1Embryonic stem cell.docx

1、1Embryonic stem cellEmbryonic stem cellEmbryonic stem cells (ES cells) are pluripotent stem cells derived from the inner cell mass of the blastocyst, an early-stage embryo. Human embryos reach the blastocyst stage 45 days post fertilization, at which time they consist of 50150 cells. Because isolati

2、ng the embryoblast or inner cell mass (ICM) results in the death of the fertilized human embryo, this raises ethical issues.What are stem cells, and why are they important?Stem cells have the remarkable potential to develop into many different cell types in the body during early life and growth. In

3、addition, in many tissues they serve as a sort of internal repair system, dividing essentially without limit to replenish other cells as long as the person or animal is still alive. When a stem cell divides, each new cell has the potential either to remain a stem cell or become another type of cell

4、with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell.Stem cells are distinguished from other cell types by two important characteristics. First, they are unspecialized cells capable of renewing themselves through cell division, sometimes after long periods of in

5、activity. Second, under certain physiologic or experimental conditions, they can be induced to become tissue- or organ-specific cells with special functions. In some organs, such as the gut and bone marrow, stem cells regularly divide to repair and replace worn out or damaged tissues. In other organ

6、s, however, such as the pancreas and the heart, stem cells only divide under special conditions.Until recently, scientists primarily worked with two kinds of stem cells from animals and humans: embryonic stem cells and non-embryonic somatic or adult stem cells. The functions and characteristics of t

7、hese cells will be explained in this document. Scientists discovered ways to derive embryonic stem cells from early mouse embryos nearly 30 years ago, in 1981. The detailed study of the biology of mouse stem cells led to the discovery, in 1998, of a method to derive stem cells from human embryos and

8、 grow the cells in the laboratory. These cells are called human embryonic stem cells. The embryos used in these studies were created for reproductive purposes through in vitro fertilization procedures. When they were no longer needed for that purpose, they were donated for research with the informed

9、 consent of the donor. In 2006, researchers made another breakthrough by identifying conditions that would allow some specialized adult cells to be reprogrammed genetically to assume a stem cell-like state. This new type of stem cell, called induced pluripotent stem cells (iPSCs), will be discussed

10、in a later section of this document.Stem cells are important for living organisms for many reasons. In the 3- to 5-day-old embryo, called a blastocyst, the inner cells give rise to the entire body of the organism, including all of the many specialized cell types and organs such as the heart, lung, s

11、kin, sperm, eggs and other tissues. In some adult tissues, such as bone marrow, muscle, and brain, discrete populations of adult stem cells generate replacements for cells that are lost through normal wear and tear, injury, or disease. Given their unique regenerative abilities, stem cells offer new

12、potentials for treating diseases such as diabetes, and heart disease. However, much work remains to be done in the laboratory and the clinic to understand how to use these cells for cell-based therapies to treat disease, which is also referred to as regenerative or reparative medicine.Laboratory stu

13、dies of stem cells enable scientists to learn about the cells essential properties and what makes them different from specialized cell types. Scientists are already using stem cells in the laboratory to screen new drugs and to develop model systems to study normal growth and identify the causes of b

14、irth defects.Research on stem cells continues to advance knowledge about how an organism develops from a single cell and how healthy cells replace damaged cells in adult organisms. Stem cell research is one of the most fascinating areas of contemporary biology, but, as with many expanding fields of

15、scientific inquiry, research on stem cells raises scientific questions as rapidly as it generates new discoveries.1. Where do stem cells come from?There are several sources of stem cells. Pluripotent stem cells can be isolated from human embryos that are a few days old. Cells from these embryos can

16、be used to create pluripotent stem cell lines cell cultures that can be grown indefinitely in the laboratory. Pluripotent stem cell lines have also been developed from fetal tissue (older than 8 weeks of development).In late 2007, scientists identified conditions that would allow some specialized ad

17、ult human cells to be reprogrammed genetically to assume a stem cell-like state. These stem cells are called induced pluripotent stem cells (iPSCs). IPSCs are adult cells that have been genetically reprogrammed to an embryonic stem celllike state by being forced to express genes and factors importan

18、t for maintaining the defining properties of embryonic stem cells. Although these cells meet the defining criteria for pluripotent stem cells, it is not known if iPSCs and embryonic stem cells differ in clinically significant ways. Mouse iPSCs were first reported in 2006, and human iPSCs were first

19、reported in late 2007. Mouse iPSCs demonstrate important characteristics of pluripotent stem cells, including expressing stem cell markers, forming tumors containing cells from all three germ layers, and being able to contribute to many different tissues when injected into mouse embryos at a very ea

20、rly stage in development. Human iPSCs also express stem cell markers and are capable of generating cells characteristic of all three germ layers.Although additional research is needed, iPSCs are already useful tools for drug development and modeling of diseases, and scientists hope to use them in tr

21、ansplantation medicine. Viruses are currently used to introduce the reprogramming factors into adult cells, and this process must be carefully controlled and tested before the technique can lead to useful treatments for humans. In animal studies, the virus used to introduce the stem cell factors som

22、etimes causes cancers. Researchers are currently investigating non-viral delivery strategies.Non-embryonic, or adult stem cells have been identified in many organs and tissues. Typically there is a very small number of multipotent stem cells in each tissue, and these cells have a limited capacity fo

23、r proliferation, thus making it difficult to generate large quantities of these cells in the laboratory. Stem cells are thought to reside in a specific area of each tissue (called a stem cell niche) where they may remain quiescent (non-dividing) for many years until they are activated by a normal ne

24、ed for more cells, or by disease or tissue injury. These cells are also called somatic stem cells.Why are embryonic stem cells important? Opponents of embryonic stem cell research often argue that embryonic stem cell research is not necessary because adult stem cells from bone marrow or umbilical co

25、rd blood cells are already curing diseases, are more likely to cure diseases and that adult stem cells would not be rejected by the immune system when transplanted. They are troubled by the cloning because they oppose creating an embryo in order to harvest its stem cells. Finally, they point out tha

26、t current methods of collecting embryonic stem cells are inefficient and cloning embryonic stem cells may not be a practical method of producing cells for transplantation purposes. These arguments are partly true but are misleading and do not justify the current restrictions of human embryonic stem

27、cell research. Let us consider some of the most frequently cited arguments for or against adult and embryonic stem cells.1. Are adult stem cells already curing diseases? Some opponents of embryonic stem cell research say that embryonic stem cells have never cured any condition while adult stem cells

28、 are already curing many diseases. Bone marrow and umbilical cord blood stem cells have been used for over two decades to treat blood-making (hematopoietic cells) disorders, such as sickle cell anemia, thalassemia, radiation or chemotherapy induced bone marrow damage, and autoimmune diseases. Bone m

29、arrow stem cell transplants may accelerate and improve healing from heart attacks (myocardial infarcts) or failing hearts (congestive heart failure). However, there is no credible evidence yet that bone marrow stem cells are replacing heart cells. Bone marrow cells may be releasing factors that help

30、 hearts heal faster. Neither bone marrow nor umbilical cord blood stem cells, or other types of adult stem cells, have cured neurological conditions, such as brain or spinal cord injury, amyotrophic lateral sclerosis, multiple sclerosis, Alzheimers disease, Parkinsons disease, or non-neurological di

31、seases such as diabetes, liver damage from hepatitis, and other currently incurable conditions. We do not know enough now to predict whether adult or embryonic stem cells would be better or which would be more effective. Many animal studies have shown beneficial effects of embryonic stem cell treatm

32、ents of animal models of diseases. Human embryonic stem cells have not yet been transplanted into humans. There are no restrictions on adult bone marrow or umbilical cord blood research at all while U.S. federal government significantly restricts funding of human embryonic stem cell research. 2. Do

33、adult stem cells circumvent the problem of immune rejection? It is true that autologous grafts, i.e. adult stem cells taken from a person and transplanted into the same person, would not be rejected by the immune system. However, autologous grafts cannot be used for many conditions. First, autologous grafts are not useful for most genetic