Archive for the ‘stem cell’ Category
- 1908 – The term “stem cell” was proposed for scientific use by the Russian histologist Alexander Maksimov (1874–1928) at congress of hematologic society in Berlin. It postulated existence of haematopoietic stem cells.
- 1960s – Joseph Altman and Gopal Das present scientific evidence of adult neurogenesis, ongoing stem cell activity in the brain; like André Gernez, their reports contradict Cajal’s “no new neurons” dogma and are largely ignored.
- 1963 – McCulloch and Till illustrate the presence of self-renewing cells in mouse bone marrow.
- 1968 – Bone marrow transplant between two siblings successfully treats SCID.
- 1978 – Haematopoietic stem cells are discovered in human cord blood.
- 1981 – Mouse embryonic stem cells are derived from the inner cell mass by scientists Martin Evans, Matthew Kaufman, and Gail R. Martin. Gail Martin is attributed for coining the term “Embryonic Stem Cell”.
- 1992 – Neural stem cells are cultured in vitro as neurospheres.
- 1997 – Leukemia is shown to originate from a haematopoietic stem cell, the first direct evidence for cancer stem cells.
- 1998 – James Thomson and coworkers derive the first human embryonic stem cell line at the University of Wisconsin–Madison.
- 2000s – Several reports of adult stem cell plasticity are published.
- 2001 – Scientists at Advanced Cell Technology clone first early (four- to six-cell stage) human embryos for the purpose of generating embryonic stem cells.
- 2003 – Dr. Songtao Shi of NIH discovers new source of adult stem cells in children’s primary teeth.
- 2004–2005 – Korean researcher Hwang Woo-Suk claims to have created several human embryonic stem cell lines from unfertilised human oocytes. The lines were later shown to be fabricated.
- 2005 – Researchers at Kingston University in England claim to have discovered a third category of stem cell, dubbed cord-blood-derived embryonic-like stem cells (CBEs), derived from umbilical cord blood. The group claims these cells are able to differentiate into more types of tissue than adult stem cells.
- August 2006 – Rat Induced pluripotent stem cells: the journal Cell publishes Kazutoshi Takahashi and Shinya Yamanaka.
- October 2006 – Scientists at Newcastle University in England create the first ever artificial liver cells using umbilical cord blood stem cells.
- January 2007 – Scientists at Wake Forest University led by Dr. Anthony Atala and Harvard University report discovery of a new type of stem cell in amniotic fluid. This may potentially provide an alternative to embryonic stem cells for use in research and therapy.
- June 2007 – Research reported by three different groups shows that normal skin cells can be reprogrammed to an embryonic state in mice.In the same month, scientist Shoukhrat Mitalipov reports the first successful creation of a primate stem cell line through somatic cell nuclear transfer
- October 2007 – Mario Capecchi, Martin Evans, and Oliver Smithies win the 2007 Nobel Prize for Physiology or Medicine for their work on embryonic stem cells from mice using gene targeting strategies producing genetically engineered mice (known as knockout mice) for gene research.
- November 2007 – Human induced pluripotent stem cells: Two similar papers released by their respective journals prior to formal publication: in Cell by Kazutoshi Takahashi and Shinya Yamanaka, “Induction of pluripotent stem cells from adult human fibroblasts by defined factors”, and in Science by Junying Yu, et al., from the research group of James Thomson, “Induced pluripotent stem cell lines derived from human somatic cells”:pluripotent stem cells generated from mature human fibroblasts. It is possible now to produce a stem cell from almost any other human cell instead of using embryos as needed previously, albeit the risk of tumorigenesis due to c-myc and retroviral gene transfer remains to be determined.
- January 2008 – Robert Lanza and colleagues at Advanced Cell Technology and UCSF create the first human embryonic stem cells without destruction of the embryo
- January 2008 – Development of human cloned blastocysts following somatic cell nuclear transfer with adult fibroblasts
- February 2008 – Generation of pluripotent stem cells from adult mouse liver and stomach: these iPS cells seem to be more similar to embryonic stem cells than the previous developed iPS cells and not tumorigenic, moreover genes that are required for iPS cells do not need to be inserted into specific sites, which encourages the development of non-viral reprogramming techniques.
- March 2008-The first published study of successful cartilage regeneration in the human knee using autologous adult mesenchymal stem cells is published by clinicians from Regenerative Sciences
- October 2008 – Sabine Conrad and colleagues at Tübingen, Germany generate pluripotent stem cells from spermatogonial cells of adult human testis by culturing the cells in vitro under leukemia inhibitory factor (LIF) supplementation.
- 30 October 2008 – Embryonic-like stem cells from a single human hair.
- 1 March 2009 – Andras Nagy, Keisuke Kaji, et al. discover a way to produce embryonic-like stem cells from normal adult cells by using a novel “wrapping” procedure to deliver specific genes to adult cells to reprogram them into stem cells without the risks of using a virus to make the change.The use of electroporation is said to allow for the temporary insertion of genes into the cell.
- 5 March 2009 Australian scientists find a way to improve chemotherapy of mouse muscle stem cells.
- 28 May 2009 Kim et al. announced that they had devised a way to manipulate skin cells to create patient specific “induced pluripotent stem cells” (iPS), claiming it to be the ‘ultimate stem cell solution’.
The patents covering a lot of work on human embryonic stem cells are owned by the Wisconsin Alumni Research Foundation (WARF). WARF does not charge academics to study human stem cells but does charge commercial users. WARF sold Geron Corp. exclusive rights to work on human stem cells but later sued Geron Corp. to recover some of the previously sold rights. The two sides agreed that Geron Corp. would keep the rights to only thre call types. In 2001, WARF came under public pressure to widen access to human stem cell technology.
These patents are now doubt as a request for review by the US Patent and Trademark Office has been filed by non-profit patent-watchdog The Foundation for Taxpayer & Conumer Rights and the Public Patent Foundation as well as molecular biologist Jeanne Loring of the Burnham Institute. According to them, two of the patents granted to WARF are invalid because they cover a technique published in 1993 for which a patent had already been granted to an Australian researcher. Another part of the challenge states that these techniques, developed by James A. Thomson, are rendered obvious by a 1990 paper and two textbooks.
The outcome of this ilegal challenge is particuarly relevant to the Geron Corp. as it can only license patents that are upheld.
Stem cell laws are the law, rules , and policy governance concerning the sources, research, and uses in treatment of stem cells in humans. These laws have been the source of much controversy and vary significanctly by country. In the European Union, stem cell research using the human embryo is permitted in Sweden, Finland, Belgium, Greece, Britain Denmark and the Netherlands; however, it is illegal in Germany, Austria, Ireland, Italy, and Portugal. The issue has similarly divided the United States, with several states enforcing a complete ban and others giving financial support. Elsewhere, Japan, India, Iran, Israel, South Korea, China, and Australia are supportive. However, New Zealand, most of Africa (except South Africa), and most of South America (except Brazil) are restrictive.
Stem cell are cells found in most, if not all, multi-cellular organisma. A common example of a stem cell is the Hematopoietic stem cell (HSC) which are multipotent stem cells that give rise to cells of the blood lineage. In contrast to multipotent stem cells, embryonic stem cells are pluripotent and are thought to be able to give rise to all cells of the body. Embryonic stem cells were isolated in mice in 1981, and in humans in 1998.
Stem cell treatments are a type of cell therapy that introduce new cells into dult bodies for possible treatment of cancer, Somatic cell nuclear transfer, diabetes, abd ither mdical conditions. Cloning also might be done with stem cells. Stem cells have been used to repair tissue damaged by disease.
Until the mid-2000s, the only source of human stem cells was donated embryos from miscarriages and abortions. In January 2007, researchers at Wake Forest University reported that “stem cells drwan from amniotic fluid donated b pregnant women hold much of the same promise as embryonic stem cells.
In 2008, the NIH, under the administration of President Bill Clinton, issued guidelines that allow federal funding of embronic stem-cell research.
The European Union has yet to issue consistent regulations with respect to stem cell research in member states. Whereas Germany, Austria, Italy, Finland, Greece, Ireland, Portugal and the Netherlands prohibit or severely restrict the use of embryonic stem cells, Sweden and the United Kingdom have created the legal basis to support this research. Belgium bans reproductive cloning but allows therapeutic cloning of embryos.France prohibits reproductive cloning and embryo creation for research purposes, but enacted laws (with a sunset provision expiring in 2009) to allow scientists to conduct stem cell research on imported surplus embryos from in vitro fertilization treatments.Germany has restrictive policies for stem cell research, but a 2008 law authorizes “the use of imported stem cell lines produced before May 1, 2007.”Italy has a 2004 law that forbids all sperm or egg donations and the freezing of embryos, but allows, in effect, using existing stem cell lines that have been imported. Sweden forbids reproductive cloning, but allows therapeutic cloning and authorized a stem cell bank.
In 2001, the British Parliament amended the Human Fertilization and Embryology Act to permit the destruction of embryos for hESC harvests but only if the research satisfies one of the following requirements:
- Increases knowledge about the development of embryos,
- Increases knowledge about serious disease, or
- Enables any such knowledge to be applied in developing treatments for serious disease.
The United Kingdom is one of the leaders in stem cell research, in the opinion of Lord Sainsbury, Science and Innovation Minister for the UK. A new £10 million stem cell research centre has been announced at the University of Cambridge.
In 2004, South Africa became the first African nation to create a stem cell bank. Prior to this, the South African government had enacted legislation maintaining a ban on reproductive cloning but authorizing the therapeutic cloning of embryos.
China prohibits human reproductive cloning but allows the creation of human embryos for research and therapeutic purposes. India banned in 2004 reproductive cloning, permitted therapeutic cloning. In 2004, Japan’s Council for Science and Technlogy Policy voted to allow scientists to conduct stem cell research for therapeutic purposes, though formal gudelines have yet to be released. The South Korean government promotes therapeutic cloning, but forbids cloning. In 1999, Israel passed legislation banning reproductive, but not therapeutic, cloning, and leading scientists. Sausi Arabia religious oficials issued a decree that sanctions the use of embryos for therapeutic and research purposes. According to the Royan Institute for Reproductive Biomedicine, Iran has some of the most liberal laws on stem cell research and cloning.
In 2006, Canada enacted a law permitting research on discarded embryos from in vitro fertilization procedures. However, it prohibits the creation of human embryos for research. Brazil has passed legisation to permit stem cell research using excess in vitro fertilized embryos that have been for at least three years.
No federal law ever banned stem cell research in the United States, but only placed restrictions on funding and use, under Congress’s power to spend.In 2001, George W. Bush implemented a policy limiting the number of stem cell lines that could be used for research. There were some state laws concerning stem cells that were passed in the mid-2000s. New Jersey’s 2004 S1909/A2840 specifically permitted human cloning for the purpose of developing and harvesting human stem cells, and Missouri’s 2006 Amendment Two legalized certain forms of embryonic stem cell research in the state. On the other hand, Arkansas, Indiana, Louisiana, Michigan, North Dakota and South Dakota passed laws to prohibit the creation or destruction of human embryos for medical research.
During Bush’s second term, in July 2006, he used his first Presidential veto on the Stem Cell Research Enhancement Act. The Stem Cell Research Enhancement Act was the name of two similar bills, and both were vetoed by President George W. Bush and were not enacted into law. New Jersey congressman Chris Smith wrote a Stem Cell Therapeutic and Research Act of 2005, which made some narrow exceptions, and was signed into law by President George W. Bush.
President Barack Obama signed into law on February 17, 2009, which amongst many other provisions, funds research into new lines of embryonic stem cells. The Omnibus Appropriations Act of 2009 contains provisions on embryonic stem cell research.
Australia is partially supportive (exempting reproduntive cloning yet allowing research on embryonic stem cells that are derived from the process of IVF). New Zealand, however, restricts stem cell research.
The stem cell controversy is the ethical debate centered only on reearch involving the creation, usage and destruction of human embryos. Most common, this contoversy focuses on embryonic stem cells.
Since stem cell have the ability to differentiate into any type of cell, they offer something in the development of medical treatments for a wide range of conditions. Treatments that have been proposed include treatment for physical trauma, degenerative conditions, and genetic diseases (in combination with gene therapy). Yet further treatments using stem cells could potentially be developed thanks to their ability to repair extensive tissue damage.
Great levels of success and potential have been demostrated from research using adult stem cells. Earlier this ear, the FDA approved the firat human clinical trials using embryonic stem cells. Some researchers are of the opinion that the differentiation potential of embryonic stem cells in broader than most adult stem cells. Embryonic stem cells can become all cell types of the body because they are Pluripotent. Adult stem cells are generally limited to differentiating into different cell types of their tissue origin. However, some evidence sugests taht adult stem cell plasticity may exist, incrasing the number of cell tpes a given adult stem cell can become. In addition, embryonic stem cells are considered more useful for nervous system therapies, because researchers have struggled to identify and isolate neural progenitors from adult tissues. Embryonic stem cells, however, might be rejected by the immune system - a problem which wouldn’t occur if the patient received his or her own stem cells.
Some stem cell researchers are working to develop techniques of isolating stem cells that are as potent as embryonic stem cells, but do not require a human embryo.
Some believe that human skin cells can be coaxed to “defferrentiate” and devert to an embryonic state. Researchers at Harvard Universuty, led by Kevin Eggan, have attempted to transfer the nucleus of a somatic cell into an existing embryonic stem cell, thus creating a new stem cell line. Another study pulished in August 2006 also indicates that differentiated cells can be reprogrammed to an embryonic-like state by introducing four specific factors, resulting in induced pluripotent stem cells.
Researchers at Advanced Cell Technology, led by Robert Lanza, reported the successful derivation of a stem cell line using a process similar to preimplantation genetic diagnosis, inwhich a single blastomere is extracted from a blastocyst. At least 2007 meeting of the International Society for Stem Cell Research (ISSCR), Lanza announced that his team had succeeded in producing three new stem cell lines without destroying the parent embryos. “These are the first human embryonic cell lines in existence that didn’t result from the destruction of an embtyo. “Lanza is currently in discussions with the National Institutes of Health (NIH) to determine whether the new technique sidesteps U.S. restrictions on federal funding for ES cell research.
Accordig to a January 9, 2007 Daily Telegraph (London) article reporting on a statement by Dr. Anthony Atala of Wake Forest University, the fluid surrounding the fetus has been found to contain stem cells, that, when utilized correctly, “can be differentiated towards cell types such as fat, bone, muscle, blood vessel, nerve and liver cells”, according to the article. The extraction of this fluid is not thought to harm the fetus in any way. “Our hope is that these cells will provide a valuable resource for tissue repair and for engineered organs as well,” said Dr Atala.
The status of the human embryo and human embryonic stem cell research is a controversial issue as, with the present state of technology, the creation of a human embryonic stem cell line requires the destruction of a human embryo. Stem cell debates have motivated and reinvigorated the pro-life movement, whose members are concerned with the rights and status of the embryo as an early-aged human life. They believe that embryonic stem cell research instrumentalizes and violates the sanctity of life and is tantamount to murder. The fundamental assertion of those who oppose embryonic stem cell research is the belief that human life is inviolable, combined with the fact that human life begins when a sperm cell fertilizes an egg cell to form a single cell.
A portion of stem cell researchers use embryos that were created but not used in vitro fertility treatments to derive new sterm cell lines. Most of these embryos are to be destroyed, or stored for long periods of time, long past their viable storage life. In the United States alone, there have been estimates of at least 400,000 such embryos. This has led some opponents of abortion, such as Senator Orrin Hatch, to support human embryonic stem cell research.
Medical researchers widely submit that stem cell research has the potential to dramatically alter approaches to understanding and treating diseases, and to alleviate suffering. In the future, most medical researchers anticipate being able to use technologies derived from stem cell research to treat a variety of diseases and impairments. Spinal cord injuries and Parkinson’s disease are two examples that have been championed by high-profile media personalities (for instance, Christopher Reeve and Michael J. Fox). The anticipated medical benefits of stem cell research add urgency to the debates, which has been appealed to by proponents of embryonic stem cell research.
In August, 2000, The U.S. National Institutes of Health’s Guidelines stated:
“…research involving human pluripotent stem cells…promises new treatments and possible cures for many debilitating diseases and injuries, including Parkinson’s disease, diabetes, heart disease, multiple sclerosis. burns and spinal cord injuries. The NIH believes the potential medical benefits of human pluripotent stem cell technology are compelling and worthy of persuit in accordance with appropriate ethical standards.
In 2006, researchers at Advanced Cell Technology of Worcester, Mass., succeeded in obtaining stem cells from mouse embryos without destroying the embryos. If this technique and its reliability are improved, it would alleviate some of the ethical concerns related to embryonic stem cell research.
Another technique announced in 2007 may also defuse the longstanding debate and controversy. Research teams in the United States and Japan have developed a simple and cost effective method of reprogramming human skin cells to function much like embryonic stem cells by introducing artificil viruses.. While extracting and cloning stem cells is complex and extremely expensive, the newly discovered method of reprogramming cells is much cheaper. However, the technique may disrupt the DNA in the new stem cells, resulting in damaged and cancerous tissue. More research will be required before non-cancerous stem cells can be created.
Embryonic stem cells have the potential to grow indefinitely in a laboratory environment and can differentiate into almost all types of bodily tissue. This makes embryonic stem cells a prospect for cellular therapies to treat a wide range of diseases.
Human potential and humanity
This argument often goes hand-in-hand with the utilitarian argument, and can be presented in several forms:
Embryos are not equivalent to human life while they are still in capable of surviving outside the womb (i.e. they only have the potential for life)
More than a third of zygotes do not implant after conception. Thus, far more embryos are lost due to chance than are proposed to be used for embryonic stem cell research or treatments.
Blastocysts are a cluster of human cells that have not differentiated into distict organ tissue; making cells of the inner cell mass no more “human” than a skin cell.
Some parties contend that embryos are not humas, believing that the life of Homo sapiens only begins when the heartbeat develops, which is during the 5th week of pregnancy, or when the brain begins developing activity, which has been detected at 54 days after conception.
In vitro fertilization (IVF) generates large numbers of unused embryos (e.g.70,000 in Australian alone). Many of these thousands of IVF embryos are slated for destruction. Using them for scientific research utilizes a resource that would otherwisw be wasted.
While the destruction of human embryos is required to establish a stem cell line, no new embryos have to be destroyed to work with existing stem cell lines. It would be wasteful not to continue to make use of these cell lines as a resource.
Abortions are legal in many countries and jurisdictions. A logical argument follows that if these embryos are being destroyed anyway, why not use the, for stem cell research or treatments?
This is usually presented as a caounter-argument to using adult stem cells as am alternative that doesn’t involve embryonic destruction.
Embryonic stem cells make up a significant proportion of a developing embryo, while adult stem cells exist as minor populations within a mature individual (e.g. in every 1,000 cells of the bone marrow, only 1 will be a usable stem cell). Thus, embryonic stem cells are likel to be easier to isolate and grow ex vivo than adult stem cells.
Embryonic stem cells divide more rapidly than adult stem cells, potentially making it easier to regenerate large numbers of cells for therapeutic means. In contrast, adult stem cell might not divide fast enough to offer immediate treatment.
Embryonic stem cells have greater plasticity, potentially allowing them to treat a wider range of diseases.
Adult stem cells from the patient’s own body might not be effective in treatment of genetic disorders. Allogeneic embryonic stem cell transplantation (i.e. from a healthy donor) may be more practical in these cases than gene therapy of a patient’s own cell.
DNA abnormalities found in adult stem cells that are caused by toxins and sunlight may make them poorly suited for treatment.
Embryonic stem cells have been shown to be effective in treating heart damage in mice.
Embryonic stem cels have the potential to cure chronic and degenerative disease which current medicine has been unable to effectively treat.
Beginning of life
Before the primitive streak is formed when the embryo attaches to the uterus at approximately 14 days after fertilization, a single fertilized egg can split in two to form identical twins, or a pair of embryos that would have resulted in fraternal twins can fuse together and develop into one person (a tetragametic chimera). Since a fertilized egg has the potential to be two individuals or half of one, some believe it can only be considered a potential person, not an actual one. Those who subscribe to this belief then hold that destroying a blastovyst for embryonic stem cells is ethical.
Value of life
The deliberate destruction of a human embro is typically interpreted as being incompatible with Roman Catholic doctrine. Based upon these interpretations, some Catholics have suggested that human bastocysts are inherently valuable and should not be voluntarily destroyed.
Viability is another standard under which embryos and fetuses have been regarded as human lives. In the United States, the 1973 Supreme Court case of Roe v.Wade concluded that viability determined the permissibility of abortions performed for reasons other than the protection of the woman’s health, defining viability as the point at which a fetus is “potentially able to live outside the mother’s womb, albeit with artificial aid.” The point of viability was 24 to 28 weeks when the case was decided and has since moved to about 22 weeks due to advancement in medical technology.
This argument is used by opponents of embryonic destruction as well as researchers specializing in adult stem cell research.
It is often claimed by pro-life supporters that the use of adult stem cells from sources such as umbilical cord blood has consistently produced more promising results than the use of embryonic stem cells.Furthermore, adult stem cell research may be able to make greater advances if less money and resources were channeled into embryonic stem cell research.
Adult stem cells have already produced therapies, while embryonic stem cells have not.Moreover, there have been many advances in adult stem cell research, including a recent study where pluripotent adult stem cells were manufactured from differentiated fibroblast by the addition of specific transcription factors. Newly created stem cells were developed into an embryo and were integrated into newborn mouse tissues, analogous to the properties of embryonic stem cells.
This argument remains hotly debated on both sides. Those critical of embryonic stem cell research point to a current lack of practical treatments, while supporters argue that advances will come with more time and that breakthroughs cannot be predicted.
Stated views of groups
Governmental policy stances in Europe
Austria, Denmark, France, Germany, and Ireland do not allow the production of embryonic stem cell lines, but the creation of embryonic stem cell lines is permitted in Finland, Greece, the Netherlands, Swedenm and the United Kingdom.
Governmantal Policy debate in the United States
In 1973, Roe v. Wade legalized abortion in the United States. Five years later, the first successful human in vitro fertilization resulted in the birth of Louise Brown in England. These developments prompted the federal government to create regulations barring the use of federal funds for research that experimented on human embryos. In 1995, the NIH Human Embryo Research Panel advised the administration of President Bill Clinton to permit federal funding for research on embryos left over from in vitro fertility treatments and also recommended federal funding of research on embryos specifically created for experimentation. In response to the panel’s recommendations, the Clinton administration, citing moral and ethical concerns, declined to fund research on embryos created solely for research purposes, but did agree to fund research on left-over embryos created by in vitro fertility treatments. At this point, the Congress intervened and passed the Dickey Amendment in 1995 (the final bill, which included the Dickey Amendment, was signed into law by Bill Clinton) which prohibited any federal funding for the Department of Health and Human Services be used for research that resulted in the destruction of an embryo regardless of the source of that embryo.
In 1998, privately funded research led to the breakthrough discovery of Human Embryonic Stem Cells (hESC). This prompted the Clinton Administration to re-examine guidelines for federal funding of embryonic research. In 1999, the president’s National Bioethics Advisory Commission recommended that hESC harvested from embryos discarded after in vitro fertility treatments, but not from embryos created expressly for experimentation, be eligible for federal funding. Even though embryos are always destroyed in the process of harvesting hESC, the Clinton Administration decided that it would be permissible under the Dickey Amendment to fund hESC research as long as such research did not itself directly cause the destruction of an embryo. Therefore, HHS issued its proposed regulation concerning hESC funding in 2001. Enactment of the new guidelines was delayed by the incoming George W. Bush administration which decided to reconsider the issue.
President Bush announced, on August 9, 2001 that federal funds, for the first time, would be made available for hESC research on currently existing embryonic stem cell lines. President Bush authorized research on existing human embryonic stem cell lines, not on human embryos under a specific, unrealistic timeline in which the stem cell lines must have been developed. However, the Bush Administration chose not to permit taxpayer funding for research on hESC cell lines not currently in existence, thus limiting federal funding to research in which “the life-and-death decision has already been made”. The Bush Administration’s guidelines differ from the Clinton Administration guidelines which did not distinguish between currently existing and not-yet-existing hESC. Both the Bush and Clinton guidelines agree that the federal government should not fund hESC research that directly destroys embryos.
Neither Congress nor any administration has ever prohibited private funding of embryonic research. Public and private funding of research on adult and cord blood stem cells is unrestricted.
U.S. Congressional response
In April 2004, 206 members of Congress signed a letter urging President Bush to expand federal funding of embryonic stem cell research beyond what Bush had already supported.
In May 2005, the House of Representatives voted 238-194 to loosen the limitations on federally funded embryonic stem-cell research — by allowing government-funded research on surplus frozen embryos from in vitro fertilization clinics to be used for stem cell research with the permission of donors — despite Bush’s promise to veto the bill if passed. On July 29, 2005, Senate Majority Leader William H. Frist (R-TN), announced that he too favored loosening restrictions on federal funding of embryonic stem cell research. On July 18, 2006, the Senate passed three different bills concerning stem cell research. The Senate passed the first bill (Stem Cell Research Enhancement Act), 63-37, which would have made it legal for the Federal government to spend Federal money on embryonic stem cell research that uses embryos left over from in vitro fertilization procedures.On July 19, 2006 President Bush vetoed this bill. The second bill makes it illegal to create, grow, and abort fetuses for research purposes. The third bill would encourage research that would isolate pluripotent, i.e., embryonic-like, stem cells without the destruction of human embryos.
In 2005 and 2007, Congressman Ron Paul introduced the Cures Can Be Found Act, with 10 cosponsors. With an income tax credit, the bill favors research upon non embryonic stem cells obtained from placentas, umbilical cord blood, amniotic fluid, humans after birth, or unborn human offspring who died of natural causes; the bill was referred to committee. Paul argued that hESC research is outside of federal jurisdiction either to ban or to subsidize.
Bush vetoed another bill, the Stem Cell Research Enhancement Act of 2007,which would have amended the Public Health Service Act to provide for human embryonic stem cell research. The bill passed the Senate on April 11 by a vote of 63-34, then passed the House on June 7 by a vote of 247-176. President Bush vetoed the bill on July 19, 2007.
On March 9, 2009, President Obama repealed a ban enacted under President Bush, thus allowing federal funds to be applied beyond what was authorized for funding under the previous president. Two days after Obama reversed the ban, the President then signed the Omnibus Appropriations Act of 2009, which still contained the long-standing Dickey-Wicker provision which bans federal funding of “research in which a human embryo or embryos are destroyed, discarded, or knowingly subjected to risk of injury or death;”the Congressional provision effectively prevents federal funding being used to create new stem cell lines by many of the known methods. So, while scientists might not be free to create new lines with federal funding, President Obama’s policy allows the potential of applying for such funding into research involving the hundreds of existing stem cell lines as well as any further lines created using private funds or state-level funding. The ability to apply for federal funding for stem cell lines created in the private sector is a significant expansion of options over the limits imposed by President Bush, who restricted funding to the 21 viable stem cell lines that were created before he announced his decision in 2001.
In 2005 the NIH funded $607 million worth of stem cell research, of which $39 million was specifically used for hESC. Sigrid Fry-Revere has argued that private organizations, not the federal government, should provide funding for stem-cell research, so that shifts in public opinion and government policy would not bring valuable scientific research to a grinding halt
In 2005 the State of California took out 3 billion dollars in bond loans to fund embryonic stem cell research in that state.
Stem cell treatments are a type of genetic medicine that introduce new cells into damaged tissus in order to treat a disease or injury. Many medical researchers believe that stem cell treatments have the potential to change the face of human disease and alleviate suffering. The ability of stem cells to self -renew and give to subsequent generations that can differentiate offers a large potential to culture tissues that can replace diseased and damaged tissues in the body, without the risk or rejection and side effects.
A number of stem cell treatments exist, although most are stil experimental and/or costly, with the notable exception of bone marrow transplantation. Medical researchers anticipate one day being able to use technologies derived from adult and embryonic stem cell research to treat cancer, Type 1 diabetes mellitus, Parkinson’s disease, Huntington’s disease, cardiac failure, muscle damage and neurolohical disorders, along with many others.
More research is needed concerning both stem cell behavior and the mechanisms of the diseases they could be used to treat before most of these experimental treatments become realities.
For over 30 years, bone marrow, and more recently, umbilical cord blood stem cells have been used to treat cancer patients with conditions such as leukemia and lymphoma. During chemotherapy, most growing cells are killed by the cytotoxic agents. These agents not only kill the leukemia or neoplastic cells, but also the haematopoietic stem cells within the bone marrow. It is this side effect of the chemotherapy that the stem cell trancplant attempts to reverse; the donor’s healthy bone marrow reintroduces functional stem cells to replace those lost in the treatment.
Stroke and traumatic brain injury lead to cell death, characterized by a loss of neurons and oligodendrocytes within the brain. Healthy adult brains neural stem cells, these divide and act to maintain gneral stem cell numbers or become progenitor cells. In healthy adult animals, progenitor cells migrate within the brain and function (thesense of smell). Interestingly, in pregnancy and after injury, this system appears to be regulated by growth factors and can increase the rate at which new brain matter is formed. In the case of brain injury, although the reparative process appears to initiate, substantial recovery is rarely observed in adults, suggesting a lack of robustness.
Stem cells may also be used to treat brain degeneration, such as in Parkinson’s and Alzheimer’s disease.
Research injecting neural (adult) stem cells into the brains of dogs has shown to be very successful in treating cancerous tumors. With traditional techniques brain cancer is almost impossible to treat because it spreads so rapidly. Researchers at the Harvard Medical School induced intracranial tumours in rodents. Then, they injected human neural stem cells. Within days the cells had migrated into the cancerous area and produced cytosine deaminase, an enzyme that converts a non-toxic pro-drug into a chemotheraputic agent. As a result, the injected substance was able to reduce tumor mass by 81 percent. The stem cells neither differentiated nor turned tumorigenic. Some researchers believe that the key to finding a cure for cancer is to inhibit cancer stem cells, where the cancer tumor originates. Currently, cancer treatments are designed to kill all cancer cells, but throug this method, researchers would be able to develop drugs to specifically target these stem cells.
Spinal cord injury
A team of Korean researchers reported on November 25, 2004, that they had transplanted multipotent adult stem cells from an umbilical cord blood to a patient suffering from a spinal cord injury and that she can now walk on her own, without difficulty. The patient had not been able stand up for roughly 19 years. For the unprecedented clinical test, the scientists isolated adult stem cells from umbilical cord blood and then injected them the damaged part of the spinal cord.
According to the October 7, 2005 issue of The Week, Universit of California researchers human embronic stem cells into paralyzed mice, which resulted in the mice regaining the ability to move and walk four months later. The researchers discovered upon dissecting the mice that the stem cells regenerated not only the neurons, but also the cells of the myelin sheath, a layer of cells which insulates neural impulses and speeds them up, facilitating communication with the brain (damage to which is often the cause of neurological injury in humans).
In January 2005, researchers at the University of Wisconsin-Madison differentiated human blastocyst stemm cells into neural stem cells, then into the beginnings of motor neurons, and finally into spinal motor neuron cells, the cell type that, in the human body, transmits messages from the brain to the spinal cord. The newly generated motor neurons exhibited elctrical activity, the signature action of neurons. Lead researchers Su-Chun Zhang described the process as “you need to teach the blastocyst stem cells to change step by step, where each step has different conditions and a strict window of time.”
Transforming blastocyst stem cells into motor neurons had eluded researchers for decades. The next step wil be to eluded researchers for decades. The next step will be to test if the newly generated neurons can communicate with other cells when transplanted into a living animal; the first test will be in chicken embryos. Su- Chun said their tril-and-error study helped them learn how motor neuron cells, which are key to the nervous system, develop in the first place. The new cells could be used to treat diseases like Lou Gehrig’s disease, muscular dystrophy, and spinal cord injuries.
Several clinical trials targeting heart disease have shown that adult stem cell therapy is safe and effective, ad is equally efficient in old as well as recent infarcts. Adult stem cell therapy for heart disease was commercially available on at least five continents at the last count (2007)
Possible mechanisms are:
Generation of heart muscle cells
Stimulation of growth of new blood vessels that repopulate the heart tissue
Secretion of growth factors, rather than actually incorporating into heart
Assistance via some other mechanism
It may be possible to have adult bone marrow cells differentiate into heart muscle cells.
Haematopoiesis (blood cell formation)
The specificity of the human immune cell repertoire is what allows the human body to defend itself from rapidly adapting antigens. However, this system is a hot spot for degradation upon the pathogenesis of disease, and because of the critical role that it plays in organismal defense, its degradation is often fatal to the system as a whole. Diseases of hematopoietic cells are called hematopathology. The specificity of the immune cells is what allows them to recognize foreign antigens, causing further challenges in the treatment of immune disease. Identical matches between donor and recipient must be made for successful transplantation treatments, while matches are uncommon, even between first-degree relativesm Research using both hamatopoietic adult stem cells and embryonic stem cells has contributes great insight into possible mechanisms and methods of treatment for many of these aliments.
Fully mature human red blood clls may be generated ex vivo by hematopoietic stem cells (HSCs), which are precursors of red blood cells. In this process, HSCs are grown together with stromal cells, creating an environment that mimics the conditions of bone marrow, the natural site of red blood cell growth. Erythropoietin, a growth factor, is added, coaxing the stem cells to complete terminal differentiation into red blood cells. Further research into this technique should have potential benefits to gene therap, blood transfusion, and topical medicine.
Hair follicles also contain stem cells, and some researchers predict research on these folicle stem cells may lead to successes in treating baldness through “hair multiplication”, also known as “hair cloning”. This treatment is expected to work through taking stem cells from existing follicles, multiplying them in cultures, and implanting the new follicles into the scalp. Later treatments may be able to simply signal follicle stem cells to give off chemical signals to nearby follicle which have shrunk during the aging process, which in turn respond to these signals by regenerating and once again making healthy hair.
In 2004, scientists at King’s College London discovered a way to sultivate a complete tooth in mice and were able to grow them stand-alone in the laboratory. Researchers are confident that this technology can be used to grow live teeth in human patients.
In theory, stem cells taken from the patient could be coaxed in the lab into turning into a tooth but which, when implanted in the gums, will rise to a new tooth, which would be expected to take two months to grow. I t will fuse with the jawbone and release chemicals that encourage nerves and blood vessels to connect with it. The process is similar to what happens when humans grow their original adult teth.
Many challenges remain, however, before stem cells could be a chioce for the replacement of missing teeth in the future.
There has been success in re-growing cochlea hair cells with the use of stem cells.
Blindness and vision impairment
Since 2003, researchers have succesfully transplanted retinal stem cells into damaged eyes to restore vision. Using emvryonic stem cells, scientists are able to grow a thin sheet of totiponent stem cells in the laboratory. When these sheets are transplanted over the damaged retina, the stem cells stimulate renewed repair, eventually restoring vision. The latest such development was in June 2005, when researchers at the Queen Victoria Hospital of Sussex, England were able to restore the sight of forty patients using the same technique. The group, led by Dr. Sheraz Daya, was able to successfully use adult stem cells obtained from rounds of trials are ongoing.
In April 2005, doctors in the UK transplanted corneal stem cells from an organ donor to the cornea of Deborah Catlyn, a woman who was blinded in one eye when an acid was thrown in her eye at a nightclub. The cornea, which is the transparent window of the eye, is a particularly suitable site for transplants. In fact, the first successful human transplant was a cornea transplant. The cornea has the remarkable property that it does not contain any blood vessels, making it relatively easy to transplant. The ajority of corneal transplants caried out today are due to a degenerative disease called keratoconus.
The University Hospital of New Jersey claims a success rate growing the new cells from transplanted stem cells varies from 25 percent to 70 percent.
In 2009, researchers at the University of Pittsburgh Medical center demostrated that stem cells collected from human corneas can response in mice with corneal damage.
Amyotrophic lateral sclerosis
Stem cell have cured rates with an Amyotrophic lateral sclerosis-like disease. The rats were injested with a virus to kill the spinal cord motor nerves related to leg movement, succeeded b injections of stem cells into their spinal cords. These migrated (passed through many layers of tissues) to the sites of injury where they were able to regenerate the dead nerve cells restoring the rats which were once gain able to walk.
Graft vs. host disease and Crohn’s disease
Phase iii clinical trials expected to end in second-quarter 2008 were conducted by Osiris Therapeutics using their indevelopment product Prochymal, derived from adult bone marrow. The target disorders of this therapeutic are graft-versus-host disease and Chron’s disease.
Neural and behavirol birth defects
A team of researchers led by Prof. Joseph Yanai were able to reverse learning deficits in the offspring of pregnant mice who were exposed to heroin and the pesticide organophosphate. This was done by direct neural stem cell transplantation into the brains of the offspring. The recovery was almost 100 percent, as proved in behavioral tests in which the treated animals improved to normal behavior and learning scores after the transplantation. On the molecular level, brain chemistry of the treated animals was also restored to normal. Through the work, which was supported by the US National Institutes of Health, the US-Israel Binational Science Foundation and the Israel anti-drug authorities, the researchers discovered that the stem cells worked even in cases where most of the cells died out in the host brain.
The scientist found that before they die the neural stem cells succeed in inducing the host brain to produce large numbers of stem cells which repair the damage. These findings, which answered a major question in the stem cell research community, were published earlier this year in the leading journal, Molecular Psychiatry. Scientists are now developing procedures to administer the neural stem cells in the least invasive way possible-probably via blood vessels, making terapy practical and clinically feasible. Researchers also plan to work on developing methods to take cells from the patient’s own body, turn them into stem cells, and then transplant them back into the patient’s blood via the blood stream. Aside from decreasing the chances of immunological rejection, the approach will also eliminate of stem cells from human embryos.
Diabetes patients lose the function of their insulin-producing beta cells of their pancreas. Human embryonic stem cells may be grown in cell culture and stimulated to form insulin-producing cells that can be transplanted into the patient.
However, success depends on developing procedures in all required steps:
Have the cells proliferate and generate sufficient amount of tissue
Differentiation into the right cell type.
Survival of the cells in the recipient (prevention of transplant rejection)
Integration with the surrounding tissue in the body
Function appropriately in long-term.
Clinical case reports in the treatment of orthopedis conditions have been reported. To date, the focus in the literature for musculoskeletal care appears to be on mesenchymal stem cells. Centeno et al. have published MRI evidence of increased cartilage and meniscuc volume in individual human subjects. the results of trials including human subject. the result of trials including more patients are yet to be published making it reports. A newly published safety study published by the same group shows good safety and less complications than surgical care in a large study group 0f 227 patients over a 3-4 year period.
In one experimental method in regenerative medicine, stem cells are used to stimulate the growth of human tissues. In an adult, wounded tissue is most often replaced by scar tissue, which is characterized in the skin by disorganized collagen structure, loss of hair follicles and irregular vascular structure. In the case of wounded fetal tissue, however, wounded tissue is replaced with normal tissue through the activity of stem cells. A possible method for tissue regeneration in adults is to place adult stem cell “seeds” inside a tissue bed :soil” in a wound bed and allow the stem cells to stimulate differentiation in the tissue bed cells. This method elicits a regenerative response more similar of fetal wound-healing than adult scar tissue formation. Researchers are still investigating different aspects of the “soil” tissue that are conductive to regeneration.
Human embryonic stem cells have been stimulated to form Spermatozoa-like cells, yet still slightly damaged or malformed. It could potentially treat azoospermia.
Stem Cell therapy has been researched in various clinical trils for a variety of treatments.
The U.S. National Institutes of Health performs a variety of stem cell clinical trils to the public and post these at their website: http://www.clinicaltrials.gov/c2/results? term+stem+cell. This website allows those interested in alternative treatments to check the availabilities and locations of clinical trials being performed all over the country.
On January 23, 2009, the US Food and Drug Administration gave clearance to Geron Corporation for the first clinical trial of an embryonic stem cell-based therapy on humans. The trial will evaluate the drug GRNOPC1, containing progenitor cels, on patienrs with acute spinal cord injury.
Stem cell use in animals
Veterinary research can contribute to human medicine
Research currently conducted of horses, dogs, and cats can benefit the development of stem-cell treatments in veterinary medicine, but may also contribute to developing those in human medicine for a range injuries and diseases such as myocardial infarction, stroke, tendon and ligament damage, osteoarthritis, osteochondrosis and muscular systrophy. Research into using stem cells for therapeutic purposes generally reflects human medical needs, but the high degree of frequency and severity of certain injuries in racehorses has put veterinary medicine at the forefront of this novel regenerative approach. Companion animals may be superior models than typical mouse models for human diseases.
Veterinary research has developed regenerative treatment models, particularly involving mesenchymal stem cells
Veterinary applications of stem cell therapy as a means of regenerating new tissue as an alternative to scar (less functional tissue) formation have developed from research that has been conducted since 1998 using adult-derived mesenchymal stem cells to treat animals with injuries or defects affecting bone, cartilage, ligaments and/or tendons. Because mesenchymal stem cells can differentiate into the cells that make up bone, cartilage, tendons, and ligaments (as well as muscle, fat, and possibly other tissues), they have been the main type of stem cells studied in the treatment of diseases affecting these tissues. The two main sources of mesenchymal stem cells used are adipose tissue or bone marrow. Because an animal’s immune system mounts a detrimental response to transplanted cells in general, except in the case of cells from a very closely genetically related individual, therapeutic stem cells are most often derived from the patient prior to therapy.These are termed autologous stem cells. In surgical repair of bone fractures in dogs and sheep, veterinarians have found that grafting mesenchymal stem cells from a genetically different donor of the same species, termed allogeneic mesenchymal stem cells, does not elicit an immunological response in the patient and can be used to help regenerate bone tissue in major bony fractures and defects. Stem cells can help speed the repair of bone fractures and defects that would normally require extensive grafting and mesenchymal stem cell use in surgical implants may actually be superior to traditional grafting techniques.Treating tendon and ligament injuries in horses using stem cells, whether derived from adipose tissue or bone-marrow, has support in the veterinary literature.Although more specific characterization and localization studies of the stem-cell containing fractions used in regenerative medicine have been identified as necessary in the veterinary literature, there is scientific evidence supporting that stem cells can improve healing by five main means: 1) providing an antiinflammatory effect, 2) homing to damaged tissues and recruiting other cells, such as endothelial progenitor cells, that are necessary for tissue growth, 3) supporting tissue remodeling over scar formation, 4) inhibiting apoptosis, and 5) differentiating into bone, cartilage, tendon, and ligament tissue.
The significance of stem cell microenvironments
To regenerate bone, stem cells must be in a carrier system that provides the appropriate context: a scaffold, upon which the introduced stem cells develop, the minerals needed to develop properly into functional bone, and growth factors that signal to the mesenchymal stem cell to differentiate into bone cells. Whether the stem cells are to heal bone or any other type of tissue, the context or microenvironment in which a group of introduced stem cells is placed is essential for effective healing, not only to provide growth factors and other chemical signals that guide appropriate differentiation of the mesenchymal stem cells, but also to ensure that they remain directed to the appropriate site and are able to emit their appropriate signals and make appropriate cell contacts. This aids healing in three ways: 1) helping the formation of new blood cells from endothelial progenitor cells, which are different type of stem cells that need to be in the regenerative cell mixture or available in the nearby host tissue; 2) preventing programed cell death or apoptosis of cells at the damaged site; and 3) reducing inflammation. Often platelet-rich plasma is used in conjunction with bone-marrow derived stem cells as a matrix which supplies growth factors and the scaffold needed to induce tissue regeneration. Alternatively, adipose tissue contains not only mesenchymal stem cells, but also other diverse types of cells that can provide the microenvironment that supports tissue regeneration without additional factors.
Sources of autologous (patient-derived) mesenchymal stem cells
Autologous stem cells intended for regenerative therapy are either taken from the patient’s bone marrow or from adipose tissue. The number of stem cells applied to the damaged tissue is important for effective therapy. For this reason, stem cells derived from bone marrow aspirates, which are normally in numbers too small to elicit a regenerative effect, are cultured in specialized laboratories to expand their numbers to be in the millions before use in regenerative therapy. Although adipose-derived tissue also needs processing prior to use in regnerative therapy, the time-consuming culturing like that needed currently for bone marrow derived mesenchymal stem cells, is not required, thus reducing the time between collection and implantation in autologous stem cell treatments.Although mesenchymal stem cells from any source have the potential to differentiate into a diverse range of tissues expert opinions vary as to which source is preferable in particular applications. Some have expressed bone-marrow derived stem cells are particularly preferred for bone, cartilage, ligament, and tendon repair; while others find the ease of collection and the multi-cellular microenvironment already present in adipose-derived stem cell fractions make fat the preferred source.
Currently Available Treatments for Horses and Dogs Suffering from Orthopedic Conditions
Autologous or allogeneic stem cells are currently used as an adjunctive therapy in the surgical repair of some types of fractures in dogs and horses. Autologous stem cell-based treatments for ligament injury, tendon injury, osteoarthritis, osteochondrosis, and sub-chondral bone cysts have been commercially available to practicing veterinarians to treat horses since 2003 in the United States and since 2006 in the United Kingdom. Autologous stem-cell based treatments for tendon injury, ligament injury, and osteoarthritis in dogs have been available to veterinarians in the United States since 2005. Over 3000 privately-owned horses and dogs have been treated with autologous adipose-derived stem cells. The efficacy of these treatments has been shown in double-blind clinical trials for dogs with osteoarthritis of the hip and elbow and horses with tendon damage The efficacy of using stem cells, whether adipose-derived or bone-marrow derived, for treating tendon and ligament injuries in horses has support in the veterinary literature.
Developments in Stem Cell Treatments in Veterinary Internal Medicine
Currently, research is being conducted to develop stem cell treatments for: 1) horses suffering from COPD, neurologic disease, and laminitis; and 2) dogs and cats suffering from heart disease, liver disease, kidney disease, neurologic disease, and immune-mediated disorders.
Stem cells were tested on rats to see if it would cure the ALS disease (see above under ALS for more information).
There is wide spread controversy over the use of embryonic stem cells. This controversy is over the technique used to create new embryonic stem cell lines, which often requires the destruction of the blastocyst.
Opposition to the use of human embryonic stem cells in research is often based on philosophical, moral or religious objections. At present there are many alternative sources for stem cells which have achieved considerable success when used as medical therapies. These alternatives do not require the destruction of an embryo, such as the use of umbilical cord blood, milky teeth stem cells, bone marrow stem cells or using genetic stimuli cue skin cells to become toti-potent. It is further argued that such methods have a proven track record of safety and efficacy, eliminating the need to use the more controversial embryonic stem cells.
Stem cell treatments around the world
Stem cell research and treatment is currently being practiced at a clinical level in the People’s Republic of China. The Ministry of Health of the People’s Republic of China has permitted the use of stem cell therapy for conditions beyond those approved of in Western countries such as the United States, United Kingdom, and Australia.
Stem cell therapies provided in China utilize umbilical cord stem cells. The stem cells are then expanded in centralized blood banks before being used in stem cell treatments. State-funded companies based in the Shenzhen Hi-Tech Industrial Zone claim to treat the symptoms of numerous disorders with adult stem cell therapy. Hospitals throughout eastern China provide numerous therapies to patients in coordination with the stem cell providers. These companies’ therapies are currently focused on the treatment of neurodegenerative and cardiovascular disorders.
It was reported that Dr. Geeta Shroff of New Delhi treats patients with terminally ill and medically incurable problems. Though having controversial figure, her patients have got very high success rate compared to conventional medicine which shows next to no hope of curing incurable problems. She recently filed a patent with World Intellectual Property Organization and European Patent Office
A stem cell line is a family of constantly-dividing cells, the product of a single parent group of stem cells. They are obtained from human or animal tissues and can replicate for long periods of time in vitro (“witin glass”, or, commonly, “in the lab”, in an artificial environment). They are frequently used for research relating to embryonic stem cells or cloning entire organisms.
Adult stem cell lines isolated from mature tissues are commonly used in stem cell research, as are cells isolated from umbilical cord blood. However, these cells have a genetic imprint of the host they were taken from, thus limiting their therapeutic use in genetic disorders. Also, adult stem cells are not totipotent or pluripotent like embryonic stem cells, but rather more specialized cells that are multipotent. Another source of stemc cells is the iPS (induced pluripotent stem) cell. This process involves reversing the differentiating cell signals that cause a stem cell to specialize. In this way, a somatic cell can be worked backwards into a stem cell. One added advantage of this type of stem is that the pluripotent cell has the same DNA as the donor and can be used therapeutically towards that end without painful bone marrow and spinal cord extraction techniques.
Typically, stem cells have been maintained using tissue culture methods that essentially date from the 1950′s. In particular, they are often “fed” using mouse embryonic fibroblasts (“feeder cells”) while being simultaneously suspended in a nutrient sollution (“media”). However, many scientists are recognizing the importance of using media that is completely free of animal ingredients. This not only liberates cell lines from animal feeder cells, but also brings the in vivo therapeutic use of stem cells one step closer to reality.
Nonmammalian Study model
One interesting study model that yields research results that ca be subsequently tested on mammalian systems, is the fruit fly Drosophila (1).
Induced pluripotent stem cells, commonly abbreviated as iPS cells are a type of pluripotent stem cell artificially derived from a non-pluripotent cell, typically an adult somatic cell, by inducing a “forced” expression of certain genes.
Induced Pluripotent Stem Cells are believed to be identical to natural pluripotent stem cells, such as embryonic stem (ES) cells in many respects, such as the expression of certain stem cell genes and proteins, chromatin methylation patterns, doubling time, embryoid body formation, teratoma formation, viable chimere formation, and potency and differentiability, but the full extent of their relation to natural pluripotent stem cells is being assessed.
IPSCs were first produced in 2006 from mouse cells and in 2007 from human cells. This has been cited as an important advancement in stem cell research, as it may allow researchers to abtain pluripotent stem cells, which are important in research and potentially have therapeutic uses, without the controversial use of embryos.
Depending on the ethods used, reprogramming of adult cells to obtain iPSCs may pose significant risks that could limit its use in humans. For example, if viruses are used to genomically alter the cells, the expression of cancer-causing genes or oncogenes may potentially be triggered. In February 2008, in ground-breaking findings published in the journal Cell, scientists announced the discovery of a technique that could remove oncogenes after the induction of puripotency, thereby increasing the potential use of iPS cells in human diseases. Even more recently, in April 2009, the group of Sheng Ding in La Jolla, California, could show that the generation of iPS cells was possible without any genetic alteration of the adult cell: A repeated treatment of the cells with certain proteins channeled into the cells via poly-arginine anchors was sufficient to induce pluripotency. The acronym given for those iPS is piPS (protein-induced pluripotent stem cells).
Production of iPSCs
iPS cells are typically derived by transfection of certin stem cell-associated genes into non-pluripotent cells, such as adult fibroblast. Transfection is typically achieved through viral vectors, such as retroviruses. Tranfected genes include the master transcriptional regulators Oct-3/4 (Pouf51) and Sox2, although it is suggested that other genes enhanced the efficiency of induction. After 3-4 weeks, small numbers of transfected cells begin to become morphologically and biochemically similar to pluripotent stem cells, and are typically through morphological selection, doubling time, or through a reporter gene and antibiotic selection.
Induced pluripotent stem cells were first generated by Shinya Yamanaka’s team at Kyoto University, Japan in 2006. Yamanaka used genes that had been identified as particularly important in embryonic stem cells (ESCc), and used retroviruses to transfect mouse fibroblast with a selection of those genes. Eventually, four key pluripotency genes essential for the production of pluripotent stem cells were isolated; Oct-3/4, SOX2, c-Myc, and Klf4. Cells were isolated by antibiotic selection of Fbx15+ cells. However, this iPS line showed DNA methylation errors compared to original patterns in ESC lines and failed to produce viable chimeras if injected into developing embryos.
Second generation in mice
In June 2007, the same group published a breakthrough study along with two other independent research groups from Harvard, MIT, and the University of California, Los Angeles, showing successful reprogramming of mouse fibroblasts into iPS and even producing viable chimera. These cell lines were also derived from mouse fibroblasts by retroviral mediated reactivation of the same four endogenous pluripotent factors, but the researchers now selected a different marker for detection. Instead of Fbx15, they used Nanog which is an important gene in ESCs. DNA methylation patterns and production of viable chimeras (and thereby contributing to subsequent germ-line production) indicated that Nanog is a major determinant of cellular pluripotency.
Unfortunately, one of the four genes used (namely, c-Myc) is oncogenic, and 20% of the chimeric mice developed cancer. In a later study, Yamanaka reported that one can create iPSCs even without c-Myc. The process takes longer and is not as efficient, but the resulting chimeras didn’t develop cancer.
Human induced pluripotent stem cells
In November 2007, a milestone was achieved by creating iPS from adult human cells; two independent research teams’ studies were released – one in Science by James Thomson and Junying Yu at University of Wisconsin–Madisonand another in Cell by Shinya Yamanaka and colleagues at Kyoto University, Japan. With the same principle used earlier in mouse models, Yamanaka had successfully transformed human fibroblasts into pluripotent stem cells using the same four pivotal genes: Oct3/4, Sox2, Klf4, and c-Myc with a retroviral system. Thomson and colleagues used OCT4, SOX2, NANOG, and a different gene LIN28 using a lentiviral system.
The viral transfection systems used to insert the genes at random locations in the host’s genome created concern for potential therapeutic applications of these iPSCs, because the created cells might be prone to form tumors. Members of both teams considered it therefore necessary to develop new delivery methods. To overcome these dangers, Konrad Hochedlinger and his Harvard University research team successfully used an adenovirus to transport the requisite four genes into the DNA of skin and liver cells of mice, resulting in cells identical to embryonic stem cells.] Since the adenovirus does not combine any of its own genes with the targeted host, the danger of creating tumors is eliminated, although the Hochedlinger method has not yet been tested on human cells. Yamanaka has since demonstrated reprogramming can be accomplished via plasmid without any virus transfection system at all, although at very low efficiencies.
The Times reported on 2009 March 2
Teams led by Keisuke Kaji, of the University of Edinburgh, and Andras Nagy, of the University of Toronto, have now collaborated to develop a new approach to creating IPS cells that does not involve viruses.
The procedure might eventually prove itself suitable for therapeutic use. Even more recently, in April 2009, the group of Sheng Ding in La Jolla, California, could show that the generation of iPS cells was possible without any genetic alteration of the adult cell: a repeated treatment of the cells with certain proteins channeled into the cells via poly-arginine anchors was sufficient to induce pluripotency.The expression of pluripotency induction genes can also be increased by treating somatic cells with FGF2 under low oxygen conditions.
Genes of induction
The generation of iPS cells is crucial on the genes used for the induction.
Oct-3/4 and certain members of the Sox gene family (Sox1, Sox2, Sox3, and Sox15) have been identified as crucial transcriptional regulators involved in the induction process whose absence makes induction impossible. Additional genes, however, including certain members of the Klf family (Klf1, Klf2, Klf4, and Klf5), the Myc family (C-myc, L-myc, and N-myc), Nanog, and LIN28, have been identified to increase the induction efficiency.
- Oct-3/4 (Pou5f1): Oct-3/4 is one of the family of octamer (“Oct”) transcription factors, and plays a crucial role in maintaining pluripotency. The absence of Oct-3/4 in Oct-3/4+ cells, such as blastomeres and embryonic stem cells, leads to spontaneous trophoblast differentiation, and presence of Oct-3/4 thus gives rise to the pluripotency and differentiation potential of embryonic stem cells. Various other genes in the “Oct” family, including Oct-3/4′s close relatives, Oct1 and Oct6, fail to elicit induction, thus demonstrating the exclusiveness of Oct-3/4 to the induction process.
- Sox family: The Sox family of genes is associated with maintaining pluripotency similar to Oct-3/4, although it is associated with multipotent and unipotent stem cells in contrast with Oct-3/4, which is exclusively expressed in pluripotent stem cells. While Sox2 was the initial gene used for induction by Yamanaka et al., Jaenisch et al., and Thomson et al., other genes in the Sox family have been found to work as well in the induction process. Sox1 yields iPS cells with a similar efficiency as Sox2, and genes Sox3, Sox15, and Sox18 also generate iPS cells, although with decreased efficiency.
- Klf family: Klf4 of the Klf family of genes was initially identified by Yamanaka et al. and confirmed by Jaenisch et al. as a factor for the generation of mouse iPS cells and was demonstrated by Yamanaka et al. as a factor for generation of human iPS cells. However, Thomson et al. reported that Klf4 was unnecessary for generation of human iPS cells and in fact failed to generate human iPS cells. Klf2 and Klf4 were found to be factors capable of generating iPS cells, and related genes Klf1 and Klf5 did as well, although with reduced efficiency.
- Myc family: The Myc family of genes are proto-oncogenes implicated in cancer. Yamanaka et al. and Jaenisch et al. demonstrated that c-myc is a factor implicated in the generation of mouse iPS cells and Yamanaka et al. demonstrated it was a factor implicated in the generation of human iPS cells. However, Thomson et al., Yamanaka et al., and unpublished work by Johns Hopkins University reported that c-myc was unnecessary for generation of human iPS cells. Usage of the “myc” family of genes in induction of iPS cells is troubling for the eventuality of iPS cells as clinical therapies, as 25% of mice transplanted with c-myc-induced iPS cells developed lethal teratomas. N-myc and L-myc have been identified to induce instead of c-myc with similar efficiency.
- Nanog: In embryonic stem cells, Nanog, along with Oct-3/4 and Sox2, is necessary in promoting pluripotency. Therefore, it was surprising when Yamanaka et al. reported that Nanog was unnecessary for induction although Thomson et al. has reported it is possible to generate iPS cells with Nanog as one of the factors.
- LIN28: LIN28 is an mRNA binding protein expressed in embryonic stem cells and embryonic carcinoma cells associated with differentiation and proliferation. Thomson et al. demonstrated it is a factor in iPS generation, although it is unnecessary.
The generated iPSCs were remarkably similar to naturally-isolated pluripotent stem cells (such as mouse and human embryonic stem cells, mESCs and hESCs, respectively) in the following respects, thus confirming the identity, authenticity, and pluripotency of iPSCs to naturally-isolated pluripotent stem cells:
- Cellular biological properties
- Morphology: iPSCs were morphologically similar to ESCs. Each cell had round shape, large nucleolus and scant cytoplasm. Colonies of iPSCs were also similar to that of ESCs. Human iPSCs formed sharp-edged, flat, tightly-packed colonies similar to hESCs and mouse iPSCs formed the colonies similar to mESCs, less flat and more aggregated colonies than that of hESCs.
- Growth properties: Doubling time and mitotic activity are cornerstones of ESCs, as stem cells must self-renew as part of their definition. iPSCs were mitotically active, actively self-renewing, proliferating, and dividing at a rate equal to ESCs.
- Stem cell markers: iPSCs expressed cell surface antigenic markers expressed on ESCs. Human iPSCs expressed the markers specific to hESC, including SSEA-3, SSEA-4, TRA-1-60, TRA-1-81, TRA-2-49/6E, and Nanog. Mouse iPSCs expressed SSEA-1 but not SSEA-3 nor SSEA-4, similarly to mESCs.
- Stem Cell Genes: iPSCs expressed genes expressed in undifferentiated ESCs, including Oct-3/4, Sox2, Nanog, GDF3, REX1, FGF4, ESG1, DPPA2, DPPA4, and hTERT.
- Telomerase activity: Telomerases are necessary to sustain cell division unrestricted by the Hayflick limit of ~50 cell divisions. hESCs express high telomerase activity to sustain self-renewal and proliferation, and iPSCs also demonstrate high telomerase activity and express hTERT (human telomerase reverse transcriptase), a necessary component in the telomerase protein complex.
- Pluripotency: iPSCs were capable of differentiation in a fashion similar to ESCs into fully differentiated tissues.
- Neural differentiation: iPSCs were differentiated into neurons, expressing βIII-tubulin, tyrosine hydroxylase, AADC, DAT, ChAT, LMX1B, and MAP2. The presence of catecholamine-associated enzymes may indicate that iPSCs, like hESCs, may be differentiable into dopaminergic neurons. Stem cell-associated genes were downregulated after differentiation.
- Cardiac differentiation: iPSCs were differentiated into cardiomyocytes that spontaneously began beating. Cardiomyocytes expressed TnTc, MEF2C, MYL2A, MYHCβ, and NKX2.5. Stem cell-associated genes were downregulated after differentiation.
- Teratoma formation: iPSCs injected into immunodeficient mice spontaneously formed teratomas after nine weeks. Teratomas are tumors of multiple lineages containing tissue derived from the three germ layers endoderm, mesoderm and ectoderm; this is unlike other tumors, which typically are of only one cell type. Teratoma formation is a landmark test for pluripotency.
- Embryoid body: hESCs in culture spontaneously form ball-like embryo-like structures termed “embryoid bodies”, which consist of a core of mitotically active and differentiating hESCs and a periphery of fully differentiated cells from all three germ layers. iPSCs also form embryoid bodies and have peripheral differentiated cells.
- Chimeric mice: hESCs naturally reside within the inner cell mass (embryoblast) of blastocysts, and in the embryoblast, differentiate into the embryo while the blastocyst’s shell (trophoblast) differentiates into extraembryonic tissues. The hollow trophoblast is unable to form a living embryo, and thus it is necessary for the embryonic stem cells within the embryoblast to differentiate and form the embryo. iPSCs were injected by micropipette into a trophoblast, and the blastocyst was transferred to recipient females. Chimeric living mouse pups were created: mice with iPSC derivatives incorporated all across their bodies with 10%-90% chimerism.
- Tetraploid complementation: iPS cells from mouse fetal fibroblasts injected into tetraploid blastocysts (which themselves can only form extra-embryonic tissues) can form whole, non-chimeric, fertile mice, although with low success rate.
- Epigenetic reprogramming
- Promoter demethylation: Methylation is the transfer of a methyl group to a DNA base, typically the transfer of a methyl group to a cytosine molecule in a CpG site (adjacent cytosine/guanine sequence). Widespread methylation of a gene interferes with expression by preventing the activity of expression proteins or recruiting enzymes that interfere with expression. Thus, methylation of a gene effectively silences it by preventing transcription. Promoters of pluripotency-associated genes, including Oct-3/4, Rex1, and Nanog, were demethylated in iPSCs, demonstrating their promoter activity and the active promotion and expression of pluripotency-associated genes in iPSCs.
- Histone demethylation: Histones are compacting proteins that are structurally localized to DNA sequences that can effect their activity through various chromatin-related modifications. H3 histones associated with Oct-3/4, Sox2, and Nanog were demethylated, indicating the expression of Oct-3/4, Sox2, and Nanog.
Amniotic stem cells are multipotent stem cells extracted from amniotic fluid.
In fact amniotic fluid contains a considerable quantity of stem cells, that are multipotent and able to differentiate into various tissues and are useful for future human applications.
Amniotic stem cells can differentiate into various types tissue as nerve, muscle, bone, and all other human cells: so are pluripotent like embryonic stem cells.
All over the world, universities and research institutes are studying amniotic fluid to discover all the qualities of amniotic stem cells, and scientist such as Anthony Atala, Paolo De Coppi and Giuseppe Simoni have discovered important results.
From an ethical point of view, stem cells from amniotic fluid can solve a lot of problems. First, because it’s possible to catch amniotic stem cells without destroying embryos, and second because they are not necessary to choose an alternative between donor or autologous use.
Amniotic stem cells banks
Thanks to recent studies, also supported by Lombardy italian region lead by Roberto Formigoni, it is possible to converse the stem cells extracted from amniotic fluid in private stem cells banks.
The first US amniotic stem cells bank opened in Medford, MA, by Biocell Center Corporation.
Adult stem cells are undifferentiated cells, found throughout the body after embryonic development, that multiply by cell division to replenish dying cells and regenerate damaged tissues. Also known as somatic stem cells (from Greek Σωματικóς, meaning of the body), they can be found in juvenile as well as adult animals and humans.
Scientific interest in adult stem cells has centered on their ability to divide or self-renew indefinitely, and generate all the cell types of the organ from which they originate, potentially regenerating the entire organ from a few cells. Unlike embryonic stem cells, the use of adult stem cells in research and therapy is not considered to be controversial as they are derived from adult tissue samples rather than destroyed human embryos. They have mainly been studied in humans and model organisms such as mice and rats.
Stem cell division and same. A – stem cells; B – progenitor cell; C – differentiated cell; 1 – symmetric stem cell division; 2 – asymmetric stem cell division; 3 – progenitor division; 4 – terminal differentiation
The rigorous definition of a stem cell requuires that it processes two properties:
- Self-renewal which is the ability to go through numerous cycles of cell division while maintaining the undifferentiated state.
- multipotency or multidifferentiative potential which is the ability to generate progeny of several distinct cell typer, (for example glial cells are restricted to producing a single-cell type. However, some researchers do not consider multipotency to be essential, and believe that unipotent self-renewing stem cells can exist.
These properties can be illustrated with relative ease in vitro, using methods such as clonogenic assays, where the progeny of a single cell is characterized, however, it is known that in vitro cell culture conditions can alter the behavior of cells. Proving that a particular subpopilation of cells possesses stem cell properties in vivo is challenging, and so considerable debate exists as to whether some proposed stem cell populations in the adult are indeed stem cells.
To ensure self-renewal, stem cells undergo two types of cell division (see Stem cell division and differentiation diagram). Symmetric division gives rise to two identical daughter cells, both endowed with stem cell properties, whereas assymmetric division produces only one stem cell and a progenitor cell with limited self-renewal potential. Asymmetric division is the process of a cell splitting into another cell and en essential cell fat, or a lipid, this lipid will bond to a free cell and reproduce. Progenitors can go through several rounds of cell division before finally differentiating into a mature cell. It is believed that the molecular distinction between symmetric and asymmetric divisions lies in differential segragation of cell membrane proteins (such as receptors) between the daughter cells.
Adult stem cells express transporters of the ATP-binding cassette family that actively pump a diversity of organic molecules out of the cell. Many pharmaceuticals are exported by these transportrs conferring multidrug resistance onto the cell. This complicates the design of drugs, for instance neural stem cell targeted therapies for the treatment of clinical depression.
Adult stem cell research has ben focused on uncovering the general molecular mechanisms that control their self-renewal and differentiation.
The transcriptional repressor Bmi-1 is one of the Polycomb-group proteins that was discovered as a common oncogene. activated in lymphoma and later shown to specifically regulate HSCs. The role of Bmi-1 has also been illustrated in neural stem cells.
The Notch pathway has been known to devlopmental biologists for decades. Its role in control of stem cell proliferation has now been demonstrated for several cell types including haematopoietic, neural and mammary stem cells.
These developmental pathways are also strongly implicated as stem cell regulators.
Under special conditions tissue-specific adult stem cells can generate a whole spectrum of cell-types of other tissues, even crossing germ layers. This phenomenon is referred to as stem cell transdifferentiation or plasticity. It can be inducd by modifying the growth medium when stem cells are cultured in vitro or tranplanting them to an organ of the body different from the one they were originally isolated from. There is yet no consensus among biologists on the prevlalence and physiological and therapeutic relevance of stem cell plasticity.
Hematopoietic stem cells
Hematopoietic stem cells are found in the bone marrow and give to the blood cell types.
Mammary stem cells
Mammary stem cells provide the source of cells for growth of the mammary gland during puberty and gestation and play an important role in carcinogenesis of the breast. Mammary stem cells have been isolated from human and mouse tissue as well as from cell lines derived from the mammary gland. Single such cells can give rise to both the luminal and myoepithelial cell types of the gland, and have been shown to have the ability to regenerate the entire organ in mice.
Mesenchymal stem cells
Mesenchymal stem cells (MSCs) are of stromal origin and may differentiate into a variety of tissues. MSCs have been isolated from placenta, adipose tissue, lung, bone marrow and blood, Wharton’s jelly from the umbilical cord, and teeth (perivascular niche of dental pulp and periodontal ligament). MSCs are attractive for clinical therapy due to their ability to differentiate, provide trophic support, and modulate innate immune response.
Endothelial stem cell
Endothelial stem cells are multipotent stem cells. They are one of three types of stem cells to be found in bone marrow.
Neural stem cells
The existence of stem cells in the adult brain has been prostulated following the discovery that the process of neurogenesis, the birth of new neurons, continues into adulthood in rats. The presence of stem cells in the mature primate brain was first reported in 1967. It has since been shown that new neurons are generated in adult mice, songbirds and primates, including humans. Normally, adult neurogenesis is restricted to two areas of the brain – the subventricular zone, which lines the lateral ventricles, and the dentate gyrus of the hippocampal formation. Although the generation of new neurons in the hippocampus is well established, the presence of tue self-renewing stem cells there has been debated. Under certain circumstances, such as following tissue damage is ischemis, neurogenesis can be induced in other brain regions including the neocortex.
Neural stem cells are commonly cultured in vitro as so called neuropheres – floating heterogeneous aggregates of cells, containing a large proportion of stem cells. They can be propagated for extended periods of time and differentiated into both neuronal and glia cells, and therefore behave as stem cells. However, some recent studies suggest that this behaviour is induced by the culture onditions in progenitor cells, the progeny of stem cell division that normally undergo a strictly limited number of replication ccles in vivo. Furthermore, neurosphere-derived cells do not behave as stem cells when transplanted back into the brain.
Neural stem cells share many properties with haematopoietic stem cells (HSCs). Remarkably, when injected into the blood, neurosphere-derived cells differentiate into various cell types of the immune system.
Olfactory adult stem cells
Olfactory adult stem cells have been successfully hrvested from the human olfactory mucosa cells, which are found in the lining of the nose and are involved in the sense of smell. If they are given the right chemical environment these cells have the same ability as embryonic stem cells to develop into many different cell types. Olfactory stem cells hold the potential for therapeutic applications and, in contrast to neural stem cells, can be harvested with ease without harm to the patient. This means they can be easily obtained from all individuals, including older patients who might be most in need of stem cell therapies
Neural crest stem cells
Hair follocles contain two types of stem cells, one of which appears to represent a remnant of the stem cells of the embryonic neural crest. Similar cells have been found in the gastrointestinal tract, sciatic nerve, cardiac outflow tract and spinal and sympathetic ganglia. These cells can generate neurons, Schwann cells, myofibroblast, chondocytes and melanocytes.
Multipotent stem cells with a claimed equivalency to embryonic stem cells have been derived from spermatogonial progenitor cells found in the testicles of laboratory mice by scientists in Germany and the United Kingdom confirmed the same capability using celss from the testicles og humans. The extracted stem cells are known as human adult germline stem cells.
Multipotent stem cells have also been derived from germ cells found in human testicles.
Adut stem cell therapies
The therapeutic potential of adult stem cells is the focus of much scientific research, due to their ability to be harvested from the patient . In common with embryonic stem cells, adult stem cells have the ability to differentiate into more than one cell type, but unlike the former they are often restricted to certain types or “lineages”. The ability of a differentiated stem cell of one lineage to ptoduce cells of a different lineage is called transdifferentiation. Some types of adult stem cells are more capable of transdifferentiation than others, and for many there is no evidence that such a transformaation is possible. Consequently, adult stem therapies require a stem cell source of the specific lineage needed, and harvesting and/or culturing them up to the numbers required is a challenge.
Pluripotent stem cells, i.e. cells that can give rise to any fetal or adult cell tupe, can be found in a number of tissues, including umbilical cord blood. Using genetic reprogramming, pluripotent stem cells equivalent to embronic stem cells have been derived from human adult skin tissue. Other adult stem cells are multipotent, meaning they are restricted in the types of cell they can become, and are generally referred to by their tissue origin (such as mesenchymal stem cell, adipose-derived stem cell, endothelial stem cell, etc). A great deal of adult stem cell research has focused on investigating their capacity to divide or self-renew indefinitely, and their potential for differentiation. In mice, pluripotent stem cells can be directly generated from adult fibroblast cultures.
Adult stem cell treatments have been used for many years to successfully treat leukemia and related bone/blood cancers utilizing bone marrow transplants. The use of adult stem cells in research and therapy is not considered as controversial as the use of embryonic stem cells, because the production of adult stem cells does not require the destruction of an embryo. Consequently, the majority of US government funding provided for research in this field is restricted to supporting adult stem cell research.
Early regenerative applications of adult stem cells has focused on intravenous delivery of blood progenitors known as Hematopetic Stem Cells (HSC’s). Other early commercial applications have focused on Mesenchymal Stem Cells (MSC’s). For both cell lines, direct injection or placement of cells into a site in need of repair may be the preferred method of treatment, as vascular delivery suffers from a “pulmonary first pass effect” where intravenous injected cells are sequestered in the lungs. Clinical case reports in orthopedic applications have been published. Wakitani has published a small case series of nine defects in five knees involving surgical transplantation of mesenchymal stem cells with coverage of the treated chondral defects.Centeno et al. have reported high field MRI evidence of increased cartilage and meniscus volume in individual human clinical subjects. Many other stem cell based treatments are operating outside the US, with much controversy being reported regarding these treatments as some feel more regulation is needed as clinics tend to exaggerate claims of success and minimize or omit risks.
First transplanted human organ grown from adult stem cells
In 2008 the first full transplant of a human organ grown from adult stem cells was carried out by Paolo Macchiarini, at the Hospital Clínic of Barcelona on Claudia Castillo, a Colombian female adult whose trachea had collapsed due to tuberculosis. Researchers from the University of Padua, the University of Bristol, and Politecnico di Milano harvested a section of trachea from a donor and stripped off the cells that could cause an immune reaction, leaving a grey trunk of cartilage. This section of trachea was then “seeded” with stem cells taken from Ms. Castillo’s bone marrow and a new section of trachea was grown in the laboratory over four days. The new section of trachea was then transplanted into the left main bronchus of the patient. Because the stem cells were harvested from the patient’s own bone marrow Professor Macchiarini did not think it was necessary for her to be given anti-rejection (immunosuppressive) medication and when the procedure was reported four months later in The Lancet, the patient’s immune system was showing no signs of rejecting the transplant
Adult stem cells and cancer
In recent years, acceptance of the concept of adult stem cells has increased. There is now a theory that stem cells reside in many adult tissues and that these unique reservoirs of cells are not only responsible for the normal reparative and regenerative processes, but are also considered to be a prime target for genetic and epigenetic changes, culminating in many abnormal conditions including cancer
Fetal stem cells are primitive cell types found in the organs of fetuses. The classification of fetal stem cells remain unclear and this type of stem cell is currently often grouped into an adult stem cell. However, a more clear distinction between the two cell types appears necessary.