DEFINITIONS AND GENERAL CONCEPTS ABOUT STEM
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.
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.