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Human Embryonic Stem Cells
[1]
By: Wu, Ke Keywords: Stem cells [2] Differentiation [3] Human development [4]
Stem cells are undifferentiated cells that are capable of dividing for long periods of time and
can give rise to specialized cells under particular conditions. Embryonic stem cells [5] are a
particular type of stem cell derived from embryos. According to US National Institutes of Health
[6] (NIH), in humans [7], the term ?embryo? applies to a fertilized egg [8] from the beginning of
division up to the end of the eighth week of gestation [9], when the embryo becomes a fetus [10].
Between fertilization [11] and the eighth week of gestation [9], the embryo undergoes multiple
cell divisions. At the eight-cell stage, roughly the third day of division, all eight cells are
considered totipotent, which means the cell has the capability of becoming a fully developed
human being. By day four, cells begin to separate and form a spherical layer which eventually
becomes the placenta [12] and tissue that support the development of the future fetus [10]. A
mass of about thirty cells, called the inner cell mass [13], forms at one end of the sphere and
eventually becomes the body. When the sphere and inner cell mass [13] are fully formed,
around day 5, the pre-implantation [14] embryo is referred to as a blastocyst [15]. At this point the
cells in the inner cell mass [13] have not yet differentiated, but have the ability to develop into
any specialized cell type that makes up the body. This property is known as pluripotency [16].
As of 2009, embryonic stem cells [17] refer to pluripotent cells that are generally derived from
the inner cell mass [13] of blastocysts.
In November 1998, two independent publications announced the first successful isolation and
culture of pluripotent human stem cells [5]. While working at the Wisconsin National Primate
Research Center [18], located at the University of Wisconsin-Madison, James A. Thomson [19]
and his team of researchers cultured human embryonic stem cells [17] from the inner cell mass
[13] of donated embryos originally produced for in vitro fertilization [20]. The characteristics of the
cultured cells were consistent with previously identified features in animal stem cells [5]. They
were capable of long-term self-renewal and thus could remain undifferentiated for long
periods of time; they had particular surface markers; and they were able to maintain a normal
and stable karyotype [21]. Thomson?s team also observed derivatives of all the three germ
layers?endoderm, mesoderm [22], and ectoderm [23]. Since the three germ layers [24] precede
differentiation [25] into all the cell types in the body, this observation suggested that the cultured
cells were pluripotent. The team published ?Embryonic Stem Cell Lines Derived from Human
Blastocysts,? in the 6 November Science issue. Soon afterwards, a research team led by
John D. Gearhart at the Johns Hopkins School of Medicine, published ?Derivation of
Pluripotent Stem Cells from Cultured Human Primordial Germ Cells? in Proceedings of the
National Academy of Science. The paper detailed the process by which pluripotent stem cells
[5] were derived from gonadal ridges [26] and mesenteries [27] extracted from aborted five-to-nine
week old human embryos. Gearhart and his team noted the same observations as
Thomson?s team. Despite coming from different sources, according to NIH, the resultant cells
seem to be the same.
The largest source of blastocysts for stem cell research comes from in vitro [28] fertilization [11]
(IVF) clinics. Used for reproductive purposes, IVF usually produces an abundance of viable [29]
blastocysts. Excess blastocysts are sometimes donated for research purposes after obtaining
informed consent [30] from donors. Another potential method for producing embryonic stem
cells [17] is somatic cell nuclear transfer [31] (SCNT). This has been successfully done using
animal cells. The nucleus [32] of a differentiated adult cell, such as a skin cell, is removed and
fused with an enucleated egg [33], an egg [33] with the nucleus [32] removed. The egg [33], now
containing the genetic material from the skin cell, is believed to be totipotent and eventually
develops into a blastocyst [15]. As of mid-2006, attempts to produce human embryonic stem
cells [17] using SCNT have been unsuccessful. Nonetheless, scientists continue to pursue this
method because of the medical and scientific implications of embryonic stem cells [17] lines
with an identical genetic makeup to particular patients. One problem faced in tissue
transplants is immune rejection, where the host body attacks the introduced tissue. SCNT
would be a way to overcome the incompatibility problem by using the patient?s own somatic
cells.
Recent discoveries in cultivating human embryonic stem cells [17] may potentially lead to major
advancements in understanding human embryogenesis [34] and medical treatments.
Previously, limitations in access and environmental control have stunted research initiatives
aimed at mapping out the developmental process. Insights into differentiation [25] factors may
lead to treatments into such areas as birth defects [35]. Manipulation of the differentiation [25]
process may then lead to large supplies of stem cells [5] for cell-based therapies [36] on patients
with Parkinson?s disease, for example. In theory adult stem cells [37] can also be cultivated for
such purposes, but isolating and identifying adult stem cells [37] has been difficult and the
prospects for treatment are more limited than using embryonic stem cells [17].
Despite the potential benefits that may come about through human embryonic stem cell
research [38], not everyone in the public embraces it. Several ethical debates surround this
newly developing research field. Much of the debate stems from differing opinions on how we
should view embryos: is an embryo a person? Should an embryo be considered property?
Ethical concerns in embryonic stem cell research [38] include destroying human blastocysts,
laws surrounding informed consent [30], and particularly for SCNT, misapplication of
techniques for reproductive cloning [39]. For the latter concern, SCNT does produce a
blastocyst [15] which contains stem cell ?clones? of an adult cell, but the desired application is
in growing replacement tissues. Still, a portion of the public fears the hypothetical ?one day,?
when someone decides to use SCNT to develop and raise a human clone.
The public debate continues, advancing along with the changes in the field. As of 2006, public
opinion polls showed that majority of religious and non-religious Americans now support
embryonic stem cell research [38], but opinions remain divided over whether it is legitimate to
create or use human blastocysts solely for research.
Sources
1. Maienschein, Jane. Whose View of Life?: Embryos, Cloning, and Stem Cells.
Cambridge, MA: Harvard University Press [40], 2003.
2. Stem Cell Basics. Bethesda, MD: National Institutes of Health [6], US Department of
Health and Human Services [41]. http://stemcells.nih.gov/info/basics/defaultpage [42]
(Accessed February 3, 2009).
3. Understanding Stem Cells: An Overview of the Science and Issues from the National
Academies. National Academy of Sciences [43], National Academy of Engineering,
Institute of Medicine, and the National Research Council [44].
http://dels.nas.edu/bls/stemcells/basics.shtml [45] (Accessed March 9, 2009).
Stem cells are undifferentiated cells that are capable of dividing for long periods of time and
can give rise to specialized cells under particular conditions. Embryonic stem cells are a
particular type of stem cell derived from embryos. According to US National Institutes of
Health (NIH), in humans, the term "embryo" applies to a fertilized egg from the beginning of
division up to the end of the eighth week of gestation, when the embryo becomes a fetus.
Between fertilization and the eighth week of gestation, the embryo undergoes multiple cell
divisions. At the eight-cell stage, roughly the third day of division, all eight cells are considered
totipotent, which means the cell has the capability of becoming a fully developed human
being. By day four, cells begin to separate and form a spherical layer which eventually
becomes the placenta and tissue that support the development of the future fetus. A mass of
about thirty cells, called the inner cell mass, forms at one end of the sphere and eventually
becomes the body. When the sphere and inner cell mass are fully formed, around day 5, the
pre-implantation embryo is referred to as a blastocyst. At this point the cells in the inner cell
mass have not yet differentiated, but have the ability to develop into any specialized cell type
that makes up the body. This property is known as pluripotency. As of 2009, embryonic stem
cells refer to pluripotent cells that are generally derived from the inner cell mass of blastocysts.
Subject
Stem cells. [46] Stem Cells [47]
Topic
Processes [48] Reproduction [49]
Publisher
Arizona State University. School of Life Sciences. Center for Biology and Society. Embryo
Project Encyclopedia.
Rights
© Arizona Board of Regents Licensed as Creative Commons Attribution-NonCommercialShare Alike 3.0 Unported (CC BY-NC-SA 3.0) http://creativecommons.org/licenses/by-ncsa/3.0/
Format
Articles [50]
Last Modified
Wednesday, September 25, 2013 - 15:53
DC Date Accessioned
Thursday, May 10, 2012 - 14:06
DC Date Available
Thursday, May 10, 2012 - 14:06
DC Date Created
2010-09-13
DC Date Issued
Thursday, May 10, 2012
DC Date Created Standard
Monday, September 13, 2010 - 07:00
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Links:
[1] https://embryo.asu.edu/pages/human-embryonic-stem-cells
[2] https://embryo.asu.edu/keywords/stem-cells
[3] https://embryo.asu.edu/keywords/differentiation
[4] https://embryo.asu.edu/keywords/human-development
[5] https://embryo.asu.edu/search?text=stem%20cells
[6] https://embryo.asu.edu/search?text=National%20Institutes%20of%20Health
[7] https://embryo.asu.edu/search?text=humans
[8] https://embryo.asu.edu/search?text=fertilized%20egg
[9] https://embryo.asu.edu/search?text=gestation
[10] https://embryo.asu.edu/search?text=fetus
[11] https://embryo.asu.edu/search?text=fertilization
[12] https://embryo.asu.edu/search?text=placenta
[13] https://embryo.asu.edu/search?text=inner%20cell%20mass
[14] https://embryo.asu.edu/search?text=implantation
[15] https://embryo.asu.edu/search?text=blastocyst
[16] https://embryo.asu.edu/search?text=pluripotency
[17] https://embryo.asu.edu/search?text=embryonic%20stem%20cells
[18] https://embryo.asu.edu/search?text=Wisconsin%20National%20Primate%20Research%20Center
[19] https://embryo.asu.edu/search?text=Thomson
[20] https://embryo.asu.edu/search?text=in%20vitro%20fertilization
[21] https://embryo.asu.edu/search?text=karyotype
[22] https://embryo.asu.edu/search?text=mesoderm
[23] https://embryo.asu.edu/search?text=ectoderm
[24] https://embryo.asu.edu/search?text=germ%20layers
[25] https://embryo.asu.edu/search?text=differentiation
[26] https://embryo.asu.edu/search?text=gonadal%20ridges
[27] https://embryo.asu.edu/search?text=mesenteries
[28] https://embryo.asu.edu/search?text=in%20vitro
[29] https://embryo.asu.edu/search?text=viable
[30] https://embryo.asu.edu/search?text=informed%20consent
[31] https://embryo.asu.edu/search?text=somatic%20cell%20nuclear%20transfer
[32] https://embryo.asu.edu/search?text=nucleus
[33] https://embryo.asu.edu/search?text=egg
[34] https://embryo.asu.edu/search?text=embryogenesis
[35] https://embryo.asu.edu/search?text=birth%20defects
[36] https://embryo.asu.edu/search?text=cell-based%20therapies
[37] https://embryo.asu.edu/search?text=adult%20stem%20cells
[38] https://embryo.asu.edu/search?text=embryonic%20stem%20cell%20research
[39] https://embryo.asu.edu/search?text=cloning
[40] https://embryo.asu.edu/search?text=Harvard%20University%20Press
[41]
https://embryo.asu.edu/search?text=US%20Department%20of%20Health%20and%20Human%20Services
[42] http://stemcells.nih.gov/info/basics/defaultpage
[43] https://embryo.asu.edu/search?text=National%20Academy%20of%20Sciences
[44] https://embryo.asu.edu/search?text=National%20Research%20Council
[45] http://dels.nas.edu/bls/stemcells/basics.shtml
[46] https://embryo.asu.edu/library-congress-subject-headings/stem-cells
[47] https://embryo.asu.edu/medical-subject-headings/stem-cells
[48] https://embryo.asu.edu/topics/processes
[49] https://embryo.asu.edu/topics/reproduction
[50] https://embryo.asu.edu/formats/articles