Stem Cells
Central of the three processes in an
organism are
Stem cells: development, repair of adult tissue and cancer.
What are stem cells?
Stem cells are unspecialized cells that have two defining
properties: the
ability to differentiate into other cells and the ability to
self-regenerate.
The ability to differentiate is the
potential to
develop into other cell types. A totipotent stem cell (e.g. fertilized egg) can develop into
all cell types including the embryonic membranes. A pleuripotent stem cell can
develop into cells from all three germinal
layers (e.g. cells from the inner
cell mass). Other cells can be oligopotent, bipotent or unipotent depending on their
ability to develop into few, two or one other cell type(s).2
Self-regeneration is the ability of stem cells to divide and
produce more stem cells. During early development, the cell
division is
symmetrical i.e. each cell divides to gives
rise to daughter cells each with
the same potential. Later in development, the cell divides asymmetrically with
one of the daughter cells produced also a stem cell and the other a more differentiated
cell.
The zygote is the ultimate stem cell. It is totipotent with the
ability to produce all the cell types of the species including the trophoblast
and the embryonic membranes. Development begins when the zygote undergoes
several successive cell divisions, each resulting in a doubling of the cell
number and a reduction in the cell size. At the 32- to 64-cell stage each cell
is called a blastomere.The blastomeres stick together to form a tight ball of
cells called a morula. Each of these cells retains totipotential. The next
stage is the blastocyst which consists of a hollow ball of cells; trophoblast
cells along the periphery develop into the embryonic membranes and placenta
while the inner cell mass develops into the fetus. Beyond the blastocyst stage,
development is characterized by cell migration in addition to cell division.
The gastrula is composed of three germ layers: the ectoderm, mesoderm and
endoderm. The outer layer or ectoderm gives rise to the future nervous system
and the epidermis (skin and associated organs such as hair and nails). The
middle layer or mesoderm gives rise to the connective tissue, muscles, bones
and blood, and the endoderm (inner layer) forms the gastrointestinal tract of
the future mammal.
Early in embryogenesis, some cells migrate to the primitive
gonad or genital ridge. These are the precursors to the gonad of the organism
and are called germinal cells. These cells are not derived from any of the
three germ layers but appear to be set aside earlier.
Stem cells in late development
As development proceeds, there is a loss of potential and a gain
of specialization, a process called determination. The cells of the germ layers
are more specialized than the fertilized egg or the blastomere. The germ layer
stem cells give rise to
progenitor cells (also known as progenitors or precursor
cells). For example, a cell in the endoderm gives rise to a primitive gut cell
(progenitor) which can further divide to produce a liver cell (a terminally
differentiated cell).
Role of Progenitor Cells in Development
While there is consensus in the literature
that a progenitor is a partially specialized type of stem cell, there are
differences in how progenitor cell division is described. For instance,
according to one source, 3 when a stem cell
divides at least one of the daughter cells it produces is also a stem cell;
when a progenitor cell undergoes cell division it produces two specialized
cells. A different source, 2 however, explains
that a progenitor cell undergoes asymmetrical cell division, while a stem cell
undergoes symmetrical cell division.
The apparent inconsistency of these two versions illustrates the
diversity and complexity of progenitor cells and their role in differentiation.
This diversity is reflected
More abstracts about the Stem Cells