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Generally, stem cells are categorized according to their source, as either adult or embryonic. The type of stem cell
most often discussed in the news is an embryonic stem cell. During fertilization, a sperm cell unites with an egg cell,
and begins to reproduce by dividing into different cells. These cells begin to arrange themselves into an outer ring of
cells that enclose an inner cell mass called a blastocyst. Researchers have collected these inner cells and discovered
that they can be made to develop into many types of specialized cells in the body .
These cells from the inner cell mass contain embryonic stem cells, which is an accurate term because they do come from the first
stages of an embryo. But once these cells are removed from the inner cell mass, they are not able to develop into an infant. Adult
stem cells are indifferentiated cells found among differentiated cells of a specific tissue and are mostly multipotent cells. They
are already being used in treatments for over one hundred diseases and conditions.
Until recently it was thought that each of these cells could produce just one particular type of cell—this is called
differentiation. However in the past few years, evidence has been gathered of stem cells that can transform into several
different forms. Bone marrow stem cells are known to be able to transform into liver, nerve, muscle and kidney cells.
Embryonic stem cell research is a less-developed field and is considered by many researchers to have greater potential
as the basis of treatments. Embryonic stem cells are cultured cells obtained from the inner mass cells of a blastocyst. Research
with embryonic stem cells is controversial because it requires destruction of embryos same like abortion, which to many people
are human beings, meaning that destroying an embryo for any reason is morally unacceptable.
In more technical terms, there are three types of stem cells:
Totipotent stem cells are cells, which are capable of forming every type of body cell. Each totipotent cell could replicate
and differentiate and become a human being. All cells within the early embryo are totipotent up until the 16-cell stage or so.
A single Totipotent stem cell can grow into an entire organism and even produce extra-embryonic tissues.
Totipotency is the ability of a single cell, usually a stem cell, to divide and produce all the differentiated
cells in an organism, or even extraembrionic tissues. For example, a plant cutting can be used to grow an entire plant.
Human development begins when a sperm fertilizes an egg and creates a single totipotent cell. In the first hours after
fertilization, this cell divides into identical totipotent cells. Approximately four days after fertilization and after
several cycles of cell division, these totipotent cells begin to specialize. Totipotent cells have total potential. They
specialize into pluripotent cells that can give rise to most, but not all, of the tissues necessary for fetal development.
Pluripotent stem cells cannot grow into a whole organism, but they are able to differentiate into cells derived from
any of the three germ layers. In cell biology, a pluripotent cell is one able to differentiate into many cell types. In
the members of Kingdom Animalia, pluripotent stem cells which can develop into any of the three major tissue types:
endoderm (interior gut lining), mesoderm (muscle, bone, blood), and ectoderm (epidermal tissues and nervous system).
Pluripotent stem cells can eventually specialize in any bodily tissue, but they cannot themselves develop into a human being.
Multipotent (also called unipotent) stem cells can only become some types of cells: e.g. blood cells, or bone cells. Although
limited in number, but multipotent stem cells can give rise to several other cell types. An example of multipotent cells is
hematopoietic cells—blood stem cells that can develop into several types of blood cells, but cannot develop into brain cells.
At the end of the long chain of cell divisions that make up the embryo are terminally differentiated cells—cells that are
considered to be permanently committed to a specific function.
Scientists have long held the opinion that differentiated cells cannot be altered or caused to behave in any way other than
the way in which they have been naturally committed. However, new research has even called that assumption into question. In
recent stem cell experiments, scientists have been able to persuade blood stem cells to behave like neurons, or brain cells.
Scientists now believe that stem cell research could reveal far more vital information about our bodies than was previously
known. There is also continuing research to see if it is possible to make multipotent cells into pluripotent types.
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