The Wnt antagonist Dickkopf-1 mobilizes vasculogenic progenitor cells via activation of the bone marrow endosteal stem cell niche

Therapeutic mobilization of vasculogenic progenitor cells is a novel strategy to enhance neovascularization for tissue repair. Prototypical mobilizing agents such as granulocyte colony-stimulating factor mobilize vasculogenic progenitor cells from the bone marrow concomitantly with inflammatory cells. In the bone marrow, mobilization is regulated in the stem cell niche, in which endosteal cells such as osteoblasts and osteoclasts play a key role. Because Wnt signaling regulates endosteal cells, we examined whether the Wnt signaling antagonist Dickkopf (Dkk)-1 is involved in the mobilization of vasculogenic progenitor cells. Using TOP-GAL transgenic mice to determine activation of beta-catenin, we demonstrate that Dkk-1 regulates endosteal cells in the bone marrow stem cell niche and subsequently mobilizes vasculogenic and hematopoietic progenitors cells without concomitant mobilization of inflammatory neutrophils. The mobilization of vasculogenic progenitors required the presence of functionally active osteoclasts, as demonstrated in PTPepsilon-deficient mice with defective osteoclast function. Mechanistically, Dkk-1 induced the osteoclast differentiation factor RANKL, which subsequently stimulated the release of the major bone-resorbing protease cathepsin K. Eventually, the Dkk-1-induced mobilization of bone marrow-derived vasculogenic progenitors enhanced neovascularization in Matrigel plugs. Thus, these data show that Dkk-1 is a mobilizer of vasculogenic progenitors but not of inflammatory cells, which could be of great clinical importance to enhance regenerative cell therapy.     

供体骨髓来源干细胞在骨髓嵌合小鼠肠上皮中定植和分化研究

目的 观察骨髓移植后供体骨髓来源干细胞（bone marrow derived stem cells，BMDSCs）是否能够在受体肠上皮中定植并分化为肠上皮细胞。方法 使用密度为1．073g／mL的Percoll分离GFP转基因小鼠的骨髓有核细胞。利用^60Co放射源对C57BL／6小鼠进行10Gy致死剂量照射，照射后4h经尾静脉移植绿色荧光蛋白（green fluorescence protein，GFP）转基因小鼠骨髓细胞建立嵌合小鼠模型，移植后1月和6月取小肠标本石蜡包埋。使用GFP抗体鉴定GFP阳性供体细胞在小肠上皮的表达情况，并使用GFP抗体和细胞角蛋白（PCK）抗体进行双标以判断BMDSCs是否分化上皮细胞，同时结合绒毛蛋白（Villin）和GFP的免疫荧光双标及连续切片染色肠道特异转录因子CDX2和GFP来分析骨髓来源细胞是否分化为肠道功能性上皮细胞。结果供体骨髓来源细胞在嵌合鼠的骨髓和小肠上皮层中均有定植，且小肠上皮中GFP阳性细胞能够同时表达PCK、Villin和CDX2。结论 供体骨髓来源干细胞可以在骨髓嵌合小鼠的肠上皮定植，并分化为具有正常功能的小肠上皮细胞。     

VEGF<Subscript>165</Subscript> expression in the tumor microenvironment influences the differentiation of bone marrow-derived pericytes that contribute to the Ewing’s sarcoma vasculature

We previously demonstrated that bone marrow (BM) cells migrate to Ewing's tumors and differentiate into endothelial cells within the tumor vasculature. Recent evidence suggests that the roles of BM cells in tumors are more diverse. We investigated whether non-endothelial cell types critical for tumor vessel development are also derived from migrated BM cells. We utilized BM transplantation with GFP(+) transgenic mice as BM donors and nude mice as recipients to track the fate of migrated BM cells. After engraftment, we injected recipient mice either subcutaneously or intramuscularly with Ewing's sarcoma cells. We labeled functional tumor vessels using intravascular perfusion staining with tomato lectin. We assessed BM cell recruitment/differentiation within the tumor microenvironment using immunohistochemistry. Ewing's tumors contained BM-derived cells that had differentiated into endothelial cells lining perfused tumor vessels. A substantial fraction of recruited BM cells also resided in the vessel vicinity and expressed desmin and PDGFR-beta, indicating smooth muscle cell differentiation. In order to further characterize the role of stem/progenitor cells in Ewing's sarcoma, we sorted Tie2(-) BM cells from Tie2-GFP transgenic mice and then injected them intravenously into Ewing's tumor-bearing mice. Tie2(-) BM progenitors migrated to Ewing's tumors and differentiated into Tie2(+) cells occupying a perivascular residence and expressing alpha-smooth muscle actin, desmin and PDGFR-beta, as well as VEGFR-2. We did not observe differentiation of Tie2(-) cells into Tie2(+) perivascular cells in VEGF(165)-inhibited TC/siVEGF(7-1) tumors. The differentiation of Tie2(-) BM cells into Tie2(+) cells in parental but not VEGF(165)-inhibited tumors indicates that the tumor microenvironment may influence the differentiation pathway.     

Hematopoietic stem cells and endothelial cell precursors express Tie-2, CD31 and CD45

Early theories of tumor angiogenesis suggested that preexisting vessels surrounding the tumor were the principal source of the tumor vasculature but recent evidence suggests that endothelial progenitor cells (EPC) migrate from the marrow play an important role in developing the tumor blood supply. In a mouse model, in which the vascularization of a transplantable tumor was studied after bone marrow (BM) transplantation, we show that cells that express Tie-2, Sca-1, CD31 and CD45 function as both BM EPC and primitive hematopoietic stem cells. BM cells from transgenic mice expressing green fluorescent protein (GFP) under the control of the endothelial lineage-specific Tie-2 promoter (Tie-2 /GFP) were used to reconstitute irradiated (12 Gy) wild-type mice. Five donor BM cell populations were studied: (1) whole BM; (2) Sca-1-enriched BMC; (3) GFP/Tie-2+, Sca-1+ BMC; (4) GFP/Tie-2-, Sca-1+ BMC and (5) Sca-1-depleted BMC. After 4 weeks, the mice were injected with Tg.AC tumor cells. Three weeks later, sections from the tumors were stained for CD31 and examined for Tie-2-driven GFP expression. BM-derived endothelial cells were found only in mice transplanted with bone marrow containing populations of Tie-2+, Sca-1+ cells. As few as 3500 of these cells were sufficient to radioprotect lethally irradiated mice. Thus, we conclude that a rare subset of BMC (approximately 4 x 10(-3)%) with the putative properties of hemangioblasts have an active Tie-2 promoter. Selection of Tie-2+Sca-1+ BMC enriches for marrow-derived EPCs that participate in tumor angiogenesis and cells that can provide hematopoietic reconstitution of marrow-ablated mice.     

Rescue of ATPa3-deficient murine malignant osteopetrosis by hematopoietic stem cell transplantation in utero

Autosomal recessive osteopetrosis (ARO) is a paradigm for genetic diseases that cause severe, often irreversible, defects before birth. In ARO, osteoclasts cannot remove mineralized cartilage, bone marrow is severely reduced, and bone cannot be remodeled for growth. More than 50% of the patients show defects in the osteoclastic vacuolar-proton-pump subunit, ATP6a3. We treated ATP6a3-deficient mice by in utero heterologous hematopoietic stem cell (HSC) transplant from outbred GFP transgenic mice. Dramatic phenotype rescue by GFP osteoclasts was obtained with engraftment, which was observed in most cases. Engraftment survived for variable periods. Recipients were not immunosuppressed, and graft-versus-host disease was not observed in all pups born after in utero treatment. Thus, differentiation of unmatched HSC transplanted in utero is sufficient to prevent fatal defects in ARO and may prevent complications of ARO unresponsive to conventional bone marrow transplantation. The presence of defective cells is not a barrier to the rescue of the phenotype by donor HSC.     

Hematopoiesis is severely altered in mice with an induced osteoblast deficiency

We previously reported a transgenic mouse model expressing herpesvirus thymidine kinase (TK) gene under the control of a 2.3-kilobase fragment of the rat collagen alpha1 type I promoter (Col2.3 Delta TK). This construct confers lineage-specific expression in developing osteoblasts, allowing the conditional ablation of osteoblast lineage after treatment with ganciclovir (GCV). After GCV treatment these mice have profound alterations on bone formation leading to a progressive bone loss. In addition, treated animals also lose bone marrow cellularity. In this report we characterized hematopoietic parameters in GCV-treated Col2.3 Delta TK mice, and we show that after treatment transgenic animals lose lymphoid, erythroid, and myeloid progenitors in the bone marrow, followed by decreases in the number of hematopoietic stem cells (HSCs). Together with the decrease in bone marrow hematopoiesis, active extramedullary hematopoiesis was observed in the spleen and liver, as measured by an increase in peripheral HSCs and active primary in vitro hematopoiesis. After withdrawal of GCV, osteoblasts reappeared in the bone compartment together with a recovery of medullary and decrease in extramedullary hematopoiesis. These observations directly demonstrate the role of osteoblasts in hematopoiesis and provide a model to study the interactions between the mesenchymal and hematopoietic compartments in the marrow.     

Host-derived circulating cells do not significantly contribute to cardiac regeneration in heterotopic rat heart transplants

OBJECTIVES: The aim of this study was to investigate the contribution of host-derived circulating cells to cardiac repair after tissue damage using the model of heterotopic heart transplantation between transgenic recipient rats expressing green fluorescent protein (GFP) and wild-type donors. METHODS: Unlabeled donor rat hearts, some of which underwent prolonged cold ischemia pretreatment, were transplanted into the abdominal cavity of GFP+ transgenic recipient rats and were analyzed 15 and 90 days after surgery. An additional experimental group underwent heart transplantation following administration of granulocyte-colony stimulatory factor (G-CSF) to mobilize bone marrow cells. RESULTS: Most transplants contained GFP+ mature cardiomyocytes. However, systematic counting in the transplants showed that the proportion of GFP+ cardiomyocytes was only 0.0005% to 0.008% of all cardiomyocytes. These relative proportions did not change after G-CSF treatment, despite evidence for sustained marrow cell mobilization. Confocal image analysis showed that the majority of GFP+ cardiomyocytes contained a high number of nuclei, suggesting that these cells may derive from fusion events. Very rarely, small GFP+ undifferentiated cells, expressing GATA-4, were also identified. Occasionally, GFP+ endothelial cells, but not smooth muscle cells, were detected in blood vessels of some transplants. CONCLUSIONS: Our results demonstrate that cardiomyocytes expressing a host transgenic marker are detectable in heterotopic heart transplants; however, they do not significantly contribute to repopulation of the damaged myocardium.     

Special proliferative sites are not needed for seeding and proliferation of transfused bone marrow cells in normal syngeneic mice.

The widely held view that transfused bone marrow cells will not proliferate in normal mice, not exposed to irradiation or other forms of bone marrow ablation, was reinvestigated. Forty million bone marrow cells from male donors were given to female recipients on each of 5 consecutive days, 5 to 10 times the number customarily used in the past. When the recipients were examined 2-13 weeks after the last transfusion, donor cells were found to average 16-25% of total marrow cells. Similar percentages of donor cells were found when variants of the enzyme phosphoglycerate kinase determined electrophoretically were used for identification of donor and recipient cells. Evidence is presented that the proportion of donor cells is compatible with a linear dependence on the number of cells transfused over the range tested--i.e., 20-200 million bone marrow cells injected intravenously. Special proliferative sites thus do not appear to be required.     

骨髓干细胞移植促进ctnt^R141W转基因小鼠心肌组织再生

目的观察骨髓干细胞移植对ctnt^R141W转基因小鼠心脏组织结构的重建和病变心脏的泵血功能的影响。方法放射线9Gy照射ctnt^R141W转基因小鼠,将绿色荧光蛋白转基因小鼠的骨髓细胞尾静脉注射入ctnt^R141W转基因小鼠体内,替换ctnt^R141W转基因小鼠自身的骨髓细胞。流式细胞仪分析移植后的ctnt^R141W转基因小鼠绿色荧光骨髓干细胞所占的百分比。用免疫荧光双染方法探测移植后的ctnt^R141W转基因小鼠心肌组织中绿色荧光心肌细胞所占的比例,M型超声检测ctnt^R141W转基因小鼠心脏功能变化。结果相对于野生型对照小鼠ctnt^R141W转基因小鼠心肌组织中绿色荧光心肌细胞显著增加,左心室内径缩小,室壁变厚,收缩期末期和舒张末期左室容积缩小,心脏泵血功能增强。结论移植后的骨髓干细胞参与ctnt^R141W转基因小鼠心肌组织的损伤后重建,有助于改善病变心脏的泵血功能。研究提示扩张型心肌病心肌组织损伤重建增加,移植后的骨髓干细胞在受损心肌组织再生过程中起重要作用。     

Primary endothelial cells isolated from the yolk sac and para-aortic splanchnopleura support the expansion of adult marrow stem cells in vitro

The embryonic origin and development of hematopoietic and endothelial cells is highly interdependent. We hypothesized that primary endothelial cells from murine yolk sac and para-aortic splanchnopleura (P-Sp) may possess the capacity to expand hematopoietic stem cells (HSCs) and progenitor cells ex vivo. Using Tie2-GFP transgenic mice in combination with fluorochrome-conjugated monoclonal antibodies to vascular endothelial growth factor receptor-2 (Flk1) and CD41, we have successfully isolated pure populations of primary endothelial cells from 9.5-days after coitus (dpc) yolk sac and P-Sp. Adult murine bone marrow Sca-1+c-Kit+lin- cells were cocultured with yolk sac or P-Sp Tie2-GFP+Flk-1+CD41- endothelial cell monolayers for 7 days and the total number of nonadherent cells increased 47- and 295-fold, respectively, and hematopoietic progenitor counts increased 9.4- and 11.4-fold, respectively. Both the yolk sac and P-Sp endothelial cell cocultures facilitated long-term (> 6 months) HSC competitive repopulating ability (2.8- to 9.8-fold increases, respectively). These data suggest that 9.5-dpc yolk sac- and P-Sp-derived primary Tie2-GFP+Flk-1+CD41- endothelial cells possess the capacity to expand adult bone marrow hematopoietic progenitor cell and HSC repopulating ability ex vivo.     

Origin of hemopoietic stromal progenitor cells in chimeras

Intravenously injected bone marrow cells do not participate in the regeneration of hemopoietic stromal progenitors in irradiated mice, nor in the curetted parts of the recipient's marrow. The hemopoietic stromal progenitors in allogeneic chimeras are of recipient origin. The adherent cell layer (ACL) of long-term cultures of allogeneic chimera bone marrow contains only recipient hemopoietic stromal progenitors. However, in ectopic hemopoietic foci produced by marrow implantation under the renal capsule and repopulated by the recipient hemopoietic cells after irradiation and reconstitution by syngeneic hemopoietic cells, the stromal progenitors were of implant donor origin, as were stromal progenitors of the ACL in long-term cultures of hemopoietic cells from ectopic foci. Our results confirm that the stromal and hemopoietic progenitors differ in origin and that hemopoietic stromal progenitors are not transplantable by the intravenous route in mice.     

骨髓间充质干细胞在小鼠肝脏局部分化为肝细胞的实验研究

目的 探讨小鼠骨髓间充质干细胞（bone marrow mesenchymal stem cells，BMSCs）在肝内分化为肝细胞的可行性。方法腹腔注射丙烯醇致雌性C57BL／6小鼠全肝损伤后局部接受雄性同品系绿色荧光蛋白（green fluorescence protein，GFP）转基因小鼠骨髓间充质干细胞移植。移植后3周取骨髓嵌合小鼠肝脏做冰冻切片，荧光显微镜下观察GFP阳性细胞在肝内分布，石蜡切片行GFP抗体和白蛋白（albumin，Alb）抗体免疫组化双标法检测GFP阳性细胞在受体小鼠肝内的分化情况。结果移植后3周小鼠肝内可见GFP阳性细胞，并可见GFP和Alb双阳性细胞。结论异体骨髓间充质干细胞可在小鼠肝内分化为表达Alb的肝细胞，为肝组织的再生和修复提供了新思路，显示了其在肝病治疗中的应用潜力。     

Osteonectin-null mutation compromises osteoblast formation,maturation, and survival

Osteonectin, also known as SPARC (secreted protein acidic and rich in cysteine) or BM-40, is one of the most abundant noncollagenous proteins in bone. Analysis of osteonectin-null mice revealed that osteonectin is necessary for the maintenance of bone mass and normal remodeling, as osteonectin-null mice have decreased osteoblast number and bone formation rate. Cultures of bone marrow stromal cells and osteoblasts from control and osteonectin-null mice were used to determine the cellular basis for the mutant phenotype. We found that marrow stroma from osteonectin-null mice contains fewer osteoblastic precursors than that of control mice, and the osteonectin-null mutation did not affect the proliferation rate of stromal cells or osteoblasts. Whereas osteonectin-null cells could adopt an osteoblastic phenotype, a smaller proportion of these cells expressed markers of a fully differentiated osteoblast. Mutant cells exhibited decreased formation of mineralized nodules, as well as diminished expression of osteocalcin mRNA and response to PTH. Furthermore, osteonectin-null cells showed an increased tendency to form adipocytes, with enhanced expression of the adipocytic markers adipsin and CCAAT/enhancer binding protein delta. Osteonectin-null cells were also more susceptible to environmental stresses. These data indicate that osteonectin is important for osteoblast formation, maturation, and survival.     

Flk1+ cells derived from mouse embryonic stem cells reconstitute hematopoiesis in vivo in SCID mice

OBJECTIVE: Embryonic stem (ES) cells are pluripotent and can differentiate into any cell type, including the hematopoietic lineage. We examined whether hematopoietic progenitor cells derived from ES cells reconstitute hematopoiesis in irradiated SCID mice. MATERIALS AND METHODS: ES cells (E14.1, H2K(b)) were cultured for 4 days in semisolid medium containing methylcellulose. Irradiated SCID mice were used as recipients of hematopoietic progenitor cells. Cell surface antigen expression was analyzed by flow cytometry. The spleens of the recipient mice were studied by hematoxylin and eosin staining and immunohistochemical staining. RESULTS: After cell culture of ES cells in methylcellulose for 4 days, the cells expressing Flk1 (VEGF receptor 2), a tentative marker of hemangioblasts, were increased, whereas cells expressing CD31 (PECAM-1) and E-cadherin (nonmesodermal adhesion molecule) were dramatically reduced. Flk1+ cells expressed c-kit predominantly. Circulating leukocytes and thrombocytes were increased in irradiated SCID (H2K(d)) mice transplanted with ES cell-derived Flk1+ cells compared with vehicle-injected control mice. H2K(b+) and VE-cadherin(+) vascular endothelial cells were prominent in spleens of the recipient mice. Flow cytometric analysis demonstrated that H2K(b+) cells were increased in the bone marrow of recipient mice. In addition, Flk1+ cells accompanying enhanced c-kit expression preferentially repopulated in the bone marrow, and leukopoiesis and thrombopoiesis of the recipient mice were evident. CONCLUSION: The Flk1+ hematopoietic cells derived from ES cells reconstitute hematopoiesis in vivo and may become an alternative donor source for bone marrow transplantation.     

Donor hematopoietic cells from transgenic mice that express GFP are immunogenic in immunocompetent recipients

OBJECTIVE: To study the immunogenicity of hematopoietic cells marked with green fluorescence protein (GFP) while avoiding the potentially confounding effects of viral gene transduction, marked cells from GFP+ transgenic mice were tracked after transplantation into unconditioned immunocompetent recipients. MATERIALS AND METHODS: Marrow was harvested from GFP+ transgenic mice that had been crossed onto a BALB/cByJ background. Unconditioned marrow transplantation involved infusion of sex-matched or sex-mismatched cells into female BALB/cByJ hosts. Engraftment and contribution to circulating nucleated blood cells were compared to recipients of donor cells that were not GFP-marked. Donor cells were detected by flow cytometry (GFP) and fluorescence in situ hybridization (FISH) for Y-chromosome sequences. RESULTS: Donor cells from mice of the same genetic background that did not express GFP were detected for more than four-weeks in unconditioned recipients. In contrast, GFP-marked cells in the blood peaked at one-week, declined to undetectable levels by two-weeks and were not detected in the marrow at sacrifice. In sex-mismatched studies, detection of male GFP+ donor cells by FISH yielded levels similar to those observed by flow cytometry, in contrast to the levels detected for many weeks in mice infused with male cells that did not express GFP. In immunocompetent recipients immunized with irradiated GFP-expressing cells, rechallenge with GFP+ cells resulted in the accelerated loss of donor cells. CONCLUSION: Donor marrow cells from GFP+ transgenic mice were lost after infusion into unconditioned immunocompetent mice and sensitization studies infer an immunologic mechanism. These results are similar to studies of virally transduced cells. Thus, infusion of cells with optimum engraftment potential could not compensate for the loss of donor cells due to immunogenicity.     

An improved mouse Sca-1+ cell-based bone marrow transplantation model for use in gene- and cell-based therapeutic studies

This study sought to develop a murine bone marrow transplantation strategy that would yield consistently high levels of long-term engraftment without significant morbidity and mortality. Hematopoietic stem cell (HSC)-enriched Sca-1+ cells were used for transplantation because of their propensity of homing to bone marrow. Green fluorescent protein (GFP)-expressing transgenic mice were used as donors. Murine Sca-1+ cells were enriched 13-fold from whole bone marrow with immunomagnetic column chromatography. Retroorbital injections yielded highly reproducible and higher levels of engraftment compared with tail vein injections. The combination of W41/W41 recipient mice and sublethal irradiation preconditioning produced long-term engraftment with minimal morbidity and mortality. A 24-hour delay between the sublethal irradiation and transplantation did not affect the efficiency and level of engraftment, but provided flexibility with respect to the timing of transplantation. Based on these findings, a mouse Sca-1+ cell-based strategy, involving the retroorbital injection of Sca-1+ cells into sublethally irradiated, myelosuppressed W41/W41 recipient mice within 24 h after irradiation, was developed. Transplantation of lentiviral vector-transduced wild-type Sca-1+ cells expressing GFP by this strategy led to consistently high levels of long-term engraftment. In summary, this murine Sca-1+ cell-based strategy could be used in studies of HSC-based gene or cell therapies.     

Estradiol increases hematopoietic stem and progenitor cells independent of its actions on bone

Hematopoietic stem and progenitor cells reside in vascular and endosteal niches in the bone marrow. Factors affecting bone remodeling were reported to influence numbers and mobilization of hematopoietic stem cells. We therefore analyzed the effects of estradiol acting anabolic on bone integrity. Here we observe that estradiol increases progenitor cell numbers in the vascular but not in the endosteal compartment independent of its estrogen receptor α-dependent anabolic bone effects. Hematopoietic progenitors capable of reconstituting lethally irradiated mice are increased by enhanced cell cycle entry, leading to a diminished long-term reconstitution potential after serial transplantation. We demonstrate that estradiol action on stromal cells potently favors hematopoietic progenitor/stem cell frequency accompanied by enhanced expression of cell adhesion molecules. Finally, estradiol treatment enhances retention of hematopoietic stem cells in the vascular niche of the bone marrow. We describe for the first time the mechanism of estrogen action on hematopoietic stem and progenitor cells.     

Hepatocyte growth factor mobilizes and recruits hematopoietic progenitor cells into liver through a stem cell factor‐mediated mechanism

Aims: Although bone marrow cells are reported to migrate to the liver under circumstances of severe liver injury, the bone marrow cell type and the mechanisms in this process, remain to be clarified. We examined the involvement of hepatocyte growth factor (HGF) in this process and the cell type of migrated hematopoietic cells by HGF. Methods: The CD34⁺ cells and colony forming cells in the peripheral blood were examined in HGF transgenic, recombinant HGF‐administered, and HGF‐expressing adenovirus‐administered mice. The cell type mobilized by HGF was examined by the percentages of donor cells in the peripheral blood of the recipient mice transplanted with Lin^‐c‐kit⁺Sca‐1⁺CD34⁺ cells and those with Lin^‐c‐kit⁺Sca‐1⁺CD34^‐ cells. Expression of stem cell factor (SCF) was examined after the addition of HGF in MS‐5 stromal cells. The numbers of the cells which were mobilized from bone marrow and recruited into liver by HGF were assessed using green fluorescence fluorescent (GFP)‐chimera mice. Results: Mobilized CD34+ cells and colony forming cells in the peripheral blood were increased by HGF treatment. The cells mobilized by HGF were mostly Lin^‐c‐kit⁺Sca‐1⁺CD34⁺ cells. Recruitment of bone marrow cells into liver was not suppressed in MMP‐9‐/‐ mice. Expression of SCF was induced by HGF in MS‐5 stromal cells. However, expression of CXCR4, SDF‐1, MMP‐9 or VCAM‐1 was not changed. The numbers of GFP‐positive cells in liver 1 month after treatment by HGF was greater than that by G‐CSF. Conclusion: The results of the present study suggest that HGF mobilizes and recruits hematopoietic progenitor cells from bone marrow into the liver through SCF‐mediated mechanism.