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Embryonic Stem Cells Developments

A major development in research came in May 2003, when researchers announced that they had successfully used Embryonic stem cells to produce human egg cells. These egg cells could potentially be used in turn to produce new Stem Cells. If research and testing proves that artificially created egg cells could be a viable source for Embryonic stem cells, they noted, then this would remove the necessity of starting a new Embryonic stem cell line with the destruction of a Blastocyst. Thus, the controversy over donating human egg cells and blastocysts could potentially be resolved, though a Blastocyst would still be required to start each cycle.

 

The online edition of Nature Medicine published a study on January 23, 2005 which stated that the Human Embryonic Stem CellsCulture Medium used to grow the cells, for example, mouse cells and other animal cells. The nonhuman cell-surface sialic acid can compromise the potential uses of the Embryonic stem cells in humans, according to scientists at the University of California, San Diego.

A study was published in the online edition of Lancet Medical Journal on March 8, 2005 that detailed information about a new stem-cell line which was derived from human embryos under completely cell- and serum-free conditions. This event is significant because exposure of existing human embryonic stem-cell lines to live animal cells and serum risks contamination with pathogens that could lead to human health risks. After more than 6 months of Undifferentiated Proliferation, these cells retained the potential to form derivatives of all three embryonic Germ layers both In vitro and in teratomas. These properties were also successfully maintained (for more than 30 passages) with the established stem-cell lines. (Lancet Medical Journal)

Recently, in California , researchers have injected Embryonic stem cells into mice as they developed in the womb. Upon maturing, it was found that some of the human ESCs had survived and two months after injection, the researchers found that the ESCs had undertaken ?the characteristics of mouse cells?

Scientists in Australia have grown human prostate in mice using Embryonic stem cells. In a world first, the scientists combined human Embryonic stem cells with mouse prostate cells, and used a mouse as the host to grow the human prostate. The researchers were able to show that it was also functioning as a human prostate. Doctors world wide would now be able to use this prostate as a model for studying prostate cancer and disease, and to produce future drugs. available for federally funded research are contaminated with nonhuman molecules from the

 

Stem Cells 101

 

Embryonic Stem Cells History

Embryonic stem cells were first derived from mouse embryos in 1981 by two independent research groups (Evans & Kaufman and Martin). The breakthrough in embryonic stem cell research came in November 1998 when a group led by James Thomson at the University of Wisconsin-Madison first developed a technique to isolate and grow the cells derived from human blastocysts. Normally, Blastocyst-stage embryos that are left over after successful In Vitro Fertilization would not be used but be destroyed.

 

Scientists are only allowed to use these discarded blastocysts after assessment by specialized committees that thoroughly check the research goals of these scientists. Of course, scientific research using those blastocysts may be conducted when it contributes to a better understanding of how to generate cells and tissues that can cure a patient?s disease or be used to treat severe injuries. Currently, in the United States , scientific research is limited by a ban on the use of federal funds for research with Embryonic stem cells derived after August 9th, 2001.

Embryonic stem cell researchers are currently attempting to grow the cells in the laboratory (i.e. in culture flasks: ?In Vitro?) beyond the first stages of cell development. It is important to make sure the Embryonic stem cells are fully differentiated into the desired cell type (i.e. tissue) before they are transplanted into the patient, as Undifferentiated Embryonic stem cells may develop into a tumor after transplantation. Further more, scientists are trying to develop techniques to prevent rejection of implanted cells by the patient (i.e. host-versus-graft response).

One of the possibilities to prevent rejection is by creating embryonic stem cell clones that are genetically identical to the patient. This can be achieved by fusing an egg cell, the nucleus (containing the genetic material: DNA) of which is removed, with a patient?s cell. The fused cell produced (containing only the DNA of the patient) is allowed to grow to the size of a few tens of cells, and Stem Cells are then extracted. Because they are genetically compatible with the patient, the patient?s immune system will not reject differentiated cells derived from these Embryonic stem cells. More commonly, they are obtained for research purposes from uncloned blastocysts, such as those discarded from In vitro Fertilization clinics. Such cells might be rejected if transplanted into a patient, as they do not contain identical genetic information. A possible solution for this is to derive as many well-characterized embryonic stem cell lines from different genetic and ethnic backgrounds and use the cell line that is most similar to the patient; treatment can then be tailored to the patient, minimizing the risk of rejection.

 

Stem Cells 101

Embryonic Stem Cells Research

Embryonic stem cells (ESCs) are Stem Cells derived from inner mass cells of a human Embryo (sometimes called a Blastocyst, which is an early stage Embryo - approximately 1 week old in humans - consisting of 50-150 cells). Embryonic stem cells are Totipotent, meaning they are able to grow (i.e. differentiate) into all derivatives of the three primary Germ layers: Ectoderm, Endoderm and Mesoderm. In other words, they can develop into each of the more than 200 cell types of the adult body as long as they are specified to do so. This characteristic property distinguishes Embryonic stem cells from adult Stem Cells or progenitor cells, the latter two of which only have the capacity to form a limited number of different cell types. Because of their unique combined abilities of unlimited expansion and pluripotency, Embryonic stem cells potentially are the ultimate source for Regenerative medicine and tissue replacement after injury or disease. To date, no medical treatments have been successfully derived from embryonic stem cell research as the first Human Embryonic Stem Cell Line has only been reported in 1998.

 

Stem Cells 101

 

Definition

Research using Embryonic stem cells remains at the zenith of stem cell science because, unlike somatic cells, Embryonic stem cells are Pluripotent. However, research using Stem Cells derived from the human Embryo is still in the basic research phase, as these Stem Cells were first isolated in 1998 (at least for humans), whereas adult Stem Cells have been studied since the 1960s. Research with Embryonic stem cells derived from humans is controversial because, in order to start a stem cell ?line? or lineage, the destruction of a human Embryo and/or Therapeutic Cloning is usually required. Some believe this to be a slippery slope to Reproductive Cloning and tantamount to the objectification of a potential human being.

In an attempt to overcome these moral, political and ethical hurdles, medical researchers have been experimenting with alternative techniques of obtaining Embryonic stem cells by extraction, which does not involve Cloning and/or the destruction of a human Embryo. Cancer Stem Cells arising through malignant transformation of adult Stem Cells are proposed to be the source of some or all tumors and cause metastasis and relapse of the disease. The stem cell origin of leukemias is well established.

The role of Stem Cells in other tumors is under intensive investigation. Cord Blood Stem Cells are derived from the blood of the placenta and umbilical cord after birth. Since 1988 these Cord blood stem cells have been used to treat Gunther?s disease, Hunter syndrome, Hurler syndrome, Acute lymphocytic leukemia and many more problems occurring mostly in children. Umbilical cord blood use has become so common that there are now umbilical cord blood banks that accept donations from parents. It is collected by removing the umbilical cord, cleansing it and withdrawing blood from the umbilical vein.

This blood is then immediately analyzed for infectious agents and the tissue-type is determined. The cord blood is processed and depleted of red blood cells before being stored in liquid nitrogen for later use, at which point it is thawed, washed of the cryoprotectant, and injected through a vein of the patient. This kind of treatment, where the Stem Cells are collected from another donor, is called allogeneic treatment. When the cells are collected from the same patient on whom they will be used, it is called autologous and when collected from identical individuals (i.e. homozygous twin), it is referred to as syngeneic.

 

Stem Cells 101

Sources Stem Cell Primer

Stem Cells are also categorized according to their source, as either adult, embryonic, cancer or Cord Blood Stem Cells.

 

Adult Stem Cells are Undifferentiated cells found among differentiated cells of a specific tissue and are mostly Multipotent cells. They are more accurately called somatic (Greek σωμα sōma = body) Stem Cells, because they need not come from adults but can also come from children or umbilical cords. Particularly interesting are adult Stem Cells termed "spore-like cells". They are present in all tissues[2] and seem to survive long time periods and harsh conditions.

Embryonic stem cells are cultured cells obtained from the Undifferentiated inner mass cells of an early stage human Embryo (sometimes called a Blastocyst, which is an Embryo that is between 50 to 150 cells). Embryonic stem cell research is "thought to have much greater developmental potential than adult Stem Cells," according to the National Institutes of Health.[3] Research using Embryonic stem cells remains at the zenith of stem cell science because, unlike somatic cells, Embryonic stem cells are Pluripotent. However, research using Stem Cells derived from the human Embryo is still in the basic research phase, as these Stem Cells were first isolated in 1998 (at least for humans), whereas adult Stem Cells have been studied since the 1960s.[4] Research with Embryonic stem cells derived from humans is controversial because, in order to start a stem cell 'line' or lineage, the destruction of a human Embryo and/or Therapeutic Cloning is usually required. Some believe this to be a slippery slope to Reproductive Cloning and tantamount to the objectification of a potential human being. In an attempt to overcome these moral, political and ethical hurdles, medical researchers have been experimenting with alternative techniques of obtaining Embryonic stem cells by extraction, which does not involve Cloning and/or the destruction of a human Embryo.

Cancer Stem Cells arising through malignant transformation of adult Stem Cells are proposed to be the source of some or all tumors and cause metastasis and relapse of the disease.[5] The stem cell origin of leukemias is well established[6]. The role of Stem Cells in other tumors is under intensive investigation.

Cord blood stem cells are derived from the blood of the placenta and umbilical cord after birth. Since 1988 these Cord blood stem cells have been used to treat Gunther's disease, Hunter syndrome, Hurler syndrome, Acute lymphocytic leukemia and many more problems occurring mostly in children. Umbilical cord blood use has become so common that there are now umbilical cord blood banks that accept donations from parents. It is collected by removing the umbilical cord, cleansing it and withdrawing blood from the umbilical vein. This blood is then immediately analyzed for infectious agents and the tissue-type is determined. The cord blood is processed and depleted of red blood cells before being stored in liquid nitrogen for later use, at which point it is thawed, washed of the cryoprotectant, and injected through a vein of the patient. This kind of treatment, where the Stem Cells are collected from another donor, is called allogeneic treatment. When the cells are collected from the same patient on whom they will be used, it is called autologous and when collected from identical individuals (i.e. homozygous twin), it is referred to as syngeneic.

 

Stem Cells 101

 

Potency Types

Stem cell potency specifies the ameliorative potential of the cell type.

Totipotent Stem Cells are produced from the fusion of an egg and sperm cell. Cells produced by the first few divisions of the fertilized egg cell are also Totipotent. These cells can differentiate into any type of cell without exception.

Pluripotent Stem Cells are the descendants of Totipotent cells and can differentiate into any cell type except for Totipotent Stem Cells.

Multipotent Stem Cells can produce only cells of a closely related family of cells (e.g. hematopoeietic Stem Cells differentiate into red blood cells, white blood cells, platelets etc.).

Unipotent cells can produce only one cell type, but have the property of self-renewal which distinguishes them from non-Stem Cells.

 

Stem Cells 101

 

What Are The Potential Uses Of Human Stem Cells And The Obstacles That Must Be Overcome Before These

There are many ways in which human stem cells can be used in basic research and in clinical research. However, there are many technical hurdles between the promise of stem cells and the realization of these uses, which will only be overcome by continued intensive stem cell research.

 

Studies of human embryonic stem cells may yield information about the complex events that occur during human development. A primary goal of this work is to identify how undifferentiated stem cells become differentiated. Scientists know that turning genes on and off is central to this process. Some of the most serious medical conditions, such as cancer and birth defects, are due to abnormal cell division and differentiation. A better understanding of the genetic and molecular controls of these processes may yield information about how such diseases arise and suggest new strategies for therapy. A significant hurdle to this use and most uses of stem cells is that scientists do not yet fully understand the signals that turn specific genes on and off to influence the differentiation of the stem cell.

Human stem cells could also be used to test new drugs. For example, new medications could be tested for safety on differentiated cells generated from human pluripotent cell lines. Other kinds of cell lines are already used in this way. Cancer cell lines, for example, are used to screen potential anti-tumor drugs. But, the availability of pluripotent stem cells would allow drug testing in a wider range of cell types. However, to screen drugs effectively, the conditions must be identical when comparing different drugs. Therefore, scientists will have to be able to precisely control the differentiation of stem cells into the specific cell type on which drugs will be tested. Current knowledge of the signals controlling differentiation fall well short of being able to mimic these conditions precisely to consistently have identical differentiated cells for each drug being tested.

Perhaps the most important potential application of human stem cells is the generation of cells and tissues that could be used for cell-based therapies. Today, donated organs and tissues are often used to replace ailing or destroyed tissue, but the need for transplantable tissues and organs far outweighs the available supply. Stem cells, directed to differentiate into specific cell types, offer the possibility of a renewable source of replacement cells and tissues to treat diseases including Parkinson's and Alzheimer's diseases, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis, and rheumatoid arthritis.

For example, it may become possible to generate healthy heart muscle cells in the laboratory and then transplant those cells into patients with chronic heart disease. Preliminary research in mice and other animals indicates that bone marrow stem cells, transplanted into a damaged heart, can generate heart muscle cells and successfully repopulate the heart tissue. Other recent studies in cell culture systems indicate that it may be possible to direct the differentiation of embryonic stem cells or adult bone marrow cells into heart muscle cells (Figure 4).

In people who suffer from type I diabetes, the cells of the pancreas that normally produce insulin are destroyed by the patient's own immune system. New studies indicate that it may be possible to direct the differentiation of human embryonic stem cells in cell culture to form insulin-producing cells that eventually could be used in transplantation therapy for diabetics.

To realize the promise of novel cell-based therapies for such pervasive and debilitating diseases, scientists must be able to easily and reproducibly manipulate stem cells so that they possess the necessary characteristics for successful differentiation, transplantation and engraftment. The following is a list of steps in successful cell-based treatments that scientists will have to learn to precisely control to bring such treatments to the clinic. To be useful for transplant purposes, stem cells must be reproducibly made to:

  • Proliferate extensively and generate sufficient quantities of tissue.

  • Differentiate into the desired cell type(s).

  • Survive in the recipient after transplant.

  • Integrate into the surrounding tissue after transplant.

  • Function appropriately for the duration of the recipient's life.

  • Avoid harming the recipient in any way.

Also, to avoid the problem of immune rejection, scientists are experimenting with different research strategies to generate tissues that will not be rejected.

To summarize, the promise of stem cell therapies is an exciting one, but significant technical hurdles remain that will only be overcome through years of intensive research.

 

Stem Cells 101

What Are The Similarities And Differences Between Embryonic And Adult Stem Cells?

Human embryonic and adult stem cells each have advantages and disadvantages regarding potential use for cell-based regenerative therapies. Of course, adult and embryonic stem cells differ in the number and type of differentiated cells types they can become. 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 of origin. However, some evidence suggests that adult stem cell plasticity may exist, increasing the number of cell types a given adult stem cell can become.

 

Large numbers of embryonic stem cells can be relatively easily grown in culture, while adult stem cells are rare in mature tissues and methods for expanding their numbers in cell culture have not yet been worked out. This is an important distinction, as large numbers of cells are needed for stem cell replacement therapies.

A potential advantage of using stem cells from an adult is that the patient's own cells could be expanded in culture and then reintroduced into the patient. The use of the patient's own adult stem cells would mean that the cells would not be rejected by the immune system. This represents a significant advantage as immune rejection is a difficult problem that can only be circumvented with immunosuppressive drugs.

Embryonic stem cells from a donor introduced into a patient could cause transplant rejection. However, whether the recipient would reject donor embryonic stem cells has not been determined in human experiments.

 

Stem Cells 101

What Are Adult Stem Cells?

An adult stem cell is an undifferentiated cell found among differentiated cells in a tissue or organ, can renew itself, and can differentiate to yield the major specialized cell types of the tissue or organ. The primary roles of adult stem cells in a living organism are to maintain and repair the tissue in which they are found. Some scientists now use the term somatic stem cell instead of adult stem cell. Unlike embryonic stem cells, which are defined by their origin (the inner cell mass of the blastocyst), the origin of adult stem cells in mature tissues is unknown.

 

Research on adult stem cells has recently generated a great deal of excitement. Scientists have found adult stem cells in many more tissues than they once thought possible. This finding has led scientists to ask whether adult stem cells could be used for transplants. In fact, adult blood forming stem cells from bone marrow have been used in transplants for 30 years. Certain kinds of adult stem cells seem to have the ability to differentiate into a number of different cell types, given the right conditions. If this differentiation of adult stem cells can be controlled in the laboratory, these cells may become the basis of therapies for many serious common diseases.

The history of research on adult stem cells began about 40 years ago. In the 1960s, researchers discovered that the bone marrow contains at least two kinds of stem cells. One population, called hematopoietic stem cells, forms all the types of blood cells in the body. A second population, called Bone Marrow Stromal Cells, was discovered a few years later. Stromal Cells are a mixed cell population that generates bone, cartilage, fat, and fibrous connective tissue.

Also in the 1960s, scientists who were studying rats discovered two regions of the brain that contained dividing cells, which become nerve cells. Despite these reports, most scientists believed that new nerve cells could not be generated in the adult brain. It was not until the 1990s that scientists agreed that the adult brain does contain stem cells that are able to generate the brain's three major cell types—astrocytes and oligodendrocytes, which are non-neuronal cells, and neurons, or nerve cells.

 

A. Where Are Adult Stem Cells Found And What Do They Normally Do?

adult stem cells have been identified in many organs and tissues. One important point to understand about adult stem cells is that there are a very small number of stem cells in each tissue. Stem cells are thought to reside in a specific area of each tissue where they may remain quiescent (non-dividing) for many years until they are activated by disease or tissue injury. The adult tissues reported to contain stem cells include brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin and liver.

Scientists in many laboratories are trying to find ways to grow adult stem cells in cell culture and manipulate them to generate specific cell types so they can be used to treat injury or disease. Some examples of potential treatments include replacing the dopamine-producing cells in the brains of Parkinson's patients, developing insulin-producing cells for type I diabetes and repairing damaged heart muscle following a heart attack with cardiac muscle cells.

 

B. What tests are used for identifying adult stem cells?

Scientists do not agree on the criteria that should be used to identify and test adult stem cells. However, they often use one or more of the following three methods: (1) labeling the cells in a living tissue with molecular markers and then determining the specialized cell types they generate; (2) removing the cells from a living animal, labeling them in cell culture, and transplanting them back into another animal to determine whether the cells repopulate their tissue of origin; and (3) isolating the cells, growing them in cell culture, and manipulating them, often by adding growth factors or introducing new genes, to determine what differentiated cells types they can become.

Also, a single adult stem cell should be able to generate a line of genetically identical cells—known as a clone—which then gives rise to all the appropriate differentiated cell types of the tissue. Scientists tend to show either that a stem cell can give rise to a Clone of cells in cell culture, or that a purified population of candidate stem cells can repopulate the tissue after transplant into an animal. Recently, by infecting adult stem cells with a virus that gives a unique identifier to each individual cell, scientists have been able to demonstrate that individual adult stem cell clones have the ability to repopulate injured tissues in a living animal.

 

C. What Is Known About Adult Stem Cell Differentiation?

As indicated above, scientists have reported that adult stem cells occur in many tissues and that they enter normal differentiation pathways to form the specialized cell types of the tissue in which they reside. Adult stem cells may also exhibit the ability to form specialized cell types of other tissues, which is known as Transdifferentiation or plasticity.

Normal differentiation pathways of adult stem cells. In a living animal, adult stem cells can divide for a long period and can give rise to mature cell types that have characteristic shapes and specialized structures and functions of a particular tissue. The following are examples of differentiation pathways of adult stem cells (Figure 2).

  • Hematopoietic stem cells give rise to all the types of blood cells: red blood cells, B lymphocytes, T lymphocytes, natural killer cells, neutrophils, basophils, eosinophils, monocytes, macrophages, and platelets.

  • Bone marrow stromal cells (Mesenchymal Stem Cells) give rise to a variety of cell types: bone cells (osteocytes), cartilage cells (chondrocytes), fat cells (adipocytes), and other kinds of connective tissue cells such as those in tendons.

  • neural stem cells in the brain give rise to its three major cell types: nerve cells (neurons) and two categories of non-neuronal cells—astrocytes and oligodendrocytes.

  • Epithelial stem cells in the lining of the digestive tract occur in deep crypts and give rise to several cell types: absorptive cells, goblet cells, Paneth cells, and enteroendocrine cells.

  • Skin stem cells occur in the basal layer of the epidermis and at the base of hair follicles. The epidermal stem cells give rise to keratinocytes, which migrate to the surface of the skin and form a protective layer. The follicular stem cells can give rise to both the hair follicle and to the epidermis.

Adult stem cell plasticity and transdifferentiation. A number of experiments have suggested that certain adult stem cell types are pluripotent. This ability to differentiate into multiple cell types is called plasticity or transdifferentiation. The following list offers examples of adult stem cell plasticity that have been reported during the past few years.

  • Hematopoietic stem cells may differentiate into: three major types of brain cells (neurons, oligodendrocytes, and astrocytes); skeletal muscle cells; cardiac muscle cells; and liver cells.

  • Bone marrow stromal cells may differentiate into: cardiac muscle cells and skeletal muscle cells.

  • Brain stem cells may differentiate into: blood cells and skeletal muscle cells.

Current research is aimed at determining the mechanisms that underlie adult stem cell plasticity. If such mechanisms can be identified and controlled, existing stem cells from a healthy tissue might be induced to repopulate and repair a diseased tissue (Figure 3).

 

D. What Are The Key Questions About Adult Stem Cells?

Many important questions about adult stem cells remain to be answered. They include:

  • How many kinds of adult stem cells exist, and in which tissues do they exist?

  • What are the sources of adult stem cells in the body? Are they "leftover" embryonic stem cells, or do they arise in some other way? Why do they remain in an undifferentiated state when all the cells around them have differentiated?

  • Do adult stem cells normally exhibit plasticity, or do they only transdifferentiate when scientists manipulate them experimentally? What are the signals that regulate the proliferation and differentiation of stem cells that demonstrate plasticity?

  • Is it possible to manipulate adult stem cells to enhance their proliferation so that sufficient tissue for transplants can be produced?

  • Does a single type of stem cell exist—possibly in the bone marrow or circulating in the blood—that can generate the cells of any organ or tissue?

  • What are the factors that stimulate stem cells to relocate to sites of injury or damage?

Stem Cells 101

What Are The Unique Properties Of All Stem Cells?

Stem cells differ from other kinds of cells in the body. All stem cells—regardless of their source—have three general properties: they are capable of dividing and renewing themselves for long periods; they are unspecialized; and they can give rise to specialized cell types.

 

Scientists are trying to understand two fundamental properties of stem cells that relate to their Long-Term Self-Renewal:

  1. why can embryonic stem cells proliferate for a year or more in the laboratory without differentiating, but most adult stem cells cannot; and

  2. what are the factors in living organisms that normally regulate stem cell Proliferation and self-renewal?

Discovering the answers to these questions may make it possible to understand how cell proliferation is regulated during normal embryonic development or during the abnormal cell division that leads to cancer. Importantly, such information would enable scientists to grow embryonic and adult stem cells more efficiently in the laboratory.

Stem cells are unspecialized. One of the fundamental properties of a stem cell is that it does not have any tissue-specific structures that allow it to perform specialized functions. A stem cell cannot work with its neighbors to pump blood through the body (like a heart muscle cell); it cannot carry molecules of oxygen through the bloodstream (like a red blood cell); and it cannot fire electrochemical signals to other cells that allow the body to move or speak (like a nerve cell). However, unspecialized stem cells can give rise to specialized cells, including heart muscle cells, blood cells, or nerve cells.

Stem cells are capable of dividing and renewing themselves for long periods. Unlike muscle cells, blood cells, or nerve cells—which do not normally replicate themselves—stem cells may replicate many times. When cells replicate themselves many times over it is called proliferation. A starting population of stem cells that proliferates for many months in the laboratory can yield millions of cells. If the resulting cells continue to be unspecialized, like the parent stem cells, the cells are said to be capable of long-term self-renewal.

The specific factors and conditions that allow stem cells to remain unspecialized are of great interest to scientists. It has taken scientists many years of trial and error to learn to grow stem cells in the laboratory without them spontaneously differentiating into specific cell types. For example, it took 20 years to learn how to grow human embryonic stem cells in the laboratory following the development of conditions for growing mouse stem cells. Therefore, an important area of research is understanding the signals in a mature organism that cause a stem cell population to proliferate and remain unspecialized until the cells are needed for repair of a specific tissue. Such information is critical for scientists to be able to grow large numbers of unspecialized stem cells in the laboratory for further experimentation.

Stem cells can give rise to specialized cells. When unspecialized stem cells give rise to specialized cells, the process is called Differentiation. Scientists are just beginning to understand the signals inside and outside cells that trigger stem cell differentiation. The internal signals are controlled by a cell's genes, which are interspersed across long strands of DNA, and carry coded instructions for all the structures and functions of a cell. The external signals for cell differentiation include chemicals secreted by other cells, physical contact with neighboring cells, and certain molecules in the Microenvironment.

Therefore, many questions about stem cell differentiation remain. For example, are the internal and external signals for cell differentiation similar for all kinds of stem cells? Can specific sets of signals be identified that promote differentiation into specific cell types? Addressing these questions is critical because the answers may lead scientists to find new ways of controlling stem cell differentiation in the laboratory, thereby growing cells or tissues that can be used for specific purposes including cell-based therapies.

Adult stem cells typically generate the cell types of the tissue in which they reside. A blood-forming adult stem cell in the bone marrow, for example, normally gives rise to the many types of blood cells such as red blood cells, white blood cells and platelets. Until recently, it had been thought that a blood-forming cell in the bone marrow—which is called a hematopoietic stem cell—could not give rise to the cells of a very different tissue, such as nerve cells in the brain. However, a number of experiments over the last several years have raised the possibility that stem cells from one tissue may be able to give rise to cell types of a completely different tissue, a phenomenon known as Plasticity. Examples of such plasticity include blood cells becoming neurons, liver cells that can be made to produce insulin, and Hematopoietic Stem Cells that can develop into heart muscle. Therefore, exploring the possibility of using adult stem cells for cell-based therapies has become a very active area of investigation by researchers.

* Page citation: Stem Cell Basics: What are the unique properties of all stem cells? . In Stem Cell Information [World Wide Web site]. Bethesda, MD: National Institutes of Health, U.S. Department of Health and Human Services, 2006

 

Stem Cells 101

 

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