Understanding Stem Cells part 52

Some people believe that the creation of chimeras involving human cells for medical research is morally acceptable as long as the chimera has no level of human conciousness. Therefore research in which it is possible for human stem cells to produce part of an animal's brain should be conducted with great care. The National Academies guidelines prohibit the introduction  of human cells into the blastocyst of a non-human primate, or the introduction of any animal and human cells into a human blastocyst. The guidelines also prohibit the breeding of human animal chimeras in the unlikely event that any human genetic material would be contained in their reproductive cells.

Understanding Stem Cells part 51

The ethics of human-animal chimeras?

Chimeras are organisms composed of cells or tissues from more than one individual. Chimeras have been produced for research for many years, but when human and animal cells are mixed in the laboratory, there is a clear need for heightened ethical consideration. Cells from different organisms can be combined either in the early developmental stages (for example, introducing human cells into a mouse blastocyst to observe certain developmental processes) or after an individual is fully developed (for example, implanting human stem cells-derived pancreatic cells into a mouse to test their ability to function in a living body). Chimeras are considered essential for advancing stem cells research to viable therapies, since no therapy can be tested in humans without research in animals first,

Understanding Stem Cells part 50

Reproductive cloning, such as the process that was used to create Dolly the sheep, also uses the nuclear transfer technique. However, instead of removing the inner cell mass to derive a stem cell line, the blastocyst is implanted into the uterus and allowed to develop fully. In 2002, the National Academies issued the report Scientific and Medical Aspects of Human Reproductive Cloning, which concluded "Human reproductive cloning should not now be practiced. It is dangerous and likely to fail."

Understanding Stem Cells part 49

The relationship of stem cell research to reproductive cloning

Although cloning and stem cell research are often lumped together in the context of ethical debates, the goals and results of the two are very different. The common factor between current attempts at reproductive cloning and stem cell research is a laboratory technique called nuclear transfer. Using nuclear transfer, scientists can create blastocysts containing stem cells that are "clones" of a single adult cell by inserting the genetic material from an adult cell (for example, skin cell) into an egg whose  nucleus has been removed.  Scientists hope that they could derive stem cells from the cells inside such blastocysts and grow replacement tissues that are genetically matched to specific patients, thus offering patients a safer alternative to traditional transplants.

Understanding Stem Cells part 48

Many traditions emphasize obligations to heal the sick and ease suffering-goals for which embryonic stem cell research holds great potential-and favor embryonic stem cell research for this reason. Several religious  groups are currently involved in internal discussions about the status of the human embryo and have not yet established official opinions on the matter. Public opinion polls suggest that the majority of both religious and-nonreligious people support embryonic stem cell research, although public opinions seems divided about the creation or use of human blastocysts  solely for research.

Understanding Stem Cells part 47

Is an embryo a person?

The controversy over embryonic stem cell research touches on some of the same fundamental questions that society has grappled with in the debates over contraception, abortion, and in vitro fertilization. The questions at the center of the controversy concern the nature of early human life and the legal and moral status of the human embryo. Embryonic stem cell research often involves removing the inner cell mass from "excess" blastocysts that are unneeded by couples who have completed their fertility treatment. This prevents those blastocysts from continuing to develop. Although such blastocysts would likely be discarded (and thus destroyed) by the clinics in any case, some believe that this does not make it morally acceptable to use them for research or therapeutic purposes. They believe that the life of a human being begins at the moment of conception and that society undermines a commitment of human equality and to the protection of vulnerable individuals if blastocysts are used for such purposes. Some cultures and religious traditions oppose the use of human life as a means to some other end, no matter how noble that end might be. Other traditions support embryonic stem cell research because they believe that the embryo gains the moral status of human being only after a few weeks or months of development.

Understanding Stem Cells part 46

Scientists and society as a whole must consider the ethical implications of stem cell research. As the science advances, it is essential  that scientists; religious, moral and  political leaders; and society as a whole continue to evaluate and communicate about the ethical implications of stem cell research.

Understanding Stem Cells part 45

Stem cells may also revolutionize traditional chemical medicine. Because embryonic stem cells can continue to divide for long periods of time and produce a variety of cell types, they could provide a valuable source of human cells for testing drugs or measuring the effects of toxins  on normal tissues without risking the health of a single human volunteer. In the future, thousands of compunds could be quickly tested on a wide assortment of cell types derived from stem cells, making drug discovery more efficient and cost effective.

Understanding Stem Cells part 44

The role of stem cells in basic research

Stem cells offer opportunities for scientific advances that go far beyond regenerative medicine. They offer a window for addressing many of biology's most fundamental questions. Watching embryonic stem cells give rise to specialized cells is like peeking into the earliest development of the many tissues and organs of the human body. Stem cell research may help clarify the role genes play in human development and how genetic mutations affect normal processes. They can be used to study how infectious agents invade and attack human cells, to investigate the genetic and environmental factors that are involved in cancer and other diseases, and to decipher what happens during aging.

Understanding Stem Cells part 43

Cancer: Getting to the root of the problem

Why are some cancers so hard to eliminate, even after many rounds of chemotherapy? The answer may lie in a few abnormal stem cells. Cancerous stem cells were first identified in 1997 when a research group from the University of Toronto  transferred a few blood stem cells from human leukemia patients into mice and watched leukemia develop in the mice. Stem cell-like cells have also recently been found in breast and brain tumors. Like normal stem cells, tumor stem cells exist in very low numbers, but they can replicate and give rise to a multitude of cells. Unlike normal stem cells, however, cancerous stem cells lack the controls that tell them when to stop dividing. Traditional chemotherapy kills  off the majority of the tumor cells, but if any of the cancerous stem cells survive the treatment, the cancer may return. Research into the differences in gene expression between normal and tumor stem cells may lead to treatments where the root of the problem-the cancer stem cell-is targeted.

Understanding Stem Cells part 42

Possible fix for diabetes?

In people who suffer from from type I diabetes, the beta cells of the pancreas that normally produce insulin are destroyed by the patient's overactive immune system. Without insulin, the cells of the body cannot take up glucose and they starve. Patients with type I diabetes require insulin injection several times a day for their entire lives. The only current cure is a pancreatic transplant from a recently deceased donor, but the demand for transplants far outweighs the supply. While adult stem cells have not yet been found in the pancreas, scientists have made progress transforming embryonic stem cells into insulin-producing cells. Combining beta-cell transplants with methods to "fix" the patient's immune system-including chemotherapy to destroy malfunctioning immune system cells and blood transplants to replenish healthy white blood cells-could offer great hope for many people suffering with type 1 diabetes.

Understanding Stem Cells part 41

Because Parkinson's disease results from the loss of one specific type of nerve cells offer a very tangible possibility for treatment. Researchers have recently learned how to differentiate embryonic stem cells into the specific type of brain cell that is lost in Parkinson's disease. They have also succesfully transplanted adult nerve stem cells into rat brains. When this technique is proven to be effective and save, transplantation of stem cells into the brains of patients  may one day allow doctors to reverse the burden of Parkinson's disease and restore control of movement. Another strategy currently under study is the addition of chemicals or growth factors that aim to induce the patient's own stem cells to repair the damaged nerves without needing to grow and transplant stem cells.

Understanding Stem Cells part 40

There are also many legal and social questions that must be addressed before stem cell-based therapies become clinically available. Legal issues that will affect stem cell applications include how to address intelectual property concerns and how to apply and enforce diverse and sometimes conflicting state and national laws. Social issues include concerns about the destruction of embryos, the distribution of the benefits of the research, and the protection of both physical and privacy interests of egg and sperm donors and clinical research subjects.

Understanding Stem Cells part 39

Are the promises of stem cell therapies realistic?

The list of medical achievements stem cells could offer seems to be expanding at an incredible pace. The role of stem cells in medicine is already very real, but there is a danger of exaggerating the promise of new medical developments. What tend to be "over-promised" are not only the potential out-comes of both embryonic and adult stem cell research, but also the time scales that are involved. The basic research needed to develop viable therapeutic options in a lengthy process that may extend over many years and decades. Even after science has moved from basic research to developing medical applications, it still takes many years to thoroughly test those        applications and demonstrate that they are safe to prescribe for patients. This is true for all medical treatments, including the development of new drugs, procedures, and medical equipment, and is not specific to the living cell therapies made possibly by stem cell research.

Understanding Stem Cells part 38

Possible future treatment for Parkinson's disease?

When most people reach for a pen, their body acts in one smooth and controlled movement. This is because the instant a person thinks of grabbing the pen, a series of nerve cells  fire in an orchestrated symphony from the brain to the muscles responsible for that action. For the movement to  be precise and smooth, all the nerve cells in the "grabbing-the-pen networking" must function properly, including cells that tell unneeded muscle to stay still. In Parkinson's disease, the brain cells responsible for keeping unneeded mucles from moving degenerate and die. This results in progressively more dramatic and uncontrolled movements, tremors and spasms. To date, there is no cure for Parkinson's disease because one has figured out a way to bring back the specialized nerve cells that have died.

Understanding Stem Cells part 37

Traditionally, doctors treated severe burns by transplanting sections of skin from undamaged areas of the body onto the burned areas, but if doctors could not find enough unharmed skin to cover the burned areas, the patient could die. Now, scientists can grow vast sheets of new skin by culturing the stem cells from small pieces of a healthy skin. This practice, which is a type of tissue engineering, has become routine for treating burn victims. Recently, scientists have identified other types of stem cells in hair follicles and deeper layers of the skin. The inclusion of these new stem cells into enginered skin should help create more natural-looking skin transplants in the future.

Understanding Stem Cells part 36

The changed face of skin grafts

For many years, scientists have been harnessing the regenerative capabilities of human skin to treat victims of severe burns using skin transplants. Skin transplants are possible because of the existence of stem cells located just  under the top layer of skin. Every day, thousands of new skin cells are produced to replace those that have been shed. When someone suffers severe burns that destroy the source of these stem cells, their skin can no longer regenerate on its own.

Understanding Stem Cells part 35

More recently, scientists have developed ways to derive hematopoietic stem cells from the blood contained in the umbilical cord and placenta at birth. The stem cells isolated from a person's own umbilical cord  blood and placenta, if used for therapies later in life, would be less likely to cause an "internal war" within the recipient's body. They are also more accessible than the stem cells in bone marrow because the extraction of this blood  poses no risk to the mother or infant.

Understanding Stem Cells part 34

In the past, the only way to use hematopoietic stem cells for therapies was through bone marrow transplants. Extracting bone marrow is an uncomfortable and invasive procedure, and in order for a transplant to work, the donor and recipient must be genetically similar. If they are too genetically different, the blood cells produced from the transplanted marrow may recognize the patient's body as foreign and fight against the patient's own cells and organs. Additionally, the patient's immune system may reject the transplant, causing a dangerous "war" within the patient's body.

Understanding Stem Cells part 33

Blood stem cells

After scraping a knee or donating blood, the body replenish the blood cells that are lost by drawing on a small number of semi-specialized hematopoietic  stem cells contained in the blood and bone marrow. For decades, scientists have been using this type of stem cells to treat patients with diseases such as leukemia, sickle cell anemia, bone marrow damage, and some metabolic disorders and immunodeficiencies where the body has lost its ability to replenish its own set of healthy  blood cell types, from infection-fighting white blood cells to blood-clotting platelets. Preliminary results have suggested that they  may also be able to produce other cell types not found in blood, but this is not yet proven.

Understanding Stem Cells part 32

Why stem cell research is being pursued?

Right now, only a few diseases are treatable with stem cell therapies because scientists can only regenerate a few types of tissues. However, the success of the most established stem cells-based therapies-blood and skin transplants-gives hope that someday stem cells will allow scientists to develop therapies for a variety of diseases previously thought to be incurable. Many major diseases are caused by the loss of a single  type of cell or tissue. For example, type I diabetes (juvenile onset) is caused by the loss of the insulin-producing cells of the pancreas, and its treatment is limited to merely alleviating the symptoms. Finding a cure for such diseases would be much easier if scientists could simply re-grow the missing or damaged cells and implant them into patients.

Understanding Stem Cells part 31

Causing an adult cell to act like an embryonic stem cell. During development, as cells become more and more specialized, they gradually lose the ability to turn on the genes that allow embryonic stem cells to be so versatile. The silencing of these genes seems to be responsible for keeping specialized cells specialized and limiting the differentiation capacities of adult stem cells. By "reprogramming" adult stem cells so that they can turn on the genes that allow versatility, scientists hope to cause them to revert to a more flexible state. It is even possible that scientists could one day "reprogram" any cell, not only stem cells. However, research in this area is in the early stages and scientists may be many years away from making an adult cell as versatile as an embryonic stem cell.

Understanding Stem Cells part 30

The creation of embryonic stem cells through a process called alter nuclear transfer (ANT). In this variation of the nuclear transfer technique, scientists create a blastocyst whose genetic material has been changed so that further development and implantation into the uterus is not possible. It aims to create embryo-like entities that are not truly embryos but that can be a source of pluripotent stem cells. ANT, so far only tested with mouse blastocysts, could allow the creation of embryonic stem cells without destroying a viable human blastocyst. Some who object to embryonic stem cell research support ANT because the resulting blastocyst could never develop into a full human being and therefore would not have the moral status of a human embryo. However, this procedure is objectionable to some because they believe that it involves the creation of an imperfect blastocyst that is designed to be destroyed.

Understanding Stem Cells part 29

Cells collected from the morula, the developmental stage prior to the blastocyst. The morula, a solid ball of about 16-30 cells, seems able to sustain the loss of a few cells without developmental damage so that the remaining cells can continue to develop. Cell extraction from the morula is already being used in some clinics to screen for for genetic disorders in embryos produced by in vitro fertilization. Researchers have recently shown that cells isolated from a mouse morula can give rise to embryonic stem cells while the remaining morula cells develop into a healthy mouse. However, this process may still be morally objectionable to some because of the chance of harm to the morula, and because the long-term effects of removing cells from a morula are not yet known.

Understanding Stem Cells part 28

Alternatives to using embryos in stem cell research

To address ethical concerns about the destruction of blastocysts, scientists are trying to find new ways of obtaining stem cells that behave like embryonic stem cells but don't require harming a blastocyst. As the science progresses, ethical issues surrounding these alternatives may also arise.

Understanding Stem Cells part 27

Organisms that contain cells or tissues from another individual of the same or a different species are called chimeras. A common example of a chimera is a mouse that has been injected with some human cells so that it can be used for studying a human disease or testing a new drug. A person who has had a blood transfusion or a person who has received a heart valve transplant from a pig is technically a chimera, as well. The making of chimeras for research has unique ethical implications that have been the topic of discussions among scientists, ethicists and the public, especially when the chimeras contain both human and animal cells.

Understanding Stem Cells part 26

Animal studies can also reveal how human cells differentiate during normal development. For example, scientists may implant human stem cells into a developing mouse to observe the processes involved in building and organizing the different tissue types that make up the human body. Scientists can also trace the development and progression of certain diseases within an animal. By implanting human stem cells that lead to a particular disease into a mouse blastocyst, scientists can observe when and how the afflicted cells begin to show signs of disease and can test drugs that might prevent that proces

Understanding Stem Cells part 25

The role of animals in stem cell research

For medical research, as well as for research that explores the basic process in the development of organisms and diseases, scientists often rely on animals. Implanting human cells into animals such as mice has long been common practice in order to test the safety and effectiveness of new drugs, procedures, and medical devices before clinical testing in human volunteers. For stem cell research, scientists use animals to make sure the stem cells are able to incorporate into the tissue, do not cause any harmful consequences, and function in concert with the rest of the body. For example, before using stem cells to replace the pancreatic cells that are destroyed by type I diabetes in humans, scientists will transplant human stem cells into a mouse to see whether the stem cells yield healthy, insulin-producing cells. If their methods prove successful in mice, scientists may eventually apply the technology to developing treatments for diabetes in humans.

Understanding Stem Cells part 24

It will be essential that scientists are sure that stem cells have fully differentiated before they can use them for medical application. If completely undifferentiated stem cells (such as embryonic stem cells) are implanted directly into an organism, they can cause a type of tumor called teratoma, which scientists have observed in experiments using mice. Semi-specialized adult stem cells and differentiated cells  derived from embryonic stem cells are unlikely to cause teratomas.

Understanding Stem Cells part 23

Once they have established a stem cell line, scientists start the process of causing the stem cells to differentiate into specialized cell types. The cellular environment in which stem cells naturally reside provides scientists with clues about how to make them differentiate in a culture dish. For example, in the bone marrow, where the blood stem cells reside, bone cells send physical and chemical signals that tell the blood stem cells when to differentiate. Scientists are just beginning to understand these signals and have developed ways to mimic the natural processes in cell cultures. Usually, the technology involves adding certain proteins to the cell culture and, in some cases, introducing specific genes into the stem cells.

Understanding Stem Cells part 22

Culturing cell lines and stimulating stem cells to differentiate

Cell culture is a term that refers to the growth and maintenance of cells in a controlled environment outside of an organism. A successful stem cell culture is one that keeps the cell healthy, dividing, and unspecialized. The culturing of stem cells is the first step in establishing a stem cell line-a propogating collection of genetically identical cells. Cell lines are important because they provide a long-term supply of multiplying cells that can be shared among scientists for research and therapy.

Understanding Stem Cells part 21

How do scientists  know when they have found a stem cell? Every cell displays an array of proteins on its surface; different cell types have different proteins. Scientists can use these surface proteins as "markers" that characterize individual cell types-a type of "molecular ID". For example, using molecules that recognize and attach to specific surface proteins and that can fluoresce under certain wavelengths of light, scientists can visually tell the difference between a blood stem cell and a mature white blood cell. Unfortunately, not all stem cells can now be identified in this manner because scientists have  not yet identified markers for all stem cell types. Scientists also identify stem cells by observing their behavior in the laboratory: stem cells must be able to remain unspecialized and self-renew for long periods of time. Scientists believe that there might be more types of adult stem cells than the handful that have already been identified, but finding them is a difficult process.

Understanding Stem Cells part 20

Identifying stem cells

As early as 1961, scientists knew that adult bone marrow contained cells that could make all of the blood cell types. But it wasn't until 1988 that those stem cells were isolated as pure populations. Why did it take so long? The techniques for identifying stem cells have only recently been developed. Partly, this is because adult stem cells are, by their very nature, inconspicuous  in shape, size and function. They also tend to hide deep in tissue and are present only in very low numbers, making their identification and isolation like finding a needle in a haystack.

Understanding Stem Cells part 19

Working with the stem cells

The day-to-day work that goes on in the laboratories across the country studying stem cells begins with developing ways to identify stem cells, culture cell lines, and stimulate stem cells to differentiate. Once these first steps have been achieved, work on animals plays an important role in furthering basic research and developing medical applications. This work is necessary to form the foundation of knowledge that will point the way to medical advances.

Understanding Stem Cells part 18

Adult stem cells

Adult stem cells are hidden deep within organs, surrounded by millions of ordinary cells, and may help replenish some of the body's cells when needed. In fact, some adult stem cells are currently being used in therapies. They have been found in several organs that need a constant supply of cells, such as the blood, skin and lining of the gut, and have also been found in suprising places like brain, which is not known to readily replenish its cells. Unlike embryonic stem cells, adult stem cells are already somewhat specialized. For example, blood stem cells normally only give rise to the many types of blood cells, and nerve stem cells can only make the various types of brain cells. Recent research however, suggest that some adult stem cells might be more flexible than previously  thought, and may be made to produce a wider variety of cell types. For example, some experiments have suggested that blood stem cells isolated from adult mice may also be able to produce liver, muscle and skin cells, but these results are not yet proven and have not been demonstrated with human cells. Nevertheles, scientists are working on finding a way to stimulate adult stem cells, or even other types of adult cells, to be more versatile. If they succeed, it could provide another source of unspecialized stem cells.

Understanding Stem Cells part 17

Although using nuclear transfer to produce stem cells is not the same as reproductive cloning, some are concerned about the potential misapplication of the technique for reproductive cloning purposes. Other ethical considerations include egg donation, which requires informed consent, and the possible destruction of blastocysts.

Understanding Stem Cells part 16

Scientists believe that if they are able to use nuclear transfer to derive human stem cells, it could allow them to study the development and progression of specific diseases by creating stem cells containing the genes responsible for certain disorders. In the future, scientists may also be able to create "personalized" stem cells that contain only the DNA of a specific patients. The embryonic stem cells created by nuclear transfer would be genetically matched to a person needing a transplant, making it far less likely that the patient's body would reject the new cells that it would be with traditional tissue transplant procedures.

Understanding Stem Cells part 15

Producing embryonic stem cells using nuclear transfer is not the same as reproductive cloning

The use of nuclear transfer to develop disease-specific stem cells can be called research cloning, and the use of this technique for personalized tissue transplants is sometimes called therapeutic cloning. These terms must be carefully distinguished from reproductive cloning, in which the intent is to implant a cloned embryo in a female's womb and allow it to develop fully into an individual. This was the technique by which Dolly the sheep was made and is now widely used for reproductive cloning in animals, In humans, however,  reproductive cloning has been actively    discouraged by most in the scientific community.

Understanding Stem Cells part 14

Nuclear transfer:

The process called nuclear transfer offers another potential way to produce embryonic stem cells. In animals, nuclear transfer has been accomplished by inserting the nucleus of an already differentiated adult cell-for example, a skin cell-into a donated egg that has had its nucleus removed. Ths egg, which now contains the genetic material of the skin cell, is then stimulated to form a blastocyst from which embryonic stem cells can be derived. The stem cells that are created in this way are therefore copies or "clones" of the original adult cell because their nuclear DNA matches that of the adult cell.

Understanding Stem Cells part 13

The in vitro fertilization (IVF) technique could potentially also be used to produce blastocysts specifically for research purposes. This would facilitate the isolation of stem cells with specific genetic traits necessary for the study  of particular disease. For example, it may be possible to study the origins of an inherited disease like cytic fibrosis using stem cells made from egg and sperm donors who have this disease. The creation of stem cells specifically for research using IVF is, however, ethically problematic for some people because it involves intentionally creating a blastocyst that will never develop into a human being.

Understanding Stem Cells part 12

Sources of Embryonic Stem Cells

In Vitro Fertilization:

The largest potential source of blastocysts for stem cell research is from in vitro fertilization (IVF) clinics. The process of IVF requires the retrieval of a woman's eggs via a surgical procedure after undergoing an intensive regimen of "fertility drugs," which stimulate her ovaries to produce multiple mature eggs. When IVF is used for reproductive purposes, doctors typically fertilize all of the donated eggs in order to maximize their chance of producing a viable blastocyst that can be implanted in the womb. Because not all the fertilized eggs are implanted, this has resulted in a large bank of "excess" blastocysts that are currently stored in freezers around the country. The blastocysts stored in IVF clinics could prove to be a major source  of embryonic stem cells for use in medical research. However, because most of these blastocysts were created before the advent of stem cell research, most donors were not asked for their permission to use these left-over blastocysts for research.

Understanding Stem Cells part 11

Some find embryonic stem cell research to be morally objectionable, because when scientists remove the inner cell mass, the blastocyst no longer has the potential to become a fully developed human being.

Understanding Stem Cells part 10

Scientists can induce embryonic stem cells to replicate themselves in an undifferentiated state for very long periods of time before stimulating them to create specialized cells. This means that just a few embryonic stem cells can build a large bank of stem cells to be used in experiments. However, such undifferentiated stem cells could not be used directly for tissue transplants because they can cause a type of tumor called a teratoma. To be used for therapies, embryonic stem cells would first need to be differentiated into specialized cell types.

Understanding Stem Cells part 9

When the blastocyst is used for stem cell research, scientists remove the inner cell mass and place these cells in a culture dish with a nutrient-rich liquid where they give rise to embryonic stem cells. Embryonic stem cells seem to be more flexible than stem cells found in adults, because they have the potential to produce every cell type in the human body. They are also generally easier to collect, purify and maintain in the laboratory than adult stem cells.

Understanding Stem Cells part 8

In normal development, the blastocyst would implant in the wall of the uterus to become embryo and continue developing into a mature organism. Its outer cells would begin to differentiate into the progressively more specialized cell types of the body.

Understanding Stem Cells part 7

Embryonic stem cells

A blastocyst (BLAST-oh-sist), is a pre-implantation embryo that develops 5 days after the fertilization of an egg by a sperm. It contains all the material necessary for the development of a complete human being. The blastocyst is a mostly hollow sphere of cells that is smaller that the period at the end of this sentence. In its interior is the inner cell mass, which is composed of 30-34 cells that are referred to by scientists as a pluripotent because they can differentiate into all of the cell types of the body. In common usage, "embryo" can refer to all stages of development from fertilization until a somewhat ill-defined stage when it is called a fetus. Scientists use terms such as "morula" and "blastocyst" to refer ti precise, specific stages of pre-implantation development. In order to be as precise as possible.

Understanding Stem Cells part 6

Stem cells are found in all of us, from the early stages of human developmen tto the end of life. All stem cells may prove useful for medical research, but each of the different types has both promise and limitations. Embryonic stem cells, which can be derived from a very early stage in human development, have the potential to produce all of the body's cell types. Adult stem cells, which are found in certain tissues in fully developed humans, from babies to adults, may be limited to producing only certain types of specialized cells. Recently, scientists have also identified stem cells in umbilical cord blood and the placenta that can give rise to the various types of blood cells.

Understanding Stem Cells part 5

Over the past two decades, scientists have been gradually deciphering the processes by which unspecialized stem cells become the many specialized cell types in the body. Stem cells can regenerate themselves or produce specialized cell types. This property makes stem cells appealing for scientists  seeking to create medical treatments that replace lost or damaged cells.

Understanding Stem Cells part 4

What is a stem cell?

Ultimately, every cell in the human body can be traced back to a fertilized egg that came into existence from the union of egg and sperm. But the body is made up of over 200 different types of cells, not just one. All of these cell types come from a pool of stem cells in the early embryo. During early development, as well as later in life, various types of stem cells give rise to the specialized  or differentiated cells that carry out the specific functions of the body, such as skin, blood, muscle and nerve cells.

Understanding Stem Cells part 3

Stem cells research is being pursued in the hope of achieving major medical breakthroughs. Scientists are striving to create therapies that rebuild or replace damaged cells with tissues grown from stem cells and offer hope to people suffering from cancer, diabetes, cardiovascular disease, spinal-cord injuries and many other disorders. Both adult and embryonic stem cells may also provide a route  for scientists to develop valuable new methods of drug discovery and testing. They are also powerful tools for doing the research that leads to a better understanding of the basic biology of the human body.