Report 5 of the Council on Scientific Affairs (A-03)
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Cloning and Stem Cell Research

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Also see the AMA's  Genetics and molecular medicine  Web site.

Background of Report

American Medical Association (AMA) policy on stem cells, the use of somatic cell nuclear transfer technology (SCNT), and cloning has evolved over the last decade in response to scientific and technical advances. Although current policy provides useful guidance on some issues, it is conflicted on others (see below).

Additionally, Resolution 511, introduced by the Texas Delegation and referred to the Board of Trustees at the 2002 AMA Annual Meeting, asked:

Therefore, the CSA agreed to provide an update on scientific advances in stem cell research and, in conjunction with a companion report offered by the Council on Ethical and Judicial Affairs (CEJA Report 132, Cloning for Biomedical Research), clarify AMA policy in this area. A Glossary of Terms used in this update is provided at the end of this report.

Introduction

Stem cells are a unique population of unspecialized cells characterized by their ability to continuously renew themselves for long periods of time through cell division. While stem cells have been isolated from both embryonic and adult tissues, they differ in several properties including the ability to differentiate into specialized cell lineages. Much of the initial biology of stem cells was based on isolates from murine embryos. The ability to isolate human embryonic stem (ES) cells has rekindled expectations that these cells will play a major role in regenerative medicine but at the same time has raised new concerns about the use of human embryos in biomedical research.

Human ES cells are commonly derived from unused fertilized eggs (supernumerary embryos) donated by in vitro fertilization clinics with informed consent of the donor. Alternatively, stem cells have also been obtained from embryos generated from unfertilized eggs using a technique called somatic cell nuclear transfer (SCNT). Initially, SCNT technology was designed to produce embryos from which immunologically compatible stem cells could be derived for use in treating human diseases (therapeutic cloning). However, recent advances in the technology have prompted concerns about embryos formed by SCNT being misused for generating human clones (reproductive cloning).

Methods

Literature searches were conducted in the MEDLINE database for English-language articles published between 1998 and April 2003 using the search terms stem cells in conjunction with human and cloning. The CSA report, "Cloning and Embryo Research" (CSA Report 7, A-99), was used to identify references prior to 1999. The World Wide Web was searched for articles relating to stem cell research. Scientists from biotechnology companies involved in adult (OsirisTherapuetics, Baltimore, MD) and embryonic (Geron Corporation, Menlo Park, CA) stem cell research were also consulted to gain an industry perspective on this technology.

Summary of Current AMA Policy on Stem Cells, Cloning, and SCNT [See Editor's Note]

Relevant policies and ethical opinions (AMA Policy Database) are:

• E-2.147 Human Cloning (I-99)
• H-460.917 Science, Policy Implications, and Current AMA Positions Regarding Embryonic/Pluripotent Stem Cell Research and Funding (CSA Rep 15, I-99)
• H-460.937 Cloning and Human Embryo Research (BOT Rep. 13, A-95; CSA Rep.7, A-99)
• H-140.930 The Ethics of Human Cloning (CEJA-2, A-99)
• H-460.925 Scientific Implications of Somatic Cell Transfer Technology (Res 11, A-98)

AMA policy opposes the cloning of human beings at this time, and the use of SCNT technology for reproductive cloning. AMA policy also strongly endorses federal funding for research involving human ES cells (Policy H-460.917). Specific elements are based on support of the 1999 National Bioethics Advisory Commission (NBAC) report, Ethical Issues in Human Stem Cell Research. This report recommended federal support for stem cell research involving fetal cadavers, or embryos remaining after infertility treatments, but opposed creation of embryos (by in vitro fertilization or SCNT) solely for the purpose of research on human ES cells. This latter view is in contrast to Policy H-460.925, which encourages application of SCNT for uses other than human reproduction including medical therapeutic procedures, and to Policy H-460.937, which recognizes that some areas of human embryo research (eg, studies directed at improving the likelihood of a successful pregnancy, preimplantation genetic diagnostic studies, and research on the fertilization process) may be acceptable for receiving federal funds (including embryos created expressly for the purposes of research). Additionally, opposition to SCNT technology for the purposes of biomedical research as noted in Policy H-460.917 may be inappropriate in light of recent scientific data indicating that embryos created via SCNT technology can be a potent source for derivation of pluripotent stem cells. Coupled with other new data indicating that stem cells derived from adult sources may not be pluripotent, our AMA’s overarching support for the NBAC report must be revisited. The pluripotency of stem cells derived from embryonic sources has repercussions in numerous areas of biomedical research, including the possibility of developing antigenically identical tissues/organs for treatment of a range of disorders (repair of damaged spinal cords, Parkinson’s disease, tissue/organ transplantation); cancer research and treatment; and basic developmental and cellular biology, to name only a few.

Background

Stem cells were first postulated to exist more than 40 years ago in the bone marrow of cancer patients¹ but were not successfully isolated in vitro until approximately 20 years later.² Since that time, various aspects of stem cell research have provided a rich source of scientific debate on issues ranging from their therapeutic potential to the manner in which they can be isolated and genetically manipulated. Stem cells are a unique population of cells that are characterized by their capacity for long-term self-renewal in culture while retaining the ability to differentiate into specialized cells depending on intrinsic factors (genetic) and the external microenvironment.³ They were initially thought to reside only in embryonic tissue or adult tissue exhibiting self-renewing properties (eg, bone marrow and skin) but have been recently identified in a variety adult tissues including brain and blood vessels.

Regardless of their origin, stem cells hold tremendous promise for treating a wide variety of human diseases as well as answering some basic biological questions regarding development. Therapeutically, stem cells can be used in regenerative medicine to replace diseased or damaged tissue. Disorders such as diabetes, Parkinson’s, and cardiovascular disease are ideally suited for such treatments since they involve losses of specific cell populations that could be repaired using cell-based transplant therapies. The ability to genetically manipulate stem cells means they can also be used as vehicles to deliver genes or proteins in gene therapy. The potential to selectively differentiate large numbers of stem cells into specific cell types makes them attractive for use in drug discovery and toxicology screening efforts as well. Finally, stem cells represent a powerful new tool that can be used to define pathways and identify novel components involved in lineage specification, cellular senescence, fertility/reproduction, and immune rejection.

Embryonic Stem Cells

The first stem cells were isolated from a murine teratoma, a gonadal tumor composed of a mixture of embryonal carcinoma (EC) cells and somatic tissue.⁴ Stem cells derived from these tumors were capable of forming a variety of somatic and germ-line tissues, a capacity referred to as pluripotency. Much of their initial value was based on their use as a model for cellular differentiation to define mechanisms that regulate embryonic development and the formation of distinct cell lineages. Subsequent derivation of stable EC cell lines derived from mice⁵ and humans⁶ enabled further identification of cell surface antigens,⁷ transcription factors,⁸ and signaling pathways⁹ involved in differentiation of specific cell lineages.

Because EC cells were derived from malignant carcinomas, efforts were initiated to isolate stem cells from other sources that would exhibit more genetic stability. Two new populations of stem cells were identified in murine embryos that were capable of forming cells derived from all 3 embryonic germ layers (ectoderm, mesoderm, and endoderm). These new pluripotent stem cells were isolated from 2 different embryonic sources. The first population, referred to as embryonic stem (ES) cells,¹⁰ were isolated from the pre-implantation embryo (blastocyst) while the second was isolated from the genital ridge of the post-implantation embryo (fetus) and called embryonic germ (EG) cells. Recently, human ES¹¹ and EG¹² cells have been isolated and found to differ in a number of aspects including colony morphology, growth rates, and surface antigens.¹³ Interestingly, prominent differences have also been observed between human and murine ES cells, indicating they are not identical. For example, human ES cells can be induced to differentiate into extraembryonic cell lineages (trophoblast) in culture while murine ES cells tend to form more disorganized masses under similar conditions.¹⁴

Human ES cells are derived from a small group of about 30 cells that form the inner cell mass (ICM) of the blastocyst-stage embryo. Embryos used to generate ES cells are derived from unused fertilized oocytes (supernumerary embryos) donated with informed consent by individuals who have undergone in vitro fertilization. Fertilized oocytes are cultured for approximately 5 to 6 days in vitro until they form a hollow sphere of cells called a blastocyst. The ICM is harvested from blastocysts and plated in a Petri dish onto a layer of mitotically inactivated murine embryonic fibroblasts (Figure 1 [PDF, 261KB, requires Adobe® Reader®]). The fibroblastic feeder layer provides a substrate for adhesion and secretes various nutrients that promote proliferation and prevent differentiation. Concerns over possible contamination of human ES cells with murine viruses or other cellular byproducts have prompted recent efforts to develop culturing methods that do not require feeder layers.¹⁵ Individual cell colonies arising in these cultures are then separated and passaged repeatedly until genetically identical (clonal) cell lines have been established. Clonal ES cell lines are typically propagated for at least 6 months to ensure they remain undifferentiated, retain the ability to self-renew, and exhibit genetic (karyotypic) stability. In addition, they are commonly tested for other properties associated with embryonic cells (eg, expression of specific surface markers and transcription factors) as well as pluripotency. A similar process is used to derive EG cell lines; however, the initial cell mass used for culturing is isolated from the gonadal ridge of a 5- to 10-week-old fetus obtained from the therapeutic termination of a pregnancy.

Much of the initial biology of ES and EG cells was established using stem cells derived from mammalian sources, especially mice. Murine stem cells, while useful for providing therapeutic proof-of-concept in pre-clinical animal models, are not suitable for clinical applications. Recent successes in isolating human stem cells, however, provide the necessary tools for researchers to confirm their therapeutic potential. The ability to isolate pluripotent human ES and EG cells has enabled several ES cell lines to be established.¹⁶ These lines exhibit properties consistent with stem cells including genetic stability after repeated passage in culture (>8 months), expression of high levels of telomerase activity, displaying of surface markers associated with undifferentiated stem cells, and pluripotency. These ES cell lines have also proven invaluable for defining biological properties unique to human stem cells and developing methodologies that will enhance their utility in cell-based therapies. For example, human cell lines have been used to identify growth factors needed to direct differentiation of stem cells into specific cell types.¹⁷ In addition, these lines have been used to develop protocols that allow genetic manipulation of stem cells using either viral vectors¹⁸ or homologous recombination.¹⁹ More recently, a protocol was developed using single-stranded oligodeoxynucleotides that enabled specific single base pair alterations to be introduced into the genomes of ES cells.²⁰ Finally, culturing conditions were described recently for inducing human ES cells to form trophoblasts and other extraembryonic cell lineages that will allow a better understanding of placental development and function.²¹

Recent legislation restricting the use of human embryos in research has limited access of U.S. laboratories to human ES cells. Currently, 71 human stem cell lines have been approved for use in federally funded research.²² However, only 16 of these cell lines are available at the present time.²³ Unresolved issues associated with licensing and patent rights have hindered the availability of many of these cell lines. In addition, concerns are growing among scientists that many of the unavailable cell lines have not been thoroughly characterized or developed to the degree required for research purposes. While the National Institutes of Health (NIH) feels these cell lines are adequate for current basic research needs, serious doubts remain that they will be sufficient to meet the needs of any future clinical trials for cell-based therapies. Generation of additional human ES cell lines is still possible in U.S. laboratories as long as it is not supported by federal funding but pending legislation could criminalize this type of research as well as prevent importation of cell lines developed outside this country. Several countries including Israel, Singapore, South Korea, Sweden, and the United Kingdom still permit the production of new human ES cell lines from cloned embryos.²⁴ The fear among U.S. researchers is that much of the work conducted on human stem cells will shift to these countries if federal legislation is approved banning the generation of new ES cells in the United States.

The development of human ES cells represents a significant breakthrough in stem cell research. Successful application of this technology in treating human disorders, however, will ultimately depend on demonstrated efficacy in animal models and clinical trials. Therapeutic efficacy in these models will require not only successful incorporation of transplanted cells into target tissue but also functional integration into existing systems. Evidence suggesting that stem cells can meet these requirements is found in several recent studies describing their use in treating various animal models of neurodegenerative disorders. For example, human EC cell transplants were demonstrated to promote significant functional recovery in a rodent model of spinal cord trauma.²⁵ In a separate study, human EC cells delayed motor dysfunction in a mouse model of familial amyotrophic lateral sclerosis (ALS).²⁶ Recent work with human ES cells demonstrated that they could be differentiated into neural precursors in vitro and then transplanted into neonatal mouse brain where they assimilated into various brain regions forming both neurons and glial cells.²⁷

Murine ES cells have demonstrated efficacy in other animal models as well. In one study, cells transplanted into rat spinal cord 9 days after traumatic injury were able to not only survive, migrate, and differentiate into neurons and glia but also resulted in some functional recovery.²⁸ Finally, a recent study demonstrated how murine ES cells can be genetically manipulated in culture to form a specific neuronal cell type that when transplanted into a rat model of Parkinson’s disease was able to repopulate a region of the brain and provide functional recovery.²⁹ A similar protocol using genetic manipulation and subsequent in vitro differentiation of murine ES cells was also used to restore function in a rodent model of immunodeficiency.³⁰ A generalized summary of the research performed with human stem cells is presented in the Table.

Table: Summary of research on human stem cells

Somatic Cell Nuclear Transfer

One of the potential hurdles associated with the use of stem cells in therapeutic applications is the problem of immune rejection. Although a number of human ES cell lines have been developed, their clinical use as an allogenic transplant would presumably necessitate chronic immunosuppression of the recipient. This concern prompted a recent study in which the cell surface expression of major histocompatibility (MHC) antigens was characterized in human ES cell lines.³¹ While only low levels of MHC-1 proteins were detected in undifferentiated cells, a significant induction of these proteins was observed in differentiated cells following exposure to interferons, raising the question of whether a similar phenomenon could occur following transplantation. To avoid this possible immune response, a technique has emerged that may enable generation of stem cells that are specifically tailored for an individual. The technique, somatic cell nuclear transfer (SCNT), involves reprogramming of a donor oocyte so that it becomes immunologically compatible with a designated recipient. The process involves removing the nuclear material from an unrelated donor oocyte (enucleation) and replacing it with the nucleus from a recipient’s somatic cell (nuclear transfer). The DNA injected into the enucleated oocyte can be in the form of a nucleus (eg, from a fibroblast obtained via skin biopsy) or an entire cell, typically a small specialized ovarian cell called a cumulus cell is used (Figure 2 [PDF, 261KB]). The engineered oocyte is then incubated in a culture dish with media that induce cell division (mitosis). The reprogramming of an oocyte is a poorly understood process that involves activation of genes needed for early development and suppression of genes associated with differentiation. The developing embryo is cultured in vitro for 5 to 6 days until the blastocyst is formed and then stem cells are harvested from the ICM.

SCNT was initially developed to generate clones for agricultural purposes and has been successfully used to generate cloned cattle³² and sheep.³³ This technique was subsequently used for research purposes to generate murine embryos³⁴ from which pluripotent ES cells were successfully derived.³⁵ Recently, attempts have been made in a commercial laboratory (Advanced Cell Technology, Worcester, MA) to produce ES cells derived from human embryos generated using SCNT technology.³⁶ In these experiments, donated human ooctyes were injected with either the nucleus from adult skin cells (fibroblast) or an ovarian cumulus cell. The engineered oocytes all formed early-stage embryos but none progressed to the stage where ICM cells could be harvested.³⁷ Differences in the organization of the mitotic spindle apparatus in primates and humans indicate that SCNT techniques may have to be further optimized in order to more efficiently produce human embryonic stem cells.³⁸

The most well publicized use of SCNT technology was the cloning of the sheep, Dolly.³⁹ She was the product of an embryo generated by replacing the nucleus from an oocyte with the DNA from an adult mammary cell. The sudden realization by the public that engineered embryos could be carried to term and produce viable offspring raised concerns about possible misuse of SCNT to generate cloned human beings (reproductive cloning). These concerns are based primarily on ethical issues since at the present time, somatic cell cloning in mammals is a highly inefficient process with <1% of the embryos generated by nuclear transfer surviving gestation. The reasons for this inefficiency remain unclear although abnormalities in placental development appear to play a role.⁴⁰ Questions on the genetic stability of embryos derived by SCNT have also arisen based on the premature death of Dolly and cloned mice.⁴¹ More recent studies analyzing gene expression in murine ES cells generated from SCNT suggest that abnormalities affecting development and lifespan may result from inadequate nuclear reprogramming, anomalies in gene imprinting, the process of SCNT itself, and the nature of the donor nucleus.⁴²

The inability to produce viable human embryos suitable for stem cell harvesting by SCNT has prompted researchers to look for other ways to induce human oocytes to divide without being fertilized by sperm or nuclear transfer. A technique known as parthenogenesis has demonstrated some limited success in certain mammalian oocytes. This technique utilizes eggs harvested at a point in their maturation cycle when they still retain a full set of genes and then subjects them to artificial stimulation to induce cell division. Stem cells isolated from murine embryos generated by parthenogenesis have been shown to produce a variety of tissues.⁴³ Parthenogenic activation of human oocytes has been described recently, but embryo development ceased at a stage prior to ICM formation.⁴⁴ The inability to produce viable human embryos would appear to involve only technical issues since this same laboratory has subsequently reported the isolation of pluriopotent stem cells from nonhuman embryos generated by parthenogenesis.⁴⁵

SCNT technology is also being utilized in more basic research applications to understand molecular and cellular events underlying human diseases. For example, a group at the Institute for Cancer/Stem Cell Biology and Medicine at the Stanford University Medical Center has announced plans to generate human and murine stem cell lines via SCNT that contain DNA mutations associated with specific human diseases. These lines will allow scientists to study mechanisms by which disease-causing mutations affect cellular function and development.

Adult Stem Cells

Stem cells were initially thought to be present only in embryonic tissue and adult tissue that exhibited a capacity to regenerate, such as skin or liver. However, recent studies have demonstrated that stem cells also exist in many types of adult tissue previously thought to contain only postmitotic cells incapable of self-renewal. For example, stem cells have been isolated in several adult tissues including nervous, adipose, placental, breast, muscle, and blood vessels. One of the main reasons these progenitors were so difficult to detect was because that they exist in such small numbers and can remain quiescent (non-dividing) for long periods of time until they are activated by injury or disease.

Unlike ES cells, the origin of adult stem cells is unknown and the question remains whether they are independent populations or remnants of their embryonic counterparts. Additionally, controversy exists as to whether stem cells isolated from certain adult tissues actually originated in these tissues or represent temporary residents that were derived from a larger pool of circulating stem cells. For example, stem cells isolated from adult murine muscle were initially reported to be capable of differentiating into cells from all the major blood lineages.⁴⁶ Subsequent studies demonstrated that these stem cells were incapable of forming myogenic cells when cultured in vitro, suggesting they had originated in the hematopoietic system and therefore did not represent an actual population of pluripotent myogenic stem cells.⁴⁷

The notion of a highly plastic adult stem cell was initially contrary to the belief held by many in the field that adult stem cells were limited to forming progeny of tissue from which they were derived. Recent work indicates that adult stem cells might exhibit more plasticity and be capable of forming cells from other tissues (multipotent). Stem cells isolated from tissues such as the brain,⁴⁸ skin,⁴⁹ and bone⁵⁰ have been reported to contribute to various unrelated lineages (transdifferentiation). Unfortunately, in many of these studies the inability to definitively demonstrate that stem cells differentiate into other cell types has rendered these results controversial. More substantial evidence for the existence of multipotent stem cells has emerged for mesenchymal stem cells derived from adult bone marrow (Figure 2 [PDF, 261KB]). Murine mesenchymal stem cells, referred to as multipotent adult progenitor cells (MAPCs), have been shown to form cells from different tissues when grown in vitro and appeared to respond to environmental cues and form tissue-specific cell types when engrafted into various adult tissues.⁵¹ For example, murine MAPCs were recently demonstrated to transdifferentiate into functionally competent pancreatic islet cells when systemically injected into irradiated mice.⁵² In light of the many controversial studies surrounding adult stem cell plasticity, researchers in the field are now calling for more rigorous experimental standards be applied to ensure that results are not artifacts of laboratory treatments. One such artifact that has gained prominence of late is the ability of pluripotent stem cells to spontaneously fuse with differentiated cells in culture and adapt a differentiated phenotype.⁵³ This phenomenon has been cited as a reasonable alternative explanation for the plasticity observed in previous studies demonstrating that stem cells isolated from the central nervous system (CNS) can form hematopoietic⁵⁴ and myogenic⁵⁵ cell lineages. Efforts are now underway to determine how frequently cell fusion occurs in vivo and if stem cells are directly involved in this process.

While adult stem cell plasticity remains controversial, the enthusiasm for potential use of these cells in repairing diseased tissue remains resolute. The most prominent advances in therapeutic application of adult stem cells to human disease is in the field of cardiovascular research. For example, adult bone marrow stem cells have been reported to regenerate cardiomyocytes and induce angiogenesis in a rodent model of myocardial infarction.⁵⁶ Results from 2 small clinical studies substantiated these results in human patients who had suffered myocardial infarctions. Significant improvements in cardiac function were observed 3 months after autologous transplant of mononuclear bone marrow cells into ischemic heart tissue that were consistent with enhanced myogenesis⁵⁷ and angiogenesis.⁵⁸ Larger clinical studies are needed to verify these results and answer such questions as the identity of the bone marrow cells effecting repairs and the extent to which tissue was actually repaired.

The use of adult stem cells may avoid the ethical concerns associated with ES cells but does not circumvent potential graft-versus-host rejection responses caused by using heterologous tissues. Recent studies with human bone marrow stromal cells (BMSC) indicate that these cells may not be as immunogenic as initially feared and therefore suitable for allogenic transplantation. In fact, in vitro experiments suggest these cells actually suppress T-cell proliferation and thereby avoid/minimize rejection.⁵⁹ The question remains whether these cells will retain these immunosuppressive properties once they have differentiated in the target tissue. Work with nonhuman primate mesenchymal stem cells confirmed that these types of adult stem cells exhibit characteristics suited for allogenic cell-based therapies including ability to differentiate into multiple mesenchymal cell lineages,⁶⁰ undergo clinical scale expansion, disseminate to a variety of tissues following systemic injection,⁶¹ and to be used as a delivery vehicle for gene therapy.⁶²

Finally, the presence of stem cells in adult tissues such as the brain has raised the question of whether their endogenous regenerative capacity can be harnessed to effect localized self-repair. This type of therapeutic approach would involve inducing and/or enhancing intrinsic signals involved in directing the migration, differentiation, and functional integration of stem cells into regions of the brain damaged by disease or trauma. A recent study provided encouraging results for this approach. Infusion of growth factors into the brains of rats following an ischemic event was found to augment the migratory and proliferative capacities of neural stem cells in damaged brain regions.⁶³

Cord Blood Stem Cells

The controversies surrounding human embryo research have heightened interest in developing adult stem cells for therapeutic uses. Unfortunately, as noted above, adult cells do not appear to have the same plasticity as ES cells, potentially limiting their utility. Another nonembryonic source of multipotent stem cells gaining more attention in recent years is umbilical cord blood (UCB) cells. While these cells have been recognized as a convenient source of stem cells for many years,⁶⁴ the vast majority of clinical applications have involved treating hematopoietic disorders associated with malignant and nonmalignant cancers in both children and adults.⁶⁵ Their value in these types of therapeutic applications has increased even further lately with reports that hematopoietic stem cells can remain viable after being stored frozen for >15 years.⁶⁶

Recently, another population of stem cells has been identified in umbilical cords, specifically in the gelatinous connective tissue (Wharton’s jelly) comprising the cord matrix.⁶⁷ The identity of these cells remains unclear but their ability to form neuronal lineages is similar to that described for mesenchymal stem cells, such as those derived from adult bone marrow stroma.⁶⁸ Interestingly, previous reports have demonstrated that peripherally administered human UCB cells preferentially migrate to the CNS and reduce neurological deficits in animal models of traumatic brain injury,⁶⁹ ischemic stroke,⁷⁰ and ALS.⁷¹ The ability of human UCB cells to delay the onset of neurodegenerative symptoms and ultimate death of transgenic ALS mice has prompted the Institute of Cellular Medicine in Atlanta, Georgia, to begin offering a similar therapy for ALS patients who otherwise have no treatment or cure available.⁷²

Related Advances in Stem Cell Research

In addition to the basic and clinical research being conducted directly on stem cells, a number of recent advances in related technologies have occurred that will facilitate their use in therapeutic applications. For example, a noninvasive imaging technique has been described for monitoring the migrational dynamics of stem cells after implantation into rodent brains. Magnetic resonance imaging (MRI) has provided sufficient temporal and spatial resolution in brain tissue to confirm that stem cells labeled with an MRI contrasting agent exhibited a directed migration toward an ischemic lesion located contralateral to the site of implantation.⁷³ The ability to observe the migration patterns of stem cell transplants in living tissue will undoubtably lead to a better understanding of factors that direct their movement and induce differentiation.

A second advance associated with the stem cell field involves the use of endothelial stem cells as an index to assess cardiovascular risk in patients without known cardiovascular disease. High-resolution ultrasonic analysis of brachial arteries in 45 healthy men revealed an inverse correlation between the number of circulating endothelial progenitors and risk factors for cardiovascular disease.⁷⁴ These results not only suggest a novel use of stem cells but indicate that endothelial stem cells may help to maintain normal function in mature blood vessels and that loss of this function leads to abnormal vasoreactivity.

Finally, researchers at the University of Michigan Comprehensive Cancer Center recently announced they have isolated stem cells from human breast cancer tissue.⁷⁵ This discovery is unique in that stem cells have been isolated from blood-related cancers only and not from solid tumors. While these tumor-inducing stem cells comprise only a small percentage of the tumor, they may explain why metastatic breast cancers are able to regenerate following chemotherapy. Further characterization of these stem cells will allow therapies to be developed that will focus on their destruction.

Summary

The existence of stem cells in embryonic, postnatal, and adult tissues is well established. Numerous questions remain on the origin of many of these stem cell populations and their ability to regenerate the various cell lineages in the human body. One of the most prominent questions that needs to be addressed is whether stem cells derived from adult tissues have the same regenerative potential as those derived from embryonic sources. Until that question can be answered, most researchers agree that work must continue on both embryonic and adult stem cells. Although science may appear at times to advance in leaps and bounds, it relies on a thorough understanding of basic mechanisms before breakthroughs are achieved. As demonstrated by the number of recent references in this report, the pace at which new discoveries are being made in the stem cell field is rapidly accelerating. Studies describing the clinical application of stem cell technology are starting to appear and continue to generate optimism that these cells will one day play a major role in preventing and alleviating human disease. The need for SCNT to generate immunologically compatible stem cells remains a subject of speculation given its inefficiencies and ethical concerns. In addition, this need may be minimized if embryonic and adult stem cells prove to be less immunogenic than initially thought. Most scientists agree that research must be conducted in parallel on both adult and ES cells since each has advantages and disadvantages (eg, plasticity, longevity, expansion, immune compatibility). For any particular disease, both embryonic and adult stem cells may have to be evaluated to determine which is most efficacious. Clearly, the similarities and differences between these types of stem cell populations must be better understood for their full potential to be realized.

RECOMMENDATIONS (Adopted AMA Policy)

The following statement, recommended by the Council on Scientific Affairs, was adopted by the AMA House of Delegates as AMA policy at the 2002 AMA Annual Meeting.

The AMA: (1) supports biomedical research on multipotent stem cells (including adult and cord blood stem cells); (2) supports the use of somatic cell nuclear transfer technology in biomedical research (therapeutic cloning); (3) opposes the use of somatic cell nuclear transfer technology for the specific purpose of producing a human child (reproductive cloning); (4) encourages strong public support of federal funding for research involving human pluripotent stem cells; and (5) will continue to monitor developments in stem cell research and the use of somatic cell nuclear transfer technology. (Policy)

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Adult stem cell. An undifferentiated cell found in differentiated tissue that can renew itself and (with certain limitations) differentiate to yield all the specialized cell types of the tissue from which it originated.

Blastocyst: A preimplantation embryo of about 150 cells. The blastocyst consists of a sphere made up of an outer layer of cells (trophectoderm), a fluid-filled cavity (blastocoel), and a cluster of cells on the interior (inner cell mass).

Bone marrow stromal cells (BMSC): A stem cell found in bone marrow that generates bone, cartilage, fat, and fibrous connective tissue.

Cell-based therapies: Treatment in which stem cells are induced to repair damaged or depleted adult cell populations or tissues.

Clone: A line of cells that is genetically identical to the originating cell; in this case, a stem cell.

Cumulus cell: Specialized cell that clings to ovum after ovulation that nurtures developing egg in ovary.

Differentiation: The process whereby an unspecialized early embryonic cell acquires the features of a specialized cell such as a heart, liver, or muscle cell.

Directed differentiation: Manipulating stem cell culture conditions to induce differentiation into a particular cell type.

Ectoderm: Upper, outermost layer of a group of cells derived from the inner cell mass of the blastocyst that forms skin nerves and brain.

Early embryo (pre-embryo): The term used to describe the preimplantation embryo that is biologically defined as the stages of development from fertilization until the appearance to the primitive streak (approximately 14 days after fertilization).

Embryo: The developing organism from the time of fertilization until the end of the eighth week of gestation, when it becomes known as a fetus.

Embryonic germ (EG) cells: Cells found in a specific part of the embryo/fetus called the gonadal ridge that normally develop into mature gametes.

Embryonic stem (ES) cells: Primitive (undifferentiated) cells derived from inner cell mass of a blastocyst-stage embryo that have the potential to become a wide variety of specialized cell types (pluripotent).

Embryonic stem cell line: Embryonic stem cells, which have been cultured under in vitro conditions that allow proliferation without differentiation for months to years.

Endoderm: Lower layer of a group of cells derived from the inner cell mass of the blastocystm that forms lungs and digestive organs.

Feeder layer: Cells used in co-culture to maintain pluripotent stem cells. Cells usually consist of mouse embryonic fibroblasts.

Fertilization: The process whereby male and female gametes unite.

Fetus: A developing human from usually two months after conception to birth.

Genomic imprinting: Epigenetic modifications of DNA or proteins surrounding DNA (e.g. histones) that result in parent-specific expression or repression of genes in offspring.

Hematopoietic stem cell: A stem cell from which all red and white blood cells develop.

Homologous recombination: Technique used to introduce exongenous genetic material into recipient nucleus based on DNA homology.

In vitro: Literally, "in glass"; in a laboratory dish or test tube; an artificial environment.

In vitro fertilization (IVF): An assisted reproduction technique in which fertilization is accomplished outside the body.

Inner cell mass (ICM): The cluster of cells inside the blastocyst. These cells give rise to the embryonic disk of the later embryo and, ultimately, the fetus.

Long-term self-renewal: The ability of stem cells to renew themselves by dividing into the same non-specialized cell type over long periods (many months to years) depending on the specific type of stem cell.

Mesenchymal stem cells: Cells from the immature embryonic connective tissue. A number of cell types come from mesenchymal stem cells, including chondrocytes, which produce cartilage.

Mesoderm: Middle layer of a group of cells derived from the inner cell mass of the blastocyst; it gives rise to bone, muscle, and connective tissue.

Microenvironment: The molecules and compounds such as nutrients and growth factors in the fluid surrounding a cell in an organism or in the laboratory, which are important in determining the characteristics of the cell.

Multipotent: Ability of a single cell to develop into many different cell types of the body. Developmental potential of a multipotent stem cell is more restricted than pluripotent or totipotent stem cell.

Neural stem cell: A stem cell found in adult neural tissue that can give rise to neurons, astrocytes, and oligodendrocytes.

Parthenogenesis: Form of nonsexual reproduction in which unfertilized ovum is artificially induced to develop into an embryo.

Passage: A round of cell growth and proliferation n cell culture.

Plasticity: The ability of stem cells from one tissue to generate the differentiated cell types of another tissue.

Pluripotent: Ability of a single cell to develop into different cell types derived from each of the embryonic lineages (ectoderm, mesoderm, or endoderm) but cannot develop into an embryo on its own..

Proliferation: Expansion of a population of cells by the continuous division of single cells into two identical daughter cells.

Reproductive cloning: A term used to describe the process of generating a human embryo via somatic cell nuclear transfer for the specific purpose of creating a cloned human being.

Regenerative or reparative medicine: A treatment in which stem cells are induced to differentiate into the specific cell type required to repair damaged or depleted adult cell populations or tissues.

Somatic cell nuclear transfer (SCNT): Technique by which a somatic cell nucleus is transplanted into an ovum whose own nucleus has been removed (enucleated). Process also referred to as nuclear transfer or transplantation.

Stromal cells: Non-blood cells derived from blood organs, such as bone marrow or fetal liver, which are capable of supporting growth of blood cells in vitro. Stromal cells that make this matrix within the bone marrow are also derived from mesenchymal stem cells.

Subculturing: The process of growing and replating cells in tissue culture for many months.

Supranumerary embryo: Embryo originally generated for the purpose of assisted reproduction by in vitro fertilization (INF) techniques but subsequently not used.

Surface markers: Surface proteins that are unique to certain cell types, which are visualized using antibodies or other detection methods.

Teratoma: A tumor composed of multiple tissues including tissues not normally found in organ in which it arises. Neoplasms frequently occur in ovary and testis. Produced experimentally in animals by injecting pluripotent stem cells, in order to determine the stem cells' abilities to differentiate into various types of tissues.

Therapeutic cloning: A term used to describe the process of generating a human embryo via somatic cell nuclear transfer for the specific purpose of obtaining stem cells for use in regenerative medicine.

Totipotent: Ability of a single cell to differentiate into any type of cell in the body (somatic, germ, or extraembryonic) and thus capable of forming a new organism (e.g. fertilized ovum).

Transdifferentiation: The process by which stem cells from one tissue are able to differentiate into cells of another tissue.

Trophoblast: The extraembryonic tissue responsible for implantation, developing into the placenta, and controlling the exchange of oxygen and metabolites between mother and embryo.
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