Mesenchymal stem versus stromal cells: International Society for Cell & Gene Therapy (ISCT®) Mesenchymal Stromal Cell committee position statement on nomenclature.

The International Society for Cell & Gene Therapy (ISCT®) Mesenchymal Stromal Cell (ISCT MSC) committee offers a position statement to clarify the nomenclature of mesenchymal stromal cells (MSCs). The ISCT MSC committee continues to support the use of the acronym "MSCs" but recommends this be (i) supplemented by tissue-source origin of the cells, which would highlight tissue-specific properties; (ii) intended as MSCs unless rigorous evidence for stemness exists that can be supported by both in vitro and in vivo data; and (iii) associated with robust matrix of functional assays to demonstrate MSC properties, which are not generically defined but informed by the intended therapeutic mode of actions.

[Adipose-derived stromal cells: history, isolation, immunomodulatory properties and clinical perspectives]. / Les cellules stromales mésenchymateuses du tissu adipeux : historique, isolement, propriétés immunomodulatrices et perspectives cliniques.

Over the last decade, the clinical use of adipose-derived stromal/stem cells (ASC) in regenerative medicine is rapidly increasing. ASC belong to the mesenchymal stromal cells initially obtained from the bone marrow. Their limited differentiation capacity in vivo into functional mature cells has led to a reassessment of their mechanisms of action. One of the major clinical interests appears related to paracrine effects through a temporary production of trophic and immunomodulatory factors. Our purpose is to provide a review on the latest knowledge in the field of ASC, mechanisms of action, mainly immunomodulatory/immunosuppressive properties, methods of obtention, with a focus on clinical perspectives particularly in the field of cellular therapy and fat grafting technique in plastic surgery.

Mesenchymal stem cells for clinical application.

Mesenchymal Stem Cells/Multipotent Marrow Stromal Cells (MSC) are multipotent adult stem cells present in all tissues, as part of the perivascular population. As multipotent cells, MSCs can differentiate into different tissues originating from mesoderm ranging from bone and cartilage, to cardiac muscle. Conflicting data show that MSCs could be pluripotent and able to differentiate into tissues and cells of non-mesodermic origin as neurons or epithelial cells. Moreover, MSCs exhibit non-HLA restricted immunosuppressive properties. This wide range of properties leads to increasing uses of MSC for immunomodulation or tissue repair. Based on their immunosuppressive properties MSC are used particularly in the treatment of graft versus host disease, For tissue repair, MSCs can work by different ways from cell replacement to paracrine effects through the release of cytokines and to regulation of immune/inflammatory responses. In regenerative medicine, trials are in progress or planed for healing/repair of different tissue or organs as bone, cartilage, vessels, myocardium, or epithelia. Although it has been demonstrated that ex-vivo expansion processes using fetal bovine serum, recombinant growth factors (e.g. FGF2) or platelet lysate are feasible, definitive standards to produce clinical-grade MSC are still lacking. MSCs have to be produced according GMP and regulation constraints. For answering to the numerous challenges in this fast developing field of biology and medicine, integrative networks linking together research teams, cell therapy laboratories and clinical teams are needed.

3D environment on human mesenchymal stem cells differentiation for bone tissue engineering.

In this work a novel method was developed to create a three dimensional environment at a cellular level for bone tissue engineering. Biphasic calcium phosphate (BCP) particles of 140-200 microm were used in association with human mesenchymal stem cells (hMSCs). The cells seeded on these particles adhered and proliferated more rapidly in the first day of culture compared to culture on plastic. Analyses of hMSCs cultured without osteogenic factors on BCP particles revealed an abundant extracellular matrix production forming 3-dimensional (3D) hMSCs/BCP particles constructs after few days. Bone morphogenetic 2 (BMP-2), bone sialoprotein (BSP) and ALP gene expression using real time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) confirmed that expression profiles were modified by the culture substrate while the addition of osteogenic medium enhanced bone markers expression. These results indicate that BCP particles alone are able to induce an osteoblastic differentiation of hMSCs that might be of interest for bone tissue engineering.

Cell-assisted lipotransfer: Friend or foe in fat grafting? Systematic review and meta-analysis.

Autologous fat grafting is a common procedure for soft-tissue reconstruction but is associated with a graft resorption rate ranging from 20% to 80%. To improve the fat graft survival rate, a new technique, called cell-assisted lipotransfer (CAL), was developed. With CAL, fat is injected along with adipose-derived stromal cells that are assumed to improve fat survival rate. We conducted an evidence-based meta-analysis to evaluate the efficacy and safety of CAL as compared with conventional autologous fat grafting (non-CAL). The databases MEDLINE (via PubMed), Cochrane Library, EBSCO, Web of Science, and EMBASE were searched for reports of clinical trials, case series, and cohorts available from 2008 to 2016. We conducted a meta-analysis of the efficacy of CAL with data analysis concerning fat survival rate. The incidence of complications and the need for multiple procedures were evaluated to determine the safety of CAL. We identified 25 studies (696 patients) that were included in the systematic review; 16 studies were included in the meta-analysis to evaluate the efficacy of CAL. The fat survival rate was significantly higher with CAL than non-CAL (64% vs. 44%, p < .0001) independent of injection site (breast and face). This benefit of CAL was significant for only injection volumes <100 ml (p = .03). The two groups did not differ in frequency of multiple procedures after fat grafting, but the incidence of complications was greater with CAL than non-CAL (8.4% vs. 1.5%, p = .0019). The CAL method is associated with better fat survival rate than with conventional fat grafting but only for small volumes of fat grafting (<100 ml). Nonetheless, the new technique is associated with more complications and did not reduce the number of surgical procedures needed after the first fat grafting. More prospective studies are required to draw clinical conclusions and to demonstrate the real benefit of CAL as compared with common autologous fat grafting.

Intramyocardial transplantation of mesenchymal stromal cells for chronic myocardial ischemia and impaired left ventricular function: Results of the MESAMI 1 pilot trial.

BACKGROUND: The MESAMI 1 trial was a bicentric pilot study designed to test the feasibility and safety of intramyocardially injected autologous bone marrow-derived mesenchymal stromal cells (MSCs) for the treatment of ischemic cardiomyopathy. METHODS AND RESULTS: The study included 10 patients with chronic myocardial ischemia, left ventricular (LV) ejection fractions (EFs) of ≤35%, and reversible perfusion defects who were on stable optimal medical therapy and were not candidates for revascularization. MSCs (mean: 61.5×10(6) cells per patient) were injected into 10-16 viable sites at the border of the LV scar via a NOGA-guided catheter. Both primary endpoints, feasibility (successful harvest, expansion, and injection of autologous MSCs) and safety (absence of severe adverse events [SAEs]) were met in all 10 patients at the 1-month follow-up time point, and none of the SAEs reported during the full 2-year follow-up period were attributable to the study intervention. The results of secondary efficacy endpoint analyses identified significant improvements from baseline to Month 12 in LVEF (29.4±2.0% versus 35.7±2.5%; p=0.003), LV end-systolic volume (167.8±18.8mL versus 156.1±28.6mL; p=0.04), 6-min walk test and NYHA functional class. CONCLUSIONS: Our results suggest that autologous MSCs can be safely administered to the hearts of patients with severe, chronic, reversible myocardial ischemia and impaired cardiac function and may be associated with improvements in cardiac performance, LV remodeling, and patient functional status. A randomized, double blind, multicenter, placebo-controlled clinical trial (MESAMI 2) will evaluate the efficacy of this treatment approach in a larger patient population. CLINICAL TRIAL REGISTRATION: Unique identifier: NCT01076920.

High-dose salvage chemotherapy without bone marrow transplantation for adult patients with refractory Hodgkin's disease.

PURPOSE: For patients with Hodgkin's disease (HD) who do not achieve complete response (CR), who experience a relapse within the first year of CR, and for those who have two or more relapses, the outcome is poor. Salvage chemotherapy regimens at conventional doses produce a CR rate that ranges from 10% to 50% and a 5-year disease-free survival (DFS) between 10% and 25%. On the other hand, high-dose chemotherapy regimens given in combination with bone marrow transplantation (BMT) produce a CR rate that ranges from 40% to 80% and a 3-year DFS of approximately 40%. We report the 5-year results of a prospective study in patients with refractory HD who were treated with three courses of intensive chemotherapy without BMT. PATIENTS AND METHODS: Thirty-nine adult patients with refractory HD were treated with three courses of intensive chemotherapy. Each cycle of chemotherapy comprised vindesine 1 mg/m2/d in continuous intravenous (IV) infusion from day 1 to day 5; Adriamycin (doxorubicin; Roger Bellon Laboratories, Neuilly, France) 40 mg/m2/d in continuous IV infusion from day 1 to day 3; carmustine 140 mg/m2/d at day 3; etoposide 200 mg/m2/d from day 3 to day 5; and methylprednisolone 120 mg/m2/d from day 1 to day 5. After the third cycle of chemotherapy, irradiation (20 Gy) was performed whenever possible and depended on previous irradiation. RESULTS: At the end of the treatment, 31 patients (79%) were in CR. Among these patients, 10 relapsed after a median time of 3 months. The overall 5-year survival rate was 46%. The freedom from progression (FFP) and the freedom from treatment failure (FFTF) rates were 48% and 43%, respectively. The main toxicities were hematologic (neutropenia and thrombocytopenia) and digestive. Four patients died due to treatment-related complications (two from septic shocks, one from respiratory insufficiency, and one from posttransfusional AIDS). CONCLUSION: The results of this study seem to be comparable to those results obtained with high-dose chemotherapies with autologous BMT.

Effects of cryopreservation on the adherent layer of human long-term bone marrow cultures: study of cell phenotypes.

This report describes the effects of cryopreservation on the adherent layer of human long-term bone marrow cultures (HLTBMC). Stromal cells are believed to be the most important cells of medullar microenvironment to regulate hematopoiesis. To study effects of cryopreservation, we compared the cell phenotypes of adherent layers of fresh and frozen-thawed bone marrows. To characterize stromal cells we used monoclonal antibodies reacting with components of these cells (CGA-7 alpha SM and gamma SM actin isoforms; HHF-35, all muscle actin isoforms; BMS-1, stromal cell lysosomes). The other components studied were: fibronectin (BMS-2 monoclonal antibody) and hematopoietic cells (monoclonal antibodies against CD45, CD33, and CD14). Results show a decrease of cells positive for CGA-7, HHF-35, and BMS-1, in adherent layer of HLTBMC of frozen-thawed bone marrows. Expression of BMS-2 is unchanged, and CD45 and CD14-positive cells proportionately increased. These results are consistent with an impairment of stromal cell proliferation in frozen-thawed marrows, without impairment of most stromal cell functions. The difference between stromal cell and hematopoietic cell kinetics seems to be an additional fact suggesting a different origin for both cell populations.

Nontransformed colony-derived stromal cell lines from normal human marrows. III. The maintenance of hematopoiesis from CD34+ cell populations.

Nontransformed stromal colony-derived cell lines (CDCLs) consist of a pure stromal cell population that differentiates following a vascular smooth-muscle cell repertoire. Here we study the maintenance of hematopoiesis by this cell population. We show that CDCLs allow the generation for several weeks of stroma-adherent colonies (comprising a cobblestone area) from CD34+, CD34+/CD38+, and CD34+/CD38- cells. Stroma-adherent colony-forming cells (CFCs) from CD34+/CD38- cells reach a maximum at week 4 and limiting dilution analysis gives a frequency of 1 per 10 cells seeded; in contrast to this, CFCs from CD34+/CD38+ cells are optimal by week 2 and the frequency is then only 1 per 120 cells seeded. Stroma-adherent colonies comprise hematopoietic cells from all lineages except the T lymphocytic, with a majority of granulomonocytes. CDCLs also allow the amplification of granulomonocytic colony-forming units (CFU-GMs), since cumulative outputs of CFU-GMs by week 6 are 190 and 8 times that observed at culture inception for the CD34+/CD38- and CD34+/CD38+ cell populations, respectively. Our results suggest that stromal cells from CDCLs allow the maintenance of primitive hematopoietic precursors and induce their proliferation and differentiation. This study underscores the potential role of one of the microenvironmental cell populations, that of myoid cells, in the regulation of hematopoietic precursor behavior.

Nontransformed colony-derived stromal cell lines from normal human marrows. II. Phenotypic characterization and differentiation pathway.

In a previous report, we described a method to generate cell lines derived from stromal colonies (colony-derived cell lines [CDCL]). In a first step, colonies were obtained from plating cells from adherent layers of human long-term bone marrow cultures (LTBMC) in methyl-cellulose in the presence of interleukin-1 beta (IL-1 beta) (20 U/mL) and tumor necrosis factor-alpha (TNF-alpha) (200 U/mL). In a second step, cell lines were derived from individual colonies cultured in liquid medium with 20 ng/mL basic fibroblast growth factor (bFGF). In this report, we describe the phenotype of cells from more than 100 cell lines. CDCL did not contain cells of hematopoietic origin, which indicates that the culture system did not allow the growth of hematopoietic precursors. CDCL did not contain endothelial-like cells, similarly to primary adherent layers. CDCL comprised stromal cells, as defined by membrane antigens recognized by monoclonal antibodies 6-19, Stro-1 and 1B10. These cells belonged to the family of connective tissue-forming cells since they synthesized interstitial collagens I, III, and V; nonplasmatic EDa+ and EDb- fibronectin; tenascin; and chondroitin-sulfate. Study of the time course of CDCL showed that the lines differed from one another according to their proliferative capacity: 32% of CDCL grew quickly, yielding about 10,000 cells in 10 days; 36% of CDCL grew slowly, yielding 1000 cells in 10 days; and the remaining 32% had intermediate proliferative capacity. For CDCL with a high proliferative capacity, a distinctive differentiation pattern could be described. At culture, inception cells from the lines were vimentin and laminin. Over time, several cytoskeletal and extracellular matrix (ECM) proteins indicative of vascular smooth muscle differentiation were expressed, including: the alpha-SM-actin isoform; the actin-binding proteins, smooth muscle myosin-heavy chain (SMMHC), SM1; h-caldesmon; calponin; gelsolin; and the ECM proteins, collagen IV and elastin. In full-grown lines, cells were similar to immature, intimal, vascular smooth muscle cells as found beneath the endothelium in adult aortas. Because of the coupling between proliferation and differentiation, the differentiation pattern seems to be under genetic control. However, since the coupling was not stringent during the whole lifespan of the lines, it is possible that cytokines are also involved, ensuring autocrine regulation of CDCL development.

Nontransformed colony-derived stromal cell lines from normal human marrows. I. Growth requirement and myelopoiesis supportive ability.

We report a new method for generating nontransformed human stromal cell lines with a replicative potential of 20 to 25 doublings yielding 10(6) to 3 x 10(7) cells after 4 to 6 weeks. Cells from week-3 to -6 adherent layers of human long-term bone marrow cultures (LTBMC) were plated in methylcellulose in the presence of 20 U/mL interleukin-1 beta (IL-1 beta) and 200 U/mL tumor necrosis factor-alpha (TNF-alpha). After 2 to 3 weeks, we obtained 180 +/- 14 colonies per 10(5) cells seeded. These well-delineated colonies with a dense central core consisted of up to several hundred tightly packed, identical, large refractile cells. Colonies were determined to be clones by sequential examination of the cultures and the linear relationship between the number of colonies counted and cells seeded. Colony-derived cell lines (CDCL) were developed by seeding individual colonies in long-term culture medium (LTCM) supplemented with 20 ng/mL basic fibroblast growth factor (bFGF). The selection of colonies yielding lines with high proliferative capacity was due to the presence of IL-1 beta and TNF-alpha in the semisolid medium. The most effective concentrations for clonal selection were 200 U/mL TNF-alpha and 20 U/mL IL-1 beta. The growth of CDCL in liquid culture depended on the presence of bFGF, with the most effective concentration at 20 ng/mL. CDCL were able to maintain the output of colony-forming units granulocyte/macrophage and burst-forming unit-erythrocyte (CFU-GM and BFU-E) for several weeks from cocultured CD34+ marrow cells. The weekly CFU-GM and BFU-E output from weeks 2-5 was at least the same as observed when using passaged adherent layers. CDCL represent a progenitor cell population for stromal cells that may prove a suitable model for the study of the relationship between marrow stromal cells and hematopoiesis.

Characteristics of megakaryocyte colony formation in normal individuals and in primary thrombocythemia: studies using an optimal cloning system.

Colony formation by megakaryocyte (MK) progenitors was studied in 36 normal individuals and in 26 patients with primary thrombocythemia (PT) using an improved plasma clot cloning system. MK colonies were identified by immunoperoxidase staining using a monoclonal antibody against human platelet glycoprotein IIb/IIIa complex. In normal individuals, a frequency of 194 +/- 23 MK colony-forming units (CFU-MK) per 5 X 10(5) bone marrow nonadherent mononuclear cells and 11 +/- 4 CFU-MK per 5 X 10(5) blood mononuclear cells was found. These CFU-MK grew as large (greater than 20 cells), mean (11-20 cells), small (3-10 cells), or mix-MK colonies (at least two MK) that comprised 15%, 23%, 62%, or 5% of all MK colonies in bone marrow and 0%, 8%, 92%, or 1% in peripheral blood, respectively. In PT, several abnormalities of MK colony formation were observed: 1) increased circulating CFU-MK numbers, 2) increased mix-MK colony formation, 3) spontaneous MK colony formation without phytohemagglutinin-stimulated leukocyte-conditioned medium and normal serum, 4) decreased proportion of larger MK colonies (greater than 11 cells) in PT bone marrow, and 5) failure of PT plasma or serum to stimulate MK colony formation by normal marrow cells in a normal fashion. These results indicate some of the characteristics of quantitative and qualitative abnormalities of in vitro megakaryocytopoiesis in PT.

Inhibitory effect of highly purified human platelet beta-thromboglobulin on in vitro human megakaryocyte colony formation.

The effect of highly purified human beta-thromboglobulin (beta TG), a glycoprotein present in platelet alpha granules, was tested on human bone marrow in vitro megakaryocyte (MK) colony formation. A concentration of 5 micrograms/ml of beta TG induced a 50% reduction in the number of MK colonies, and concentrations of 10 and 20 micrograms/ml completely inhibited MK growth. This inhibition was of importance for MKs because a higher concentration of beta TG (10 micrograms/ml) was needed to obtain a nonsignificant decrease in erythroblastic progenitors (erythroid burst-forming units, BFU-E), and no inhibition of granulocyte-macrophage colony-forming units (CFU-GM) was observed. beta TG acts mainly on maturation of MKs. These results indicate that beta TG could play a role in the physiological regulation of platelet production by MKs.

ING1b negatively regulates HIF1α protein levels in adipose-derived stromal cells by a SUMOylation-dependent mechanism.

Hypoxic niches help maintain mesenchymal stromal cell properties, and their amplification under hypoxia sustains their immature state. However, how MSCs maintain their genomic integrity in this context remains elusive, since hypoxia may prevent proper DNA repair by downregulating expression of BRCA1 and RAD51. Here, we find that the ING1b tumor suppressor accumulates in adipose-derived stromal cells (ADSCs) upon genotoxic stress, owing to SUMOylation on K193 that is mediated by the E3 small ubiquitin-like modifier (SUMO) ligase protein inhibitor of activated STAT protein γ (PIAS4). We demonstrate that ING1b finely regulates the hypoxic response by triggering HIF1α proteasomal degradation. On the contrary, when mutated on its SUMOylation site, ING1b failed to efficiently decrease HIF1α levels. Consistently, we observed that the adipocyte differentiation, generally described to be downregulated by hypoxia, was highly dependent on ING1b expression, during the early days of this process. Accordingly, contrary to what was observed with HIF1α, the absence of ING1b impeded the adipogenic induction under hypoxic conditions. These data indicate that ING1b contributes to adipogenic induction in adipose-derived stromal cells, and thus hinders the phenotype maintenance of ADSCs.

Prognostic factors for survival after high-dose therapy and autologous stem cell transplantation for patients with relapsing Hodgkin's disease: analysis of 280 patients from the French registry. Société Française de Greffe de Moëlle.

High-dose therapy with autologous stem cell transplantation (ASCT) has been widely proposed for patients with relapsed Hodgkin's disease (HD). From 1982 to 1993, we selected (from the French registry for bone marrow transplantation) 280 patients, who underwent ASCT for relapsed HD after initial treatment including chemotherapy. Patient characteristics at diagnosis were: sex ratio (M/F): 1.5; median age: 30 years (5-59 years), stage I, II: 43%; III, IV: 57%; 32% had chemotherapy, 68% chemo+ radiotherapy. All patients achieved complete remission after first-line therapy and subsequently relapsed. The median interval between diagnosis and high-dose therapy was 34 months. First relapse occurred in 78% of the patients at a median end-of-treatment to relapse time of 18 months. All patients received salvage chemotherapy before high-dose therapy, and the median time between relapse and high-dose therapy was 5 months. After this regimen, 84% of the patients were considered to have chemosensitive relapse. Conditioning regimens were: BEAM: 60%; CBV/BEAC: 26%. Transplant-related mortality was 6%. With a median follow-up of 3 years after high-dose therapy, overall and progression-free survivals at 4 years were 66 and 60%, respectively. Neither the conditioning regimen nor the stem cell source affected survival. Good prognostic factors for survival were: chemosensitivity of relapse (P < 0.01) and first relapse vs further relapse (P < 0.05). For 214 patients in first relapse, other significant factors for survival were: end-of-treatment to relapse interval < 12 months (P < 0.05) and nodal vs extranodal relapse (P < 0.001). These two prognostic factors were used to validate a prognostic model with three significantly different subgroups: 0 (n = 59), 1 (n = 125), or 2 factors (n = 30) with 4-year survival, respectively, at 93, 59 and 43% (P < 0.001). Salvage therapy can be tailored in patients with relapsing HD: conventional treatment in the good prognosis group (0 factor), high-dose therapy after response to second line regimen (1 factor) and more intensive treatments for the bad prognosis group (2 factors).

Relevance of in vitro studies of drug-induced agranulocytosis. Report of 14 cases.

Bone marrow colony forming unit-granulocyte macrophage (CFU-GM) cultures of 14 patients after neutrophil recovery from drug-induced agranulocytosis (median 12 weeks) were performed in the presence of 20 different drugs and/or acute-phase serum (APS) obtained during agranulocytosis. In 10 cases, drugs involved in agranulocytosis in vivo caused a significant inhibition of CFU-GM growth in vitro in comparison with normal cultures without drug. Three types of direct toxicity are suggested: (i) a decrease in the rate of mitosis; (ii) a destruction of cells (cytotoxic effect); or (iii) a blockage in progenitor mitosis (cytostatic effect). A humoral mechanism was suggested in 1 case because of enhanced inhibition with APS. In 4 cases no effect of the suspected drug could be detected by in vitro studies, but all hypotheses have not been tested in these cases, particularly the possible role of APS and other substances.

Increased fibroblast colony stimulating activity (F-CSA) in serum of myeloproliferative disorders.

Sera of patients with primary myelofibrosis (PMF), primary thrombocythemia (PT), polycythaemia vera (PV) and chronic myeloid leukemia (CML) contained a significantly increased F-CSA (or F-CSAs) compared to those of normal subjects and patients with secondary thrombocytosis (ST). This F-CSA was heat sensitive and had the capacity to promote both proliferation and maturation of normal marrow fibroblast colony-forming cells (CFU-F). This F-CSA seemed to be different from human platelet derived growth factor (PDGF), tumor necrosis factor (TNF) and fibroblast growth factor (FGF) from bovine brain. This F-CSA might be of importance in the pathogenesis of bone marrow fibrosis in myeloproliferative disorders.

In vitro study of translocation t(4;11) acute leukemia. Sequential range of phenotype.

The nature of translocation t(4;11) acute leukemia cells has been widely discussed over the past few years. Many authors report their phenotypic heterogeneity, ranging from apparently common acute lymphoblastic leukemia antigen-positive to monoblastic leukemia, through "promiscuous" phenotype. We studied in vitro phenotypic modulation of three typical cases after induction by 12-O-tetradecanoyl phorbol 13 acetate. The initial phenotype was different in each case, but all of them exhibited changes in morphologic shape, cytochemistry, and immunophenotype; common features appeared after induction by 12-O-tetradecanoyl phorbol 13 acetate, including esterase activity, expression of CD-9, CD-15, and CD-18 surface antigens, and monocyte/macrophage morphology. In all cases a B-associated surface antigen, CD-19, persisted. During clinical evolution, some previously reported cases have shown a karyotypic and phenotypic transformation. In one of our cases this phenomenon correlated with in vitro phenotypic modulation of initial blast population. Furthermore, clinical relapses and in vitro modulation always seem to evolve toward a more "mature" phenotype. Those results support the "promiscuous lineage" hypothesis, and point out the usefulness of in vitro studies to express the myeloid potential of this category of acute leukemia, which can be regarded as a model of early hematopoietic differentiation.