Stem Cells
I. Introduction:
Stem
cells are a class of undifferentiated cells that are able to differentiate into
specialized cell types. These cells have the remarkable property to get developed
into many different cell types in the body during early life and growth. They
may even show unlimited division resulting in internal repair and prevention
from injury in some cases. As these cells are undifferentiated, each cell has a
choice either to be an undifferentiated cell (stem cell) or get converted into
other type of specialized cell having specific function like a nerve cell, a
blood cell or any other cell for that matter.
Stem cells are different
from other cells as they are the cells which are unspecialized but can renew
themselves or can take up a special function under certain conditions. Their properties
of differentiation also depend on type of organs or tissue they are present in.
For instance in organs like bone marrow, Stem cells divide regularly too repair
and replace the dead and worn out cells while in organs such as pancreas and
the heart, stem cells only divide under special conditions.
Commonly, stem cells come from two main sources:
- Embryos formed during the blastocyst phase of embryological development (embryonic stem cells) and
- Adult tissue (somatic or adult stem cells).
Both
types are generally characterized by their potential to differentiate into
different cell types (such as skin, muscle, bone, etc.). Scientists discovered ways to
derive embryonic stem cells from early mouse embryos more than 30 years ago, in
1981. The detailed study of the biology of human stem cells however came into
existence as late as, in 1998. Embryos used in these studies were created for
reproductive purposes. When they were no longer needed for that purpose, they
were donated for research with the informed consent of the donor. Afterwards
scientists have found many applications of these stem cells and their banking.
One such application is to completely reprogram some specialized cells to get
converted into stem cell like state, which can be a revolution. These stem
cells as are induced, so named induced pluripotent stem cells.
In the 3- to 5-day-old embryo, called a blastocyst, the
inner cells give rise to the entire body of the organism, including all of the
many specialized cell types and organs such as the heart, lungs, skin, sperm,
eggs and other tissues. In some adult tissues, such as bone marrow, muscle, and
brain, discrete populations of adult stem cells generate replacements for cells
that are lost through normal wear and tear, injury, or disease.
Due to these unique
properties, stem cells have great futuristic aspects as they can be used to
treat many diseases like Diabetes, thallesemia and much other deadly disease.
Although these aspects still need to be researched heavily and yet to be tested
properly but theoretically it is indeed a possibility.
Laboratory
studies of stem cells enable scientists to learn about the cells’ essential
properties and what makes them different from specialized cell types.
Scientists are already using stem cells in the laboratory to screen new drugs
and to develop model systems to study normal growth and identify the causes of
birth defects.
Embryonic
stem cells
Embryonic
stem cells are derived from a four- or five-day-old human embryo that is in the
blastocyst phase of development. Sexual reproduction begins when a male's sperm
fertilizes a female's ovum (egg) to form a single cell called a zygote. The
single zygote cell then undergo a series
of divisions, forming 2, 4, 8, 16 cells, etc. After four to six days - before
implantation in the uterus - this mass of cells is called a blastocyst. The
blastocyst consists of an inner cell mass (embryoblast) and an outer cell mass
(trophoblast). The outer cell mass becomes part of the placenta, and the inner
cell mass is the group of cells that will differentiate to become all the
structures of an adult organism. This latter mass is the source of embryonic
stem cells - totipotent cells (cells with total potential to develop into any
cell in the body). In a normal
pregnancy, the blastocyst stage continues until implantation of the embryo in
the uterus, at which point the embryo is referred to as a fetus. This usually
occurs by the end of the 10th week of gestation after all major organs of the
body have been created.
Somatic stem cells
Adult or
somatic stem cells exist throughout the body after embryonic development and
are found inside of different types of tissue. These stem cells have been found
in tissues such as the brain, bone marrow, blood, blood vessels, skeletal
muscles, skin, and the liver. They remain in a quiescent or non-dividing state
for years until activated by disease or tissue injury.
Adult
stem cells can divide or self-renew indefinitely, enabling them to generate a
range of cell types from the originating organ or even regenerate the entire
original organ. It is generally thought that adult stem cells are limited in
their ability to differentiate based on their tissue of origin, but there is
some evidence to suggest that they can differentiate to become other cell
types.
Stem cell cultures
Stem
cells are either extracted from adult tissue or from a dividing zygote in a
culture dish. Once extracted, scientists place the cells in a controlled
culture that prohibits them from further specializing or differentiating but
usually allows them to divide and replicate. The process of growing large
numbers of embryonic stem cells has been easier than growing large numbers of
adult stem cells, but progress is being made for both cell types.
Stem cell lines
Once
stem cells have been allowed to divide and propagate in a controlled culture,
the collection of healthy, dividing, and undifferentiated cells is called a
stem cell line. These stem cell lines are subsequently managed and shared among
researchers. Once under control, the stem cells can be stimulated to specialize
as directed by a researcher - a process known as directed differentiation.
Embryonic stem cells are able to differentiate into more cell types than adult
stem cells.
Potency
Stem
cells are categorized by their potential to differentiate into other types of
cells. Embryonic stem cells are the most potent since they must become every
type of cell in the body. The full classification includes:
- Totipotent - the ability to differentiate into all possible cell types. E.g. zygote formed at egg fertilization and the first few cells that result from the division of the zygote.
- Pluripotent - the ability to differentiate into almost all cell types.
E.g. embryonic stem cells and cells that are derived from the
mesoderm, endoderm, and ectoderm germ layers that are formed in the beginning
stages of embryonic stem cell differentiation.
- Multipotent - the ability to differentiate into a number of closely related cells. e.g. hematopoietic stem cells that can become red and white blood cells or platelets.
- Oligopotent - the ability to differentiate into a few cells. e.g. include (adult) lymphoid or myeloid stem cells.
- Unipotent - the ability to only produce cells of their own type.E.g. muscle stem cells.
Embryonic
stem cells are considered pluripotent instead of totipotent because they do not
have the ability to become part of the extra-embryonic membranes or the
placenta.
