Stem Cells and Tissue Regeneration When Is a Stem Cell Really a Stem Cell?

Bone Marrow Transplantation (2003) 32, S7–S11 & 2003 Nature Publishing Group All rights reserved 0268-3369/03 $25.00 www.nature.com/bmt Stem Cells and Tissue Regeneration When is a stem cell really a stem cell?

Gerald J Spangrude

Departments of Oncological Sciences, Pathology, and Medicine, Division of Hematology, University of Utah, Salt Lake City, UT, USA

Summary: years after this dramatic insight, we now understand that the ability of such transplants to reconstitute hematopoiesis While bone marrow transplantation has long been following radiation depends upon the presence of extremely established as an effective approach to the clinical rare stem cells found predominantly in the bone marrow management of a variety of malignant and nonmalignant but capable of mobilization into peripheral tissues via the diseases, the future application of pluripotent stem cells blood vascular system.2 in transplant settings promises to deliver this therapy to a After many years shrouded in mystery and controversy, much broader range of indications. In this review, I the characteristics of blood stem cells were gradually summarize the emerging field of embryonic stem cell revealed through novel assays3–5 and methods for isolation biology in the context of potential clinical applications and of these rare cells.6,7 We now understand that the definition regulatory issues. of a stem cell must include the two essential characteristics Bone Marrow Transplantation (2003) 32, S7–S11. of self-renewal (cellular division maintains stem cell doi:10.1038/sj.bmt.1703936 potential) and multipotency (differentiation into function- Keywords: embryonic stem cell; primordial germ cell; ally distinct lineages). To complicate matters, it is clear that adult stem cell; hematopoietic stem cell progenitor cells, which are multipotent but lack self- renewal potential, are often difficult to distinguish from true stem cells.8 Finally, at least some confusion persists in the tissue stem cell field, where unipotent precursor cells which maintain a tissue through a self-renewing process are In recent years, data from numerous experimental studies often considered stem cells. has suggested that the potential uses of stem cells in The general field of stem cell biology has been the subject medicine may reach far beyond bone marrow transplanta- of intense public interest in recent years for several reasons. tion. How applicable is recent research to modern First, the demonstration that recipients of bone marrow medicine, and how soon might we expect to see stem cell transplants harbor donor-derived cells in a variety of tissues therapies applied to tissue engineering problems? These and has radically changed our expectations for the applications of this type of therapy,9 even though many questions have other questions are explored in this special issue of Bone 10 Marrow Transplantation. It is altogether fitting that a been raised by these interesting findings. Second, the discussion of the therapeutic potentials of stem cell therapy derivation of totipotent human stem cells from both be grounded in our field, being the first to apply stem cell embryonic and fetal sources has introduced a potential therapy to the clinical management of acquired and new source of tissue for engineering applications. Equally inherited diseases. But what is a stem cell? In the context importantly, this new technology marks the genesis of a new of bone marrow transplantation, we understand the answer level of conflict between science and religion that surpasses to this question in a concrete and functional sense because that raised by older questions of creationism vs evolution. of decades of research and clinical applications that grew The potential use of stem cells derived from adult tissues out of the need to understand the effects of ionizing introduces yet another side to this complex story. How are radiation on biological systems. In the years following the we to define when a stem cell is a stem cell? It is in this vein Second World War, a considerable amount of scientific that I examine a few of the historical aspects of stem cell effort was focused on the prevention and treatment of biology in order to better understand where we have come radiation sickness. From these studies came the observation from at this stage in the development of the stem cell field. that transplants of spleen or bone marrow cells contribute 1 to cellular recovery following lethal radiation. Almost 50 Embryonic stem cells: a timeline

Lewis11 has correctly identified the origins of the stem cell biology field in the work of Leroy Stevens, a developmental Correspondence: Dr GJ Spangrude, Division of Hematology RM biologist who identified frequent testicular tumors arising 4C416, University of Utah, 30 N 1900 East, Salt Lake City, UT spontaneously in strain 129mice at the Jackson Labora- 84132-2408, USA tories. This work was published to little fanfare beginning When is a stem cell really a stem cell? GJ Spangrude S8 in 1958.12 However, the curiosity of Mintz and Illmensee mutagenesis,16,17 an entirely new approach to the investiga- led to a startling observation. When malignant teratocarci- tion of complex mammalian biological systems. Today, it is noma cells were mixed into developing mouse embryos, the difficult to imagine a world in which the genome could not environment of the embryo harnessed the unregulated be mutated in a specific manner. The true power of stem growth of the tumor and directed these cells to proper cell biology was revealed to the world at large with the channels of proliferation and differentiation.13 The result announcement that the transfer of nuclei derived from was chimeric mice in which a significant portion of the adult somatic cells into enucleated oocytes produced, at a body mass was derived from the teratocarcinoma. This low frequency, viable offspring clonally derived from the startling discovery was viewed at the time as evidence for donor of the nuclei.18 environmental regulation of malignant growth, but the potential of these cells was certainly not overlooked by developmental biologists. Embryonic stem cell lines were Mouse to man derived from the inner cell mass of mouse blastocysts in 1981,14,15 as shown in Figure 1. These cells were adapted for The successful application of targeted mutagenesis in the growth in culture without differentiation, but could mouse was not the only useful application of embryonic differentiate into mesoderm, endoderm, and ectoderm in stem cell lines. A variety of investigators utilized these vitro and in vivo. The derivation of embryonic stem cell lines cell lines to model the development of the early embryo in was rapidly exploited to give birth to the field of targeted culture systems, and successfully recapitulated several

Figure 1 From the initial descriptions of the ability of testicular carcinoma cells to produce pluripotent embryonic stem cells, these cells have since been derived from blastocysts as well as the primordial germ cells in the developing genital ridge in both mouse and man. Figure courtesy of Terese Winslow, used with permission of the artist.

Bone Marrow Transplantation When is a stem cell really a stem cell? GJ Spangrude S9 aspects of embryogenesis. When the application of in vitro characterized example of adult stem cells is that of the fertilization to the clinical problem of infertility resulted in hematopoietic system. the birth of the first test-tube baby in 1978, the stage was set for the eventual derivation of human embryonic stem cells from embryos fertilized in vitro but not implanted into a Hematopoietic stem cells: paradigms for stem cell biology womb.19 Since these early embryos are frozen in quantities that exceed clinical need, large banks of fertilized embryos The limited lifespan of most blood cells demands that a destined for destruction now exist around the world as a continual source of these cells be assured throughout life. It consequence of the widespread application of in vitro is likely for this reason that the hematopoietic system has so fertilization. Some of these embryos have been cultured to readily lent itself to applications involving clinical trans- derive embryonic stem cell lines: however, the derivation of plantation. Indeed, the challenges faced by cells utilized in cell lines in addition to those already in existence has been bone marrow transplantation are not so different than the deemed unnecessary and will not be supported by federal normal physiologic role played by these cells during funding agencies in the United States. maintenance of hematopoiesis over the lifetime of the A second approach to the application of stem cell normal mammal. Compared to the hematopoietic system, technology in humans utilizes tissue derived from the other tissues of the adult mammal display relatively limited genital ridge of aborted first trimester fetuses (Figure 1).20 potential for replacement from endogenous stem cells in These cells, which normally develop into mature gametes, response to tissue injury. Furthermore, no other tissue is can be cultured under specific conditions to produce cell characterized by such a wide variety of different cell lines with all known characteristics of blastocyst-derived lineages, which all arise from a common stem cell in a embryonic stem cells but lacking apparent tumorigenic developmental process that continues throughout life. The potential. This combination of multipotential differentia- ability to model many of these differentiation pathways tion in the absence of tumor formation has lead to the under controlled conditions in vitro, and the availability of proposed use of these cells in clinical trials to treat spinal recombinant proteins that select or direct differentiation cord injury, Parkinson’s disease, and other cell-based along specific lineages makes the hematopoietic system the therapies. With the specter of the cloning of human beings premier paradigm for the field of stem cell biology. looming before us, the National Academy of Sciences initiated a comprehensive analysis of this brave new Plasticity world.21 The current state of federal funding will support the utilization of fetal-derived embryonic germ cells in The concept of stem cell plasticity refers to the phenom- clinical applications, most likely because the derivation and enon of trans-differentiation, which is the ability of an use of these cells avoids some of the concerns raised by the adult stem cell from one tissue to differentiate as a concept of frozen embryos as sources of embryonic stem specialized cell type of another tissue. A recent study cells. Embryonic germ cells are unable to be implanted into showed that neural stem cells were capable of regenerating a surrogate mother to produce a genetically normal human, blood lineages in transplant recipients,22 and the field unlike the embryos formed during in vitro fertilization. As rapidly advanced as examples of muscle, epithelium, liver, such, the only embryos that might be formed by embryonic and other tissues derived from heterologous stem cells germ cells would be genetic mosaics of the germ cell and (usually bone marrow derived) were reported.23 With few a blastocyst in which such cells might be introduced, or exceptions, these studies involved transplantation of large would be the product of a somatic cell nuclear transfer. numbers of cells, leaving open the possibility that distinct Since the latter process can be performed using a wide classes of stem cells were responsible for regeneration of variety of cell types, the embryonic germ cell provides no the different tissues. Furthermore, the magnitude of tissue special advantage in this sense. replacement has often been minor, suggesting that this approach to tissue engineering will require extensive optimization in order to be clinically useful. Finally, Adult stem cells technical artefacts24 and difficulty in reproducing some of the reported findings25 suggest that caution is indicated in Undifferentiated cells that are found in a differentiated interpreting many of the experiments. The concept that adult tissue are considered adult stem cells, particularly stem cells derived from adult tissues will provide a viable when these cells contribute to ongoing tissue maintenance alternative to the embryo as a source of material for tissue or repair. These cells may be capable of self-renewal, but do engineering is far from validated. not replicate indefinitely in culture. Adult stem cells may differentiate to produce progenitor, precursor, and mature cells, but these activities are usually limited to the cells Stem cells as therapeutic agents contained in the tissue of origin. Adult stem cells usually comprise a small minority of the total tissue mass, and as The ability of stem cells to provide a self-renewing source of such are usually quite difficult to identify and isolate. Adult normal differentiated cells has been extensively exploited in stem cells have been described in regenerating tissues such the bone marrow transplantation field. Applications include as the liver, epithelium, and muscle, as well as in tissues like the treatment of marrow failure syndromes, leukemia and the brain, which previously was thought not to possess lymphoma, certain inherited blood disorders and auto- extensive regenerative properties. By far, the most well- immune diseases.

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Bone Marrow Transplantation When is a stem cell really a stem cell? GJ Spangrude S11 25 Morshead CM, Benveniste P, Iscove NN et al. Hematopoietic Institutes of Health, 2001 http://www.nih.gov/news/stemcell/ competence is a rare property of neural stem cells that may scireport.htm depend on genetic and epigenetic alterations. Nat Med 2002; 8: 27 Zerhouni E. Stem cell research. Senate Appropriations Sub- 268–273. committee on Labor. HHS, Education: Washington, DC, 2002 26 Department of Health and Human Services. Stem cells: http://www.nih.gov/about/director/092502sctestimony.htm Scientific Progress and Future Research Directions. National

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