Experimental Hematology
Volume 38, Issue 7 , Pages 540-547, July 2010

Hematopoietic stem cell origin of connective tissues

  • Makio Ogawa

      Affiliations

    • Department of Pathology and Laboratory Medicine, Medical University of South Carolina, Charleston, SC., USA
    • Research Service, Department of Veterans Affairs Medical Center, Charleston, SC., USA
    • Corresponding Author InformationOffprint requests to: Makio Ogawa, M.D., Ph.D., Department of Veterans Affairs Medical Center, 109 Bee Street, Charleston, SC 29401-5799
    • ,
  • Amanda C. LaRue

      Affiliations

    • Department of Pathology and Laboratory Medicine, Medical University of South Carolina, Charleston, SC., USA
    • Research Service, Department of Veterans Affairs Medical Center, Charleston, SC., USA
    • ,
  • Patricia M. Watson

      Affiliations

    • Department of Medicine, Medical University of South Carolina, Charleston, SC., USA
    • ,
  • Dennis K. Watson

      Affiliations

    • Department of Pathology and Laboratory Medicine, Medical University of South Carolina, Charleston, SC., USA
    • Department of Biochemistry and Molecular Biology, Medical University of South Carolina, Charleston, SC., USA

Received 6 April 2010; received in revised form 6 April 2010; accepted 8 April 2010. published online 21 April 2010.

Article Outline

Connective tissue consists of “connective tissue proper,” which is further divided into loose and dense (fibrous) connective tissues and “specialized connective tissues.” Specialized connective tissues consist of blood, adipose tissue, cartilage, and bone. In both loose and dense connective tissues, the principal cellular element is fibroblasts. It has been generally believed that all cellular elements of connective tissue, including fibroblasts, adipocytes, chondrocytes, and bone cells, are generated solely by mesenchymal stem cells. Recently, a number of studies, including those from our laboratory based on transplantation of single hematopoietic stem cells, strongly suggested a hematopoietic stem cell origin of these adult mesenchymal tissues. This review summarizes the experimental evidence for this new paradigm and discusses its translational implications.

 

Connective tissue is involved in the maintenance of structure and functional support of organs and tissues. It is usually classified into “connective tissue proper,” consisting of loose and dense (fibrous) connective tissues, and “specialized connective tissues.” The loose connective tissue holds organs in place and attaches epithelial tissue to other underlying tissues. Tendons and ligaments represent the dense connective tissue, consisting of large amounts of closely packed collagenous fibers. In both loose and dense connective tissues, the principal cellular element is fibroblasts. Specialized connective tissues consist of blood, adipose tissue, cartilage, and bone. It has been generally believed that all cellular elements of connective tissue, including fibroblasts, adipocytes, chondrocytes, and bone cells, are generated solely by mesenchymal stem cells (MSCs) 1, 2, 3, 4, 5, 6, 7, while blood cells are produced by hematopoietic stem cells (HSCs). Mesenchyme, or mesenchymal connective tissue, represents the part of the embryonic mesoderm that consists of loosely packed, nonspecialized cells embedded in a gelatinous ground substance, from which connective tissues and hematolymphatic systems develop. The term mesenchymal stem cells was coined by Caplan [8] in 1991 to describe a population of cells in the adult bone marrow that can be stimulated to differentiate into bone, cartilage, muscle, marrow stroma, tendon, fat, and a variety of other connective tissues in culture. Clarification of the lineage potentials of MSCs by Friedenstein and his associates, however, had begun prior to Caplan's report. In their classic series of studies, Friedenstein et al. 9, 10, 11 had demonstrated that fibroblasts and fibroblast colony-forming units (CFU-F) are the precursors for bone cells and chondrocytes. Because large numbers of MSCs can be generated in culture, MSCs were thought to be useful for “tissue-engineering” purposes [12], as exemplified by a number of clinical trials 13, 14. As such, this popular dogma has stipulated a distinct and separate repertoire of differentiation/reconstituting potentials for HSCs and MSCs. A series of studies, including those in our laboratory based on single HSC transplantation, however, supported the concept that the multiple connective tissues are generated by HSCs. This review summarizes our observations and discusses relevant prior literature. These data jointly support the new paradigm that is diagrammatically presented in Figure 1.

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Single HSC transplantation 

Several years ago, at the height of stem cell plasticity controversies, we began single HSC transplantation to obtain unequivocal conclusions on the tissue-reconstituting capability of HSCs. We used transgenic enhanced green fluorescent protein (EGFP) mice [15] as donor mice and CD 45.1 C57BL/6 mice as lethally irradiated recipient mice. We reasoned that, in order to study the full differentiation potentials of HSCs, it is necessary to generate mice exhibiting high-level, multilineage engraftment from a single HSC. In our hands, however, direct transplantation of individual lineage (Lin)− Sca-1+ c-kit+ CD34− cells [16] was not sufficiently efficient for producing mice with high-level, multilineage engraftment. In order to raise the efficiency, we took advantage of the cell-cycle dormancy of HSCs and devised a method consisting of single-cell deposition and short-term cell culture of putative HSCs 17, 18. Details of the original method are presented elsewhere in this issue [19]. Briefly, Lin− Sca-1+ c-kit+ CD34− cells or Lin− Sca-1+ CD34− side population cells [20] from EGFP mice were individually deposited into the wells of Corning (Corning, NY, USA) 96-well tissue culture plates and, after confirmation of the presence of a single cell per well, cultured for 1 week in the presence of steel factor (c-kit ligand) and interleukin-11 (IL-11) or a combination of steel factor and granulocyte colony-stimulating factor (G-CSF). Earlier, we had observed that both IL-11 [21] and G-CSF [22] support proliferation of cell-cycle dormant primitive multipotential progenitors in culture. The majority of HSCs in the steady-state bone marrow are dormant in cell cycle and do not begin cell division until a few days after initiation of cell culture. Therefore, transplantation of clones consisting of ≤20 cells detected after 1-week incubation significantly raised the efficiency of generating mice with high-level multilineage engraftment 17, 18. Two months to 1 year after cell transplantation, nucleated peripheral blood cells from these mice were analyzed for hematopoietic engraftment and only the mice revealing high-level, multilineage engraftment by donor EGFP+ cells were selected for analysis of tissue reconstitution. In order to exclude the possibility that the observed results from clonally engrafted mice were artifacts of short-term cell culture, we also carried out transplantation of 100 uncultured Lin−, Sca-1+, c-kit+, CD34− cells and made similar observations to those seen in clonally engrafted mice. In most of the studies, we excluded the possibility of cell fusions by carrying out male-to-male or female-to-male transplantation and analyzing the number of Y-chromosomes in the EGFP+ cells. In the first series of investigations, we discovered that several types of fibroblasts/myofibroblasts are derived from HSCs via nonfusion mechanisms as summarized in a recent review [23].

HSC origin of fibroblasts/myofibroblasts 

Fibroblasts and myofibroblasts are important in the steady-state physiology of many organs and tissues. In general, they confer the structural integrity of the tissues and support the proliferation and differentiation of other cell classes, such as epithelial cells. Tissue fibroblasts play a key role in growth factor secretion, matrix deposition, and matrix degradation and are, therefore, also important in many pathological processes. For example, fibroblasts are critical to the inflammatory response and its control at the time of tissue injury. Fibroblasts participate in wound healing by producing extracellular matrix proteins, responding to and synthesizing cytokines, chemokines, and other mediators of inflammation (for review see 24, 25). In addition, fibroblasts can be activated to become myofibroblasts, which, armed with myosin and α-smooth muscle actin, exert contractile force to reduce the size of the wound. Uncontrolled proliferation and/or activation of these cells, however, results in tissue fibrosis, often with devastating consequences. In addition to the ubiquitous type of fibroblasts, there are a number of myofibroblasts with defined tissue-specific functions. For example, contractile myofibroblasts, such as glomerular mesangial cells in the kidney, hepatic stellate cells, and pericytes, function as regulators of blood flow. An example of even more specialized myofibroblasts are the interstitial cells of Cajal in the intestines that control intestinal motility. Readers are referred to several reviews 26, 27 for a list of myofibroblasts and their functions.

The first type of myofibroblasts we detected in the mice engrafted with single HSC was glomerular mesangial cells of the kidney [17]. Although the location in the kidney and morphology of the EGFP+ cells suggested that the cells were mesangial cells, confirmation of their identity was provided by the ability of the EGFP+ cells to contract upon exposure to angiotensin II. Next, we discovered that brain microglial cells and perivascular cells are of HSC origin and observed that induction of stroke by ligation of middle cerebral artery enhances recruitment of the EGFP+ microglial cells to the injury site [18]. Morphological and immunohistochemical properties of the perivascular cells were consistent with the cells being pericytes rather than endothelial cells. We then identified HSC-derived fibroblasts (cancer-associated fibroblasts) associated with transplantable murine melanoma and Lewis lung carcinoma [28]. The EGFP+ cells in the tumors were identified to be fibroblasts by their distinct morphology and expression of procolagen1α1 messenger RNA (mRNA). A subpopulation of the EGFP+ fibroblastic cells expressed α−smooth muscle actin, indicating that they were myofibroblasts. Again, prevalent in the specimens were the EGFP+ pericyte-like perivascular cells present on the tumor vascular structure and admixed with tumor cells. Similar to our findings in the brain, simultaneous staining for CD31 expression clearly established that the EGFP+ cells were indeed perivascular cells and not endothelial cells. Our failure to observe generation of endothelial cells by bone marrow cells was in agreement with two earlier independent reports 29, 30. In other studies, we found that fibrocytes and other unidentified mesenchymal-type cells in the spiral ligament of the inner ear are of HSC origin [31]. Inner ear fibrocytes are known to play critical roles in the homeostasis of inner ear ion and fluid channels and are important for the health of the hair cells. Inner ear fibrocytes are classified into five types based on location, morphology, and histochemical properties. EGFP+ cells were seen among all five types of fibrocytes [31]. We also discovered that fibroblasts/myofibroblasts in the adult heart valves are derived from HSCs [32]. More recently, investigators in two other laboratories have described HSC origin of fibroblasts/myofibroblasts in other tissues using single HSC transplantation. Their studies showed that fibroblasts/myofibroblasts seen at the site of myocardial infarction [33] and liver stellate cells, a type of specialized myofibroblasts, are derived from HSCs [34].

We also documented the HSC origin of fibroblasts in culture by growing fibroblasts from EGFP+ bone marrow cells of clonally engrafted mice [35]. EGFP+ bone marrow cells from these mice, incubated in fibronectin-coated tissue culture dishes or flasks in the presence of 10% fetal bovine serum and 10% mouse serum [36] generated adherent cells exhibiting the morphology of fibroblasts described three decades ago by Friedenstein and his associates 9, 37. The EGFP+ cells had prominent clear nuclei and spindle-shaped or pleomorphic cytoplasm. They also expressed mRNAs for procollagen 1αI, fibronectin, vimentin, and discoidin domain receptor type 2 (DDR2). CFU-F, precursors of fibroblasts 9, 11, were also documented in the bone marrow of the clonally engrafted mice [35]. Earlier, Penn et al. [38] described that a population of cells in the murine CFU-F−derived colonies was positive for Mac-1 and F4/80. Together, these findings in cell culture were consistent with the results of transplantation studies and strongly suggested that most, if not all, fibroblasts/myofibroblasts are derived from HSCs.

HSC origin of adipocytes 

Adipose tissues, scattered throughout many organs, play a critical role in energy balance. When excess calories are available, the adipose tissues grow larger via increases in both the size and number of adipocytes. Because mature adipocytes cannot divide, hyperplasia is achieved by recruitment of preadipocytes. It has been generally believed that preadipocytes are derived from MSCs in the bone marrow 2, 3. Regarding the pathway of adipocytic differentiation from MSCs, much evidence has suggested that fibroblasts or “fibroblastic” cells are the intermediate between MSCs and preadipocytes (see reviews 39, 40). Two-way conversion between human adipocytes and fibroblasts has been documented in culture [41]. These findings strongly suggested that adipocytes are closely related to, if not derived from, fibroblasts. After demonstration of an HSC origin of a number of fibroblasts/myofibroblasts, we next tested the hypothesis that adipocytes are also derived from HSCs using clonal transplantation and primary culture [42]. Adipose tissues harvested from clonally engrafted mice showed EGFP+ adipocytes that stained positive for leptin, perilipin, and fatty acid−binding protein 4. A diet containing rosiglitazone, a peroxisome proliferator−activated receptor-γ agonist, increased the number of the EGFP+ adipocytes. When EGFP+ bone marrow cells from clonally engrafted mice were cultured under adipogenic conditions, all of the cultured cells stained positive with Oil Red O and Sudan Black B and exhibited the presence of abundant mRNA for multiple adipocyte genes. Finally, clonal culture and sorting based on Mac-1 expression of hematopoietic progenitors suggested that adipocytes are derived from HSCs via progenitors for monocytes/macrophages. Pertinent to our findings are two conflicting results from bone marrow transplantation studies. First, Crossno et al. [43] reported that transplanted bone marrow cells generate new adipocytes and that both a high-fat diet and administration of rosiglitazone induce hyperplasia of EGFP+ adipocytes. A year later, Koh et al. [44] refuted this observation and concluded that what appeared to be adipocytes in the adipose tissues were macrophages. Our observations, based on single-HSC transplantation [42], supported Crossno's conclusion and further extended their studies to identify the bone marrow progenitor of the adipocytes, the HSC.

HSC origin of osteochondrocytes 

There are a number of studies suggesting an inverse close relationship between adipogenesis and osteogenesis 45, 46. Clinically, increased adipose mass in the bone marrow is associated with primary osteoporosis [47]. Studies of cell lines in culture also demonstrated reciprocal regulation of adipogenesis and osteogenesis 48, 49. Although molecular mechanisms regulating the differentiation of adipocytes and osteocytes are being elucidated 45, 46, one important regulator of this relationship appears to be macrophage colony-stimulating factor (M-CSF). Overexpression of M-CSF increases adipose mass [50] and deficiency of M-CSF [51] or its receptor [52] is associated with osteopetrosis. Postnatal administration of neutralizing anti−M-CSF antibody induces osteopetrosis and decreases adipocyte size [53]. This inverse but close relation between adipogenesis and osteogenesis indicated the possibility that osteogenesis is also a function of HSCs. More direct evidence for HSC origin of bone cells resulted from bone marrow transplantation studies in mice. It was shown that transplantation of 3000 side population cells that are highly enriched for HSCs generated osteoblasts in vivo [54]. Dominici et al. [55] transplanted marrow cells that had been transduced with GFP-expressing retroviral vector and observed a common retroviral integration site in clonogenic hematopoietic cells and osteoprogenitors from each of the recipient mice. In this issue of Experimental Hematology, we report our transplantation and microcomputerized tomography studies using a mouse model of osteogenesis imperfecta (OI). Transplantation of 50 Lin− Sca-1+ c-kit+ CD34− EGFP+ bone marrow cells to an OI mouse ameliorated the bone pathologies, while control OI mice showed steady exacerbations of the disease processes during the same observation periods [56]. Together, these observations strongly suggest that bone cells are derived from HSCs.

Relation with MSCs 

The premise that fibroblasts/myofibroblasts, adipocytes, chondrocytes, and bone cells are derived from HSCs directly challenges the long-held belief that these lineages originate in MSCs. MSCs, however, are poorly defined and their physiology remains virtually unknown, despite significant current academic and commercial interest in their potential therapeutic applications. An overwhelming majority of the studies of MSCs were carried out in culture and most in vivo experiments are site-directed transplantation experiments. Experiments based on systemic transplantation showed primarily cell lodging in the lungs [57] and low-level engraftment at the site of injuries [13]. In an excellent review in 2004, Javazon et al. [58] described “Characteristics of MSCs differ among laboratories and species, and there is no specific marker or combination of markers that identify MSCs either in vivo or in vitro. In addition, there are no quantitative assays to assess the presence of MSCs in any given population. Therefore, MSCs are currently defined by a combination of physical, morphologic, phenotypic, and functional properties, many of which are clearly non-physiologic.” Some of these points were reiterated in a more recent review [4]. Reflecting this lack of evidence for the stem cell nature of MSCs, a position article [59] published by the International Society for Cellular Therapy recommended that the term MSCs represent multipotent mesenchymal stromal cells. We have grown fibroblasts from the EGFP+ bone marrow cells of a mouse engrafted by a single HSC [35] and induced adipocytic differentiation in all EGFP+ fibroblasts [42]. Investigators in other laboratories have noticed biochemical and functional similarities between fibroblasts and MSCs 60, 61. Together, these findings strongly suggest that MSCs are very similar, if not identical, to the fibroblasts that are derived from HSCs in culture. If so, it would explain the scarcity of evidence for stem cell nature in MSCs and the fact that site-directed transplantation of MSCs yields better tissue reconstitution than systemic infusion.

Differentiation of HSCs to mesenchymal lineages 

The mechanisms of differentiation of HSCs to individual mesenchymal lineages remain to be clarified. First, it needs to be addressed whether there are common mesenchymal precursors through which individual mesenchymal (e.g., fibroblast, adipocye, ostocyte, and chondrocyte) lineages differentiate or the individual lineages are derived directly from HSCs independently from each other. For both models, the next question to be elucidated is whether the commitment process is intrinsic to the cells or controlled by external factors. If intrinsic, differentiation may be a fixed or stochastic (random) process or a combination of the two mechanisms. Regardless of the mechanisms of differentiation, there are studies to suggest a close relationship between the monocyte/macrophage lineage and fibroblasts and/or MSCs. It has been shown that a population of cells in the murine CFU-F−derived colonies was positive for Mac-1 and F4/80 [38]. Human bone marrow mesenchymal progenitor cells that are capable of adipogenic, osteogenic, and chondrogenic differentiation in culture were shown to express CD13 [62], a marker associated with granulocytes and monocytes. Human peripheral blood cells that express CD14, a surface protein preferentially expressed on monocytes and macrophages, were shown to generate multiple mesenchymal lineages, including osteoblasts, adipocytes, chondrocytes, and myocytes in culture [63]. In our laboratory, we examined the correlation between EGFP+ glomerular mesangial cells and the levels of EGFP+ B cells, T cells, or Mac-1/Gr-1+ cells in the clonally engrafted mice. EGFP+ mesangial cells were only detected in mice expressing high levels of Mac-1/Gr-1+ cells in the blood [64]. Finally, the most direct demonstration of the close relationship is our single progenitor cell culture and subsequent cohort analysis for hematopoietic and adipogenic differentiation [42]. Here, bone marrow cells that are highly enriched for hematopoietic progenitors were deposited individually into round-bottomed 96-well culture plates and incubated for 1 week in the presence of a combination of hematopoietic cytokines permissive of all lineage expression. Resulting clones were then individually divided into two aliquots and cultured under two different conditions. One aliquot was cultured in 12-well nontissue culture plates (Becton-Dickinson Biosciences, Franklin Lakes, NJ, USA) containing the same combination of hematopoietic cytokines and the other in tissue culture plates for fibroblast and adipogenic differentiation and growth. The aliquots cultured for hematopoietic growth for 10 to 12 days were individually deposited onto slides and stained with May-Grünwald Giemsa. Correlative analyses of the cohorts for hematopoietic lineage expression and adipocytic differentiation revealed a strong connection between monocyte/macrophage lineage and adipogenesis [42].

The transition of hematopoietic cells to fibroblasts/adipocytes is a gradual process. In a time-course cell culture study of fibroblast differentiation from bone marrow mononuclear cells, we observed gradual transition of CD45+ collagen I− DDR2− bone marrow cells to CD45− collagen I+ DDR2+ fibroblasts in 3 weeks [35]. During this transition, cells at the intermediate stages expressed both CD45 and collagen I. Earlier, Bucala et al. [65] had documented the presence, in circulating blood, of cells intermediate between fibroblasts and hematopoietic cells and named them fibrocytes by drawing analogy from other circulating hematopoietic cells such as erythrocytes and granulocytes. Circulating fibrocytes are CD34+ CD45+ CD11b+ and produce collagen and are considered to be precursors for tissue fibroblasts [65]. In a more recent study of a mouse model of asthma [66], PKH-26−labeled fibrocytes recruited to the airway by repeated exposure to an allergen showed gradual decline of CD34 expression and increase of the intensity of collagen I and α−smooth muscle actin. This in vivo observation is in agreement with our time-course study of transition of hematopoietic cells to fibroblasts in cell culture [35]. Also important to consider is that the time required for the transition of EGFP-labeled HSCs to mesenchymal cells in vivo depends on the physiological cellular turnover and pool size of resident progenitors of the respective tissues. Glomerular mesangial cells are generated from HSCs soon after HSC transplantation because the turnover of murine mesangial cells is about 1% per day [67]. In contrast, despite high levels of hematopoietic engraftment and analysis at 7 to 10 months after transplantation of HSCs, only a few new HSC-derived adipocytes were observable and stimulation of neoadipogenesis was required for robust generation of HSC-derived adipocyes [42]. This is due to known slow physiological turnover of adipocytes, which was estimated to be 10% per year in man, based on analysis of the integration of 14C derived from nuclear bomb tests in genomic DNA [68]. Another example of slow turnover is osteocytes. It is estimated that a complete skeletal remodeling requires 10 years in adults [69]. We have found that, for robust engraftment of donor origin osteocytes, bone fracture or use of genetic model of bone disease, such as OI, is required as presented elsewhere in this issue [56].

HSC origin of human fibroblasts 

Because our series of studies based on single-HSC transplantation provided unequivocal evidence that mouse fibroblasts/myofibroblasts are derived from HSCs [23], we next attempted to address HSC origin of human fibroblasts [70]. We first established a method for culturing fibroblasts from human peripheral blood mononuclear cells using the culture method described by Bucala et al. [65]. We then cultured peripheral blood cells from three female subjects who showed near complete hematopoietic reconstitution by transplantation of G-CSF−mobilized male peripheral blood cells and examined the gender of the resulting fibroblasts using fluorescent in situ hybridization for X and Y chromosomes. All fibroblasts derived from the circulating precursors showed the presence of Y chromosome, indicating donor BM origin of human fibroblasts. Because the mobilized peripheral blood cells may contain MSCs, this result did not address the question of whether the male fibroblasts are derived from HSCs or MSCs. To document the HSC origin of human fibroblasts, we next examined fibroblasts cultured from two patients with untreated chronic myelogenous leukemia. The fibroblasts cultured from the peripheral blood cells revealed the presence of BCR-ABL translocation. This observation provided strong evidence for the HSC origin of human fibroblasts because chronic myelogenous leukemia is a clonal disorder of the HSC. It has been reported at the annual meeting of the American Society of Hematology 2006, that transplantation of CD34+ CD38− peripheral blood cells from patients with idiopathic myelofibrosis into severely immune-incompetent mice reconstitutes bone marrow with human myelomonocytic cells associated with bone marrow fibrosis [71]. Because idiopathic myelofibrosis is also a clonal disorder of the HSC, this observation is consistent with the concept of an HSC origin of human fibroblasts. Together, these results suggest that fibrosis seen in patients with all myeloproliferative syndromes may be a part of the clonal process rather than physiological polyclonal response. This premise then raises the next interesting question about the clonality of the connective tissues of the patients with myeloproliferative disorders and their proliferative ability and functions. Although connective tissues with very slow turnover may mostly be polyclonal, some myofibroblasts with brisk turnover, such as glomerular mesangial cells, of the patients with myeloproliferative disorders may be a part of the clonal process.

There have been a number of studies from other laboratories indicating proximity of human hematopoietic cells with fibroblasts/myofibroblasts and/or other mesenchymal cells. When CD34+ human peripheral blood or cord blood cells were transplanted into nonobese diabetic/severe combined immune deficiency mice, their bone marrow showed human cell engraftment, contained 5B5+ human fibroblasts and expressed human proline hydroxylase mRNA [72], an enzyme required for collagen synthesis by fibroblasts. As described here, human mesenchymal progenitor cells have been identified from both CD13-expressing bone marrow cells [62] and CD14-positive peripheral blood cells [63]. CD13 and CD14 are markers preferentially expressed on cells of the myeloid lineage, suggesting a common precursor for the mesenchymal progenitor and granulocytes, monocytes, and macrophages. In vitro studies have also demonstrated that human CD14+ peripheral blood cells can give rise to fibrocytes when cocultured with T cells [36]. These studies and our observations in cell culture are in agreement and are strongly support the concept generated by studies of mouse models that adult human connective tissues are derived from HSCs.

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Conclusion 

This concept of HSC origin of connective tissues contradicts the current dogma that only MSCs give rise to these types of mesenchymal cells. The new paradigm suggests that transplantation of HSCs, rather than MSCs, is the choice of therapy for such genetic deficiencies of connective tissues as OI, epidermolysis bullosa, and leptin deficiency. There are indications that bone marrow transplantation is beneficial to patients with severe OI, a collagen-I deficiency 73, 74, 75. Epidermolysis bullosa is caused by collagen-VII deficiency and the pathology of a genetic mouse model was ameliorated by transplantation of unmanipulated bone marrow cells from wild-type mice, but not by nonhematopoietic bone marrow cells [76]. The genetic mouse model of leptin deficiency, B6.V-Lepob (ob/ob), develops extreme obesity caused by both hyperphagia and reduced metabolic rate. Because leptin is secreted by the adipocyte, transplantation of normal white adipose tissue that secretes leptin was shown to correct the various defects in this model [77]. We believe that early intervention with HSC transplantation will prevent development of the severe obesity. Although Alport syndrome may not be considered a disease of connective tissues, it is a genetic disease of fibroblasts/myofibroblasts and, therefore, will be treatable by HSC transplantation. Alport syndrome is caused by a deficiency in collagen IV and is characterized by renal insufficiency and deafness caused by pathological changes in the basement membranes of glomeruli and inner ear. Kidney transplantation is known to be of only limited value. As discussed here, we have shown that glomerular mesangial cells [17] and inner ear fibrocytes [31] are derived from HSCs. Glomerular mesangial cells produce collagen IV and collagen IV is abundant in cochlea. Recently, transplantation of crude BM cells was shown to be effective in reversing the renal pathologies in the autosomal Alport mice 78, 79. As presented elsewhere in this issue [56], transplantation to an OI mouse of 50 bone marrow cells that are highly enriched for HSCs ameliorated the bone pathologies, while control OI mice showed steady exacerbations of the disease processes. These observations depict only a few examples of many genetic disorders of connective tissues in which HSC transplantation is indicated. More importantly, the new paradigm heralds radically new approaches in the development of therapies for many acquired diseases and injuries of connective tissues. Although this review is limited to the HSC origin of connective tissues, it is possible that the scope of HSC plasticity extends far beyond the connective tissues. Continuous and cautious exploration of HSC plasticity based on careful studies of individual cell and tissue types may uncover much more differentiation potentials of HSCs and lead to new therapies based on HSCs for many more human ailments, both genetic and acquired.

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Conflict of Interest Disclosure 

No financial interest/relationships with financial interest relating to the topic of this article have been declared.

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PII: S0301-472X(10)00148-7

doi:10.1016/j.exphem.2010.04.005

Experimental Hematology
Volume 38, Issue 7 , Pages 540-547, July 2010

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