The Evolving Concept of a Stem Cell: Entity or Function?

Abstract

Stem cells have long been regarded as undifferentiated cells capable of proliferation, self-renewal, production of a large number of differentiated progeny, and regeneration of tissues. Generally, it has been thought that only embryonic stem cells (ES) are pluripotent, since during early development such plasticity is critical. Indeed, extensive data support this supposition and differentiation of ES cells into a range of cell types has been documented both in vitro and in vivo. By contrast, stem cells in the adult have traditionally been thought to be restricted in their differentiative and regenerative potential to the tissues in which they reside. Examples include liver cells that proliferate following partial hepatectomy, hematopoietic stem cells (HSC) that reconstitute the blood following lethal irradiation, satellite cells that repair damaged skeletal muscle, and keratinocyte precursors that participate in wound healing. In addition to repairing damage, stem cells play a key role in ongoing tissue homeostasis, for example in maintaining the blood and skin throughout life. Invariably the diagrams of the differentiation of adult stem cell progeny have been linear and irreversible, depicting an orderly progression along a well-defined path concluding in a terminally differentiated cell type. However, this view of adult stem cell potential has been challenged of late. Bone marrow (BM)-derived cells have been shown not only to replenish the blood, but also to contribute to muscle, brain, liver, heart, and the vascular endothelium. Some reports document stem cell movement in the reverse direction, suggesting that muscle or CNS-derived cells can give rise to the blood. Stromal cells in the bone marrow, which are distinct from HSC, have also been shown to yield a multitude of cell types. Although many of these cell fate transitions have been observed following tissue injury, in some cases such transitions between distinct tissue compartments have been documented in the absence of overt tissue damage. These recent findings suggest that stem cell biology may be more complex than originally anticipated. The discovery that stem cells in adults can first reside in one tissue and then contribute to another suggests a previously unrecognized degree of plasticity in stem cell function. Indeed, it now appears that cell fate changes are a natural property of stem cells and may be involved in ongoing physiological repair of tissue damage throughout life. Although the frequency in each case is still relatively low, the numerous recent unexpected findings suggest that the concept of stem cells is in a state of flux and that the commonly held view of a tissue-specific adult stem cell may need to be expanded. Accordingly, adult stem cells may not only act locally in the tissues in which they reside, but may also be recruited out of the circulation and enlisted in regeneration of diverse tissues at distal sites. Taken to an extreme, even highly specialized cell types in tissues may be capable of reversing their differentiated state and contributing to the stem cell pool, as recent studies with multinucleated muscle and differentiated CNS cells suggest. Thus, according to this novel view, at least some stem cells in adult tissues are highly plastic and amenable to change given the appropriate microenvironment. An attractive hypothesis, given the current state of knowledge, is that the concept of a stem cell is evolving. We propose that rather than referring to a discrete cellular entity, a stem cell most accurately refers to a biological function that can be induced in many distinct types of cells, even differentiated cells. The results mentioned above suggest that an expansion of the traditional view of stem cells is needed. If cells from diverse organs can migrate systemically, enter other organs and assume morphologies and functions typical of their new environment, then these changes in cell fate may not always be linear. In other words, there may be multiple sources of stem cells and routes whereby an organism can generate specific types of mature differentiated cells. A schematic of this concept is shown in Figure 1. Some stem cells can transit through the circulation, which can be envisioned as a “stem cell highway,” with access to all organs of the body. BM-derived stem cells enter different organs such as those which have been documented experimentally: heart, brain, skeletal muscle, or liver (Bittner et al. 1999Bittner R.E. Schofer C. Weipoltshammer K. Ivanova S. Streubel B. Hauser E. Freilinger M. Hoger H. Elbe-Burger A. Wachtler F. Recruitment of bone-marrow-derived cells by skeletal and cardiac muscle in adult dystrophic mdx mice.Anat. Embryol. 1999; 199: 391-396Crossref PubMed Scopus (377) Google Scholar, Brazelton et al. 2000Brazelton T.R. Rossi F.M. Keshet G.I. Blau H.M. From marrow to brain expression of neuronal phenotypes in adult mice.Science. 2000; 290: 1775-1779Crossref PubMed Scopus (1561) Google Scholar, Gussoni et al. 1999Gussoni E. Soneoka Y. Strickland C.D. Buzney E.A. Khan M.K. Flint A.F. Kunkel L.M. Mulligan R.C. Dystrophin expression in the mdx mouse restored by stem cell transplantation.Nature. 1999; 401: 390-394Crossref PubMed Scopus (1605) Google Scholar, Jackson et al. 1999Jackson K.A. Mi T. Goodell M.A. Hematopoietic potential of stem cells isolated from murine skeletal muscle.Proc. Natl. Acad. Sci. USA. 1999; 96: 14482-14486Crossref PubMed Scopus (864) Google Scholar, Krause et al. 2001Krause D.S. Theise N.D. Collector M.I. Henegariu O. Hwang S. Gardner R. Neutzel S. Sharkis S.J. 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 (2406) Google Scholar, Lagasse et al. 2000Lagasse E. Connors H. Al-Dhalimy M. Reitsma M. Dohse M. Osborne L. Wang X. Finegold M. Weissman I.L. Grompe M. Purified hematopoietic stem cells can differentiate into hepatocytes in vivo.Nat. Med. 2000; 6: 1229-1234Crossref PubMed Scopus (2095) Google Scholar, Mezey et al. 2000Mezey E. Chandross K.J. Harta G. Maki R.A. McKercher S.R. Turning blood into brain cells bearing neuronal antigens generated in vivo from bone marrow.Science. 2000; 290: 1779-1782Crossref PubMed Scopus (1661) Google Scholar). Homing signals, depicted on “billboards” near on-ramps, may result from local damage and influence the migration of stem cells to specific sites, in a manner reminiscent of white blood cell homing (Butcher 1991Butcher E.C. Leukocyte-endothelial cell recognition three (or more) steps to specificity and diversity.Cell. 1991; 67: 1033-1036Abstract Full Text PDF PubMed Scopus (2466) Google Scholar). Growth factors, depicted on neighboring “billboards,” induce stem cells to participate in the function of the organ they enter. Thus, the microenvironment, including contact with surrounding cells, the extracellular matrix (Hay 1991Hay E.D. Collagen and other matrix glycoproteins in embyrogenesis.in: Hay E.D. Cell Biology of Extracellular Matrix. Plenum Press, New York1991Crossref Google Scholar), the local milieu (Studer et al. 2000Studer L. Csete M. Lee S.-H. Kabbani N. Walikonis J. Wold B. McKay R. Enhanced proliferation, survival, and dopaminergic differentiation of CNS precursors in lowered oxygen.J. Neurosci. 2000; 20: 7377-7383Crossref PubMed Google Scholar) as well as growth and differentiation factors, is likely to play a key role in determining a stem cell's function. Within organs such as brain, liver, and muscle, it is well known that there is a resident pool of stem cells, long thought to be dedicated exclusively to the repair of the tissue in which they reside. The diagram indicates that stem cells can enter an organ via the circulation, where they can either contribute to the existing pool of stem cells within that organ, or directly generate differentiated cells. The emerging knowledge that there are cells capable of both movement between tissues and cell fate changes suggests that at least a subset of stem cells may alter their function in a manner that is more plastic and dynamic than previously thought. The evolving view of stem cells in adults raises a number of issues that are elaborated upon in this review. To provide a perspective for thinking about the recent stem cell findings, a historical account is given of how plasticity in adult differentiated cells was gradually recognized and its validity accepted. In addition, examples are reviewed of some of the different types of tissue-specific stem cells that are well documented to respond to local tissue damage and contribute to tissue regeneration in adults. New evidence is then described that stem cells in adults are capable of transit through the circulation and possess plasticity that allows them to alter their function according to their microenvironment. The need for specific markers is highlighted, as their paucity has stymied stem cell purification resulting in enrichment protocols that preclude an in-depth analysis of shared and distinct stem cell properties. Criteria are proposed for documenting a cell fate change. To increase the frequency of such changes, a better understanding of homing and growth signals is required. In this review the neuron is used as an example, since it represents a particularly remarkable and challenging cell fate transition. In the last section we discuss questions raised by the new body of stem cell research in adults. What is an adult stem cell? Why is there such a lack of stem cell markers? Is there a universal stem cell? Are adult stem cells not only tissue specific, but can they also transit between tissues via the circulation? Are stem cells in tissues a subset of cells that are specialized for stem cell function or can differentiated cells in these tissues also assume the function of a stem cell? If stem cells are a distinct population, does that imply that the stem cell state is actively maintained? If so, how are these cells prevented from differentiating? If differentiated, how might that specialized state be reversed? Finally, what is the physiological significance and what are the potential clinical consequences of the recently demonstrated plasticity of adult-derived stem cells? That differentiated adult cells can change their fate has been known for decades. However, experimental manipulations, such as tissue damage, nuclear transplantation, or cell fusion are typically required to reveal this otherwise concealed potential for cellular plasticity. For example, transdifferentiation occurs in adult vertebrates when melanin-producing iris cells become crystallin-producing lens cells after lentectomy (Eguchi 1988Eguchi G. In Regulatory Mechanisms.in: Eguchi G. Okada T.S. Saxén L. Developmental Processes. Elsevier, New York1988Google Scholar). In the case of transdifferentiation from pancreas to liver, this change can be induced by exogenous expression of a single transcription factor (Shen et al. 2000Shen C.N. Slack J.M. Tosh D. Molecular basis of transdifferentiation of pancreas to liver.Nat. Cell Biol. 2000; 2: 879-887Crossref PubMed Scopus (345) Google Scholar). Cloning experiments in amphibia first demonstrated that cells from differentiated adult tissues can yield nuclei that upon transplantation into enucleated eggs give rise to entire organisms (Gurdon 1962Gurdon J.B. The developmental capacity of nuclei taken from intestinal epithelium cells of feeding tadpoles.J. Embryol. Exp. Morphol. 1962; 10: 622-640PubMed Google Scholar). More than three decades later, cloning was achieved in mammals leading to “Dolly” the sheep (Schnieke et al. 1997Schnieke A.E. Kind A.J. Ritchie W.A. Mycock K. Scott A.R. Ritchie M. Wilmut I. Colman A. Campbell K.H. Human factor IX transgenic sheep produced by transfer of nuclei from transfected fetal fibroblasts.Science. 1997; 278: 2130-2133Crossref PubMed Scopus (710) Google Scholar). These studies provided the first evidence that in many types of cells DNA is generally not lost, even when only a subset of genes is expressed upon differentiation. Nonetheless, questions remained from such cloning experiments as to whether progression through the egg was required for the reawakening of genes that had been shut off. Experimentally induced fusion of two different types of differentiated cells to yield nondividing stable heterokaryons showed that this was not the case: previously inactive genes in “terminally” differentiated cells were induced to express protein products through exposure to a novel cytoplasmic environment. When differentiated muscle cells were fused with mature cells isolated from all three embryonic lineages (hepatocytes, keratinocytes, and fibroblasts), muscle gene expression in the nonmuscle nuclei was induced within days (Blau et al. 1983Blau H.M. Chiu C.P. Webster C. Cytoplasmic activation of human nuclear genes in stable heterokaryons.Cell. 1983; 32: 1171-1180Abstract Full Text PDF PubMed Scopus (583) Google Scholar, Blau et al. 1985Blau H.M. Pavlath G.K. Hardeman E.C. Chiu C.P. Silberstein L. Webster S.G. Miller S.C. Webster C. Plasticity of the differentiated state.Science. 1985; 230: 758-766Crossref PubMed Scopus (586) Google Scholar, Wright 1984Wright W.E. Expression of differentiated functions in heterokaryons between skeletal myocytes, adrenal cells, fibroblasts and glial cells.Exp. Cell Res. 1984; 151: 55-69Crossref PubMed Scopus (28) Google Scholar). Moreover, this expression of previously silent genes occurred in the absence of changes in chromatin requiring cell division or DNA replication (Chiu and Blau 1984Chiu C.P. Blau H.M. Reprogramming cell differentiation in the absence of DNA synthesis.Cell. 1984; 37: 879-887Abstract Full Text PDF PubMed Scopus (94) Google Scholar). Such heterokaryon results proved to be generally true of diverse cell types other than muscle (Baron and Maniatis 1986Baron M.H. Maniatis T. Rapid reprogramming of globin gene expression in transient heterokaryons.Cell. 1986; 46: 591-602Abstract Full Text PDF PubMed Scopus (141) Google Scholar, Spear and Tilghman 1990Spear B.T. Tilghman S.M. Role of alpha-fetoprotein regulatory elements in transcriptional activation in transient heterokaryons.Mol. Cell. Biol. 1990; 10: 5047-5054PubMed Google Scholar). Although the fusion of disparate cell types is experimentally forced in heterokaryons, a unifying principle has emerged. These experiments demonstrate that the differentiated state in adult mammalian cells is generally not fixed and irreversible, but instead is regulated by a dynamic active process that requires continuous regulation (Blau 1992Blau H.M. Differentiation requires continuous active control.Annu. Rev. Biochem. 1992; 61: 1213-1230Crossref PubMed Scopus (134) Google Scholar, Blau and Baltimore 1991Blau H.M. Baltimore D. Differentiation requires continuous regulation.J. Cell Biol. 1991; 112: 781-783Crossref PubMed Scopus (199) Google Scholar). At any given time, the differentiated state is dictated by the balance of regulators present in the cell. Pluripotency was long thought to be a property specific to embryonic cells. The existence of pluripotent stem cells in embryos became evident from studies of teratocarcinomas produced by ectopic injection of blastocysts into adult mice. In these cases, embryonic tissues exhibited a wide range of differentiated phenotypes including teeth and hair but in an aberrant disorganized manner. The remarkable pluripotency of the cells was shown in experiments by Mintz (Figure 2) (Dewey et al. 1977Dewey M.J. Martin Jr., D.W. Martin G.R. Mintz B. Mosaic mice with teratocarcinoma-derived mutant cells deficient in hypoxanthine phosphoribosyltransferase.Proc. Natl. Acad. Sci. USA. 1977; 74: 5564-5568Crossref PubMed Scopus (86) Google Scholar) in which the teratocarcinoma cells were isolated, genetically marked and implanted into the blastocyst of a foster mother. The resulting mice were normal, yet had chimeric mixtures of teratocarcinoma and wild-type cells in virtually every tissue of their bodies. That the ES cells could be isolated and cultured under conditions that either allowed extensive proliferation as undifferentiated cells, accumulation in floating embryoid bodies with features similar to early embryos, or differentiation as specialized cell types, led to a breakthrough in ES cell research (Evans and Kaufman 1981Evans M.J. Kaufman M.H. Establishment in culture of pluripotential cells from mouse embryos.Nature. 1981; 292: 154-156Crossref PubMed Scopus (6067) Google Scholar, Martin 1975Martin G.R. Teratocarcinomas as a model system for the study of embryogenesis and neoplasia.Cell. 1975; 5: 229-243Abstract Full Text PDF PubMed Scopus (294) Google Scholar). Adherent and nonadherent substrates, feeder layers of irradiated or mitomycin-treated cells, and specific growth factor cocktails influenced the cells to assume predominantly cardiac, skeletal, neuronal, or other highly differentiated fates. That tissue-specific stem cells reside in certain adult tissues has been clearly documented, yet their specific properties often continue to elude us because of their paucity in parent tissues, heterogeneity, and technical difficulties in identifying them and tracing their progeny. Such adult tissue stem cells are responsible for regenerating damaged tissue and maintaining tissue homeostasis, for example physiological replenishment of skin and blood cells. In some cases such as hematopoiesis, the stem cells can be highly enriched and markers that distinguish these cells have been well characterized for that purpose. In other cases in which the extensive regenerative capacity of stem cells has been documented, such as in the liver, stem cell markers still remain to be identified. Below we briefly summarize pertinent data regarding a selected sample of these tissue-specific stem cells with the aim of highlighting areas that warrant further research and placing them in the context of the new findings on non-tissue-specific stem cells described in the next section. The best characterized tissue-specific stem cells in adults are undoubtedly the pluripotent hematopoietic stem cells (HSC), which have the ability to reconstitute all cells of the blood (Lagasse et al. 2001Lagasse E. Shizuru J.A. Uchida N. Tsukamoto A. Weissman I.L. Toward regenerative medicine.Immunity. 2001; 14: 425-436Abstract Full Text Full Text PDF PubMed Scopus (115) Google Scholar, Morrison and Weissman 1994Morrison S.J. Weissman I.L. The long-term repopulating subset of hematopoietic stem cells is deterministic and isolatable by phenotype.Immunity. 1994; 1: 661-673Abstract Full Text PDF PubMed Scopus (801) Google Scholar, Weissman 2000Weissman I.L. Stem cells units of development, units of regeneration, and units in evolution.Cell. 2000; 100: 157-168Abstract Full Text Full Text PDF PubMed Scopus (1399) Google Scholar). Two classes of HSC have been identified: short-term (ST-HSC) and long-term (LT-HSC) that are capable of reconstituting the blood of mice for two and greater than six months, respectively (Jones et al. 1989Jones R.J. Celano P. Sharkis S.J. Sensenbrenner L.L. Two phases of engraftment established by serial bone marrow transplantation in mice.Blood. 1989; 73: 397-401PubMed Google Scholar, Lagasse et al. 2001Lagasse E. Shizuru J.A. Uchida N. Tsukamoto A. Weissman I.L. Toward regenerative medicine.Immunity. 2001; 14: 425-436Abstract Full Text Full Text PDF PubMed Scopus (115) Google Scholar). The majority of HSC enrichment protocols rely on fluorescence-activated cell sorting (FACS), which allows cells to be positively selected based on the expression of a set of cell surface proteins. Most protocols also use lineage depletion panels to exclude cells expressing proteins characteristic of a mature hematopoietic cell. This enrichment protocol permits the isolation of stem cell populations in which greater than 80% of the cells have the potential to reconstitute the blood (Lagasse et al. 2001Lagasse E. Shizuru J.A. Uchida N. Tsukamoto A. Weissman I.L. Toward regenerative medicine.Immunity. 2001; 14: 425-436Abstract Full Text Full Text PDF PubMed Scopus (115) Google Scholar). It has been suggested, based on statistical arguments, that individual stem cells can give rise to clones with full hematopoietic capacity (Lagasse et al. 2001Lagasse E. Shizuru J.A. Uchida N. Tsukamoto A. Weissman I.L. Toward regenerative medicine.Immunity. 2001; 14: 425-436Abstract Full Text Full Text PDF PubMed Scopus (115) Google Scholar, Weissman 2000Weissman I.L. Stem cells units of development, units of regeneration, and units in evolution.Cell. 2000; 100: 157-168Abstract Full Text Full Text PDF PubMed Scopus (1399) Google Scholar). There are, of course, some caveats. The difference between negative and low expression of many of the marker proteins used in such isolation protocols can be subtle and as a result the cells obtained by lineage depletion are not always consistent among studies. Even the most rigorous isolation protocols currently available result in heterogeneous populations that are enriched for HSC but in which some of the cells fail to demonstrate pluripotency and/or long-term reconstituting ability (Morrison and Weissman 1994Morrison S.J. Weissman I.L. The long-term repopulating subset of hematopoietic stem cells is deterministic and isolatable by phenotype.Immunity. 1994; 1: 661-673Abstract Full Text PDF PubMed Scopus (801) Google Scholar). Moreover, since most of the protein markers used to identify HSC are not known to be essential to stem cell function, the expression of these proteins may not directly correlate with stem cell potential. For example, CD34 expression on LT-HSC has been found to be reversible and dependent not only on the activation state of the cells but also the developmental age of the donor (Matsuoka et al. 2001Matsuoka S. Ebihara Y. Xu M. Ishii T. Sugiyama D. Yoshino H. Ueda T. Manabe A. Tanaka R. Ikeda Y. et al.CD34 expression on long-term repopulating hematopoietic stem cells changes during developmental stages.Blood. 2001; 97: 419-425Crossref PubMed Scopus (101) Google Scholar, Sato et al. 1999Sato T. Laver J.H. Ogawa M. Reversible expression of CD34 by murine hematopoietic stem cells.Blood. 1999; 94: 2548-2554PubMed Google Scholar). Nonetheless, the hematopoietic stem cell clearly can be highly enriched up to 10,000-fold and delivered to marrow ablated recipients to fully reconstitute the blood (Morrison and Weissman 1994Morrison S.J. Weissman I.L. The long-term repopulating subset of hematopoietic stem cells is deterministic and isolatable by phenotype.Immunity. 1994; 1: 661-673Abstract Full Text PDF PubMed Scopus (801) Google Scholar). The discovery that stem cells exist in the adult brain was quite unexpected and required years of investigation to become widely accepted. It was long thought that damage to the brain could not be repaired, as adult neurons could not be replaced. A series of studies in rats and in songbirds first revealed that neurons from adult brains could be formed anew (Altman 1969Altman J. Autoradiographic and histological studies of postnatal neurogenesis. IV. Cell proliferation and migration in the anterior forebrain, with special reference to persisting neurogenesis in the olfactory bulb.J. Comp. Neurol. 1969; 137: 433-457Crossref PubMed Scopus (1204) Google Scholar, Goldman and Nottebohm 1983Goldman S.A. Nottebohm F. Neuronal production, migration, and differentiation in a vocal control nucleus of the adult female canary brain.Proc. Natl. Acad. Sci. USA. 1983; 80: 2390-2394Crossref PubMed Scopus (731) Google Scholar). Additional studies by a number of investigators have now confirmed that mammalian adult neuronal progenitors exist and are capable of extensive cell division and self renewal (Gage 2000Gage F.H. Mammalian neural stem cells.Science. 2000; 287: 1433-1438Crossref PubMed Scopus (3933) Google Scholar, Palmer et al. 1997Palmer T.D. Takahashi J. Gage F.H. The adult rat hippocampus contains primordial neural stem cells.Mol. Cell. Neurosci. 1997; 8: 389-404Crossref PubMed Scopus (929) Google Scholar, Reynolds and Weiss 1992Reynolds B.A. Weiss S. Generation of neurons and astrocytes from isolated cells of the adult mammalian central nervous system.Science. 1992; 255: 1707-1710Crossref PubMed Scopus (4372) Google Scholar, van der Kooy and Weiss 2000van der Kooy D. Weiss S. Why stem cells?.Science. 2000; 287: 1439-1441Crossref PubMed Scopus (233) Google Scholar). Moreover, neural progenitors can migrate and home to specific sites of damage or regeneration, for instance to the olfactory bulb of rodents (Goldman and Luskin 1998Goldman S.A. Luskin M.B. Strategies utilized by migrating neurons of the postnatal vertebrate forebrain.Trends Neurosci. 1998; 21: 107-114Abstract Full Text Full Text PDF PubMed Scopus (124) Google Scholar), the hippocampus of humans (Eriksson et al. 1998Eriksson P.S. Perfilieva E. Bjork-Eriksson T. Alborn A.M. Nordborg C. Peterson D.A. Gage F.H. Neurogenesis in the adult human hippocampus.Nat. Med. 1998; 4: 1313-1317Crossref PubMed Scopus (4761) Google Scholar), and sites of tumors such as gliomas (Aboody et al. 2000Aboody K.S. Brown A. Rainov N.G. Bower K.A. Liu S. Yang W. Small J.E. Herrlinger U. Ourednik V. Black P. et al.Neural stem cells display extensive tropism for pathology in adult brain Evidence from intracranial gliomas.Proc. Natl. Acad. Sci. USA. 2000; 97: 12846-12851Crossref PubMed Scopus (957) Google Scholar). Thus, stem cells from the CNS provide a second source of well-characterized tissue-specific stem cells. In tissue culture as well as following direct injection into brains, clones from the NSC population give rise to all three major cell types of the CNS: neurons, astrocytes, and oligodendrocytes (Gage 2000Gage F.H. Mammalian neural stem cells.Science. 2000; 287: 1433-1438Crossref PubMed Scopus (3933) Google Scholar, Qian et al. 2000Qian Z. Shen Q. Goderie S.K. He W. Capela A. Davis A.A. Temple S. Timing of CNS cell generation A programmed sequence of neuron and glial cell production from isolated murine cortical stem cells.Neuron. 2000; 28: 69-80Abstract Full Text Full Text PDF PubMed Scopus (639) Google Scholar, van der Kooy and Weiss 2000van der Kooy D. Weiss S. Why stem cells?.Science. 2000; 287: 1439-1441Crossref PubMed Scopus (233) Google Scholar). Following proteolytic dissociation of adult brain tissue, populations enriched for neural stem cells (NSC) can be obtained based on differential density in a sedimentation gradient (Palmer et al. 1999Palmer T.D. Markakis E.A. Willhoite A.R. Safar F. Gage F.H. Fibroblast growth factor-2 activates a latent neurogenic program in neural stem cells from diverse regions of the adult CNS.J. Neurosci. 1999; 19: 8487-8497Crossref PubMed Google Scholar). These NSC progeny have typical morphologies, characteristic patterns of protein expression, and exhibit physiological evidence of function (Auerbach et al. 2000Auerbach J.M. Eiden M.V. McKay R.D. Transplanted CNS stem cells form functional synapses in vivo.Eur. J. Neurosci. 2000; 12: 1696-1704Crossref PubMed Scopus (83) Google Scholar). Despite the extensive characterization of these cells, currently available cell surface markers allow for only a 45-fold enrichment of neural stem cells from embryonic brain (Uchida et al. 2000Uchida N. Buck D.W. He D. Reitsma M.J. Masek M. Phan T.V. Tsukamoto A.S. Gage F.H. Weissman I.L. Direct isolation of human central nervous system stem cells.Proc. Natl. Acad. Sci. USA. 2000; 97: 14720-14725Crossref PubMed Scopus (1507) Google Scholar). Despite the fact that the liver is an organ capable of extensive regeneration, the precise source of the tissue-specific stem cells responsible for this regeneration remains unclear. In contrast to bone marrow or skin in which a relatively small population of cells undergoes massive expansion to support regeneration, liver regeneration following partial hepatectomy, in which two-thirds of the liver of a rat is removed, involves a modest proliferation by a variety of differentiated liver cells including hepatocytes, biliary epithelial cells, and endothelial cells. However, following some types of injury which compromise hepatocytes, a smaller portion of stem-like cells near the bile ducts give rise to a proliferation of oval cells that subsequently generate hepatocytes and ductular cells (Kubota and Reid 2000Kubota H. Reid L.M. Clonogenic hepatoblasts, common precursors for hepatocytic and biliary lineages, are lacking classical major histocompatibility complex class I antigen.Proc. Natl. Acad. Sci. USA. 2000; 97: 12132-12137Crossref PubMed Scopus (203) Google Scholar, Sell 2001Sell S. Heterogeneity and plasticity of hepatocyte lineage cells.Hepatology. 2001; 33: 738-750Crossref PubMed Scopus (371) Google Scholar, Thorgeirsson et al. 1993Thorgeirsson S.S. Evarts R.P. Bisgaard H.C. Fujio K. Hu Z. Hepatic stem cell compartment activation and lineage commitment.Proc. Soc. Exp. Biol. Med. 1993; 204: 253-260Crossref PubMed Scopus (93) Google Scholar). These data, taken together with morphological studies of liver regeneration following injury, have raised the possibility that the true liver stem cell with multilineage potential resides in or near the terminal bile ductules (reviewed in Theise et al. 1999Theise N.D. Saxena R. Portmann B.C. Thung S.N. Yee H. Chiriboga L. Kumar A. Crawford J.M. The canals of Hering and hepatic stem cells in humans.Hepatology. 1999; 30: 1425-1433Crossref PubMed Scopus (590) Google Scholar, Vessey and de la Hall 2001Vessey C.J. de la Hall P.M. Hepatic stem cells a review.Pathology. 2001; 33: 130-141PubMed Scopus (144) Google Scholar). The satellite cell has been defined as a quiescent mononucleated cell ensheathed under the basal lamina that surrounds multinucleated muscle fibers (Mauro 1961Mauro A. Satellite cells of skeletal muscle fibers.Biophys. Biochem. Cytol. 1961; 9: 493-495Crossref PubMed Scopus (2