Stem cells are those cells that have the capability of self-renewal and differentiation. First identified in the hematopoietic system, they are likely to be present in many other tissues. Stem cells have altered the care of individuals with hematologic, oncologic, dermatologic, ophthalmologic, and orthopedic conditions. The range of possible applications of stem cells to medicine extends beyond the conception of stem cells as replacement parts.
The evolving role of stem cells in clinical medicine is developing along at least three lines:
- Stem cells as therapy (either to replace cell lines that have been lost or destroyed, or to modify the behavior of other cells).
- Stem cells as targets of drug therapy.
- Stem cells to generate differentiated tissue for in vitro study of disease models for drug development.
TYPES OF STEM CELLS
All stem cells share two cardinal features: they are capable of self-renewal and they can differentiate. Self-renewal is the ability of cells to proliferate without the loss of differentiation potential and without undergoing senescence (biologic aging). Self-renewal does not imply that each cell division results in two exact replicas of the stem cell; daughter cells may be either stem cells or more differentiated cells. Indeed, stem cells are hypothesized to be able to divide symmetrically (in which both daughter cells are either stem cells or differentiated cells) or asymmetrically (yielding both a stem cell and a more differentiated cell).
The potency of a stem cell is defined by the types of more differentiated cells that the stem cell can make. Stem cells can be either totipotent, pluripotent, multipotent, or unipotent.
- Totipotent cells have the capability to produce all cell types of the developing organism, including both embryonic and extraembryonic (eg, placenta) tissues.
- Pluripotent cells can only make cells of the embryo proper, but make all cells of the embryo including germ cells and cells from any of the germ layers. Therefore, they can make any cell of the body.
- Multipotent cells can only make cells within a given germ layer. For example, multipotent stem cells from a mesodermal tissue like the blood can make all the cells of the blood, but cannot make cells of a different germ layer such as neural cells (ectoderm) or liver cells (endoderm).
- Unipotent cells make cells of a single cell type. An example is a germ cell stem cell that makes the cells that mature to become egg or sperm, but not other cell types.
References: UTD