Temporal response of murine pluripotent stem cells and myeloid and erythroid progenitor cells to low-dose glucan treatment

B6D2F1 female mice were intravenously administered 0.4 mg of glucan. 1, 5, 11, and 17 days later, the total nucleated cellularity (TNC) and the numbers of pluripotent hemopoietic stem cells (CFU-s), granulocyte-macrophage progenitor cells (GM-CFC), and erythroid colony-forming (CFU-e) and burst-forming (BFU-e) cells were assayed in the bone marrow and spleen. Bone marrow TNC was not altered, but splenic TNC increased approximately twofold on day 5 and remained increased on days 11 and 17 after glucan treatment. The concentrations of bone marrow and splenic CFU-s and GM-CFC both significantly increased (p less than 0.01) by 5 days after glucan administration; however, they returned to control levels by day 17. Splenic CFU-e concentration increased on days 5, 11, and 17, whereas splenic BFU-e concentration increased only on day 11 after treatment. By contrast, bone marrow CFU-e and BFU-e concentrations were either unaffected or slightly decreased by glucan treatment. When peripheral blood was assayed for CFU-s and GM-CFC, no detectable increase in the concentrations of these progenitors was noted at any time after glucan treatment. The relevance of these effects of low-dose (0.4 mg) glucan treatment is discussed with respect to previously reported effects of higher-dose (e.g., 4.0 mg) glucan treatment.     

Effects of an aplastic anemia urinary extract on mouse erythroid progenitor cells in vivo

We investigated the in vivo effects of a crude extract from the urine of aplastic anemia patients (AA urinary extract) on erythroid precursor cells in the femoral bone marrow and spleens of normal adult mice. A single intraperitoneal injection of AA urinary extract induced a significant increase in the number of splenic erythroid burst-forming units (BFU-e) and erythroid colony-forming units (CFU-e) within 24 h after injection. We then injected pure recombinant erythropoietin (Epo) equivalent to the amount present in the urinary extract. This addition increased the number of splenic CFU-e by almost the same degree as the amount induced by the AA urinary extract 24 h after injection, but failed to elicit any change in the number of splenic BFU-e. In other studies, mice were injected with the same amount of lipopolysaccharide (LPS) and/or pure Epo as that present in the AA urinary extract. Experiments with Limulus amebocyte lysate-adsorbed (endotoxin-depleted) or nonadsorbed (endotoxin-containing) AA urinary extracts showed that endotoxin contamination interfered with the increase in numbers of marrow CFU-e and enhanced the increase in splenic CFU-e numbers induced by pure Epo or Epo activity in the AA urinary extract. The number of splenic BFU-e, however, was not affected by administration of LPS and/or Epo or by adsorbed endotoxin. These data suggest that AA urinary extract contains a stimulating activity for mouse splenic BFU-e, and that this activity is not attributable to the Epo activity or endotoxin contamination within the urinary extract.     

In vivo erythropoietin requirements of regenerating erythroid progenitors (BFU-e, CFU-e) in bone marrow of mice.

Erythroid progenitors (B-8, B-4, CFU-e) in the femoral marrow of polycythemic mice were measured by in vitro culture assays after a single administration of BCNU or Myleran. BCNU reduced pluripotent stem cells to 40% and erythroid progenitors to less than 5% of normal. B-8, the earliest erythroid progenitors, regenerated without erythropoietin (Epo) completely within 5 days. At 14 days after BCNU, intermediate progenitors (B-4) attained 60% of their normal numbers and CFU-e attained approximately 30%. Daily injections of Epo promptly restored normal B-4 numbers and near-normal CFU-e numbers in BCNU-treated mice. After Myleran, CFU-s remained below 2% of normal for 14 days, and no regeneration of the B-8 occurred with or without daily Epo injections. The findings suggest that regneration of B-8 was dependent on cell inflow from the pluripotent stem cell compartment but was independent of the presence of Epo. Intermediate progenitors (B-4) required Epo and the presence of B-8 for complete and permanent regeneration. CFU-e were the most Epo-dependent of the three progenitors. B-4, recruited by Epo, required after their formation a second exposure to the hormone in order to progress into the CFU-e stage.     

Erythropoiesis in vitro. III. The role of potassium ions in erythroid colony formation.

The role of potassium as an essential promotor of erythroid progenitor growth (BFU-e & CFU-e) from normal murine hematopoietic tissues was studied. Dialyzed fetal calf serum, over a wide range of concentrations, was shown to reduce the numbers of BFU-e and CFU-e that could be cultured from normal murine bone marrow. A dose-dependent addition of 1 M KC1 restored erythroid progenitor colony growth to the levels generally seen when normal, non-dialyzed fetal calf serum was used. Furthermore, when [K] was increased in some human urinary and sheep plasma erythropoietin preparations, the number of erythroid progenitor cells cultured also increased. This influence is crucial to the differentiation of committed stem cells into the erythroid pathway and must therefore be considered in the development of serum-free growth media.     

A case of acquired pure red cell anemia studied by cloning of erythroid progenitor cells in vitro

A 70-year-old woman developed typical clinical symptoms of pure red cell anemia (PRCA) following a history of rheumatoid arthritis (RA). The patient's bone marrow erythropoietic progenitors cells were cloned in a micro agar culture system several times over a period of 11 months, revealing a diminished frequency of bone marrow erythroblasts paralleled by a markedly reduced number of CFU-e and BFU-e in vitro. No inhibitory activity in the patient's IgG fraction could be detected either by preincubation with IgG and/or rabbit complement, or in the continuous presence of IgG. Depletion of T lymphocytes from the patient's bone marrow cells led to an improved in vitro erythroid proliferation. Cytostatic therapy with cyclophosphamide clinically induced a marked increase in the bone marrow erythroblast and reticulocyte number, correlated in vitro by normalization of CFU-e levels and increase in the number of BFU-e. Nevertheless, BFU-e values never attained normal levels, which could be attributed to a reduced stem cell pool resulting from previous therapy with cyclophosphamide and/or antirheumatic drugs. Two independent factors, a reduced pool of committed stem cells as well as an autoimmune cell-mediated suppression, may both contribute to the pathomechanism of the disease in this patient.     

In vitro myelotoxicity of 2',3'-dideoxynucleosides on human hematopoietic progenitor cells

Three nucleoside analogues, 2',3'-dideoxyadenosine (ddA), 2',3'-dideoxyinosine (ddI), and 2',3'-dideoxycytosine (ddC), were evaluated for their potential myelotoxic effects to normal human hematopoietic progenitor cells. The myeloid (granulocyte-monocyte colony-forming units, CFU-gm) and erythroid (erythroid burst-forming units, BFU-e: and erythroid colony-forming units, CFU-e) committed progenitor cells were exposed to the agents for a 1-h period prior to culture in a microcapillary assay or continuously exposed during the entire culture period. Both ddA and ddI (100 microM) were mildly toxic (less than 50% colony inhibition) to human CFU-gm, BFU-e, and CFU-e following either 1-h or continuous exposures. Marrow progenitor sensitivities to ddA and ddI were indistinguishable. Colony inhibition ranged from 47% to 67% for 1-h ddC exposure (100 microM), values that were comparable to ddA and ddI. Continuous exposure to ddC was highly myelotoxic to human hematopoietic progenitors, with concentrations of 10 and 100 microM suppressing colony formation by 79%-92% and 93%-97%, respectively. These results demonstrate that 1-h and continuous exposures to ddA and ddI were similarly myelotoxic to human hematopoietic cells, whereas a 1-h exposure to ddC was equivalent to ddA and ddI, yet continuous ddC exposure was extremely toxic to marrow cell progenitors.     

Prolactin exerts hematopoietic growth-promoting effects in vivo and partially counteracts myelosuppression by azidothymidine.

Prolactin (PRL) is a neuroendocrine hormone that influences immune and hematopoietic development. The mechanism of action of this hormone in vivo remains unclear; therefore, we assessed the effects of PRL on hematopoiesis in vivo and in vitro. Normal resting mice were treated with 0, 1, 10, or 100 microg of recombinant human prolactin (rhPRL) for 4 consecutive days and euthanized on the fifth day for analysis of myeloid and erythroid progenitors in the bone marrow and spleen. Both frequencies and absolute numbers of splenic colony-forming unit granulocyte-macrophage (CFU-GM) and burst-forming unit-erythroid (BFU-e) were significantly increased in mice receiving rhPRL compared to the controls that had received saline only. Bone marrow cellularities were not significantly affected by any dose of rhPRL, but the absolute numbers and frequencies of bone marrow CFU-GM and BFU-e were augmented by rhPRL. These results suggest that rhPRL can promote hematopoiesis in vivo. Because rhPRL augments myeloid development in vivo, we examined the potential of the hormone to reverse the anemia and myelosuppression induced by azidothymidine (AZT). Mice were given rhPRL injections concurrent with 2.5 mg/mL AZT in drinking water. rhPRL partially restored hematocrits in the animals after 2 weeks of treatment and increased CFU-GM and BFU-e in both spleens and bone marrow. The experiments with AZT and rhPRL support the conclusion that the hormone increases myeloid and erythroid progenitor numbers in vivo, and they suggest that the hormone is clinically useful in reversing myelosuppression induced by AZT or other myeloablative therapies.     

Separation of erythroid progenitor cells in mouse bone marrow by isokinetic-gradient sedimentation.

Isokinetic-gradient sedimentation employing a shallow linear gradient of Ficoll in tissue culture medium was used to isolate erythroid progenitor cells (CFU-e) from mouse bone marrow. Following gradient sedimentation, 34% of the total nucleated cells and 48% of the CFU-e applied to the gradient were recovered, and three distinct modal populations of CFU-e could be distinguished. The slowest-migrating population did not require exposure to exogenous erythropoietin in order to form erythroid colonies in vitro. The other two modal populations of CFU-e required exposure to exogenous erythropoietin for differentiation. One of these, constituting 64% of the hormone-dependent CFU-e recovered, migrated with the bulk of the marrow cells, whereas the other migrated ahead of the bulk of the marrow cells. This latter population, which contained 34% of the CFU-e, was recovered with 11% of the marrow cells, representing a twofold to threefold enrichment. BFU-e migrated more slowly than the erythropoietin-dependent CFU-e. Resedimentation studies suggested that the two erythropoietin-dependent CFU-e populations were distinct modal populations. When cells from the fastest-migrating population of erythropoietin-dependent CFU-e were cocultured with unseparated marrow cells, a further twofold to threefold enhancement of erythroid colony formation was obtained. Comparison of isokinetic-gradient sedimentation with discontinuous and continuous albumin density-gradient sedimentation revealed that isokinetic-gradient sedimentation was a more efficient method than the former and a more rapid method than the latter for isolating CFU-e from mouse bone marrow.     

Immunological aspects of the anemia of rheumatoid arthritis

In order to investigate the cause of the anemia concomitant with rheumatoid arthritis (RA), we examined, using the erythroid colony assay of human bone marrow colony-forming units-erythroid (CFU-e) and burst-forming units-erythroid (BFU-e), the effects of the patients' serum and peripheral blood T lymphocytes on the CFU-e-derived colonies. The counts of erythroid colonies of RA patients were markedly lower than those of human control subjects [CFU-e: control 152.9 +/- 30.6 (n = 19), RA 51.1 +/- 13.6 (n = 7), t = 7.66567, p less than 0.01; BFU-e: control 25.2 +/- 5.9 (n = 5), RA 12.6 +/- 2.6 (n = 7), t = 4.574, p less than 0.01]. The serum from two out of seven RA patients slightly inhibited the formation of CFU-e-derived colonies of human control subjects (t = 2.31, 0.05 less than p less than 0.1); however, the serum from the other five RA patients did not significantly inhibit human control erythroid colony formation as compared with human control serum (t = 0.981, 0.3 less than p less than 0.4). On the other hand, peripheral blood T lymphocytes of the patients markedly inhibited the formation of CFU-e-derived colonies of the control subjects as compared with peripheral blood T lymphocytes from human control subjects (t = 4.24, p less than 0.01). The above-mentioned results suggest that the peripheral blood T lymphocytes of RA patients might play a role as one of the causes of the concomitant anemia of RA patients.     

Changes of the phenotype of erythroid progenitor cells (BFU-E, CFU-E) and their progenies during early steps of differentiation

Early differentiation processes of human erythroid progenitor cells (BFU-e, CFU-e) have been studied during in vitro proliferation using a panel of monoclonal antibodies with known reactivity on different levels of the erythroid cell line. Two antibodies recognizing structures on BFU-e (VIP-2b, BMA 021), two antibodies reactive with CFU-e and nucleated red cells (5F1, CLB-Ery-3) and one antibody directed against glycophorin A (VIE-G4) were used for this study. Normal human bone marrow cells were induced to proliferation in an erythroid progenitor cell assay and, after different periods of incubation, agar cultures were treated with these antibodies and complement. Thereafter, the remaining erythroid cells were incubated again to continue their proliferation with the same stimulators as before. The changes of the phenotype of BFU-e and CFU-e progenies during in vitro proliferation were determined by the reduction of colony formation in comparison with untreated control cultures. Our results indicate that the loss of HLA-DR antigens and the p45 structure is accompanied by the acquisition of structures recognized by the antibodies 5F1 and CLB-Ery-3. After 5-7 d of incubation BFU-e derived progenies exhibit the same antigenic structure as has been found for CFU-e. Glycophorin A expression could only be demonstrated at a late differentiation stage of the erythroid cell lineage.     

Further studies on the in vitro enhancement of erythroid stem cell (CFU-e and BFU-e) colony formation by ouabain

The cardiac glycoside ouabain has been shown to stimulate erythroid stem cell (CFU-e/BFU-e) colony formation while inhibiting both granulocyte/macrophage precursor cell (CFU-gm) and murine spleen stem cell (CFU-s) colony formation. We have examined the ability of ouabain to increase both CFU-e/BFU-e colony formation by performing time-delay and temperature-dependent studies. After pre-exposure of marrow cells to ouabain (10(-15) M) at temperatures of 27, 37, or 4 degrees C, erythropoietin (Ep) was added after time-delays ranging from 10 s to 60 min. The ouabain-induced increase in CFU-e/BFU-e colony formation was observed up to an Ep-delay of 20 s, after which time the enhancing effect of ouabain was diminished. This increase was also observed when marrow cells were first exposed to Ep prior to a similar delayed exposure to ouabain. The enhancement effect seen with ouabain was also temperature dependent, since at 37 degrees C an earlier time increase in CFU-e/BFU-e colony formation was observed when compared to identical studies performed at either room temperature or 4 degrees C. These studies suggest that ouabain-induced elevations in erythroid stem cell colony formation involve mechanisms that are both time and temperature dependent.     

Sensitivity of erythroid progenitor colonies to erythropoietin in azidothymidine treated immunodeficient mice

The anaemia induced by 3'-azido-3'dideoxythymidine (AZT) is poorly understood. We have used a murine model of AIDS, infection of female C57BL/6 mice with LP-BM5 murine leukaemia (MuLV) virus, to determine if AZT-induced anaemia is due, in part, to decreased responsiveness of erythropoietic precursors (BFU-e) to erythropoietin (EPO). Mice in the early stage of LP-BM5 MuLV disease were given AZT in their drinking water at 1.0 and 2.5 mg/ml. AZT produced anaemia in both groups, in a dose-dependent fashion. Despite the anaemia, the number of splenic and bone marrow BFU-e in AZT treated mice increased up to five-fold over levels observed in infected untreated animals after 15 d of treatment. Colony formation by splenic and bone marrow BFUe was stimulated at lower concentrations of EPO in mice receiving AZT for 15 d than for infected, untreated mice. By day 30, sensitivity of both splenic and bone marrow BFU-e of treated animals returned to that observed from cells of infected untreated animals. The mean plasma levels of EPO observed in AZT treated mice were appropriate for the degree of anaemia observed when compared with phenylhydrazine (PHZ) treated mice. The numbers of BFU-e and the percentage of bone marrow erythroblasts observed were comparable in AZT and PHZ treated mice with similar degrees of anaemia. However, reticulocytosis was inappropriate for the degree of anaemia observed in AZT treated infected mice. AZT-induced peripheral anaemia in the face of increased numbers of BFU-e and increased levels of plasma EPO suggest a lesion in terminal differentiation.     

Influence of the tumour promoter TPA upon erythroid burst formation in vitro

A dose dependent effect of the tumour promoter TPA on burst formation by rabbit erythroid progenitors (BFU-e) was demonstrated in cultures deficient in the early erythroid regulator burst-promoting activity (BPA). In these culture conditions the burst number was highest (193% of controls) at 10(-9)M TPA and concentrations higher than 3 x 10(-9)M TPA were inhibitory. The degree of burst enhancement by TPA and bone marrow conditioned medium as a source of BPA was similar. The addition of optimal concentrations of both TPA and BPA simultaneously to cultures resulted in no further increase in burst number. Short-term incubation of bone marrow cells with TPA failed to enhance the percentage of S-phase BFU-e under conditions in which BPA significantly increases the number of BFU-e in the cell cycle. These results indicate that the same population of BFU-e responds to TPA and BPA, but TPA does not mimic the mitogenic effect of BPA upon BFU-e.     

Factor-independent erythropoietic progenitor cells in leukemia induced by the myeloproliferative leukemia virus

The myeloproliferative leukemia virus (MPLV), a novel murine retroviral complex that does not transform fibroblasts, has been shown to cause an acute leukemia in adult mice accompanied by a progressive polycythemia. The present study demonstrates that, on in vivo inoculation, MPLV induces a rapid suppression of growth factor requirement for in vitro colony formation by both the late and the primitive erythroid progenitor cells. CFU-e-derived erythrocytic colonies developed and differentiated in semi-solid medium without the addition of erythropoietin (Epo). In addition, the formation of CFU-e colonies was not altered by the presence of specific neutralizing Epo antibodies. In the spleen, the CFU-e pool size increased rapidly up to 30-fold. By day 6 postinfection, 100% of these progenitor cells were Epo-independent. The in vivo effects of MPLV-infection on early erythroid progenitor cell compartments were examined in cultures grown for seven days. The concentration of erythroid progenitor cells was twofold elevated in spleen from MPLV-infected mice. As early as day 4 postinfection, 50% of these progenitors produced fully hemoglobinized colonies in serum-free cultures without the addition of interleukin-3 (IL-3) and Epo. Most spontaneous colonies were large and contained up to 10(5) cells per colony. They were composed of either erythroblasts only (16%) or erythroblasts and megakaryocytes (70%); few of them were multipotential (14%). In the marrow, the total number of BFU-e was reduced and only few factor-independent bursts were observed, suggesting a rapid migration of infected progenitors from marrow to spleen. Furthermore, the data show that abnormal erythropoiesis was due to the replication defective MPLV information and was not influenced by the Fv-2 locus.     

Circulating Granulocytic and Erythroid Progenitor Cells in Chronic Granulocytic Leukaemia

S ummary . We used a standard methyl cellulose method to assay erythroid progenitor cells in the blood of 35 patients with untreated CGL and of 18 normal controls. In 28 patients we simultaneously assayed granulocyte/nionocyte committed progenitor cells (CFU-c) by an agar method. Circulating erythroid burst-forming units (HFU-e) in CGL wcre increased above normal by a factor of about 180; CFU-c were increascd by a factor of about 9000. Both BFU-e and CFU-c numbers were linearly related to the total leucocyte count in individual patients but not to numbers of circulating blast cells. There was a positive correlation in individual patients between CFU-c and BFU-e numbers. Circulating BFU-e and erythroid colony-forming cells (CFU-e) were unable to proliferate in vitro in the absence of erythropoietin. We conclude that erythroid progenitor cells are involved in the'clonal expansion'that characterizes CGL, but apparently to a lesser extent than are granulocyte/monocyte progenitor cells.     

Drug-indiced agranilocytosis: Evidence for the commitment of bone marrow haematopoiesis

Clinical and haematological features of 61 patients with drug-induced agranulocytosis (63 episodes) are presented. Multiple drug consumption was a common observation and complicated the attempt to incriminate a particular drug as being aetiologically involved. Bone marrow analysis shortly after the diagnosis revealed evidence for an impairment of proliferative granulopoiesis in the majority of cases. This observation was confirmed by in vitro culturing of granuloid precursor cells (CFU-c). Moreover, the data clearly demonstrated that drug-induced agranulocytosis may not be restricted to the granulocytic series. Thrombocytosis and reticulocytosis during the recovery phase are taken as an indication for the commitment of all haemopoietic cell lineages in agranulocytosis. These observations were in accordance with cytomorphological studies and in vitro culture data of erythroid precursor cells (CFU-e, BFU-e) of bone marrow aspirates taken in the initial phase of agranulocytosis. More than 25% of the patients showed a marked erythroid depression in the marrow.     

Drug-induced agranulocytosis: evidence for the commitment of bone marrow haematopoiesis

Clinical and haematological features of 61 patients with drug-induced agranulocytosis (63 episodes) are presented. Multiple drug consumption was a common observation and complicated the attempt to incriminate a particular drug as being aetiologically involved. Bone marrow analysis shortly after the diagnosis revealed evidence for an impairment of proliferative granulopoiesis in the majority of cases. This observation was confirmed by in vitro culturing of granuloid precursor cells (CFU-c). Moreover, the data clearly demonstrated that drug-induced agranulocytosis may not be restricted to the granulocytic series. Thrombocytosis and reticulocytosis during the recovery phase are taken as an indication for the commitment of all haemopoietic cell lineages in agranulocytosis. These observations were in accordance with cytomorphological studies and in vitro culture data of erythroid precursor cells (CFU-e, BFU-e) of bone marrow aspirates taken in the initial phase of agranulocytosis. More than 25% of the patients showed a marked erythroid depression in the marrow.     

Insulin-like growth factor I stimulates human erythroid colony formation in vitro

The effects of human GH and insulin-like growth factor I on the proliferation and differentiation of erythroid progenitor cells from the bone marrow and peripheral blood of children were studied in a hormone-depleted culture system. Growth of erythroid progenitors was quantified by directly scoring colonies and by biochemical determination of the activity of a cytosolic enzyme of the heme pathway, uroporphyrinogen I synthase. In the presence of erythropoietin, high concentrations (50-100 ng/mL) of human GH induced an increase in the number of erythroid colonies (and their uroporphyrinogen I synthase activity) formed by bone marrow or peripheral blood erythroid precursors. In the same conditions, physiological concentrations of insulin-like growth factor I (0.5-1 ng/mL) stimulated erythroid cell growth and differentiation (P less than 0.03) from bone marrow or peripheral blood.     

An erythremia with acquired HbH disease and chromosomal abnormality

A 56-year-old male was admitted to the Nihon University Hospital because of general fatigue and anemia on September 21st, 1985. He had mild hepato-splenomegaly. Hematological findings showed RBC 286 x 10(4)/microliters, Hb 6.0/dl, reticulocyte count 2.5%, platelet count 9.3 x 10(4)/microliters and WBC 2,400/microliters. An erythroblast per 100 leukocytes counted in a blood film was found. Bone marrow was erythroid hyperplasia with megaloblasts. The erythroblasts were PAS positive but not ringed sideroblasts. Other laboratory data including hemolysis were all negative. This case seemed to be diagnosed as refractory anemia (RA) according to the FAB classification. Chromosomal analysis of marrow cells, however, all revealed 46, XY, 20q- at diagnosis and 46, XY, 7q- 20q- after 22 months. Furthermore, Hb electrophoresis ahd family study indicated the presence of acquired HbH disease. Neither erythroid bursts (BFU-e) nor late erythroid progenitors (CFU-e) were detected. He has had progressive anemia without proliferation of blasts for over 2 years. From these findings, we postulate that the entity of erythremia should be distinguished from RA including many heterogeneous diseases.     

Expression of CD41 and c-mpl does not indicate commitment to the megakaryocyte lineage during haemopoietic development

Haemopoietic progenitors with the phenotype expected of early megakaryocyte precursors (CD34+ CD41+) were isolated from normal human bone marrow or induced in culture from CD34+ CD41- bone marrow cells by treatment with thrombopoietin (TPO) or IL-3. We found that although this population included the majority of cells that can form CFU-MK in culture, it also contained both erythroid and myeloid progenitors. The clonogenic potential of the CD34+ CD41+-induced cells was greater than that of isolated CD34+ CD41+ cells in that the isolated cells only formed CFU-MK and BFU-e, whereas the induced cells formed myeloid colonies as well. Glycophorin was found on isolated CD34+ CD41+ cells, not on induced cells. Its presence distinguished between MK and erythroid progenitors. Separation of a CD34+ CD41+ glycophorin A+ population resulted in the isolation of a highly purified population of BFU-e. A major portion of the cells that expressed CD34+ CD41+, in either cohort, were of the erythroid lineage. True MK progenitors were present in the CD34+ population in greater proportion than in whole marrow and were further enriched amongst CD34+ populations that expressed CD41. The presence of the thrombopoietin (TPO) receptor, c-mpl, did not correlate with inducibility of the gpIIbIIIa complex since essentially all CD34+ progenitors, including the earliest identifiable human haemopoietic progenitors (CD34+ CD38- cells), expressed c-mpl mRNA detectable by PCR regardless of their ultimate fate. Thus neither the expression of CD41 nor the expression of c-mpl was predictive of commitment to the MK lineage.