DEFINITIONS AND GENERAL CONCEPTS ABOUT STEM CELLS

Stem cell: A stem cell is a cell from the embryo, fetus,or adult that has,under certain conditions,the ability to reproduce itself for long periods or,in the case of adult stem cells,throughout life of the organizm.It also can give rise to specialized cells that make up the tissues and organs of the body.

Pluripotent stem cell: A single pluripotent stem cell has the ability to give rise to types of cells that develop from three germ layers (mesoderm, endoderm, and ectoderm)from which all the cells of the body arise.
Embryonic stem cells: An embryonic stem cell is derived from a group of cells called he inner cell mass,which is part of the early embryo called blastocyst.

Embryonic germ cell: An embryonic germ cell is derived from fetal tissue.Specifically ,they are isolated from the primordial germ cells of the gonadal ridge of the 5-10 week fetus.

Differentiation: Differentiation is a process by which an unspecialized cell (such as a stem cell) becomes specialized into one of the many cells that make up the body.During differentiation, certain genes become activated and other genes become inactivated in an intricately regulated fashion. As a result, a differantiated cell develops specific structures and perform certain functions.For example, a mature,differantiated nevre cell has thin,fiber like projections that send and receive the electrochemical signals that permit the nevre cells to communicate with other nevre cells. In the laboratory,a stem cell can be manipulated to become specialized cell types (eg,hear muscle,nevre, or pancreatic cells.)and this is known as drected differantiation.

Adult stem cell: An adult stem cell is an undifferantiated cell that occurs in a differantiated tissue, renews itself, and becomes specialized to yield all of the specialized cell types of the tissue from which it originated.
Plasticity: Plasticity is the ability of an adult stem cell from one tissue to generate the specialized cell types of another tissue. For example,under specific experimental conditions,adult stem cells from bone marrow generated cells that resemble neurons and other cell types that are commonly found in the brain.Evidence suggests that given the right enviroment ,some adult stem cells are capable of being ‘genetically reprogrammed’ to generate specialized cells that are characteristic of different tissues.

Clonality or clonally derived stem cells: A cell is said to be clonally derived or to exhibit clonality if it was generated by the division of a single cell and is genetically identical to that cell. For researchers to fully understand and harness the ability of stem cells to generate replacement cells and tissues, the exact identity of those cells genetic capabilities and functional qualities must be known. Human pluripotent stem cells from embryos and fetal tissue are by their nature clonally derived but,very few studies have shown clonal properties of the cells that are developed from adult stem cells.

Progenitor or precursor cell: A progenitor or precursor cell occurs in fetal or adult tissues and is partially specialized; it divides and gives rise to differentiated cells. Researchers often distinguish precursor/progenitor cells from adult stem cells in the following way: when a stem cell divides,one of the two new cell is often a stem cell capable of replicating itself again.In contrast, when a progenitor/procursor cell divides,it can form more progenitor/precursor cells or it can form two specialized cells,neither of which is capable of replicating itsef. Progenitor /precursor cells can replace cells that are damaged or dead, thus maintaining the integrity and functions of a tissue such as liver or brain.Progenitor/precursor cells give rise to related types of cells-lymphocytes such as T cells, B cells and naturel killer cells, for example-but in their normal state do not generate a wide variety of cell types.