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NeuroMolecular Medicine Copyright © 2002 Humana Press Inc. All rights of any nature whatsoever reserved. ISSN1535-1084/02/02:233–249/$20.00
REVIEW ARTICLE
Stem Cell and Precursor Cell Therapy Jingli Cai and Mahendra S. Rao* Laboratory of Neurosciences, National Institute on Aging, 5600 Nathan Shock Drive, Baltimore, MD 21224 Received July 24, 2002; Accepted July 25, 2002
Abstract Strategies for cell replacement therapy have been guided by the success in the hematopoietic stem cell field. In this review, we discuss the basis of this success and examine whether this stem cell transplant model can be replicated in other systems where stem cell therapy is being evaluated. We conclude that identifying the most primitive stem cell and using it for transplant therapy may not be appropriate in all systems. We suggest alternative strategies such as progenitor cell replacement, inductive factors, bioengineering organs, in utero transplants, or any approach that takes advantage of the unique properties of the tissue and the stem cell type which, are more likely to provide effective functional replacement. Index Entries: Stem cells; progenitor cells; hematopoietic cells; neural stem cells; islet cells; replacement; repair; transplantation.
Introduction Stem cells have been defined as cells that demonstrate self-renewal and retain the capacity to differentiate into one or many kinds of differentiated cells (1,2). Stem cells have been classified on the basis of their differentiation capacity, region of isolation and degree of self-renewal (3–5). Cells that fulfill these criteria have been isolated from nearly all major tissues or organs, and their ability to differentiate has been demonstrated in vitro and in vivo. Stem cells in turn generate blast cells or intermediate precursors which, in general, have a more limited selfrenewal capacity and can differentiate into a subset
of cells that any particular stem cell may differentiate into. These progenitors or intermediate precursors or blast cells have also been described in multiple tissues, and can be distinguished from their stem cell counterparts by cytokine dependence, antigen expression, or functional abilities (4,6–8). The availability of cells (stem and progenitor) that can be maintained in culture and, upon transplantation, respond to environmental cues to differentiate and integrate relatively seamlessly into the host environment, has been heralded as a breakthrough that is likely to extend our therapeutic reach to diseases that were previously intractable to other methods of therapy. This promise of stem cell therapy has
*Author to whom all correspondence and reprint requests should be addressed. Email:
[email protected]
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Fig. 1. The process of bone marrow transplantation is schematized. Cells are harvested from multiple sources, partially purified, and then transplanted into a recipient where a niche has been prepared for the transplanted stem cells.
been best actualized in stem cell replacement for hematopoietic disorders (9–12). Bone marrow purified stem cells and cord blood stem cells have been routinely harvested, stored, and used therapeutically for the past forty years (13,14). Currently, thousands of bone marrow transplants are performed for disease as diverse as cancer, multiple sclerosis, and immune deficiency (11–13,15–19), and the strategy used is summarized in Figure 1. Despite the success of bone marrow transplantation, routine stem cell therapy is not available for any other tissue or organ. Fetal tissue transplants have been performed in the central nervous system (CNS) (20–23) and for pancreatic islet replacement (see, for example, refs. 24–26, reviewed in ref. 27). There is currently one report of mesenchymal stem cells being used to treat osteogenesis imperfecta (28). Progenitor cells have also been used for therapy and autologous cellular replacement of cartilage (reviewed in refs. 29,30), and skin has been offered for many years (reviewed in ref. 31). However, stem cell biologists have not been able to emulate the success that has been demonstrated in the hematopoietic stem cell replacement field. It is useful to understand the key features that made marrow replacement so successful, and to determine if this success can be replicated in other systems where stem cell replacement is being pursued.
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Hematopoietic Cell Replacement Therapy The general overall strategy, albeit with variations, for cell replacement in the hematopoietic system (Fig. 1) is to select the most primitive population of cells either from bone marrow aspirate, mobilized stem cells from peripheral blood, or from cord blood and to infuse them into a recipient whose endogenous stem cell pool has been depleted. Enough donor cells are relatively easy to obtain, and the cells infused into the bloodstream can home to the stem cell niche in the marrow to ensure lifetime replenishment of needed cells. Success of hematopoietic stem cell therapy in our opinion is predicated on several unique properties of the hematopoietic system (summarized in Table 1). Stem cells are relatively abundant and are geared to undergo sufficient self-renewal to last the life span of the individual. Stem cells are present in a specific niche and feedback loops exist to govern the total number of stem cells and the rate at which differentiated cell will be generated. Transplanted blood stem cells exhibit a homing tendency (32,33) such that when infused into blood, they will be directed to stem cell niches in bone and can integrate into a preexisting niche to replace damaged or lost stem cells. Thus, the absolute number
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Table 1 Factors That May Be Important in the Success of Hematopoietic Cell Therapy Hematopoietic stem cells Most primitive cell population Stem cell niche Autologous or matched stem cells Homing and delivery Ongoing differentiated cell turnover Positional information Connectivity and integration Numbers of cells required and markers for selection Appropriate ratios of differentiated cells
of stem cells present is appropriate to produce the appropriate numbers of differentiated progeny (reviewed in ref. 34). Finally, success has required that space, or a niche, be available or created to allow replacing stem cells to integrate adequately. Mechanisms such as radiation therapy or chemotherapy can selectively target bone marrow to create a competitive advantage for transplanted cells to repopulate an existing niche. It is also important to emphasize that because of the constant turnover of differentiated cells, one needs to replace lost stem cells with a cell that has sufficient self-renewal capacity to last the life-span of the individual. In general, this means replacing with the most primitive stem cells. Therefore, precursor cells, blast cells, or intermediate precursors have never been considered as suitable cells for cell replacement in the hematopoietic system even though they are relatively abundant, and markers to isolate specific populations of such cells exist. At best, precursor cells play a peripheral role in hematopoietic stem cell therapy for short-term replacement while the slow self-renewing stem cells generate enough differentiated cells to provide a therapeutic cure. Overall, we suggest that there has been success in the hematopoietic field because stem cells that have a lifetime self-renewal capability can be purified in sufficient numbers for transplantation. When transplanted, these stem cells can home into an appropriate niche where they can participate in ongoing feedback loops which regulate appropriate numbers of differentiated progeny that are under constant turnover. Equally important, positional cues and connectivity are not important for either hematopoietic stem cells or their differentiated progeny.
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Necessary Clearly exists Available Demonstrated Occurs Unimportant Unnecessary Available Feedback loops exist
Neural Stem Cell Therapy It is useful to examine whether the criteria that we have identified as being those that were critical for success in the field of hematopoietic stem cell replacement (Table 1) exist in other systems as well, and whether the model of obtaining the most primitive stem cells in a particular tissue or organ for replacement therapy is also appropriate in all systems where stem cell therapy is being considered. While our comments are general, we will, for consistency, use the nervous system as an example of a system where transplanting primitive stem cells may perhaps be an inappropriate strategy. Neural cell replacement therapy usually does not follow the hematopoietic model of transplant therapy. The basic procedure for neural cell replacement is outlined in Figure 2. In general, cells are present in limiting numbers and have to be amplified in culture and subsequently harvested (in some cases subsets of cells selected), while a number of cells are directly implanted into a specific region of the brain. The reason for this difference in procedures is because the nervous system differs from the hematopoietic system in several fundamental ways (summarized in Table 2). Unlike the hematopoietic system where therapy is directed at stem cells or precursor cell disorders, therapy in the nervous system is directed at cellular replacement of postmitotic cells or fully differentiated glia which are not being replaced with any regularity from stem cells or precursors. Indeed with the possible exception of the hippocampus and the olfactory bulb (reviewed in ref. 35), it is unlikely that neuronal replacement will serve as the predominant method of repair and regeneration. Differentiated cells sur-
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Fig. 2. Neural stem cells present in limiting quantities have to be cultured in vitro prior to transplantation. Often enrichment or selection is necessary to deplete unwanted cells, and no niche is prepared in the host. Table 2 Factors That May Necessitate Defining Altenative Strategies for Cell Replacement in the Nervous System Neural stem cells Most primitive cell population Stem cell niche Autologous or matched stem cells Homing and delivery Ongoing differentiated cell turnover Positional information Connectivity and integration Numbers of cells required and markers for selection Appropriate ratios of differentiated cells
vive for years and there is little evidence of ongoing neurogenesis in most regions of the brain. The blood-brain barrier is a major impediment to deliver stem cells intravenously if it is assumed that they will home into an appropriate stem cell niche. Cells need to be deposited by injection into the brain, and the identified regions where stem cells persist are small and difficult to reach by most surgical techniques. Placing stem cells in an ectopic location and hoping that they will home into the few stem cell niches is unlikely to happen in our opinion. In normal development, stem cells appear to be restricted to the subventricular zone (SVZ)/ependymal region, do not migrate any great distances, and are actively inhibited (with precursor cells or progenitor cells being the only cells
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??? Does not exist in most brain regions Unavailable at present Difficult Limited in most brain regions Critical Necessary for success Limited and controversial Appropriate regulatory loops unlikely to exist
to migrate away from the proliferating zone). Thus, it is unlikely that homing mechanisms such as those described in the hematopoietic system exist in the nervous system. We note that even in studies where extensive migration of stem cells has been reported (see for example ref. 36, reviewed in ref. 37), no directed migration to the stem cell niche has been described, even when cells have been transplanted close to the ependymal zone. Rather, migration appears to be directed towards sites of injury (38,39). Creating a suitable niche for stem cells to localize is also problematic. As is routinely done in bone marrow transplants, it is necessary to either growth arrest or kill endogenous stem cells to offer transplanted stem cells a competitive advantage to repopulate the neural stem cell niche. To date, there are
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Transplant Therapy with Human Cells no strategies that can be used to selectively destroy the endogenous neural stem cell population, which, being relatively quiescent, is resistant to the strategies used to deplete dividing stem cells. While we can theoretically place stem cells in ectopic locations, it will be important to realize that appropriate stem cell survival and differentiation cues are unlikely to exist in these regions. It is clear that such cues are critical (40), which is a possible reason why stem cells are localized to specific regions of the brain. It is also likely that appropriate regulatory feedback loops will not be present if stem cells are transplanted into ectopic locations. Differentiation then is likely to be stochastic and therefore inappropriate for cell replacement therapy under consideration. The lack of ongoing regeneration and replacement underscores an additional fundamental difference between the nervous system and hematopoietic system: i.e., the absence of ongoing cues to direct appropriate differentiation. The spinal cord is an excellent example of such a lack of cues. When stem cells are transplanted into an intact or damaged spinal cord, no neuronal replacement is seen, even though there is well-documented neuronal loss (41,42). The failure to differentiate into neurons is likely due to the environment, as transplantation of the same cells into the hippocampus will readily encourage neuronal differentiation (43). Of even greater importance are observations that positional information is critical to regulating appropriate differentiation. This positional information appears to be specified very early in development and maintained in culture. Transplantation and culture evidence have clearly demonstrated that unlike the hematopoietic system, positional information is critical to determining the identity of the differentiated progeny (44). Indeed, it has been very difficult to direct stem cells towards dopaminergic neuron differentiation unless stem cells were isolated from the midbrain (45,46). Likewise, olfactory bulb precursors will not generate dopaminergic neurons in the striatum after transplantation, even though they will make olfactory bulb neurons when transplanted into the SVZ. These positional cues appear to develop very early in development (47–49) and are maintained in culture (50, reviewed in ref. 51). These results, in our opinion, illustrate a fundamental difference between hematopoietic and neural system development, and in turn dictate possible therapeutic approaches.
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237 Migration and appropriate connectivity are also major barriers to effective replacement in the nervous system. Unlike experiences in marrow replacement, differentiated cells derived from neural stem cells have to migrate from the site of implantation to appropriate regions where they must integrate and establish proper connections with their target, which in some cases are centimeters or even meters away. In adult, pathways of normal migration no longer exist and appropriate guidance cues are absent as there is little ongoing repair and regeneration. Thus, appropriate migration of cells transplanted in the CNS is generally seen only in the hippocampus and olfactory bulb where endogenous cues and feedback loops exist. In most other brain regions where there is little ongoing regeneration, if cells are transplanted, they usually form a large clump of undifferentiated cells. Few cells migrate, and the cells that do migrate can be found in appropriate and inappropriate sites. Differentiation stochastic and appropriate or inappropriate cell type can be readily identified. For example, if neuronal precursors derived from the spinal cord are transplanted into the spinal cord in neonate or adult, cells will readily differentiate into neurons. However, the neurons will be seen in both the gray and white matter, and cholinergic neurons are present in inappropriate locations as well. Even when we see motorneuron-like differentiation, we rarely see any axons in the ventral horn, the normal site of projection, and to our knowledge, few appropriate supraspinal and interneuron connectivity has ever been demonstrated. Given these fundamental differences in the biology of the system, it appears that stem cell therapy, using the most primitive stem cell, is unlikely to be successful in the adult nervous system except in limited regions such as the hippocampus or olfactory bulb (52,53). Even in these systems, major challenges exist in terms of creating a niche for stem cells, and demonstrating adequate and ongoing replacement with cells that have the appropriate regional and positional markers.
Stem Cell Replacement in Other Systems As we believe, it is possible to generalize from the two extreme examples discussed to other systems where cell therapy may be considered. Sys-
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Cai and Rao Table 3 Stem Cells Present in Selected Tissues Cells
Ectoderm
Neural stem cell Neural crest stem cell Skin-derived stem cell
Mesoderm
Muscle-derived stem cells Circulating skeletal stem cells Processed lipoaspirate Mesenchymal stem cell
Endoderm
Umbilical cord blood stem cell Hematopoietic stem cell Fetal liver epithelial progenitor cells Facultative liver stem cells (oval cells) Embryonic renal epithelial stem cells Pancreatic islet stem cells Intestinal epithelial stem cell
Properties Self-renewing and able to differentiate into neurons, astrocytes and oligodendrocytes. Self-renewing and able to generate neurons and Schwann cells. Able to generate neural, glia, smooth muscle cells, and adipocytes. Multipotent and self-renewal. Not committed to myogenic lineage only. Multipotent with both osteogenic and adipogenic potential. Differentiate into adipogenic, chondrogenic, myogenic, and osteogenic cells. Give progenies committed to a specific phenotypic pathway in cartilage or bone tissue. Self-renewing and multipotent. Self-renewing and multipotent. Form hepatocytic cluster and generate parenchymal and bile duct cells. Bone marrow origin, generate epithelial cells within the liver, hepatocytes and bile-ductular cells. Differentiate into myofibroblasts, smooth muscle, and endothelial cells. Differentiate ex vivo into pancreatic endocrine, exocrine, and hepatic phenotype. Give rise predominantly to enterocytes, mucus-secreting Goblet cells, peptide hormone-secreting enteroendocrine cells, Paneth cells, and M cells.
Only a partial list has been compiled to illustrate that tissue specific stem cells have been isolated from all three major germ layers and selected organ systems.
tems or organs where stem cells have been isolated (see Table 3), and which are similar to the bone marrow in their lack of complex connectivity, the presence of ongoing regeneration in the adult, and a niche for stem cell integration would benefit from strategies directed at identifying and replacing with the most primitive stem cell (54). Cell therapy in diabetes is likely to follow the hematopoietic model, as little connectivity is necessary; positional information appears irrelevant and evidence of ongoing replenishment of beta cells provides some confidence that feedback regulatory loops exists (reviewed in ref. 25). Mesenchymal stem cell (MSC) replacement for connective tissue disorders is likely to be more straightforward than neural replacement, and is also likely to follow the hematopoietic model
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(reviewed in ref. 55). MSCs can be delivered intravenously. MSCs exhibit a homing tendency to the marrow, and like the hematopoietic stem cell, have a lifelong self-renewal capacity. Ongoing connective tissue repair and remodeling have been demonstrated, and positional information appears unimportant (reviewed in ref. 56). On the other hand, for organs where complex connectivity is the norm, stem cell niches do not appear to exist; there is little ongoing repair and positional information is critical; it is unlikely that replacing with, or transplanting the most primitive tissue specific stem cell, will work. Examples of such systems include the liver, heart, lung, skin, skeletal muscle, etc. In these systems, we suggest that alternate strategies will have to be considered.
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239 in Fig. 3). These gene discovery and drug screening strategies indirectly supporting cell-replacement efforts are excluded from our discussions.
Progenitor Cell Therapy Using Restricted Precursors
Fig. 3. Possible alternatives to directly transplanting stem cells for therapy are listed.
It is important to emphasize that while we do not expect transplantation of the primitive stem cell to work in every system, we do expect that stem cells will play a critical role in any cell-based therapy (see Fig. 3). Whether one uses cultured stem cells as a source of progenitor cells to develop organs in culture, to passage stem cells through animals or transplant cells in utero (see below), it is clear that we will depend on the ability of stem cells to self-renew for prolonged periods in order to obtain the necessary numbers of cells for therapy. However, we do suggest that one needs to carefully evaluate which cell therapy strategy is the most appropriate in any given tissue or organ.
Alternate Strategies Several alternate strategies can be envisaged in systems that will not benefit from replacement of the most primitive stem cell population in adult tissue. Options that can be considered include: 1) transplant of progenitor cells; 2) modulation of the endogenous precursor cell population using inductive factors; 3) transplanting cells in utero; and 4) organ generation in vitro. The advantages and disadvantages of these alternate strategies are discussed briefly below. None of these options are offered as an alternative or substitute for using stem cells, but rather as a strategy that complements stem cell therapy as successfully used in the hematopoietic model of stem cell replacement. Multipotent stem cells may also be used to enhance our understanding of the stem cell differentiation process, and multipotent stem cell lines may be used for gene discovery and drug screening efforts (summarized
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In addition to multipotent stem cells, it is possible to use more restricted precursor cells for therapy. Analysis of organ and tissue development suggests that stem cells do not generate fully differentiated cells directly. Rather, through a sequential process of differentiation, progressively more restricted progenitor populations will be generated. These cells retain the ability to divide for several generations, but appear more restricted in their developmental plasticity. Intermediate precursor or blast cells have been well characterized in the hematopoietic system, and cell surface markers that can uniquely distinguish them have been identified. Restricted precursors have also been identified in the CNS and peripheral nervous system (PNS), as well as in several other systems (summarized in Table 4). Restricted precursors that persist in the adult, particularly in the nervous tissue, appear to be the predominant population of dividing cells. Restricted precursors in general are more abundant than multipotent stem cells and most precursor cells proliferate rapidly (for limited time periods) in culture. In the CNS, endogenous astrocyte proliferation and oligodendrocyte replacement from endogenous precursors are seen throughout the neuraxis, and competitive focal replacement is possible. Precursor cells respond to injury in other systems as well (see, for example, liver regeneration or pancreatic islet regeneration). Thus, it appears that proximate cues required directing cell type, and site specific differentiation exist. Furthermore, evidence from transplantation experiments in rodents has suggested that at least for glial precursors of the CNS positional identity does not appear critical to their appropriate function. The limited ability of precursor cells to self-renew may even be advantageous since many injuries where cell transplantation is being considered as a possible therapy, are self-limiting and may not require lifetime, ongoing replacement. Migratory pathways for precursors likely exist in the adult in tissues where there is ongoing replacement with
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Cai and Rao Table 4 Precursor Cells Present in Selected Tissue. Cells Ectoderm
Mesoderm
Endoderm
Skin-derived mast cell Schwann cell precursor Keratinocyte transient amplifying cells Melanocyte precursor cells Ductular progenitor cell Hematopoietic progenitor cells Metanephrogenic mesenchyme precursor cells Adipocyte precursors Muscle precursor cells (myoblasts) Chondrocyte precursor cells Osteoprogenitor cells Fetal lung mesenchyme cells Thymus-derived myoid precursor cell Pancreas precursor cells Ureteric bud precursor cells
References (107) (108) (109) (110) (111) (112) (113) (114) (115) (116) (117) (118) (119) (120) (113)
Only a partial list has been compiled to illustrate that precursor cells have been isolated from all three major germ layers and selected organ systems. Note more than one kind of progenitor cell is usually present in any organ. References included serve as an example, and are not meant to be comprehensive.
progenitor cells, and in fact, precursor cells have been shown to undergo long distance migration (for example, ref. 39). This, however, is unlikely to be true for multipotent stem cells (see above). In several regions of the brain, neuronal progenitor replacement may be more effective than stem cell therapy. For example, we and others have noted that transplanting multipotent stem cells into the spinal cord is not sufficient to ensure neuronal differentiation. Cells will survive and migrate, but will not differentiate as appropriate cues to direct differentiation into neurons, do not persist in the adult, and do not appear to be induced after injury. The cues that appear to be lacking are those directing the transition from the stem cell stage to the precursor cell stage. However, if neuronal progenitor cells are transplanted, these cells will readily differentiate into mature neurons that will survive for prolonged periods and integrate into the host environment (20,57,58). Likewise, differentiation of myelinating oligodendrocytes from oligodendrocyte progenitors is robust and much more effective than transplanting multipotent neural stem cells. Similarly in the liver, hepatocyte replacement or transplantation appears to be more feasible than replacement with a primitive stem cell. Hepatocytes proliferate after injury and normal cues to direct site
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specific differentiation exist. Positional identity is not critical and regeneration leads to regeneration of total liver mass, but not of an anatomically normal liver (at the gross level). Hepatocytes can be delivered via the portal vein, and the cells transit the endothelial wall and can replace up to 20–30% of the liver volume. Hepatocytes can be cultured in vitro and will undergo limited, but sufficient, amplification so that enough cells are available for therapy. Overall, we argue that in some systems, many of the criteria that we have outlined as necessary for successful replacement, exist for precursor cells but not for primitive stem cells (Figure 4). In these systems, precursor cell therapy would be more appropriate. We emphasize that while precursor cells may be better than multipotent stem cells in some situations, we do not imply that all problems related to cell therapy have been resolved by opting to use progenitor cells. In the case of the nervous system, we still need to identify a reliable and reproducible source of cells that will be immunologically compatible, and to demonstrate that transplanted cells will compete effectively and provide functionally adequate replacement. In the case of hepatic cell replacement, strategies to enhance the integration of hepatic progenitors will need to be developed. If a niche is not created, then replacement is spotty and
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Fig. 4. Precursor cells obtained directly from tissue by selection or by predifferentiation of cultured stem cells can be transplanted. Precursor cells may have specific advantages over stem cells in systems where ongoing repair and regeneration are not stem cell based (see text).
inadequate (59–61). Possible solutions for these problems do exist and are under active development. An additional problem that remains unresolved is the issue of regional identity. It has been clearly demonstrated that cells appear to be regionally specified relatively early in development (even at the stem cell stage), and that these regional identities are difficult to alter. It is reasonable to assume that precursor and progenitor cells will likewise be regionally specified. Thus transplanting any cell will involve not only selecting a particular type of progenitor (for example, glial or neuronal progenitor) but also a progenitor that is rostro-caudally and dorsoventrally appropriately specified. A ventral midbrain neuronal progenitor, for example, is far more likely to generate dopaminergic neurons than is any other progenitor population. Distinguishing one regionally specified progenitor from another is not trivial, and obtaining sufficient numbers is even more problematical. Overall, however, we believe using stem cells as a source for defined populations of cells or directly isolating progenitor cells is a viable option for cell replacement. It may be better than using the most primitive stem cell in many cases (as discussed above).
Inductive Therapy A strategy for utilizing stem cell and progenitor cell populations that is distinct from the hematopoi-
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etic model of identifying the most primitive stem cells and transplanting it into an existing stem cell niche for lifetime replacement, is to consider modulating the ongoing process of differentiation in tissues where ongoing repair persists in the adult. Augmenting or modulating an ongoing process is a more realistic option in systems where we lack either sufficient control of, or adequate understanding of, the complex interactions that regulate the development of fully differentiated cells. We see inductive factors as any factors that modulate the process of cellular survival, proliferation, migration, or differentiation to enhance the net output of differentiated cells that are deficient in a particular disease (see Figure 5). Inductive factors can act at multiple stages in the differentiation process. A factor may alter the proliferative rate of the stem cell and thus augment the total number of differentiated cells, or at the other extreme, it may enhance the survival of fully differentiated cells that would have been lost (Fig. 5). The ability to grow neural stem cells and possibly other stem cell populations in culture allows us to use self-renewing potentially immortal stem cell populations as a source of cells to identify previously unknown inductive factors, and to test the relative efficacy of candidate molecules in a reliable and reproducible way. Known inductive factors can be provided by any appropriate means, and unknown factors may be assessed by taking advantage of our ability to har-
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Fig. 5. Modulation of endogenous repair by inductive factors can occur by multiple mechanisms at various stages of development. Multiple factors can be combined for synergistic effects provided the process of differentiation is well understood in each system, as is the mechanism of action of a candidate inductive factor.
ness stem cells themselves. Stem cells and progenitor cells often secrete autocrine or paracrine factors, and their survival and differentiation is regulated by factors secreted by their differentiated progeny (reviewed in ref. 62). Thus, augmenting the endogenous stem cell pool with additional cells may alter the overall balance sufficiently to enhance the net output of differentiated cells. Examples of inductive factor therapy abound and a detailed discussion of candidate inductive factors is beyond the scope of this review. Examples of stem cells themselves as inductive agents are fewer but nevertheless exist (63–65). Perhaps the best example of induction by transplantation of stem cells is the work of Dr. Snyder and colleagues. These investigators have transplanted an immortalized rodent stem cell line into the spinal cord and the striatum, and demonstrated quite dramatic behavioral improvements (66). Similar effects were also seen in olfactory ensheathing cells (67–69) and embryonic stem cells (70). These improvements, however, cannot be attributed to the stem cells differentiating into a specific type of neuron, but rather to the trophic and differentiation signals (inductive factors) provided to the endogenous precursor population. In the case of the spinal cord transplants, the inductive factor that was secreted by the stem cells has been identified as glial cell line-derived neurotrophic factor (GDNF), which is secreted at high
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levels by an engineered stem cell line (71). Dr. Blakemore and colleagues showed, several years ago, that astrocyte proliferation after injury could inhibit successful remyelination (72). They also noted that Schwann cells can overcome the astrocyte inhibition and can themselves promote myelination (73,74). Thus Schwann cells could be used to provide a barrier to astrocyte invasion and thereby allow endogenous cells to repair a demyelinated lesion. In these examples, stem and precursor cells themselves were not directly involved in generating tissue specific or site specific differentiated cells, but providing appropriate signals that modulated the ongoing repair program (serving as inductive agents). Such therapy bypasses many of the problems associated with classical therapy based on the hematopoietic model as we expect much less of stem cells to be evaluated in all systems where cell replacement is being considered.
In Utero Transplantation or Transplantation During Early Development As we have indicated earlier, cell replacement is likely to be successful in systems where a niche for stem cells exists or can be created, and where there
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Fig. 6. In utero transplantation to co-opt normal developmental cues to direct tissue specific and site specific differentiation is an alternative to stem cell therapy in the adult. The timetable of neural development illustrates the necessity of determining when transplantation will be appropriate to maximize cell integration (106).
is substantial ongoing cell proliferation, migration, and differentiation into cells that have the appropriate regional and positional markers. Such a state exists in virtually all tissues during embryonic development. Stem cells are present in larger numbers, migration and connectivity cues are present, and appropriate feedback loops exist to regulate final cell number. It is also worth pointing out that identification of antigens as self versus nonself occurs late in development, and cells transplanted prior to this stage are likely to be recognized as self and therefore will not provoke an immune response. Furthermore, the blood brain barrier is not yet fully formed, and interactions between molecules or cells delivered intravenously are more likely to transit to the brain. Thus many of the criteria we have identified as being critical to successful cell replacement exist in the embryonic and neonatal period. Development in humans is spread over a prolonged period and many techniques for in utero manipulation and surgery have been developed. Delivering cells to the embryo, while technically challenging, is feasible with current technology (75). The uterus can be readily accessed throughout most embryonic development, and specific tissues can be targeted with relative ease. Detailed timetables of organ generation are available and ultrasound or other noninvasive methods of assessing fetal devel-
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opment have been developed (76–78), enabling therapy to be targeted to the most appropriate developmental time period (see Figure 6). Surgical intervention in utero has been described (75,79,80) and the limited data that is available in rodents suggests that incorporation is robust, reliable, and reproducible (75). For example, in utero transplantation into the brain at E18 in rodents results in reliable integration (81). Ourednik and colleagues, for example, have shown integration of stem cells into the ventricular zone after intrauterine transplantation in monkeys (37). While any cell that has the capability of integrating into the tissue targeted for therapy and responding appropriately to local signals is useful for intrauterine replacement, we suggest that tissue specific stem cells will be most appropriate. It is unlikely, in our opinion, that appropriate cues to direct embryonic stem cells (ES cells) to differentiate will be available at most stages of embryonic development. Progenitor or intermediate precursors may not provide lifetime replacement and may thus be of limited benefit. Tissues in which selfrenewing stem cells exist and show lifetime selfreplenishment are the best systems in which in utero transplantation should be considered. Perhaps the biggest concern is identifying prospective patients for cell replacement therapy. It is clear that in utero
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Fig. 7. Combining the ability to grow stem cells in culture, and advances in tissue engineering provide an alternate strategy to using stem cells. Either tissue specific differentiated cells or stem cells can be combined with scaffold materials and cultured in bioreactors to generate complex three-dimensional tissue fragments or organs.
transplantation cannot be an option for diseases that manifest late in development, or cannot be unambiguously diagnosed early. However, for genetic disorders and childhood diseases, in utero stem cell therapy may be a possibility that offers a high potential for success, as it appropriates normal developmental feedback loops. Possible disease targets are osteogenesis imperfecta, glycogen storage disorders, cerebral palsy, and other disorders where candidate cells are available and the diagnosis can be made early.
Organ Generation It may be possible to harness what we have learned in stem cell biology and normal development to develop tissue and organs in vitro which can then be transplanted. Tissue engineering enthusiasts have suggested for years that, rather than transplanting single cells, it will ultimately be possible to generate entire organs—either synthetic, organic, or hybrids that could replace damaged tissue (82). The procedure of organ generation is outlined in Figure 7. Advances in biomaterial engineering and stem cell technology have made the creation of biohybrid organs feasible (83,84). Indeed, commercial products are available and biotechnology companies that
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Cai and Rao market these products have multimillion-dollar revenues. While a detailed discussion of recent breakthroughs is beyond the scope of this article, it is important to point out how some advances may expand the range of stem cell therapy. Skin, perhaps, is the best example of an organ that is synthesized from precursor cells in vitro and sold as a commercial product (31,85,86). Apligrap®, Dermagraft®, OrCel®, Alloderm®, Epicell®, etc., are commercial products routinely used in therapy. Other tissues or organs where bioengineering and specialized culturing techniques have resulted in usable products are cartilage, bone, and smooth muscle (reviewed in refs. 29,30,56,87). Osiris Therapeutics Inc. has demonstrated good bone engraftment using a carbonate scaffold, as has Orthovista (88). Urinary organs have been repaired by combining endothelium with smooth muscle using calcium biphosphate ceramics (82,89), and periodontal implants have been developed as well (90,91). The strategy used here is harvesting autologous or heterologous cells that are amplified in culture in a proprietary fashion and then grown into a tissue using specialized matrices. Tittered mixtures of cells are then resupplied to the donor for use in rebuilding damaged tissue. Vascular tissue made by combining endothelial and vascular smooth muscle cells in an elegant flow device to obtain tubes that mimic the properties of veins and arteries, have been developed and transplanted in test models to demonstrate efficacy (92–94). Investigators have succeeded in preparing corneal cells in culture. Using precise ratios of different cell types, corneas have been grown in culture which are translucent and have an inflammatory response similar to natural corneas (95–97). Overall, the ability to combine biodegradable matrices that have been assembled into threedimensional structures with dividing stem and progenitor cells to generate complex forms that closely mimic the contours and functionality of the natural structure, provide an additional dimension to the use of stem and progenitor cells in cell replacement strategies. Advances have also been made in providing hybrid biological/synthetic structures for organs and tissue where it is still impossible to generate synthetic organs. This approach of mechanical or electronic devices that signal to biological tissues, will in turn process the information. Examples of such devices are cochlear and retinal implants or
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Transplant Therapy with Human Cells nerve stimulation devices (98–101). This synthetic biological interface is one where stem cells and progenitor cells may play a useful role as well (81). An even more radical possibility is to develop organs in swine by transplantation of human stem cells or progenitor cells into an organ (102,103). Such a transplant could take place in utero or in early stages of postnatal development, or in cadaveric tissue. The organ along with the transplanted stem and progenitor cells can be harvested and then transplanted as xenoplants (or cadaveric transplants). The host immune system will in time destroy the xenobiotic cells, while sparing the matched human cells which will gradually expand to replace the destroyed cells, leaving in time a fully formed human organ that maintains the complex morphology and contours of the normal organ (104). Since we cannot as yet generate complex organs in vitro, this strategy offers a mechanism whereby organs can be generated by coopting the complex regulatory controls that biological system have evolved to generate complex three-dimensional structures. Heart valves may be an ideal test of such a strategy (105). They are relatively straightforward structures, and cells that would appropriately repopulate the valve can be grown in culture and valve transplants are routinely performed (105). Overall we suggest that combining our ability to harvest and grow stem cells and progenitor cells in culture with evolving techniques in bioengineering to provide tissue or organ replacement represents an alternative strategy to harness the power of stem cells in a manner distinct from hematopoietic cell therapy.
Summary We believe that there is no “one size fits all” cell or cell based strategy that is optimal for any system in which cell replacement is desired. The hematopoietic cell replacement strategy is the best example of successful stem cell therapy, and we can learn several important lessons from this model of cellular intervention. In tissues and organs where a stem cell niche is present, or can be created, and where there is ongoing cell replacement are best suited for therapy along the hematopoietic model. In other systems, alternative strategies need to be evaluated or are being attempted. These include progenitorcell therapy, inductive strategies, in utero cell replacement, organ generation in vitro, etc. We note that
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245 some of these approaches are already in commercial production while others are close to reality. Perhaps the one which is far from commercial application yet, paradoxically, has garnered the most attention, is neuronal replacement therapy.
Acknowledgments This work was supported by the NIA, the ALS center, and the CNS foundation. We thank all members of our laboratory for constant, stimulating discussions and the numerous suggestions that improved this manuscript. MSR acknowledges the contributions of Dr. S. Rao that made undertaking this project possible.
References 1. Cronkite E. P. and Feinendegen L. E. (1976). Notions about human stem cells. Nouv. Rev. Fr. Hematol. Blood Cells 17, 269–284. 2. Weissman I., Spangrude G., Heimfeld S., Smith L., and Uchida N. (1991). Stem cells. Nature 353, 26. 3. Kalyani A. J. and Rao M. S. (1998). Cell lineage in the developing neural tube. Biochem. Cell Biol. 76, 1051–1068. 4. Rao M. S. (1999). Multipotent and restricted precursors in the central nervous system. Anat. Rec. 257, 137–148. 5. Gage F. H. (2000). Mammalian neural stem cells. Science 287, 1433–1438. 6. Anderson D. J. (2001). Stem cells and pattern formation in the nevous system: the possible versus the actual. Neuron 30, 19–35. 7. Weissman I. L., Anderson D. J., and Gage F. (2001). Stem and progenitor cells: origins, phenotypes, lineage commitments, and transdifferentiations. Annu. Rev. Cell Dev. Biol. 17, 387–403. 8. Gage F. H. (2002). Neurogenesis in the adult brain. J. Neurosci. 22, 612–613. 9. Kohn D. B. (1999). Gene therapy using hematopoietic stem cells. Curr. Opin. Mol. Ther. 1, 437–442. 10. Craddock C. (2000). Haemopoietic stem cell transplantation: recent progress and future promise. Lancet Oncol. 1, 227–234. 11. Bordignon C. and Roncarolo M. G. (2002). Therapeutic applications for hematopoietic stem cell gene transfer. Nat. Immunol. 3, 318–321. 12. Emery D. W., Nishino T., Murata K., Fragkos M., and Stamatoyannopoulos G. (2002). Hematopoi-
Volume 2, 2002
NMM02-3,CH01,pp233-250,18pgs
1/16/03 1:44 PM
Page 246
246
13. 14. 15. 16. 17. 18. 19.
20.
21.
22. 23. 24.
25. 26.
27.
etic stem cell gene therapy. Int. J. Hematol. 75, 228–236. Myers L. W. (2001). Immunologic therapy for secondary and primary progressive multiple sclerosis. Curr. Neurol. Neurosci. Rep. 1, 286–293. Jansen J., Thompson J. M., Dugan M. J., et al. (2002). Peripheral blood progenitor cell transplantation. Ther. Apher. 6, 5–14. Bhatia V. and Porter D. L. (2001). Novel approaches to allogeneic stem cell therapy. Expert Opin. Biol. Ther. 1, 3–15. Horak D. A. and Forman S. J. (2001). Critical care of the hematopoietic stem cell patient. Crit. Care Clin. 17, 671–695. Lagasse E., Shizuru, J. A., Uchida N., Tsukamoto A., and Weissman I. L. (2001). Toward regenerative medicine. Immunity 14, 425–436. Hintzen R. Q. (2002). Stem cell transplantation in multiple sclerosis: multiple choices and multiple challenges. Mult. Scler. 8, 155–160. Kozak T. and Rychlik I. (2002). Developments in hematopoietic stem cell transplantation in the treatment of autoimmune diseases. Isr. Med. Assoc. J. 4, 268–271. Kondziolka D., Wechsler L., Goldstein S., et al. (2000). Transplantation of cultured human neuronal cells for patients with stroke. Neurology 55, 565–569. Freed C. R., Greene P. E., Breeze R. E., Tsai W. Y., DuMouchel W., Kao R., et al. (2001). Transplantation of embryonic dopamine neurons for severe Parkinson’s disease. N. Engl. J. Med. 344, 710–719. Lindvall O. (2001). Parkinson disease. Stem cell transplantation. Lancet 358 Suppl, S48. Check E. (2002). Parkinson’s patients show positive response to implants. Nature 416, 666. Shapiro A. M., Lakey J. R., Ryan E. A., Korbutt G. S., Toth E., Warnock G. L., et al. (2000). Islet transplantation in seven patients with type 1 diabetes mellitus using a glucocorticoid-free immunosuppressive regimen. N. Engl. J. Med. 343, 230–238. Soria B., Skoudy A., and Martin F. (2001). From stem cells to beta cells: new strategies in cell therapy of diabetes mellitus. Diabetologia. 44, 407–415. Yang T. Y., Oh S. H., Jeong I. K., Seo I. A., Oh E. Y., Kim S. J., et al. (2002). First human trial of pancreatic islet allo-transplantation in Korea—focus on re-transplantation. Diabetes Res. Clin. Pract. 56, 107–113. Thivolet C. (2001). New therapeutic approaches to type 1 diabetes: from prevention to cellular or gene therapies. Clin. Endocrinol. (Oxf) 55, 565–574.
NeuroMolecular Medicine
Cai and Rao 28. Horwitz E. M., Prockop D. J., Gordon P. L., et al. (2001). Clinical responses to bone marrow transplantation in children with severe osteogenesis imperfecta. Blood 97, 1227–1231. 29. Boyan B. D., Lohmann C. H., Romero J., and Schwartz Z. (1999). Bone and cartilage tissue engineering. Clin. Plast. Surg. 26, 629–645, ix. 30. Freed L. E., Martin I., and Vunjak-Novakovic G. (1999). Frontiers in tissue engineering. In vitro modulation of chondrogenesis. Clin. Orthop. S46–58. 31. Bello Y. M., Falabella A. F., and Eaglstein W. H. (2001). Tissue-engineered skin. Current status in wound healing. Am. J. Clin. Dermatol. 2, 305–313. 32. Hardy C. L. and Tavassoli M. (1988). Homing of hemopoietic stem cells to hemopoietic stroma. Adv. Exp. Med. Biol. 241, 129–133. 33. Tavassoli M. and Hardy C. L. (1990). Molecular basis of homing of intravenously transplanted stem cells to the marrow. Blood 76, 1059–1070. 34. Whetton A. D. and Graham G. J. (1999). Homing and mobilization in the stem cell niche. Trends Cell Biol. 9, 233–238. 35. Steindler D. A. and Pincus D. W. (2002). Stem cells and neuropoiesis in the adult human brain. Lancet 359, 1047–1054. 36. Taylor R. M. and Snyder E. Y. (1997). Widespread engraftment of neural progenitor and stem-like cells throughout the mouse brain. Transplant. Proc. 29, 845–847. 37. Ourednik V., Ourednik J., Flax J. D., Zawada W. M., Hutt C., Yang C., et al. (2001). Segregation of human neural stem cells in the developing primate forebrain. Science 293, 1820–1824. 38. Snyder E. Y. and Wolfe J. H. (1996). Central nervous system cell transplantation: a novel therapy for storage diseases? Curr. Opin. Neurol. 9, 126–136. 39. Aboody K. S., Brown A., Rainov N. G., et al. (2000). From the cover: neural stem cells display extensive tropism for pathology in adult brain: evidence from intracranial gliomas. Proc. Natl. Acad. Sci. USA 97, 12,846–12,851. 40. Nishino H., Hida H., Takei N., Kumazaki M., Nakajima K., and Baba H. (2000). Mesencephalic neural stem (progenitor) cells develop to dopaminergic neurons more strongly in dopamine-depleted striatum than in intact striatum. Exp. Neurol. 164, 209–214. 41. Cao Q. L., Zhang Y. P., Howard R. M., Walters W. M., Tsoulfas P., and Whittemore S. R. (2001). Pluripotent stem cells engrafted into the normal or lesioned adult rat spinal cord are restricted to a glial lineage. Exp. Neurol. 167, 48–58.
Volume 2, 2002
NMM02-3,CH01,pp233-250,18pgs
1/16/03 1:44 PM
Page 247
Transplant Therapy with Human Cells 42. Magnuson D. S., Zhang Y. P., Cao Q. L., Han Y., Burke D. A., and Whittemore S. R. (2001). Embryonic brain precursors transplanted into kainate lesioned rat spinal cord. Neuroreport 12, 1015–1019. 43. Fricker R. A., Carpenter M. K., Winkler C., Greco C., Gates M. A., and Bjorklund A. (1999). Sitespecific migration and neuronal differentiation of human neural progenitor cells after transplantation in the adult rat brain. J. Neurosci. 19, 5990–6005. 44. Englund U., Fricker-Gates R. A., Lundberg C., Bjorklund A., and Wictorin K. (2002). Transplantation of human neural progenitor cells into the neonatal rat brain: extensive migration and differentation with long-distance axonal projections. Exp. Neurol. 173, 1–21. 45. Svendsen C. N., Clarke D. J., Rosser A. E., and Dunnett S. B. (1996). Survival and differentiation of rat and human epidermal growth factor-responsive precursor cells following grafting into the lesioned adult central nervous system. Exp. Neurol. 137, 376–388. 46. Svendsen C. N. and Rosser A. E. (1995). Neurones from stem cells? Trends Neurosci. 18, 465–467. 47. Wichterle H., Garcia-Verdugo J. M., Herrera D. G., and Alvarez-Buylla A. (1999). Young neurons from medial ganglionic eminence disperse in adult and embryonic brain. Nat. Neurosci. 2, 461–466. 48. Briscoe J., Pierani A., Jessell T. M., and Ericson J. (2000). A homeodomain protein code specifies progenitor cell identity and neuronal fate in the ventral neural tube. Cell 101, 435–445. 49. McCarthy M., Turnbull D. H., Walsh C. A., and Fishell G. (2001). Telencephalic neural progenitors appear to be restricted to regional and glial fates before the onset of neurogenesis. J. Neurosci. 21, 6772–6781. 50. Na E., McCarthy M., Neyt C., Lai E., and Fishell G. (1998). Telencephalic progenitors maintain anteroposterior identities cell autonomously. Curr. Biol. 8, 987–990. 51. Rubenstein J. L. (2000). Intrinsic and extrinsic control of cortical development. Novartis Found Symp. 228, 67–75; discussion 75–82, 109–113. 52. Young M. J., Ray J., Whiteley S. J., Klassen H., and Gage F. H. (2000). Neuronal differentiation and morphological integration of hippocampal progenitor cells transplanted to the retina of immature and mature dystrophic rats. Mol. Cell. Neurosci. 16, 197–205. 53. Durbec P. and Rougon G. (2001). Transplantation of mammalian olfactory progenitors into chick
NeuroMolecular Medicine
247
54.
55.
56. 57.
58. 59.
60.
61.
62. 63. 64.
65.
hosts reveals migration and differentiation potentials dependent on cell commitment. Mol. Cell Neurosci. 17, 561–576. Alison M., Golding M., Lalani el N., and Sarraf C. (1998). Wound healing in the liver with particular reference to stem cells. Philos. Trans. R. Soc. Lond. B. Biol. Sci. 353, 877–894. Caterson E. J., Nesti L. J., Albert T., Danielson K., and Tuan R. (2001). Application of mesenchymal stem cells in the regeneration of musculoskeletal tissues. Med. Gen. Med. E1. Muschler G. F. and Midura R. J. (2002). Connective tissue progenitors: practical concepts for clinical applications. Clin. Orthop. 66–80. Sheen V. L., Arnold M. W., Wang Y., and Macklis J. D. (1999). Neural precursor differentiation following transplantation into neocortex is dependent on intrinsic developmental state and receptor competence. Exp. Neurol. 158, 47–62. Han S. S. and Fischer I. (2000). Neural stem cells and gene therapy: prospects for repairing the injured spinal cord. JAMA 283, 2300–2301. Guha C., Deb N. J., Sappal B. S., Ghosh S. S., RoyChowdhury N., and Roy-Chowdhury J. (2001). Amplification of engrafted hepatocytes by preparative manipulation of the host liver. Artif. Organs 25, 522–528. Laconi S., Pillai S., Porcu P. P., Shafritz D. A., Pani P., and Laconi E. (2001). Massive liver replacement by transplanted hepatocytes in the absence of exogenous growth stimuli in rats treated with retrorsine. Am. J. Pathol. 158, 771–777. Gordon G. J., Butz G. M., Grisham J. W., and Coleman W. B. (2002). Isolation, short-term culture, and transplantation of small hepatocyte-like progenitor cells from retrorsine-exposed rats. Transplantation 73, 1236–1243. Sommer L. and Rao M. (2002). Neural stem cells and regulation of cell number. Prog Neurobiol. 66, 1–18. Chen J., Li Y., and Chopp M. (2000). Intracerebral transplantation of bone marrow with BDNF after MCAo in rat. Neuropharmacology 39, 711–716. Li Y., Chen J., Wang L., Lu M., and Chopp M. (2001). Treatment of stroke in rat with intracarotid administration of marrow stromal cells. Neurology 56, 1666–1672. Zhao L. R., Duan W. M., Reyes M., Keene C. D., Verfaillie C. M., and Low W. C. (2002). Human bone marrow stem cells exhibit neural phenotypes and ameliorate neurological deficits after grafting into the ischemic brain of rats. Exp. Neurol. 174, 11–20.
Volume 2, 2002
NMM02-3,CH01,pp233-250,18pgs
1/16/03 1:44 PM
Page 248
248 66. Flax J. D., Aurora S., Yang C., Simonin C., Wills A. M., Billinghurst L. L., et al. (1998). Engraftable human neural stem cells respond to developmental cues, replace neurons, and express foreign genes. Nat. Biotechnol. 16, 1033–1039. 67. Ramon-Cueto A., Cordero M. I., Santos-Benito F. F., and Avila J. (2000). Functional recovery of paraplegic rats and motor axon regeneration in their spinal cords by olfactory ensheathing glia. Neuron 25, 425–435. 68. Boruch A. V., Conners J. J., Pipitone M., et al. (2001). Neurotrophic and migratory properties of an olfactory ensheathing cell line. Glia. 33, 225–229. 69. Lu J. and Ashwell K. (2002). Olfactory ensheathing cells: their potential use for repairing the injured spinal cord. Spine 27, 887–892. 70. McDonald J. W., Liu X. Z., Qu Y., Liu S., Mickey S. K., Turetsky D., et al. (1999). Transplanted embryonic stem cells survive, differentiate and promote recovery in injured rat spinal cord. Nat. Med. 5, 1410–1412. 71. Akerud P., Canals J. M., Snyder E. Y., and Arenas E. (2001). Neuroprotection through delivery of glial cell line-derived neurotrophic factor by neural stem cells in a mouse model of Parkinson’s disease. J. Neurosci. 21, 8108–8118. 72. Blakemore W. F., Crang A. J., and Curtis R. (1986). The interaction of Schwann cells with CNS axons in regions containing normal astrocytes. Acta. Neuropathol. (Berl) 71, 295–300. 73. Harvey A. R. and Plant G. W. (1995). Schwann cells and fetal tectal tissue cografted to the midbrain of newborn rats: fate of Schwann cells and their influence on host retinal innervation of grafts. Exp. Neurol. 134, 179–191. 74. Guest J. D., Rao A., Olson L., Bunge M. B., and Bunge R. P. (1997). The ability of human Schwann cell grafts to promote regeneration in the transected nude rat spinal cord. Exp. Neurol. 148, 502–522. 75. Hayashi S. and Flake A. W. (2001). In utero hematopoietic stem cell therapy. Yonsei Med. J. 42, 615–629. 76. Hill L. M., Guzick D., Fries J., Hixson J., and Rivello D. (1990). The transverse cerebellar diameter in estimating gestational age in the large for gestational age fetus. Obstet. Gynecol. 75, 981–985. 77. Harrington K. and Campbell S. (1993). Fetal size and growth. Curr. Opin. Obstet. Gynecol. 5, 186–194. 78. Lysikiewicz A., Bracero L. A., and Tejani N. (2001). Sonographically estimated fetal weight percentile as a predictor of preterm delivery. J. Matern. Fetal. Med. 10, 44–47.
NeuroMolecular Medicine
Cai and Rao 79. Agarwal S. K. and Fisk N. M. (2001). In utero therapy for lower urinary tract obstruction. Prenat. Diagn. 21, 970–976. 80. Stevens G. H., Schoot B. C., Smets M. J., et al. (2002). The ex utero intrapartum treatment (EXIT) procedure in fetal neck masses: a case report and review of the literature. Eur. J. Obstet. Gynecol. Reprod. Biol. 100, 246–250. 81. Teng Y. D., Lavik E. B., Qu X., Park K. I., Ourednik J., Zurakowski D., et al. (2002). Functional recovery following traumatic spinal cord injury mediated by a unique polymer scaffold seeded with neural stem cells. Proc. Natl. Acad. Sci. USA 99, 3024–3029. 82. Park K. D., Kwon I. K., and Kim Y. H. (2000). Tissue engineering of urinary organs. Yonsei Med. J. 41, 780–788. 83. Fuchs J. R., Nasseri B. A., and Vacanti J. P. (2001). Tissue engineering: a 21st century solution to surgical reconstruction. Ann. Thorac. Surg. 72, 577–591. 84. Zandstra P. W. and Nagy A. (2001). Stem cell bioengineering. Annu. Rev. Biomed. Eng. 3, 275–305. 85. Yang E. K., Seo Y. K., Youn H. H., Lee D. H., Park S. N., and Park J. K. (2000). Tissue engineered artificial skin composed of dermis and epidermis. Artif. Organs 24, 7–17. 86. Kuroyanagi Y., Yamada N., Yamashita R., and Uchinuma E. (2001). Tissue-engineered product: allogeneic cultured dermal substitute composed of spongy collagen with fibroblasts. Artif. Organs 25, 180–186. 87. Goble E. M., Kohn D., Verdonk R., and Kane S. M. (1999). Meniscal substitutes—human experience. Scand. J. Med. Sci. Sports 9, 146–157. 88. Pittenger M. F., Mosca J. D., and McIntosh K. R. (2000). Human mesenchymal stem cells: progenitor cells for cartilage, bone, fat and stroma. Curr. Top. Microbiol. Immunol. 251, 3–11. 89. LeGeros R. Z. (2002). Properties of osteoconductive biomaterials: calcium phosphates. Clin. Orthop. 81–98. 90. Buckley M. J., Agarwal S., and Gassner R. (1999). Tissue engineering and dentistry. Clin. Plast. Surg. 26, 657–662, x. 91. Malament K. A. (2000). Prosthodontics: achieving quality esthetic dentistry and integrated comprehensive care. J. Am. Dent. Assoc. 131, 1742–1749. 92. Alessandri G., Girelli M., Taccagni G., Colombo A., Nicosia R., Caruso A., et al. (2001). Human vasculogenesis ex vivo: embryonal aorta as a tool for isolation of endothelial cell progenitors. Lab. Invest. 81, 875–885.
Volume 2, 2002
NMM02-3,CH01,pp233-250,18pgs
1/16/03 1:44 PM
Page 249
Transplant Therapy with Human Cells 93. Nerem R. M. and Seliktar D. (2001). Vascular tissue engineering. Annu Rev. Biomed. Eng. 3, 225–243. 94. Tiwari A., Salacinski H. J., Hamilton G., and Seifalian A. M. (2001). Tissue engineering of vascular bypass grafts: role of endothelial cell extraction. Eur. J. Vasc. Endovasc. Surg. 21, 193–201. 95. Ferber D. (1999). Tissue engineering. Growing human corneas in the lab. Science 286, 2051, 2053. 96. Griffith M., Osborne R., Munger R., Xiong X., Doillon C. J., Laycock N. L., et al. (1999). Functional human corneal equivalents constructed from cell lines. Science 286, 2169–2172. 97. Germain L., Carrier P., Auger F. A., Salesse C., and Guerin S. L. (2000). Can we produce a human corneal equivalent by tissue engineering? Prog. Retin. Eye Res. 19, 497–527. 98. Humayun M. S. (2001). Intraocular retinal prosthesis. Trans. Am. Ophthalmol. Soc. 99, 271–300. 99. Raine C. H. and Martin J. (2001). Cochlear and middle ear implants: advances for the hearing impaired. Hosp. Med. 62, 664–668. 100. Kerdraon Y. A., Downie J. A., Suaning G. J., et al. (2002). Development and surgical implantation of a vision prosthesis model into the ovine eye. Clin. Experiment. Ophthalmol. 30, 36–40. 101. Rauschecker J. P. and Shannon R. V. (2002). Sending sound to the brain. Science 295, 1025–1029. 102. Tuch B. E. and Beretov J. (1994). Interaction between xenografted human fetal pancreas and liver. Transplant. Proc. 26, 3333. 103. Angioi K., Hatier R., Merle M., and Duprez A. (2002). Xenografted human whole embryonic and fetal entoblastic organs develop and become functional adult-like micro-organs. J. Surg. Res. 102, 85–94. 104. Macchiarini P., Candelier J. J., Coullin P., et al. (2000). Use of embryonic human trachea grown in nude mice to patch-repair congenital tracheal stenosis. Transplantation 70, 1555–1559. 105. Zeltinger J., Landeen L. K., Alexander H. G., Kidd I. D., and Sibanda B. (2001). Development and characterization of tissue-engineered aortic valves. Tissue Eng. 7, 9–22. 106. Lou H. (1982). Developmental Neurology. Raven Press, p. 291. 107. Kambe N., Kambe M., Kochan J. P., and Schwartz L. B. (2001). Human skin-derived mast cells can proliferate while retaining their characteristic functional and protease phenotypes. Blood 97, 2045–2052.
NeuroMolecular Medicine
249 108. Jessen K. R., Brennan A., Morgan L., Mirsky R., Kent A., Hashimoto Y., et al. (1994). The Schwann cell precursor and its fate: a study of cell death and differentiation during gliogenesis in rat embryonic nerves. Neuron 12, 509–527. 109. Lehrer M. S., Sun T. T., and Lavker R. M. (1998). Strategies of epithelial repair: modulation of stem cell and transit amplifying cell proliferation. J. Cell Sci. 111 (Pt 19), 2867–2875. 110. Silver A. F., Chase H. B., and Potten C. S. (1969). Melanocyte precursor cells in the hair follicle germ during the dormat stage (telogen). Experientia. 25, 299–301. 111. Sell S. (2001). The role of progenitor cells in repair of liver injury and in liver transplantation. Wound Repair Regen. 9, 467–482. 112. Metcalf D. (1998). Pre-progenitor cells: a proposed new category of hematopoietic precursor cells. Leukemia 12, 1–3. 113. Al-Awqati Q. and Oliver J. A. (2002). Stem cells in the kidney. Kidney Int. 61, 387–395. 114. Van R. L. and Roncari D. A. (1982). Complete differentiation in vivo of implanted cultured adipocyte precursors from adult rats. Cell Tissue Res. 225, 557–566. 115. Yiou R., Dreyfus P., Chopin D. K., Abbou C. C., and Lefaucheur J. P. (2002). Muscle precursor cell autografting in a murine model of urethral sphincter injury. BJU Int. 89, 298–302. 116. Fang J. and Hall B. K. (1997). Chondrogenic cell differentiation from membrane bone periostea. Anat. Embryol. (Berl) 196, 349–362. 117. Long M. W., Robinson J. A., Ashcraft E. A., and Mann K. G. (1995). Regulation of human bone marrow-derived osteoprogenitor cells by osteogenic growth factors. J. Clin. Invest. 95, 881–887. 118. Akeson A. L., Wetzel B., Thompson F. Y., et al. (2000). Embryonic vasculogenesis by endothelial precursor cells derived from lung mesenchyme. Dev. Dyn. 217, 11–23. 119. Oka T., Hayashi K., Nakaoka Y., Ohtsuki Y., and Akagi T. (2000). Differentiation of rat thymic myoid progenitor cell line established by coculture with human T-lymphotropic virus type-I producing human T cells. Cell Tissue Res. 300, 119–127. 120. Alpert S., Hanahan D., and Teitelman G. (1988). Hybrid insulin genes reveal a developmental lineage for pancreatic endocrine cells and imply a relationship with neurons. Cell 53, 295–308.
Volume 2, 2002