An Overview of Stem Cell Research and Regulatory Issues
2003; Elsevier BV; Volume: 78; Issue: 8 Linguagem: Inglês
10.1016/s0025-6196(11)63146-7
ISSN1942-5546
AutoresChristopher R. Cogle, Steven M. Guthrie, Ronald C. Sanders, William L. Allen, Edward W. Scott, Bryon E. Petersen,
Tópico(s)Renal and related cancers
ResumoStem cells are noted for their ability to self-renew and differentiate into a variety of cell types. Some stem cells, described as totipotent cells, have tremendous capacity to self-renew and differentiate. Embryonic stem cells have pluripotent capacity, able to form tissues of all 3 germ layers but unable to form an entire live being. Research with embryonic stem cells has enabled investigators to make substantial gains in developmental biology, therapeutic tissue engineering, and reproductive cloning. However, with these remarkable opportunities many ethical challenges arise, which are largely based on concerns for safety, efficacy, resource allocation, and methods of harvesting stem cells. Discussing the moral and legal status of the human embryo is critical to the debate on stem cell ethics. Religious perspectives and political events leading to regulation of stem cell research are presented and discussed, with special attention directed toward the use of embryonic stem cells for therapeutic and reproductive cloning. Adult stem cells were previously thought to have a restricted capacity to differentiate; however, several reports have described their plasticity potential. Furthermore, there have been close ties between the behavior of stem cells and cancer cells. True eradication of cancer will require a deeper understanding of stem cell biology. This article was written to inform medical scientists and practicing clinicians across the spectrum of medical education about the research and regulatory issues affecting the future of stem cell therapy. Stem cells are noted for their ability to self-renew and differentiate into a variety of cell types. Some stem cells, described as totipotent cells, have tremendous capacity to self-renew and differentiate. Embryonic stem cells have pluripotent capacity, able to form tissues of all 3 germ layers but unable to form an entire live being. Research with embryonic stem cells has enabled investigators to make substantial gains in developmental biology, therapeutic tissue engineering, and reproductive cloning. However, with these remarkable opportunities many ethical challenges arise, which are largely based on concerns for safety, efficacy, resource allocation, and methods of harvesting stem cells. Discussing the moral and legal status of the human embryo is critical to the debate on stem cell ethics. Religious perspectives and political events leading to regulation of stem cell research are presented and discussed, with special attention directed toward the use of embryonic stem cells for therapeutic and reproductive cloning. Adult stem cells were previously thought to have a restricted capacity to differentiate; however, several reports have described their plasticity potential. Furthermore, there have been close ties between the behavior of stem cells and cancer cells. True eradication of cancer will require a deeper understanding of stem cell biology. This article was written to inform medical scientists and practicing clinicians across the spectrum of medical education about the research and regulatory issues affecting the future of stem cell therapy. Stem cells were first recognized experimentally by Owen1Owen RD Immunogenetic consequences of vascular anastomoses between bovine twins.Science. 1945; 102: 400-401Crossref PubMed Scopus (865) Google Scholar in 1945, when he found lifelong blood chimer-ism between twin cows. He postulated that an interchange of cells between bovine twin embryos occurred as a result of conjoined blood vessels and that the interchanging cell had to be "ancestral" to the terminal erythrocyte. More than 15 years later, investigators formally tested for these ancestral blood cells by preventing radiation death in mice with bone marrow transplantation.2Till JE McCulloch EA A direct measurement of the radiation sensitivity of normal mouse bone marrow cells.Radiat Res. 1961; 14: 213-222Crossref PubMed Scopus (3230) Google Scholar, 3Becker AJ McCulloch EA Till JE Cytological demonstration of the clonal nature of spleen colonies derived from transplanted mouse marrow cells.Nature. 1963; 197: 452-454Crossref PubMed Scopus (899) Google Scholar, 4Siminovitch L McCulloch EA Till JE The distribution of colony-forming cells among spleen colonies.J Cell Comp Physiol. 1963; 62: 327-336Crossref Scopus (582) Google Scholar, 5Wu AM Till JE Siminovitch L McCulloch EA Cytological evidence for a relationship between normal hemotopoietic colony-forming cells and cells of the lymphoid system.J Exp Med. 1968; 127: 455-464Crossref PubMed Scopus (260) Google Scholar These stem cells were noted for their ability to give rise to clonal colonies of differentiated blood cells in the recipient spleen and for their ability to rescue subsequent generations of lethally irradiated mice. This multilineage recon-stitution by a self-renewing cell is a cardinal feature of stem cells. To this day, transplantation experiments like those performed in the 1960s that showed clonal, robust, and functional differentiation by a cell transplantable over many generations remain the gold standard in testing stem cells. Some stem cells have a greater capacity of self-renewal and multilineage differentiation than others (Table 1). At the time of conception, the fertilized egg (zygote) contains dividing cells (blastomeres) that form an embryo and placenta (Figure 1). These blastomeres are totipotent; they have the potential to form an entire living organism. After about 4 days, these totipotent cells begin to specialize and form into a hollow ball, the blastocyst, containing an outer shell (trophoectoderm) and a cluster of cells called the inner cell mass (ICM), from which the embryo develops. Human embryonic stem (hES) cells are derived by removing the trophoectoderm, which would normally become the placenta, and culturing cells from the ICM.6Thomson JA Itskovitz-Eldor J Shapiro SS et al.Embryonic stem cell lines derived from human blastocysts [published correction appears in Science. 1998;282:1827].Science. 1998; 282: 1145-1147Crossref PubMed Scopus (12341) Google Scholar These hES cells of the ICM are pluripotent; they are able to differentiate into tissues of all 3 germ layers but cannot produce another embryo because they are unable to give rise to the placenta and supporting tissues. Transplanting hES cells into a woman's uterus would not produce a fetus. Blasto-cysts harvested for hES derivation are usually acquired from unused in vitro fertilizations (IVFs). The hES cells express specific surface antigens, as well as OCT-4 and human telomerase, proteins associated with a pluripotent and immortal phenotype.7Henderson JK Draper JS Baillie HS et al.Preimplantation human embryos and embryonic stem cells show comparable expression of stage-specific embryonic antigens.Stem Cells. 2002; 20: 329-337Crossref PubMed Scopus (410) Google Scholar A second source of pluripotent cells is human embryonic germ cells, derived from the fetal gonadal ridge, which normally gives rise to either sperm or egg.8Shamblott MJ Axelman J Wang S et al.Derivation of pluripotent stem cells from cultured human primordial germ cells [published correction appears in Proc Natl Acad Sci U S A. 1999; 96;1162].Proc Natl Acad Sci U S A. 1998; 95: 13726-13731Crossref PubMed Scopus (1274) Google Scholar It remains to be tested whether human embryonic germ cells have the same capacities that make hES cells so important.9Shamblott MJ Axelman J Littlefield JW et al.Human embryonic germ cell derivatives express a broad range of developmentally-distinct markers and proliferate extensively in vitro.Proc Natl Acad Sci U S A. 2001; 98: 113-118Crossref PubMed Scopus (286) Google Scholar A hallmark of hES cells is their long-term self-renewal capability in culture dishes. Whereas adult stem cells divide asynchronously and eventually lose their ability to self-renew, hES cells have been cultured over many years, maintaining developmental potential, proliferative capacity, and karyotypic stability.10Amit M Carpenter MK Inokuma MS et al.Clonally derived human embryonic stem cell lines maintain pluripotency and proliferative potential for prolonged periods of culture.Dev Biol. 2000; 227: 271-278Crossref PubMed Scopus (1226) Google ScholarTable 1Cell Characteristics in Stem Cell BiologyTermDefinitionExampleTotipotentAble to produce an entire beingBlastomeresPluripotentAble to produce all tissues and self-renew indefinitelyEmbryonic stem cellMultipotentAble to produce many cell types and self-renew over the lifetime of the being and over many subsequent generations if transplantedHematopoietic stem cellProgenitorAble to produce restricted number of cell types and with limited to no capacity of self-renewalNeural stem cell Open table in a new tab Because of this unlimited self-renewal and boundless developmental potential, hES cells may serve as powerful tools to unlock important medical challenges. As a basic science tool, these cells help to identify molecular mechanisms in pluripotent cell differentiation, affording investigators a much better understanding of fetal development.11Xu RH Chen X Li DS et al.BMP4 initiates human embryonic stem cell differentiation to trophoblast.Nat Biotechnol. 2002; 20: 1261-1264Crossref PubMed Scopus (885) Google Scholar Ultimately, this understanding aims to reduce infertility, pregnancy loss, and birth defects—major health care challenges in the United States. Also, hES cells are helpful in drug development, identifying potentially embryotoxic te-ratogens.12Rohwedel J Guan K Hegert C Wobus AM Embryonic stem cells as an in vitro model for mutagenicity, cytotoxicity and embryotoxicity studies: present state and future prospects.Toxicol In Vitro. 2001; 15: 741-753Crossref PubMed Scopus (130) Google Scholar Moreover, normal cell lines derived from these pluripotent cells may serve as representative tissues for in vitro toxicity testing of medicinal compounds in development. Finally, organ regeneration from hES cells would not only halt disease progression but also could help to remodel damaged organs. Investigators are already using embryonic stem cells to create heart muscle, brain, pancreatic islet cells, and blood vessels.13Xu C Police S Rao N Carpenter MK Characterization and enrichment of cardiomyocytes derived from human embryonic stem cells.Circ Res. 2002; 91: 501-508Crossref PubMed Scopus (780) Google Scholar, 14Carpenter MK Inokuma MS Denham J Mujtaba T Chiu CP Rao MS Enrichment of neurons and neural precursors from human embryonic stem cells.Exp Neurol. 2001; 172: 383-397Crossref PubMed Scopus (423) Google Scholar, 15Assady S Maor G Amit M Itskovitz-Eldor J Skorecki KL Tzukerman M Insulin production by human embryonic stem cells.Diabetes. 2001; 50: 1691-1697Crossref PubMed Scopus (791) Google Scholar, 16Levenberg S Golub JS Amit M Itskovitz-Eldor J Langer R Endothelial cells derived from human embryonic stem cells.Proc Natl Acad Sci U S A. 2002; 99: 4391-4396Crossref PubMed Scopus (797) Google Scholar Techniques used for tissue engineering include transplanting a patient's somatic cell nucleus into an enucleated oocyte, activating the cell to mimic fertilization, culturing the totipotent cells in a dish, and then differentiating the cells into tissue of need. The resulting tissue, whether cardiac, pancreatic, hematopoietic, neural, or hepatic, is genetically identical to the patient's tissue, would not be rejected because of identical HLA antigen expression, and could be used for tissue repair. As an extension of research with embryonic stem cells, investigators have also used these pluripotent cells to produce identical offspring. Three approaches have been used to clone progeny. The first technique is blastomere separation. Splitting blastomeres at this totipotent stage leads to the development of genetically identical offspring but can produce only a limited brood due to the low cell number at the blastocyst stage. The second cloning technique is nuclear transfer of embryonic stem cell nuclei into enucleated eggs. Nuclear transfer involves preparing a recipient mature oocyte from which the chromosomes have been removed. A donor nucleus cell is then collected and either physically injected into the oocyte with a glass pipette or fused by means of electrical stimulation. In 1952, Briggs and King17Briggs R King TJ Transplantation of living nuclei from blastula cells into enucleated frogs' eggs.Proc Natl Acad Sci U S A. 1952; 38: 455-463Crossref PubMed Google Scholar were the first to describe successful embryonic cell nuclear transfer in leopard frogs. The third technique, mammalian embryonic cell nuclear transfer, is more complex, and although cattle, sheep, mice, rabbits, pigs, monkeys, and mules have been cloned, the number of viable offspring is limited because of required cesarean section and intense postnatal support.18Keefer CL Stice SL Matthews DL Bovine inner cell mass cells as donor nuclei in the production of nuclear transfer embryos and calves.Biol Reprod. 1994; 50: 935-939Crossref PubMed Scopus (89) Google Scholar, 19Wells DN Misica PM Day TA Tervit HR Production of cloned lambs from an established embryonic cell line: a comparison between in vivo- and in vitro-matured cytoplasts.Biol Reprod. 1997; 57: 385-393Crossref PubMed Scopus (228) Google Scholar, 20Wakayama T Rodriguez I Perry AC Yanagimachi R Mombaerts P Mice cloned from embryonic stem cells.Proc Natl Acad Sci U S A. 1999; 96: 14984-14989Crossref PubMed Scopus (418) Google Scholar, 21Scientists clone their first mule: horses could be next to join the clone parade of the animals. MSNBC News Web site.Available at: www.msnbc.com/news/919532.aspGoogle Scholar Almost 50 years after the report of frog cloning, Campbell et al22Campbell KH McWhir J Ritchie WA Wilmut I Sheep cloned by nuclear transfer from a cultured cell line.Nature. 1996; 380: 64-66Crossref PubMed Scopus (1474) Google Scholar reported animal cloning of live Welsh mountain lambs after nuclear transfer of cultured embryo-derived epithelial cells into enucleated sheep oocytes. Shortly thereafter, this group also reported the birth of Dolly, a lamb obtained after the nuclear transfer of a mammary gland cell from an adult ewe.23Wilmut I Schnieke AE McWhir J Kind AJ Campbell KH Viable offspring derived from fetal and adult mammalian cells [published correction appears in Nature. 1997;386:200].Nature. 1997; 385: 810-813Crossref PubMed Scopus (4049) Google Scholar Since then nuclear transfer of differentiated cells from adults has produced offspring in cattle,24Cibelli JB Stice SL Golueke PJ et al.Cloned transgenic calves produced from nonquiescent fetal fibroblasts.Science. 1998; 280: 1256-1258Crossref PubMed Scopus (1142) Google Scholar goats,25Baguisi A Behboodi E Melican DT et al.Production of goats by somatic cell nuclear transfer.Nat Biotechnol. 1999; 17: 456-461Crossref PubMed Scopus (885) Google Scholar, 26Keefer CL Keyston R Lazaris A et al.Production of cloned goats after nuclear transfer using adult somatic cells.Biol Reprod. 2002; 66: 199-203Crossref PubMed Scopus (139) Google Scholar mice,27Wakayama T Perry AC Zuccotti M Johnson KR Yanagimachi R Full-term development of mice from enucleated oocytes injected with cumulus cell nuclei.Nature. 1998; 394: 369-374Crossref PubMed Scopus (1946) Google Scholar and pigs.28Betthauser J Forsberg E Augenstein M et al.Production of cloned pigs from in vitro systems.Nat Biotechnol. 2000; 18: 1055-1059Crossref PubMed Scopus (428) Google Scholar, 29Polejaeva IA Chen SH Vaught TD et al.Cloned pigs produced by nuclear transfer from adult somatic cells.Nature. 2000; 407: 86-90Crossref PubMed Scopus (1035) Google Scholar, 30Onishi A Iwamoto M Akita T et al.Pig cloning by microinjection of fetal fibroblast nuclei.Science. 2000; 289: 1188-1190Crossref PubMed Scopus (724) Google Scholar, 31De Sousa PA Dobrinsky JR Zhu J et al.Somatic cell nuclear transfer in the pig: control of pronuclear formation and integration with improved methods for activation and maintenance of pregnancy.Biol Reprod. 2002; 66: 642-650Crossref PubMed Scopus (158) Google Scholar The importance of an adult somatic cell nucleus driving the growth and development of an entire living being corroborates that genetic material can be reversibly modified by the surrounding cytoplasm and that a wider variety of cells may be used as donor cells for cloning. However, somatic cell nuclear transfer is still inefficient (276 attempts were unsuccessful before Dolly was produced), and several challenges exist in reproductive cloning, such as techniques of oocyte enucleation, activation of eggs to mimic fertilization, and cell cycle synchronization between donor nucleus and recipient oocyte. In regard to the well-being of cloned offspring, there are conflicting reports on the effect of using adult cell nuclei; some investigators have found developmental abnormalities and early aging, whereas others have found evidence for normal growth and even cellular rejuvenation.32Hill JR Burghardt RC Jones K et al.Evidence for placental abnormality as the major cause of mortality in first-trimester somatic cell cloned bovine fetuses.Biol Reprod. 2000; 63: 1787-1794Crossref PubMed Scopus (374) Google Scholar, 33De Sousa PA King T Harkness L Young LE Walker SK Wilmut I Evaluation of gestational deficiencies in cloned sheep fetuses and placentae.Biol Reprod. 2001; 65: 23-30Crossref PubMed Scopus (178) Google Scholar, 34Ogonuki N Inoue K Yamamoto Y et al.Early death of mice cloned from somatic cells.Nat Genet. 2002; 30: 253-254Crossref PubMed Scopus (210) Google Scholar, 35Tamashiro KL Wakayama T Blanchard RJ Blanchard DC Yanagimachi R Postnatal growth and behavioral development of mice cloned from adult cumulus cells.Biol Reprod. 2000; 63: 328-334Crossref PubMed Scopus (137) Google Scholar, 36Lanza RP Cibelli JB Blackwell C et al.Extension of cell life-span and telomere length in animals cloned from senescent somatic cells.Science. 2000; 288: 665-669Crossref PubMed Scopus (379) Google Scholar Dolly, the first mammal born after somatic cell nuclear transfer (SCNT), had arthritis in atypical joint distributions for years and was killed on February 14, 2003, at 6 years of age, after being diagnosed with an incurable lung cancer. The tumor reportedly came after virus exposure, affecting both normal and cloned flock at the Roslin Institute in Scotland. Stem cell research using embryonic stem cells and cloning also has many ethical challenges, encouraging many investigators to fully explore the potential of adult stem cells. It is generally accepted that each organ of our body is in balance between degradation and repair. The liver that we were born with is not the same liver that we have when we die. Throughout life, toxic insults wound our organs, bringing about the question of what keeps the balance between destruction and construction. In adults, stem cells have been found in many tissues, such as liver, bone marrow, pancreas, and brain, maintaining this homeostasis. Moreover, some of these adult stem cells, once thought to mend only local property, also help in disaster relief of other more distant organs37Petersen BE Bowen WC Patrene KD et al.Bone marrow as a potential source of hepatic oval cells.Science. 1999; 284: 1168-1170Crossref PubMed Scopus (2196) Google Scholar, 38Grant MB May WS Caballero S et al.Adult hematopoietic stem cells provide functional hemangioblast activity during retinal neovascularization.Nat Med. 2002; 8: 607-612Crossref PubMed Scopus (619) Google Scholar (Table 239Theise ND Nimmakayalu M Gardner R et al.Liver from bone marrow in humans.Hepatology. 2000; 32: 11-16Crossref PubMed Scopus (1165) Google Scholar, 40Alison MR Poulsom R Jeffery R et al.Hepatocytes from non-hepatic adult stem cells.Nature. 2000; 406: 257Crossref PubMed Scopus (959) Google Scholar, 41Korbling M Katz RL Khanna A et al.Hepatocytes and epithelial cells of donor origin in recipients of peripheral-blood stem cells.N Engl J Med. 2002; 346: 738-746Crossref PubMed Scopus (718) Google Scholar, 42Kleeberger W Rothamel T Glockner S Flemming P Lehmann U Kreipe H High frequency of epithelial chimerism in liver transplants demonstrated by microdissection and STR-analysis.Hepatology. 2002; 35: 110-116Crossref PubMed Scopus (110) Google Scholar, 43Fogt F Beyser KH Poremba C Zimmerman RL Khettry U Ruschoff J Recipient-derived hepatocytes in liver transplants: a rare event in sex-mismatched transplants.Hepatology. 2002; 36: 173-176Crossref PubMed Scopus (69) Google Scholar, 44Grimm PC Nickerson P Jeffery J et al.Neointimal and tubulointerstitial infiltration by recipient mesenchymal cells in chronic renal-allograft rejection.N Engl J Med. 2001; 345: 93-97Crossref PubMed Scopus (244) Google Scholar, 45Poulsom R Forbes SJ Hodivala-Dilke K et al.Bone marrow contributes to renal parenchymal turnover and regeneration.J Pathol. 2001; 195: 229-235Crossref PubMed Scopus (585) Google Scholar, 46Sinclair RA Origin of endothelium in human renal allografts.BMJ. 1972; 4: 15-16Crossref PubMed Scopus (58) Google Scholar, 47Williams GM ter Haar A Parks LC Krajewski CA Endothelial changes associated with hyperacute, acute, and chronic renal allograft rejection in man.Transplant Proc. 1973; 5: 819-822PubMed Google Scholar, 48Lagaaij EL Cramer-Knijnenburg GF van Kemenade FJ van Es LA Bruijn JA van Krieken JH Endothelial cell chimerism after renal transplantation and vascular rejection.Lancet. 2001; 357: 33-37Abstract Full Text Full Text PDF PubMed Scopus (267) Google Scholar, 49Quaini F Urbanek K Beltrami AP et al.Chimerism of the transplanted heart.N Engl J Med. 2002; 346: 5-15Crossref PubMed Scopus (1124) Google Scholar, 50Laflamme MA Myerson D Saffitz JE Murry CE Evidence for cardiomyocyte repopulation by extracardiac progenitors in transplanted human hearts.Circ Res. 2002; 90: 634-640Crossref PubMed Scopus (391) Google Scholar, 51Muller P Pfeiffer P Koglin J et al.Cardiomyocytes of noncardiac origin in myocardial biopsies of human transplanted hearts.Circulation. 2002; 106: 31-35Crossref PubMed Scopus (200) Google Scholar, 52Glaser R Lu MM Narula N Epstein JA Smooth muscle cells, but not myocytes, of host origin in transplanted human hearts.Circulation. 2002; 106: 17-19Crossref PubMed Scopus (190) Google Scholar, 53Mezey E Key S Vogelsang G Szalayova I Lange GD Crain B Transplanted bone marrow generates new neurons in human brains.Proc Natl Acad Sci U S A. 2003; 100: 1364-1369Crossref PubMed Scopus (524) Google Scholar, 54Cogle CR, Jorgensen ML, Yachnis AT, et al. Long-term bone marrow derived neurons and astrocytes are not the result of cell fusion. Submitted for publication.Google Scholar, 55Weimann JM Charlton CA Brazelton TR Hackman RC Blau HM Contribution of transplanted bone marrow cells to Purkinje neurons in human adult brains.Proc Natl Acad Sci U S A. 2003; 100: 2088-2093Crossref PubMed Scopus (385) Google Scholar).Table 2Summary of Human Studies That Show Adult Stem Cell TransdifferentiationReferenceEnd-organStem cell originPlasticityTheise et al39Theise ND Nimmakayalu M Gardner R et al.Liver from bone marrow in humans.Hepatology. 2000; 32: 11-16Crossref PubMed Scopus (1165) Google ScholarLiverBone marrowYesAlison et al40Alison MR Poulsom R Jeffery R et al.Hepatocytes from non-hepatic adult stem cells.Nature. 2000; 406: 257Crossref PubMed Scopus (959) Google ScholarLiverBone marrowYesKorbling et al41Korbling M Katz RL Khanna A et al.Hepatocytes and epithelial cells of donor origin in recipients of peripheral-blood stem cells.N Engl J Med. 2002; 346: 738-746Crossref PubMed Scopus (718) Google ScholarLiverBone marrowYesKleeberger et al42Kleeberger W Rothamel T Glockner S Flemming P Lehmann U Kreipe H High frequency of epithelial chimerism in liver transplants demonstrated by microdissection and STR-analysis.Hepatology. 2002; 35: 110-116Crossref PubMed Scopus (110) Google ScholarLiverExtrahepaticYesFogt et al43Fogt F Beyser KH Poremba C Zimmerman RL Khettry U Ruschoff J Recipient-derived hepatocytes in liver transplants: a rare event in sex-mismatched transplants.Hepatology. 2002; 36: 173-176Crossref PubMed Scopus (69) Google ScholarLiverExtrahepaticNoGrimm et al44Grimm PC Nickerson P Jeffery J et al.Neointimal and tubulointerstitial infiltration by recipient mesenchymal cells in chronic renal-allograft rejection.N Engl J Med. 2001; 345: 93-97Crossref PubMed Scopus (244) Google ScholarKidneyExtrahepaticYesPoulsom et al45Poulsom R Forbes SJ Hodivala-Dilke K et al.Bone marrow contributes to renal parenchymal turnover and regeneration.J Pathol. 2001; 195: 229-235Crossref PubMed Scopus (585) Google ScholarKidney tubulesExtrarenalYesSinclair46Sinclair RA Origin of endothelium in human renal allografts.BMJ. 1972; 4: 15-16Crossref PubMed Scopus (58) Google ScholarKidney vesselsExtrarenalYesWilliams et al47Williams GM ter Haar A Parks LC Krajewski CA Endothelial changes associated with hyperacute, acute, and chronic renal allograft rejection in man.Transplant Proc. 1973; 5: 819-822PubMed Google ScholarKidney vesselsExtrarenalYesLagaaij et al48Lagaaij EL Cramer-Knijnenburg GF van Kemenade FJ van Es LA Bruijn JA van Krieken JH Endothelial cell chimerism after renal transplantation and vascular rejection.Lancet. 2001; 357: 33-37Abstract Full Text Full Text PDF PubMed Scopus (267) Google ScholarKidney vesselsExtrarenalYesQuaini et al49Quaini F Urbanek K Beltrami AP et al.Chimerism of the transplanted heart.N Engl J Med. 2002; 346: 5-15Crossref PubMed Scopus (1124) Google ScholarHeartExtracardiacYesLaflamme et al50Laflamme MA Myerson D Saffitz JE Murry CE Evidence for cardiomyocyte repopulation by extracardiac progenitors in transplanted human hearts.Circ Res. 2002; 90: 634-640Crossref PubMed Scopus (391) Google ScholarHeartExtracardiacYesMuller et al51Muller P Pfeiffer P Koglin J et al.Cardiomyocytes of noncardiac origin in myocardial biopsies of human transplanted hearts.Circulation. 2002; 106: 31-35Crossref PubMed Scopus (200) Google ScholarHeartExtracardiacYesGlaser et al52Glaser R Lu MM Narula N Epstein JA Smooth muscle cells, but not myocytes, of host origin in transplanted human hearts.Circulation. 2002; 106: 17-19Crossref PubMed Scopus (190) Google ScholarHeartExtracardiacNoMezey et al53Mezey E Key S Vogelsang G Szalayova I Lange GD Crain B Transplanted bone marrow generates new neurons in human brains.Proc Natl Acad Sci U S A. 2003; 100: 1364-1369Crossref PubMed Scopus (524) Google ScholarBrainBone marrowYesCogle et al54Cogle CR, Jorgensen ML, Yachnis AT, et al. Long-term bone marrow derived neurons and astrocytes are not the result of cell fusion. Submitted for publication.Google ScholarBrainBone marrowYesWeimann et al55Weimann JM Charlton CA Brazelton TR Hackman RC Blau HM Contribution of transplanted bone marrow cells to Purkinje neurons in human adult brains.Proc Natl Acad Sci U S A. 2003; 100: 2088-2093Crossref PubMed Scopus (385) Google ScholarBrainBone marrowYes Open table in a new tab To test cell plasticity, investigators use a variety of cell transplantation models. The basic procedure includes injecting donor cells of interest into a recipient and subsequently analyzing the recipient's organs for donor contribution. Modifications have been made to this basic procedure, sometimes resulting in conflicting reports in stem cell plasticity.56Wagers AJ Sherwood RI Christensen JL Weissman IL Little evidence for developmental plasticity of adult hematopoietic stem cells.Science. 2002; 297: 2256-2259Crossref PubMed Scopus (1310) Google Scholar, 57Castro RF Jackson KA Goodell MA Robertson CS Liu H Shine HD Failure of bone marrow cells to transdifferentiate into neural cells in vivo.Science. 2002; 297: 1299Crossref PubMed Scopus (382) Google Scholar These differences in findings may be explained by disparities in donor stem cell separation, cell cycle of transplanted cells, time from transplantation to evaluation of end-organ chimerism, type of injury eliciting plasticity, and ability of target niche to support stem cell transdifferentiation.58Habibian HK Peters SO Hsieh CC et al.The fluctuating phenotype of the lymphohematopoietic stem cell with cell cycle transit.J Exp Med. 1998; 188: 393-398Crossref PubMed Scopus (196) Google Scholar Thus, before plasticity can be confirmed, several criteria must be addressed. First, clonal repopulation should be demonstrated from the transplanted stem cell. To assert plasticity potential from a specific cell type, transplantation should not be performed with a mixture of undefined cells. In experimental animal systems, researchers are now using single cell transplant and retroviral stem cell tagging to test clonal plasticity.38Grant MB May WS Caballero S et al.Adult hematopoietic stem cells provide functional hemangioblast activity during retinal neovascularization.Nat Med. 2002; 8: 607-612Crossref PubMed Scopus (619) Google Scholar, 56Wagers AJ Sherwood RI Christensen JL Weissman IL Little evidence for developmental plasticity of adult hematopoietic stem cells.Science. 2002; 297: 2256-2259Crossref PubMed Scopus (1310) Google Scholar, 59Krause DS Theise ND Collector MI et al.Multi-organ, multi-lineage engraftment by a single bone marrow-derived stem cell.Cell. 2001; 105: 369-377Abstract Full Text Full Text PDF PubMed Scopus (2480) Google Scholar, 60Lemischka IR Clonal, in vivo behavior of the totipotent hematopoietic stem cell.Semin Immunol. 1991; 3: 349-355PubMed Google Scholar Second, a self-renewing cell should be responsible for observed plasticity. To address this challenge, hematopoietic stem cell (HSC) investigators transplant bone marrow from the first transplant recipient into a secondary transplant recipient. If donor reconstitution develops in the secondary transplant recipient, a selfrenewing stem cell is present and active. Although cells cap
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