Feasible sources and characterization of adult stem cells
Stem cells are undifferentiated cells defined by their abilities to self-renew and differentiate into mature cells. Stem cells found in fully developed tissues are defined as adult stem cells. The function of adult stem cells is the maintenance of adult tissue specificity by homeostatic cell replacement and tissue regeneration (Wagers and Weissman, 2004). Adult stem cells are presumed quiescent within adult tissues, but divide infrequently to generate a stem cell clone and a transiently-amplifying cell. The transiently-amplifying cells will undergo a limited number of cell divisions before terminal differentiation into mature functional tissue cells. The existence of adult stem cells has been reported in multiple organs; these include: brain, heart, skin, intestine, testis, muscle and blood, among others. This chapter focuses on four adult stem cell populations: hematopoietic, mesenchymal, periodontal ligament-derived, and spermatogonial (Table 1).
Hematopoietic stem cells are the most characterized adult stem cell population. They function to generate all cell lineages found in mature blood (erythroid, myeloid and lymphoid) and to sustain blood production during the entire life of an animal (Kondo et al., 2003). Adult bone marrow, umbilical cord blood and mobilized peripheral blood are sources of hematopoietic stem cells for transplantation in many blood-related diseases. Hematopoietic stem cells can be characterized by positive selection of CD34, CD45, and CD133 markers and negative selection of CD31, CD105 and CD146 markers (Tárnok et al., 2010).
Mesenchymal stem cells, also called marrow stromal cells, are another well-studied adult stem cell population. Mesenchymal stem cells were originally identified in the bone marrow, but have since been found in other systems such as adipose tissue, umbilical cord and menstrual blood (Ding et al., 2011). Mesenchymal stem cells differentiate into osteocytes, chondrocytes and adipocytes (Arita et al., 2011; Pittenger et al., 1999). Human mesenchymal stem cells can be characterized by the positive expression of CD29, CD44, CD73, CD90, CD105, CD146 and STRO-1, and the negative expression of CD31, CD34, CD45, CD49f and CD133 (Mödder et al., 2012; Tárnok et al., 2010).
|Hematopoietic stem cells||Bone marrow, umbilical cord blood, mobilized peripheral blood||(+): CD34, CD45, CD133|
|(-): CD31, CD105, CD146|
|Mesenchymal stem cells||Bone marrow, adipose tissue, umbilical cord, menstrual blood||(+): CD29, CD44, CD73, CD90, CD105, CD146, STRO-1|
|(-): CD31, CD34, CD45, CD49f, CD133|
|Periodontal ligament-derived stem cells||Periodontal ligament||Mesenchymal stem cell markers: CD29, CD44, CD73, CD90, CD105, CD146, STRO-1|
|Neural crest cell markers: p75, nestin, Slug, SOX10|
|Spermatogonial stem cells||Testis||(+): CD9, CD49f and GPR125|
Periodontal ligament, derived from the cranial neural crest, is a soft connective tissue embedded between the tooth root and the alveolar bone socket, supporting the teeth
Testicular spermatogonial stem cells are the germ-line cells for spermatogenesis, an ongoing process throughout the lifespan of the male animals. They are unipotent in nature and continuously generate differentiating daughter cells for subsequent production of spermatozoa (Fagoonee et al., 2011). Human spermatogonial stem cells can be purified by antibodies against cell surface markers CD9, CD49f and GPR125 (Conrad et al., 2008).
2. Pluripotent stem cells
Pluripotency refers to the ability of cells to self-renew and differentiate into all 3 germ layers (ectoderm, endoderm and mesoderm). Pluripotent stem cells are the origin of all somatic and germ-line cells in the developing embryo. The first pluripotent cells were derived in 1976 from a type of germ-line tumor known as a teratocarcinoma (Hogan, 1976). Embryonic stem cells, derived from the inner cell mass of a blastocyst prior to gastrulation, are still considered the gold standard for pluripotent stem cells. Even though adult cells are terminally differentiated, pluripotency has also been conferred to these cells in past studies, by the technique of somatic cell nuclear transfer (Perry, 2005), parthenogenesis of unfertilized eggs (Brevini et al., 2008), and reprogramming by cell fusion (Pralong et al., 2006). Research into adult cell pluripotency was slow to progress until a major breakthrough in 2006 brought with it the technique of “induced pluripotent stem cells”. In this process adult skin fibroblasts were induced into a pluripotent state by the forced expression of key transcription factors (OCT4, SOX2, KLF4 and c-MYC; Takahashi et al., 2007) or (OCT4, SOX2, NANOG and LIN28; Yu et al., 2007). Despite the low reprogramming efficiency, this has become a convenient method for generating new pluripotent stem cell lines for research from differentiated adult cells.
Adult stem cells are thought to be tissue-specific and only able to differentiate into progeny cells of their tissues of origin. An increasing number of studies, however, report that adult stem cells are capable of giving rise to cells of an entirely distinct lineage. The concept of adult stem cell plasticity might be explained by 5 potential mechanisms: cell fusion, trans-differentiation, de-differentiation, heterogeneous stem cell populations, or pluripotency (Wagers and Weissman, 2004). Cell-cell fusion occurs at a low frequency, but is implicated in the transplantation of bone marrow cells to liver hepatocytes, cardiomyocytes and Purkinje neurons (Alvarez-Dolado et al., 2003). In cell fusion events, the stem cells acquire the mature phenotype of the tissue they are embedded within and can be easily mistaken for correct differentiation of the transplanted cells. Trans-differentiation is a direct lineage conversion by the activation of a dormant differentiation program to alter the lineage specificity of the cell. De-differentiation is another lineage conversion phenomenon in which a tissue-specific cell spontaneously de-differentiates into a more basal multipotent cell and re-differentiates to a new lineage. While the heterogeneity of the stem cell population employed can account for some of the apparent trans-differentiation and de-differentiation events observed
3. Isolation of pluripotent adult stem cells
The expression of embryonic stem cell markers in some adult stem cells suggest a sub-population of pluripotent cells in these niches (Table 2). The common embryonic stem cell makers, such as OCT4, SOX2, NANOG, KLF4, LIN28, SSEA-1, SSEA-3, SSEA-4, TRA-1-60 and TRA-1-81, are all expressed in hematopoietic stem cells (Wang et al., 2010; Zhao et al., 2006; Zulli et al., 2008) and mesenchymal stem cells (Anjos-Afonso and Bonnet, 2007; Jaramillo-Ferrada et al., 2012; Riekstina et al., 2009; Sung et al., 2010). Similarly, expressions of most of these markers, except for LIN28, have been reported in periodontal ligament-derived stem cells, a tissue arising from the migrating cranial neural crest (Huang et al., 2009; Kawanabe et al., 2010). Previous studies show that spermatogonial stem cells also express most of the embryonic stem cell markers, except SSEA-3 and TRA-1-60 (Izadyar et al., 2008; Izadyar et al., 2011; Kanatsu-Shinohara et al., 2008; Panda et al., 2011; Zheng et al., 2009). These findings suggest that pluripotent stem cells exist as sub-populations in adult stem cell reservoirs.
The existence of cells with a defined pluripotency-associated phenotypic expression within adult tissues enables researchers to isolate and purify a homogeneous subpopulation of adult pluripotent stem cells. In fact, with the use of magnetic affinity cell sorting, adult human mesenchymal stem cells, shown to differentiate into endodermal, ectodermal and mesodermal cells, were isolated by antibody against SSEA-3 (Kuroda et al., 2010). Similarly, stem cells exhibiting the potential to generate specialized cells of the three embryonic germ layers can be isolated by positive SSEA-4 expression from human periodontal ligament (Kawanabe et al., 2010). Furthermore, human spermatogonial stem cells, sharing cellular and molecular similarities with human embryonic stem cells, can be purified by α6 integrin (CD49f) antibody (Conrad et al., 2008). Moreover, a human hematopoietic stem cell subpopulation, highly efficient in generating long-term multi-lineage grafts, can also be isolated by the same α6 integrin expression (Notta et al., 2011). In addition, stem cells from granulocyte colony-stimulating factor-mobilized human peripheral blood can divide indefinitely without reaching replicative senescence and differentiate into multiple lineages (Cesselli et al., 2009).
Recently, a cell surfaceome map of mouse embryonic stem cells and induced pluripotent stem cells was reported (Gundry et al., 2012). Previously unidentified cellular surface markers, such as CD31, CD49f, CD123 and CD326, indicated a purified population of pluripotent stem cells. Further analyses should be performed to determine the expression of these markers in different adult stem cell populations. Their presence in adult stem cell populations could facilitate the purification of homogeneous pluripotent stem cells within an otherwise heterogeneous pool of regenerative adult cells.
4. Characterization of pluripotent adult stem cells
The standard tests for pluripotency are teratoma and chimera formation assays. Teratomas can be formed when pluripotent stem cells are injected into immunodeficient animals; they consist of foci with derivatives of ectodermal, mesodermal and endodermal embryonic germs layers (Wobus et al., 1984). Chimeras can be generated when pluripotent stem cells are microinjected into mouse blastocysts and are induced to differentiate into multiple cell types during normal developmental processes (Becker et al., 1984). Teratoma formation assays can be used to test for the pluripotency of human stem cells, whereas both teratoma and chimera formation can test for the pluripotency of mouse stem cells. Spermatogonial stem cells isolated from human testis by positive expression of CD49f are able to form teratomas when injected into immunodeficient mice (Conrad et al., 2008). Mesenchymal stem cells isolated from murine bone marrow contribute to most of the somatic cell types (chimerism ranged between 0.1% and 45%) when they are singly injected into an early mouse blastocyst (Jiang et al., 2002). Moreover, human hematopoietic stem cells isolated by CD49f cell surface marker display multi-lineage chimerism when transplanted into the NOD-
Although most of the adult stem cells are unable to form teratomas in immunodeficient mice, can they still be defined as pluripotent stem cells? Considering this apparent inability as well as the variability in teratoma formation efficiency even when using a known pluripotent stem cell line, a teratoma assay might not be a suitable assay for pluripotency of adult stem cells. Instead,
5. Advantages of pluripotent adult stem cells over embryonic stem cells and induced pluripotent stem cells
Human embryonic stem cells come from the inner cell mass of human blastocysts. Therefore, embryonic stem cells used for cell therapy are allogenic; the transplanted donor cells do not originate from the recipient. This raises a concern about the immunogenic response of the host, and the need for immune-suppressive therapy concurrent with embryonic stem cell transplantation (Charron et al., 2009). Moreover, embryonic stem cell-based therapy has been hampered by the moral, legal and ethical dilemma surrounding the use of human embryos for derivation of the stem cell lines (Zarzeczny and Caulfield, 2009). Furthermore, as the gold standard of pluripotent stem cells, embryonic stem cells have the potential to form teratomas in the host. Tumorigenic potential can be reduced by differentiating the embryonic stem cells into lineage-specific progenitor cells or mature tissue cells prior to transplantation (Schwartz et al., 2012). In order to better control standards of good manufacturing practices and reduce variability as much as possible, the
Differentiated adult cells used for the generation of the induced pluripotent stem cells can be collected from the recipient body, avoiding the contentious need for a human embryo. This also circumvents the problem of immune rejection. There are technical hurdles, however, concerning generation of induced pluripotent stem cells (Hayden, 2011). Firstly, the delivery of reprogramming factors (OCT4, SOX2, NANOG, LIN28, KLF4 and c-MYC) relies on the use of viral vectors for delivery (Takahashi et al., 2007). Retroviral sequences could integrate into the DNA of the host cells, potentially disrupting the gene structure as well as resulting in an aberrant phenotypic expression. Ultimately this could result in pathological mutations and cancer formation. Alternative methods such as direct protein or small molecule delivery have been adopted, although the reprogramming efficiency of these techniques is lower than with viral vectors (Kim et al., 2009; Shi et al., 2008). Secondly, two of the reprogramming factors,
The sources of adult stem cells are multiple and feasibly obtained from various adult tissues, such as bone marrow, blood, adipose tissue, teeth and testes (Table 1). These adult stem cells can be collected from the human body at anytime throughout life. This makes them readily available and does not raise the moral and ethical issues involved with the attainment of embryonic stem cells. Moreover, pluripotent adult stem cells can easily be isolated and purified by cell surface markers, such as CD49f, SSEA-3 and SSEA4 (Conrad et al., 2008; Kuroda et al., 2010; Kawanabe et al., 2010; Notta et al., 2011). The pluripotent status of these adult stem cells is naturally acquired and does not require reprogramming by the introduction of pluripotent transcriptional factors, thus eliminating the use of viral vectors and the chance of aberrant chromosomal changes. Furthermore, transplantation of mesenchymal stem cells and periodontal ligament-derived stem cells can be autogenic or allogeneic. Immuno-suppression is not necessary since mesenchymal stem cells have strong immunomodulatory properties against alloreactivity of T lymphocytes and dendritic cells (Chen et al., 2011). Similarly, mesenchymal stem cells and periodontal ligament-derived stem cells inhibit the proliferation of peripheral blood mononuclear cells (Wada et al., 2009). Spermatogonial stem cells, however, are killed by cytotoxic T lymphocytes after transplantation (Dressel et al., 2009), whereas allogeneic hematopoietic stem cell transplantation induces graft-vs-host disease (Strober et al., 2011). Therefore, transplantation of spermatogonial stem cells and hematopoietic stem cells should only be autogenic, without the application of immunosuppressive drugs. Similar to embryonic stem cells and induced pluripotent stem cells, pluripotent adult stem cells can differentiate into specialized cells of the three germ layers. Except for spermatogonial stem cells (Conrad et al., 2008), teratoma formation was not found in pluripotent hematopoietic stem cells, mesenchymal stem cells and periodontal ligament-derived stem cells (Kuroda et al., 2010; Kawanabe et al., 2010; Notta et al., 2011). This suggests a reduction in the probabilities of tumor formation post-transplantation, and the elimination of the need to manipulate the cells into mature tissue prior to transplantation. In addition, transplanted stem cell-induced regeneration may not be due to stem cell differentiation per se (Johnson et al., 2010; Williams and Hare, 2011). Instead, a paracrine effect has been hypothesized in which the adult stem cells secrete cytokines, chemokines, or protective proteins (Bai et al., 2012; Bráz et al., 2012) that nourish the host tissue cells and facilitate the healing process. This special feature has not yet been reported with the use of embryonic stem cells or induced pluripotent stem cells in a clinical setting.
6. Potential applications of pluripotent adult stem cells
Stem cell clinical trials have advanced rapidly for a broad spectrum of diseases, such as diabetes, neurodegeneration, immune diseases, heart disease, and bone disease. In 2011, there were 123 clinical trials using mesenchymal stem cells (Trounson et al., 2011). It is predicted that stem cell therapy will eventually become the treatment of choice in regenerative medicine, especially the use of adult stem cells. As stem cell products become more wide-spread and maintained under various conditions, the need for global standardization and regulation of processes will become necessary for the viable application of these products in a clinical setting. The Food and Drug Administration regulates interstate commerce in human cells and tissue-based products under the Public Health Service Act and the Code of Federal Regulations for Food and Drugs (Lysaght and Campbell, 2011). Human cells and tissue-based products are defined as “articles containing or consisting of human cells or tissues that are intended for implantation, transplantation, infusion, or transfer into a human recipient” (Lysaght and Campbell, 2011). Human cells and tissue-based products must be: (1) minimally manipulated, (2) intended only for homologous use, (3) not combined with another article (except for water, or sterilization, preservation, or storage agents), and (4) either: (a) have no systemic or metabolic effect, or (b) be for autologous use, allogeneic use in first- or second-degree blood relative, or reproductive use.
Pluripotent adult stem cells fall under the criteria for human cells and tissue-based products as stated by the Food and Drug Administration. Unlike induced pluripotent stem cells, pluripotent adult stem cells can be minimally manipulated as their pluripotent state occurs naturally. Unlike embryonic stem cells, pluripotent adult stem cells are suited for autologous use. Similar to embryonic stem cells and induced pluripotent stem cells, pluripotent adult stem cells are able to differentiate into specialized cells of the three germ layers. In addition, embryonic stem cells and induced pluripotent stem cells have the potential to form teratomas (an unfavorable side-effect in clinical applications) although a recent study suggests that the teratoma-forming cells could be removed by the antibody against SSEA-5 (Tang et al., 2011). In contrast, most pluripotent adult stem cells do not form teratomas
If stem cell-aided regeneration is not due to stem cell differentiation to replace damaged cells (Johnson et al., 2010; Williams and Hare, 2011), pluripotent adult stem cells are favorable over embryonic stem cells and induced pluripotent stem cells. The secretion of cytokines, chemokines, and/or protective proteins from the adult stem cells could nourish the host tissue and facilitate the healing process (Bai et al., 2012; Bráz et al., 2012).
Adult stem cells are found all over the body. They can be conveniently obtained from different accessible tissues: bone marrow, blood, adipose tissue, teeth and testes. Pluripotent adult stem cells, which reside as a subpopulation within adult stem cells, can be easily isolated by pluripotent cell surface markers, such as SSEA-3, SSEA-4 and CD49f. Moreover, pluripotent adult stem cells can be characterized by their ability to differentiate into cells of 3 germ layers (ectoderm, mesoderm and endoderm) as well as by the chimera formation in xeno-transplanted mice. Pluripotent adult stem cells are better than embryonic stem cells and induced pluripotent stem cells as they are an autologous source, require minimal manipulation and do not have the ability to form teratomas. In addition, they are more appropriate to be used as a clinical product for therapeutic treatments, as a cellular replacement or secretory protein reservoir. However, there are uncertainties that still remain unanswered. Which stem cell types are optimal for regenerative medicine? What is the optimal cell number for transplantation? Should the cells be preemptively differentiated or used as is? Further research is needed to understand the mechanisms of stem cells in regenerating damaged tissues after transplantation.
Alvarez-Buylla. Alvarez-dolado M. R Pardal J. M Garcia-verdugo J. R Fike H. O Lee K Pfeffer C Lois S. J Morrisonand A 2003 Fusion of bone-marrow-derived cells with Purkinje neurons, cardiomyocytes and hepatocytes. 425 968 973
and Anjos-afonso F D Bonnet 2007Nonhematopoietic/endothelial SSEA-1+ cells define the most primitive progenitors in the adult murine bone marrow mesenchymal compartment. Blood. 109 1298 1306
Arita N. A and D Pelaez H. S Cheung 2011Activation of the extracellular signal-regulated kinases 1 and 2 (ERK1/2) is needed for the TGFβ-induced chondrogenic and osteogenic differentiation of mesenchymal stem cells. Biochem Biophys Res Commun. 405 564 569
Bai L D. P Lennon A. I Caplan A Dechant J Hecker J Kranso and A Zaremba R. H Miller 2012 Hepatocyte growth factor mediates mesenchymal stem cell-induced recovery in multiple sclerosis modelsNat Neurosci. In press.
Becker K A. M Wobus and U Conrad J Schöneich 1984 Injection of murine embryonal carcinoma cells and embryo-derived pluripotential cells into mouse blastocysts. Cell Differ. 15 195 202
Bráz J. M R Sharif-naeini D Vogt A Kriegstein A Alvarez-buylla and J. L Rubenstein A. I Basbaum 2012 Forebrain GABAergic neuron precursors integrate into adult spinal cord and reduce injury-induced neuropathic pain 74 663 675
Brevini T. A G Pennarossa and S Antonini F Gandolfi 2008 Parthenogenesis as an approach to pluripotency: advantages and limitations involved Stem Cell Rev. 4127 135
Cesselli D A. P Beltrami S Rigo N Bergamin F. D Aurizio R Verardo S Piazza E Klaric R Fanin B Toffoletto S Marzinotto L Mariuzzi N Finato M Pandolfi A Leri C Schneider and C. A Beltrami P Anversa 2009 Multipotent progenitor cells are present in human peripheral blood Circ Res. 104 1225 1234
Charron D and C Suberbielle-boissel R Al-daccak 2009 Immunogenicity and allogenicity: a challenge of stem cell therapy J Cardiovasc Transl Res. 2 130 138
Chen P. M M. L Yen K. J Liu and H. K Sytwu B. L Yen 2011 Immunomodulatory properties of human adult and fetal multipotent mesenchymal stem cells J Biomed Sci. 18:49.
Conrad S M Renninger J Hennenlotter T Wiesner L Just M Bonin W Aicher H. J Bühring U Mattheus A Mack H. J Wagner S Minger M Matzkies M Reppel J Hescheler K. D Sievert and A Stenzl T Skutella 2008 Generation of pluripotent stem cells from adult human testis 456 344 349
Ding D. C and W. C Shyu S. Z Lin 2011Mesenchymal stem cells. Cell Transplant. 20 5 14
Dressel R K Guan J Nolte L Elsner S Monecke K Nayernia and G Hasenfuss W Engel 2009 Multipotent adult germ-line stem cells, like other pluripotent stem cells, can be killed by cytotoxic T lymphocytes despite low expression of major histocompatibility complex class I molecules Biol Direct. 4:31.
Fagoonee S R Pellicano and L Silengo F Altruda 2011 Potential applications of germline cell-derived pluripotent stem cells in organ regeneration. 7 116 122
Goldstein, and K. Zhang. Gore A Z Li H. L Fung J. E Young S Agarwal J Antosiewicz-bourget I Canto A Giorgetti M. A Israel E Kiskinis J. H Lee Y. H Loh P. D Manos N Montserrat A. D Panopoulos S Ruiz M. L Wilbert J Yu E. F Kirkness J. C Izpisua Belmonte D. J Rossi J. A Thomson K Eggan G. Q Daley L. S 2011 Somatic coding mutations in human induced pluripotent stem cells 471 63 67
Gundry R. L D. R Riordon Y Tarasova S Chuppa S Bhattacharya O Juhasz O Wiedemeier S Milanovich F. K Noto I Tchernyshyov K Raginski D Bausch-fluck H. J Tae S Marshall S. A Duncan B Wollscheid R. P Wersto S Rao and J. E Van Eyk K. R Boheler 2012 A Cell Surfaceome Map for Immunophenotyping and Sorting Pluripotent Stem Cells Mol Cell Proteomics. In press.
Hayden E. C 2011 Stem cells: The growing pains of pluripotency 473 272 274
Hogan B. L 1976 Changes in the behaviour of teratocarcinoma cells cultivated in vitro. 263 136 137
Hussein S. M N. N Batada S Vuoristo R. W Ching R Autio E Närvä S Ng M Sourour R Hämäläinen C Olsson K Lundin M Mikkola R Trokovic M Peitz O Brüstle D. P Bazett-jones K Alitalo R Lahesmaa and A Nagy T Otonkoski 2011 Copy number variation and selection during reprogramming to pluripotency 471 58 62
and Ilic D J. M Polak 2011 Stem cells in regenerative medicine: introduction Br Med Bull. 98 117 126
Izadyar F F Pau J Marh N Slepko T Wang R Gonzalez T Ramos K Howerton and C Sayre F Silva 2008 Generation of multipotent cell lines from a distinct population of male germ line stem cells Reproduction. 135 771 784
Izadyar F J Wong C Maki J Pacchiarotti T Ramos K Howerton C Yuen S Greilach H. H Zhao M Chow Y. C Chow J Rao J Barritt and N Bar-chama A Copperman 2011 Identification and characterization of repopulating spermatogonial stem cells from the adult human testis Hum Reprod. 26 1296 1306
Jaramillo-ferrada P. A and E. J Wolvetang J. J Cooper-white 2012 Differential mesengenic potential and expression of stem cell-fate modulators in mesenchymal stromal cells from human-term placenta and bone marrow. J Cell Physiol. 227 3234 3242
Jiang Y B. N Jahagirdar R. L Reinhardt R. E Schwartz C. D Keene X. R Ortiz-gonzalez M Reyes T Lenvik T Lund M Blackstad J Du S Aldrich A Lisberg W. C Low and D. A Largaespada C. M Verfaillie 2002 Pluripotency of mesenchymal stem cells derived from adult marrow. 418 41 49
Johnson T. V N. D Bull D. P Hunt N Marina and S. I Tomarev K. R Martin 2010 Neuroprotective effects of intravitreal mesenchymal stem cell transplantation in experimental glaucoma Invest Ophthalmol Vis Sci. 51 2051 2059
Kanatsu-shinohara M J Lee K Inoue N Ogonuki H Miki S Toyokuni M Ikawa T Nakamura and A Ogura T Shinohara 2008 Pluripotency of a single spermatogonial stem cell in mice Biol Reprod. ; 78 681 687
Kawanabe N S Murata K Murakami Y Ishihara S Hayano H Kurosaka H Kamioka and T Takano-yamamoto T Yamashiro 2010 Isolation of multipotent stem cells in human periodontal ligament using stage-specific embryonic antigen-4 79 74 83
Kim D C. H Kim J. I Moon Y. G Chung M. Y Chang B. S Han S Ko E Yang K. Y Cha and R Lanza K. S Kim 2009 Generation of human induced pluripotent stem cells by direct delivery of reprogramming proteins 4 472 476
Kondo M A. J Wagers M. G Manz S. S Prohaska D. C Scherer G. F Beilhack and J. A Shizuru I. L Weissman 2003 Biology of hematopoietic stem cells and progenitors: implications for clinical application. Annu Rev Immunol. 21 759 806
Kuroda Y M Kitada S Wakao K Nishikawa Y Tanimura H Makinoshima M Goda H Akashi A Inutsuka A Niwa T Shigemoto Y Nabeshima T Nakahata Y Nabeshima and Y Fujiyoshi M Dezawa 2010 Unique multipotent cells in adult human mesenchymal cell populations Proc Natl Acad Sci U S A. 107 8639 8643
Lister R M Pelizzola Y. S Kida R. D Hawkins J. R Nery G Hon J Antosiewicz-bourget R. O Malley R Castanon S Klugman M Downes R Yu R Stewart B Ren J. A Thomson and R. M Evans J. R Ecker 2011 Hotspots of aberrant epigenomic reprogramming in human induced pluripotent stem cells 471 68 73
and Lysaght T A. V Campbell 2011 Regulating autologous adult stem cells: the FDA steps up 9 393 396
Mayshar Y U Ben-david N Lavon J. C Biancotti B Yakir A. T Clark K Plath and W. E Lowry N Benvenisty 2010 Identification and classification of chromosomal aberrations in human induced pluripotent stem cells 7 521 531
Mödder U. I M. M Roforth K. M Nicks J. M Peterson L. K Mccready and D. G Monroe S Khosla 2012 Characterization of mesenchymal progenitor cells isolated from human bone marrow by negative selection 50 804 810
Mrozik K S Gronthos and S Shi P. M Bartold 2010 A method to isolate, purify, and characterize human periodontal ligament stem cells. Methods Mol Biol. 666 269 284
Notta F S Doulatov E Laurenti A Poeppl and I Jurisica J. E Dick 2011 Isolation of single human hematopoietic stem cells capable of long-term multilineage engraftment 333 218 221
Panda R. P and H. K Barman C Mohapatra 2011 Isolation of enriched carp spermatogonial stem cells from Labeo rohita testis for in vitro propagation 76 241 251
Perry A. C 2005 Progress in human somatic-cell nuclear transfer. N Engl J Med. 353 87 88
Pittenger M. F A. M Mackay S. C Beck R. K Jaiswal R Douglas J. D Mosca M. A Moorman D. W Simonetti and S Craig D. R Marshak 1999 Multilineage potential of adult human mesenchymal stem cells. Science. 284 143 147
Pralong D and A. O Trounson P. J Verma 2006 Cell fusion for reprogramming pluripotency: toward elimination of the pluripotent genome. Stem Cell Rev. 2 331 340
Riekstina U I Cakstina V Parfejevs M Hoogduijn G Jankovskis I Muiznieks and R Muceniece J Ancans 2009 Embryonic stem cell marker expression pattern in human mesenchymal stem cells derived from bone marrow, adipose tissue, heart and dermis Stem Cell Rev. 5 378 386
Schwartz S. D J. P Hubschman G Heilwell V Franco-cardenas C. K Pan R. M Ostrick E Mickunas R Gay and I Klimanskaya R Lanza 2012Embryonic stem cell trials for macular degeneration: a preliminary report. Lancet. 379 713 720
Seo B. M M Miura S Gronthos P. M Bartold S Batouli J Brahim M Young P. G Robey and C. Y Wang S Shi 2004Investigation of multipotent postnatal stem cells from human periodontal ligament. Lancet. 364 149 155
Shi Y C Desponts J. T Do H. S Hahm and H. R Schöler S Ding 2008Induction of pluripotent stem cells from mouse embryonic fibroblasts by Oct4 and Klf4 with small-molecule compounds. Cell Stem Cell. 3 568 574
Slack J. M 2008Origin of stem cells in organogenesis. Science. 322 1498 1501
Strober S T. R Spitzer and R Lowsky M Sykes 2011Translational studies in hematopoietic cell transplantation: treatment of hematologic malignancies as a stepping stone to tolerance induction. Semin Immunol. 23 273 281
Sung H. J S. C Hong J. H Yoo J. H Oh H. J Shin I. Y Choi K. H Ahn S. H Kim and Y Park B. S Kim 2010Stemness evaluation of mesenchymal stem cells from placentas according to developmental stage: comparison to those from adult bone marrow. J Korean Med Sci. 25 1418 1426
Takahashi K K Tanabe M Ohnuki M Narita T Ichisaka and K Tomoda S Yamanaka 2007Induction of pluripotent stem cells from adult human fibroblasts by defined factors. Cell. 131 861 872
Tang C A. S Lee J. P Volkmer D Sahoo D Nag A. R Mosley M. A Inlay R Ardehali S. L Chavez R. R Pera B Behr J. C Wu and I. L Weissman M Drukker 2011An antibody against SSEA-5 glycan on human pluripotent stem cells enables removal of teratoma-forming cells. Nat Biotechnol. 29 829 834
Tárnok A and H Ulrich J Bocsi 2010Phenotypes of stem cells from diverse origin. Cytometry A. 77 6 10
Trounson A R. G Thakar and G Lomax D Gibbons 2011Clinical trials for stem cell therapies. BMC Med. 9:52.
Wada N D Menicanin S Shi and P. M Bartold S Gronthos 2009Immunomodulatory properties of human periodontal ligament stem cells. J Cell Physiol. 219 667 676
and Wagers A. J I. L Weissman 2004Plasticity of adult stem cells. Cell. 116 639 648
Wang J X Zhou L Cui L Yan J Liang X Cheng L Qiao Y Shi Z Han Y Cao and Y Han D Fan 2010The significance of CD14+ monocytes in peripheral blood stem cells for the treatment of rat liver cirrhosis. Cytotherapy. 12 1022 1034
and Williams A. R J. M Hare 2011Mesenchymal stem cells: biology, pathophysiology, translational findings, and therapeutic implications for cardiac disease. Circ Res. 109 923 940
Wobus A. M H Holzhausen and P Jäkel J Schöneich 1984Characterization of a pluripotent stem cell line derived from a mouse embryo. Exp Cell Res. 152 212 219
Yu J M. A Vodyanik K Smuga-otto J Antosiewicz-bourget J. L Frane S Tian J Nie G. A Jonsdottir V Ruotti R Stewart and I. I Slukvin J. A Thomson 2007Induced pluripotent stem cell lines derived from human somatic cells. Science. 318 1917 1920
and Zarzeczny A T Caulfield 2009Emerging ethical, legal and social issues associated with stem cell research & and the current role of the moral status of the embryo. Stem Cell Rev. 5 96 101
Zhao Y and H Wang T Mazzone 2006Identification of stem cells from human umbilical cord blood with embryonic and hematopoietic characteristics. Exp Cell Res. 312 2454 2464
Zheng K X Wu and K. H Kaestner P. J Wang 2009The pluripotency factor LIN28 marks undifferentiated spermatogonia in mouse. BMC Dev Biol. 9:38.
Zipori D 2005The stem state: plasticity is essential, whereas self-renewal and hierarchy are optional. Stem Cells. 23 719 726
Zulli A B. F Buxton and M Merrilees D. L Hare 2008Human diseased arteries contain cells expressing leukocytic and embryonic stem cell markers. Hum Pathol. 39 657 665