Stem cells have unique properties that other cells do not have.
First, to understand what stem cells are, you need to understand
how humans and other mammals develop:
1. Sperm fertilizes an egg and forms a single
cell called a zygote.
2. The zygote undergoes a process known as mitosis,
in which it replicates its chromosomes (which carry each
parent’s DNA) and then divides, resulting in two
identical cells. These cells are called totipotent and
have the ability to develop into a new organism. The zygote
repeats the process of mitosis for about 5 or 6 days creating
a small ball of a few hundred cells called a blastocyst.
3. The blastocyst has an outer-layer of cells called the trophoblast,
which will eventually form the protective placenta. Inside
the trophoblast is a group of cells called the inner
cell mass. The organism in a stage between zygote
to fetus is called an embryo* and the cells are called
embryonic stem cells.
At this point embryonic stem cells have the ability to
become a cell for any part of the body (nerve, muscle,
blood, etc.). This ability to become any type of cell in
the body is called pluripotent. The difference
between totipotent and pluripotent cells is only that totipotent
cells can give rise to both the placenta and the embryo.
4. As the embryo grows these pluripotent cells
develop into specialized, multipotent stem cells. Multipotent stem
cells have the ability to develop specific types of cells
(terminally differentiated cells). For
example a blood stem cell (multipotent) can develop into
a red blood cell, white blood cell or platelets (all specialized
cells). There are multipotent stem cells for all of the
different types of tissue in the body.
*Embryo: the embryo of mammals is defined as the stage
of organism between the first division of zygote and the
time it becomes a fetus through further development. For
humans, the embryo is defined as the implantation of fertilized
egg in the uterus through the eighth week of its development.
The embryo will be called a fetus after the eighth week
Stem Cells are different from other cells because:
1. They can continue to divide for long periods of time: Most
cells such as skin cells cannot replicate themselves after a certain
period of time. Stem cells are self-sustaining by replicating themselves
for a much longer period of time.
2. They are unspecialized: Specialized cells
have specific capabilities that allow them to perform certain tasks.
For example a red blood cell contains hemoglobin that allows it
to carry oxygen. Stem cells have unspecialized capability and do
not have tissue- specific structures to perform specialized functions.
3. They can give rise to specialized cells: Stem
cells go through a process called differentiation and
create special types of cells (muscle, nerve, skin, etc.).
Embryonic stem cells
Embryonic stem cells are the cells within the protective layer of the blastocyst.
They are pluripotent, which means they can develop into any of the cells
of the adult body. Researchers believe that, because they are pluripotent,
and easy to grow, they have the best potential for replacing damaged or lost
tissue or body parts.
Adult stem cells
Also known as progenitor cells or somatic stem cells,
adult stem cells are located, in small quantities, throughout the body and
generate specialized cells for the area they are located. These cells do not
renew themselves as well as embryonic stem cells. Still, if these cells are
put in a different environment, they may produce a different type of cells
from the originating cell.
Stem cell research is an active area of inquiry and scientists
are discovering new characteristics of stem cells every day. For
example, recent research indicated that multipotent stem cells
from one type of tissue (blood) might actually have the ability
to generate cells for a different type of tissue (nerve).
Scientists are continuing to search for new sources of adult stem
cells. Some of the locations where stem cells have been located
include: bone marrow, skin, liver, blood, and the brain. Some adult
stem cells, which have already been used to treat illnesses, include hematopoietic stem
cells and umbilical cord blood stem cells.
Hematopoietic stem cells
are located in the bone marrow and form blood cells. They have
been successfully used to treat blood disorders for younger patients. Umbilical
cord blood stem cells are located in the blood of the
umbilical cord after birth. Umbilical cord stem cells are similar
to hematopeitetic stem cells in adults, but they are less mature
and have much more potential to differentiate into various types
Toney. Stem cells. Detroit, Lucent Books, c2006. 128
Nancy. Stem cells: the hope and the hype. Time. v.168,
August 7, 2006: 40-46.
Robert et al. Essentials of stem cell biology. Amsterdam,
Boston, Elsevier, c2006. 548 p.
Joseph. Stem cell research: medical applications
and ethical controversy. New York, Facts on File, c2005.
178 p. (Juvenile)
Emily. From stem cell to any cell. Science News
for Kids. Oct. 19, 2005. http://www.sciencenewsforkids.org/articles/20051019
more print resources...
Search on "Stem
cells," "Stem Cells physiology"
in the Library of Congress Online
CELLS — This micrograph shows a colony of undifferentiated
human embyonic stem cells. Argonne National Laboratory Web
stem cells. National Science Foundation Web site.
depicting differentiated cells coming from embryonic stem cells. National
Institutes of Health, National Institute of General Medical Sciences
for generating embryonic stem cell cultures. USDA Center for
Biologics Evaluation and Research Web site.
Eliezer Huberman (right) and Yong Zhao discovered that the human
blood stream is a source for stem cells. Argonne National
Laboratory Web site.