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Blood is extremely important for a multicellular organism: it connects all organs and tissues, supplies them with nutrients and oxygen, removes carbon dioxide and metabolic products, maintains homeostasis, and provides protection against infections. That is why studies on blood have always drawn a great deal of attention. In ancient times, it was believed that the soul was in the blood and that it sometimes "sank into the stomach." Initially, the study of blood was limited to morphological methods, to which physiological and cellular research were added in the twentieth century. With their help, researchers established that mature blood cells are formed from a rare population of hematopoietic stem cells (HSCs), which are located in the bone marrow. The development of molecular biology methods and their combination with classical physiological ones allowed a breakthrough in understanding the structure of the hematopoietic system, which changed our understanding not only of hematopoiesis but also about the nature of adult stem cells. This review describes the molecular assays used in experimental hematology, and how their application has gradually been expanding our knowledge of blood formation and continues to provide new information about it.

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Clonal composition of human multipotent mesenchymal stromal cells (MMSCs) labeled with lentiviral vectors carrying genetic barcodes was studied. MMSCs were transduced with a cloned library of self-inactivating lentiviral vectors carrying 667 unique barcodes. At each cell culture passage, 120 cells were plated one cell per well in 96-well plates. The efficiency of cloning and labeling of the clonogenic cells was determined. DNA was extracted from the cell-derived colonies, and the barcodes were identified by Sanger sequencing. Also, DNA was extracted from the total MMSC population at each passage to analyze the diversity and representation of barcodes by deep sequencing using the Illumina platform. It was shown that the portion of MMSCs labeled with the lentiviral vectors remained stable in the passaged cells. Because of the high multiplicity of infection, the labeling procedure could decrease the proliferative potential of MMSCs. Identification of barcodes in individual cell clones confirmed the polyclonal character of the MMSC population. Clonal composition of MMSCs changed significantly with the passages due to the depletion of proliferative potential of most cells. Large clones were found at the first passage; at later passages, many small clones with a limited proliferative potential were detected in the population. The results of deep sequencing confirmed changes in the clonal composition of MMSCs. The polyclonal MMSC population contained only a small number of cells with a high proliferative potential, some of which could be stem cells. MMSCs with a high proliferative potential were detected more often in the earliest passages. In this regard, we would recommend to use MMSCs of early passages for regenerative medicine applications based on cell proliferation.

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In murine bone-marrow stromal microenvironment cells and in human multipotent mesenchymal stromal cells (MMSCs), proinflammatory cytokine interleukin-1 beta (IL-1ß) serves as a growth factor. In murine bone tissue, IL-1ß expression increases in vivo after irradiation. Here, we have presented our evaluation of the effects of exogenous IL-1ß on the expression of NF-kB transcription factors in human MMSCs and stromal layer cells of murine long-term bone marrow cultures (LTBMCs). The cytokine signaling pathway was also activated in murine LTBMC by braking electron radiation in doses of 3-12 Gy. The level of expression of genes that code for IL-1ß, IL-1ß type-I receptor and NF-kB and IKK protein families have been studied at different time points post exposure. In both human and murine stromal cells, exogenous IL-1ß led to an increase in the level of expression of its own gene, while levels of expression of NF-kB and IKK gene families were not substantially changed. Nevertheless, in human cells, a significant correlation between levels of expression of IL-1ß and all NF-kB family genes was detected. It points to a similarity in IL-1ß signal pathways in mesenchymal and hematopoietic cells, where the posttranslational modifications of NF-kB transcription factors play a major role. The irradiation of murine LTBMC resulted in a transient increase in the expression of genes that code NF-kB transcription factors and IL-1ß. These results indicate an important role of Rel, Rela, Relb, and Nfkb2 genes in the induction of IL-1ß signal pathway in murine stromal cells. An increase in IL-1ß expression after the irradiation of stromal cells may be related to both the induction of inflammation due to massive cell death and to a profound stimulation of the expression of this proinflammatory cytokine expression.

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The present study involved three patients with graft failure following allogeneic hematopoietic stem cell transplantation (allo-HSCT). We obtained multipotent mesenchymal stromal cells (MSCs) from the original hematopoietic cell donors and implanted these cells in the periosteum to treat long-term bone marrow aplasia. The results showed that in all patients endogenous blood formation was recovered 2 weeks after MSC administration. Donor MSCs were found in recipient bone marrow three and 5 months following MSC implantation. Thus, our findings indicate that functional donor MSCs can persist in patient bone marrow.

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Mouse mesenchymal stromal precursor cells were labeled with lentiviral vector in long-term bone marrow culture. We studied the fate of labeled cells in the stromal sublayer of the long-term bone marrow culture and in ectopic hemopoiesis foci formed from the labeled cultures. The incidence of labeled polypotent fibroblast CFU in sublayers of long-term bone marrow culture and in ectopic hemopoiesis foci formed from these sublayers under the renal capsule of syngeneic mice was also analyzed. It was shown that the marker gene was present in about 40% cells of the stromal sublayer and 30% fibroblast CFU and that effective gene transfer did not affect the total production of hemopoietic cells. The size of ectopic hemopoietic foci formed after implantation of labeled sublayers of the long-term bone marrow culture under the renal capsule did not differ from the control. Differentiated cells of the osseous shell in these foci carried the marker gene in 40% cases. Analysis of fibroblast CFU in these foci showed that despite the total concentration of fibroblast CFU was comparable to that in the bone marrow, the concentration of labeled fibroblast CFU was about 6%, which suggests that one more class of precursors probably exists in the hierarchy of stromal cells presumably between mesenchymal stem cells and fibroblast CFU. Our findings demonstrate the capacities of mesenchymal stem cells to self-maintenance and differentiation without losing the marker gene integrated into the genome.

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The hierarchy of stromal precursor cells was studied. Changes in the number of fibroblast CFU in foci forming in irradiated recipients were analyzed. These precursor cells are most close known descendants of mesenchymal stem cells, while inducible precursors rank lower in the hierarchy and are the cells directly enlarging the hemopoietic area in irradiated recipients.

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Transplanted myeloproliferative disease developed in mice against the background of repeated injections of granulocytic CSF was characterized using morphological and molecular biological methods. It was demonstrated that transplanted myeloproliferative disease had a non-viral nature and is probably induced by repeated injections of granulocytic CSF. Tumor cells actively populate the liver of sick animals, which leads to their rapid death. Expression of Myc, Abl, G-CSF, and MPO genes is enhanced, which is typical of myeloid neoplastic transformation.

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We developed a method for gene transfer into mesenchymal stromal cells. Lentivirus vector containing green fluorescent protein gene for labeling stromal and hemopoietic precursor cells was obtained using two plasmid sets from different sources. The vector was injected into the femur of mice in vivo and added into culture medium for in vitro infection of the stromal sublayer of long-term bone marrow culture. From 25 to 80% hemopoietic stem cells forming colonies in the spleen were infected with lentivirus vector in vivo and in vitro. Fibroblast colony-forming cells from the femoral bones of mice injected with the lentivirus vector carried no marker gene. The marker gene was detected in differentiated descendants from mesenchymal stem cells (bone cavity cells from the focus of ectopic hemopoiesis formed after implantation of the femoral bone marrow cylinder infected with lentivirus vector under the renal capsule of syngeneic recipient). In in vitro experiments, the marker gene was detected in sublayers of long-term bone marrow cultures infected after preliminary 28-week culturing, when hemopoiesis was completely exhausted. The efficiency of infection of stromal precursor cells depended on the source of lentivirus. The possibility of transfering the target gene into hemopoietic precursor cells in vivo is demonstrated. Stromal precursor cells can incorporate the provirus in vivo and in vitro, but conditions and infection system for effective infection should be thoroughly selected.

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OBJECTIVE: To evaluate the dynamics of stem cell production to hematopoiesis, the number of active stem cell clones and the lifespan of individual clones were studied. MATERIALS AND METHODS: The clonal contribution of primitive hematopoietic stem cells (HSC) responsible for long-term hematopoiesis was determined using two approaches. In one model, irradiated female mice were reconstituted with retrovirally marked male hematopoietic cells. In the second model, mice were irradiated sublethally without hematopoietic cell transplantation. In both models, bone marrow cells were serially sampled from the same mouse throughout a 12- to 20-month period and injected into irradiated recipients for analysis of day 10 colony-forming unit-spleen (CFU-S). The donor origin of CFU-S was determined by the presence of retrovirally marked cells or cells with chromosomal aberrations. RESULTS: The results of the two essentially different models show that 1) hematopoiesis is mainly the product of small clones of hematopoietic cells; 2) the lifespan of the majority of clones is only 1 to 2 months; 3) the clones usually function locally; and 4) the vast majority of the clones replace one another sequentially. Primitive HSCs capable of producing long-lived clones (about 10% among all clones), which exist during the entire life of a mouse, were detected by the radiation-marker technique only. CONCLUSION: Multiple short-living clones (at least on the level of CFU-S production) comprise the vast majority of the active stem cells in transplanted recipients or after endogenous recovery from sublethal irradiation.

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Normal haematopoietic proliferation and differentiation occur within the human bone marrow microenvironment which is comprised of stromal cells including fibroblasts, adipocytes, macrophages and endothelial cells as well as the extracellular matrix made of collagen, fibronectin, laminin, vitronectin, thrombospondin and haemonectin. All haematopoietic progenitor cells including primitive LTC-IC, multilineage CFU-mix, myeloid CFU-GM and erythroid BFU-E adhere to the heparin-binding domains of the extracellular matrix component fibronectin. Human long-term bone marrow cultures (LTHBMC) represent the best available approximation for the in vivo marrow microenvironment in which the proliferation and differentiation of haematopoietic progenitor cells depend on the presence of marrow stromal cells and their attendant matrices. Since extracellular matrix components have been shown to promote myelopoiesis in long-term murine bone marrow cultures, we have examined the effect of two main components of the extracellular matrix: fibronectin and collagen type I on myelopoiesis in LTHBMC in an effort to increase the myeloid progenitor cell production. The present study revealed different modulatory effects for these two components. Collagen significantly increased the adherent fraction of LTHBMC (p < 0.05) but always resulted in a decreased myeloid progenitor cell (CFU-GM) production throughout the whole 8 weeks of culture. On the other hand, fibronectin significantly increased the number of both non-adherent cells. CFU-GMs (p < 0.01) and to a lesser extent the number of adherent cells as well as maintaining the LTHBMC up to 14 weeks. Fibronectin has been previously shown to stimulate the development of CFU-GMs in short-term semisolid cultures and to play an active role in haematopoietic progenitor cell-microenvironment interactions. Therefore, the presence of fibronectin in LTHBMC could increase both the productivity and longevity of myelopoiesis in the system. The integration of fibronectin in the ex vivo expansion systems currently undergoing development would ensure a sustained effective cumulative production of the myeloid progenitor cells (CFU-GMs), and consequently could accelerate the rate of haematological recovery in transplanted patients.

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We describe here a technique to study the clonal contribution of primitive stem cells that account for long-term hematopoiesis in the same mouse over a 14-month period. Specifically, irradiated recipient female mice were transplanted with retrovirally marked male hematopoietic progenitors. Bone marrow was then collected repeatedly from local sites from the same mice throughout a 14-month period and injected into secondary irradiated recipients for analysis of donor retrovirally marked day-11 colony-forming unit-spleen (CFU-S-11). We have tracked the temporal in vivo fate of 194 individual CFU-S-derived cell clones in 38 mice reconstituted with such retrovirally marked bone marrow cells. Our data show that long-term hematopoiesis is maintained by a large number of simultaneously functioning small, shortlived (1 to 3 months) clones that usually grow locally with little or no dispersion between different regions of the hematopoietic system. Furthermore, the clones that disappeared were never detected again. The data suggest that normal hematopoiesis is supported by the sequential recruitment of marrow repopulating cells into a differentiation mode.

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Marker bacterial Neor gene was transduced by retroviral gene transfer into stromal precursor cells making up the hematopoietic microenvironment in murine long-term bone marrow cultures (LTBMC). Cultures were infected six times during the first 3 weeks of cultivation. At 4 weeks, the adherent cell layers (ACLs) were implanted under the renal capsule of syngeneic unirradiated and irradiated mice. Cells from newly formed ectopic foci were explanted into secondary LTBMC. ACLs containing the marker gene were detected by polymerase chain reaction. About 74% of stromal cells in ACLs contained Neor gene. The possibility of stable gene transduction into stromal precursor cells competent to transfer the hematopoietic microenvironment was established.

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Extracellular matrix (ECM) plays an important role in the regulation of hematopoiesis. The ECM obtained from murine long-term bone marrow cultures (LTBMCs) induces hematopoietic foci formation within 3 months after implantation under the murine renal capsule. The foci consist of approximately 3 x 10(6) hematopoietic cells and function for at least 11 months. The induced stroma contains transplantable precursors capable of transferring a hematopoietic microenvironment to secondary recipients, and is insensitive to the stroma-stimulating factor produced in recipient mice after irradiation. The ECM induces hematopoietic foci formation in chimeras irradiated by a dose which is lethal for most of the stromal precursors. These facts point to the differences observed between bone marrow stromal precursors and mesenchymal cells induced under the renal capsule. The foci contain bone, but its appearance is limited to early stages of foci growth, and depends on the dose of implanted ECM. Bone is not formed when the xenogeneic ECM from nonhematopoietic tissue is used as an inducer. In this case, the foci develop slowly and are observed only to the tenth month after implantation. The data obtained demonstrate a novel function of the ECM in the induction of a hematopoietic microenvironment.

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Forty-seven individual hemopoietic cell clones bearing unique radiation markers were studied in long-term bone marrow cultures. Throughout cultivation clones appeared at different times, from 1 to 12 weeks after explantation, survived during 1-10 more weeks, and were characterized by marked variability in size. Usually, the number of metaphases peculiar to an individual clone rapidly increased, achieved maximum, and then underwent a decline. Cells of reliably disappearing clones were never seen again. The experimental results provide further evidence for the model of hemopoiesis by clonal succession.

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Long-term haemopoiesis in bone marrow culture is sustained by the progeny of haemopoietic progenitor cells (HPC), which differ from CFUs by very low sensitivity to repeated hydroxyurea (HU) injections. The transit time of a haemopoietic clone from HPC to cells proliferating in culture is 6-7 weeks. The results suggest that the stem cell continuum is an expansion type compartment, members of which gradually lose their proliferative potential during differentiation.

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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.

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The CFU-s proliferative potential varied greatly during long-term cultivation. Most of the CFU-s in the cultures were represented by cells with low renewal capacity. Pre-CFU-s cells capable of producing multipotential colonies in methylcellulose, which contained CFU-s with a high proliferative potential, were identified in the culture. In cultivation of a mixture of cells of different karyotype their ratio changed rapidly from week to week. The findings were consistent with the hypothesis that haemopoietic stem cells are maintained in the culture by the products of a small number of clones which arise and decline in succession, and that pre-CFU-s, but not the CFU-s themselves, are clonogenic progenitors.

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In the early periods (7-9 days) after haemopoietic cell injection, colonies produced by CFU-s and by their progeny are identified in the spleen, while at later periods (11 days after injection) only spleen nodules produced by CFU-s persist. The increase in the suicide values of CFU-s after sublethal (2 Gy) irradiation of mice is associated with a higher proliferation rate of precursors of transitory spleen colonies, but not of CFU-s, as measured by different suicide techniques. During the log-phase of cell growth in a lethally irradiated recipient, the injected CFU-s and CFU-tr proliferate at a higher rate. Active proliferation of CFU-s and CFU-tr has been demonstrated in long-term bone-marrow cultures by the hydroxyurea in-vitro suicide assay. CFU-tr may be the cause of artifactual effects during measurement of haemopoietic stem-cell cycling by CFU-s suicide methods.

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