifying cells are obligate progenitor cells.

Facultative Progenitor Cell: A cell that exhibits differentiated features when in the quiescent state yet has the capacity to proliferate for normal tissue maintenance and in response to injury. Bronchiolar Club cells are an example of this cell type.

Classical Stem Cell Hierarchy: A stem cell hierarchy in which the adult tissue stem cell actively participates in normal tissue maintenance and gives rise to a transit-amplifying cell. Within this type of hierarchy, renewal potential resides in cells at the top of the hierarchy (i.e., the stem and transit-amplifying cell), and cells at each successive stage of proliferation become progressively more differentiated.

Nonclassical Stem Cell Hierarchy: A stem cell hierarchy in which the adult tissue stem cell does not typically participate in normal tissue maintenance but can be activated to participate in repair after progenitor cell depletion.

Rapidly Renewing Tissue: Tissue in which homeostasis is dependent on maintenance of an active mitotic compartment. Rapid turnover of differentiated cell types requires continuous proliferation of stem and/or transit-amplifying cells. A prototypical rapidly renewing tissue is the intestinal epithelium.

Slowly Renewing Tissue: Tissues in which the steady-state mitotic index is low. Specialized cell types are broadly distributed, long-lived, and a subset of these cells, the facultative progenitor cell, retain the ability to enter the cell cycle. The relative stability of the differentiated cell pool is paralleled by infrequent proliferation of stem and/or transit amplifying cells. The lung is an example of a slowly renewing tissue.

Hematopoietic Stem Cell: Cell that has the capacity for self-renewal and whose progeny differentiate into all of the different blood cell lineages including mature leukocytes, erythrocytes, and platelets.

Endothelial Progenitor Cell: Circulating cells that have the potential to proliferate and differentiate into mature endothelial cells. Studies of EPCs have been complicated by the use of the same terminology to define at least two different cell populations that have different cell surface markers, different cell sources, and different abilities to differentiate into mature endothelial cells in vitro and in vivo. There is a critical need to develop a consensus definition of EPCs with particular emphasis on the functional capabilities of these cells.

Mesenchymal Stromal (Stem) Cell (MSC): Cells of stromal origin that can self-renew and have the ability to differentiate into a variety of cell lineages. Initially described in a population of bone marrow stromal cells, they were first described as fibroblastic colony forming units subsequently as marrow stromal cells, then as mesenchymal stem cells, and most recently as multipotent mesenchymal stromal cells or MSCs. MSCs have been isolated from a wide variety of tissues, including umbilical cord blood, Wharton’s jelly, placenta, adipose tissue, and lung. The Mesenchymal and Tissue Stem Cell Committee of the International Society for Cellular Therapy has recently publisheded the minimal criteria for defining (human) MSCs. MSCs have been described to differentiate into a variety of mature cells types and may also have immunomodulatory properties.

Fibrocyte: A cell in the subset of circulating leukocytes that produce collagen and home to sites of inflammation. The identity and phenotypic characterization of circulating fibrocytes is more firmly established than that for EPCs. These cells express the cell surface markers CD34, CD45, CD13, MHC II and also express type 1 collagen and fibronectin.

Bronchiolar Stem Cell: A term applied to a rare population of toxin (i.e., naphthalene)-resistant Club cell secretory protein (CCSP)-expressing cells that localize to neuroepithelial bodies and the bronchoalveolar duct junction of the rodent lung. These cells proliferate infrequently in the steady-state but increase their proliferative rate after depletion of transit-amplifying (Club) cells. Lineage tracing studies indicate that these cells have the differentiation potential to replenish specialized cell types of the bronchiolar epithelium. Human correlates have not been identified.

Bronchioalveolar Stem Cell: A term applied to a small population of cells located at the bronchoalveolar duct junction in mice identified in vivo by dual labeling with CCSP and surfactant protein C (SPC) and by resistance to destruction with toxins (i.e., naphthalene). In culture, some of the dual-labeled cells also express Sca1 and CD34, self-renew, and give rise to progeny that express CCSP, pro-SPC, or aquaporin 5, leading to speculation that a single cell type has the capacity to differentiate into bronchiolar (Club cells) and alveolar (type 1 and 2 pneumocytes) lineages. The relationship of the cells studied in vitro to those observed by dual labeling in vivo is unclear. Human correlates have not been identified.

Adapted with permission from Reference 4.