Comparative measurements of different types of hematopoietic progenitors present in human fetal liver, cord blood, and adult marrow showed a large (up to 250-fold), stage-specific, but lineage-unrestricted, amplification of the colony-forming cell (CFC) compartment in the fetal liver, with a higher ratio of all types of CFC to long-term culture-initiating cells (LTC-IC) and a lower ratio of total (mature) cells to CFC. Human fetal liver LTC-IC were also found to produce more CFC in LTC than cord blood or adult marrow LTC-IC, and more of the fetal liver LTC-IC–derived CFC were erythroid. Human fetal liver cells regenerated human multilineage hematopoiesis in NOD/SCID mice with the same kinetics as human cord blood and adult marrow cells, but sustained a high level of terminal erythropoiesis not seen in adult marrow-engrafted mice unless exogenous human erythropoietin (Epo) was injected. This may be due to a demonstrated 10-fold lower activity of murine versus human Epo on human cells, sufficient to distinguish between a differential Epo sensitivity of fetal and adult erythroid precursors. Examination of human LTC-IC, CFC, and erythroblasts generated either in NOD/SCID mice and/or in LTC showed the types of cells and hemoglobins produced also to reflect their ontological origin, regardless of the environment in which the erythroid precursors were generated. We suggest that ontogeny may affect the behavior of cells at many stages of hematopoietic cell differentiation through key changes in shared signaling pathways.

HEMATOPOIESIS IS FIRST detectable in the extravascular regions of the developing human fetal liver during week 6 of gestation, which then remains the major site of hematopoiesis until birth.1 In addition to morphologically recognizable hematopoietic cells, the human fetal liver has been shown to contain the full hierarchy of more primitive progenitors detectable by functional endpoints. These include erythroid, megakaryopoietic, granulopoietic, and multilineage colony-forming cells (CFC) detectable in semisolid culture assays,2-7 their more primitive precursors, referred to as long-term culture-initiating cells (LTC-IC) because they are able to proliferate and differentiate in vitro for more than 5 weeks on stromal feeder layers,7 and transplantable stem cells able to engraft xenogeneic8-11 as well as allogeneic recipients.12 Comparisons of analogous cell types in murine fetal tissues and adult marrow have shown differences in their rate of turnover,13 surface phenotype,14,15 rate of regeneration posttransplant,16-21 responsiveness to specific growth factors,22-27 and differentiation programs executed by their lineage-restricted progeny.28-30 Previous studies have provided some indication that similar differences between fetal and adult hematopoietic cells of human origin also exist.31-37 

In vivo, the fetal liver is characterized by a predominance of terminally differentiating erythroid cells. This could reflect an intrinsically increased probability of pluripotent fetal liver stem cells to commit to this pathway or the acquisition by their erythroid-committed progenitors of features that favor expansion of their differentiating progeny in vivo. To investigate these possibilities, we examined the erythropoietic potential and types of hemoglobin produced by the erythroid progeny of very primitive (uncommitted) human fetal liver cells stimulated to differentiate under a variety of conditions either in vivo (in NOD/SCID mice) and/or in vitro (in CFC and LTC-IC assays). We then compared the results with parallel measurements for cells derived from newborn and adult sources of human hematopoietic cells (ie, cord blood and adult marrow).

MATERIALS AND METHODS

Human cells.

Human bone marrow cells were either aspirate samples obtained from normal individuals donating marrow for allogeneic marrow transplantation or were from cyropreserved cadaveric marrow samples obtained from the Northwest Tissue Center (Seattle, WA). Cord blood samples were obtained from mothers undergoing cesarean delivery of normal, full-term infants and livers were from 14- to 21-week-old aborted fetuses. The age of the embryo was determined by a foot length measurement. (There was no evidence of a consistent change in any parameter assessed in cells obtained from fetal livers over this range in gestational age and, accordingly, data from different fetal livers have been pooled.) For all human samples, approved institutional procedures for obtaining informed consent were followed. Single-cell suspensions were obtained from the human fetal livers by pushing the sample through a coarse sieve. The low-density (< 1.077 g/mL) cells were then isolated by density centrifugation on Ficoll-Hypaque (Pharmacia Biotech, Uppsala, Sweden). To obtain a population that was further enriched in CD34+ cells, cells bearing lineage markers expressed by different types of mature cells (lin+cells) were removed using a StemSep column according to the suppliers’ directions (StemCell Technologies, Vancouver, British Columbia, Canada).

Animals.

NOD/LtSz-scid/scid (NOD/SCID) mice38 were bred and maintained in microisolators under defined sterile conditions in the animal facility of the British Columbia Cancer Research Centre (Vancouver, British Columbia, Canada). Six- to 8-week-old mice were sublethally irradiated with 350 cGy from a 137Cs source the day before being intravenously injected with human cells. In some experiments, mice were injected intraperitoneally 3 times a week for 2 weeks from day 15 to day 30 posttransplant with the following combination of human growth factors: 10 μg per mouse of Steel factor (SF; Amgen, Thousand Oaks, CA), 6 μg per mouse of interleukin-3 (IL-3; Novartis, Basel, Switzerland), 6 μg per mouse of granulocyte-macrophage colony-stimulating factor (GM-CSF; Novartis), and 10 U per mouse of erythropoietin (Epo; StemCell) per injection. These mice were killed 2 hours after the last injection of growth factors. Cells were flushed from the shafts of the 4 hind leg long bones of all mice using a syringe and 21-g needle prefilled with cold Hanks’ balanced salt solution supplemented with 5% fetal calf serum (HF; StemCell) plus 5% pooled normal human serum (HF/5% HS), and a single-cell suspension was obtained by gentle aspiration.

Flow cytometry.

For immunophenotyping of human cells, suspensions were first incubated for 10 minutes at 4°C with HF/5% HS supplemented with 3 mg/mL of an antimouse Fc receptor antibody (2.4 G2) to block Fc receptors and prevent nonspecific antibody binding39 and were then labeled for 30 minutes at 4°C with antihuman CD34-fluorescein isothiocyanate (FITC; 8G12)40 and CD19-phycoerythrin (PE) and CD20-PE (Becton Dickinson [BD], San Jose, CA), or antihuman CD71-PE (OKT-9), CD45-PE, CD15-FITC, and CD66b-FITC (Pharmingen, Mississauga, Ontario, Canada), or glycophorin-A-FITC (10F7) and antimouse Ter-119-PE (Pharmingen), or CD41 (3H2) only, or CD34-FITC (8G12) and CD38-PE (BD), as described.41,42 Various phenotypes within the viable (propidium iodide-negative [PI], Sigma Chemical Co, St Louis, MO) fraction were determined using a FACSort (BD) and LYSIS II software (BD). Assessment of CD34+ cells was further restricted to cells with medium-to-high forward light scattering (FSC) and low side light scattering (SSC) properties. Positivity in all cases was defined as fluorescence that exceeded 99.98% (ie, >5 such events in 20,000 analyzed) of that obtained with irrelevant isotype-matched control antibodies labeled with the corresponding fluorochromes. For in vitro assays of human progenitor activity, cells labeled with both antihuman CD45-PE and CD34-FITC were isolated by fluorescence-activated cell sorting (FACS) as described.41,42 

For intracellular staining and analysis of human cells producing different hemoglobins (Hb), similar appearing, well-hemoglobinized, erythroblast-containing colonies generated in direct methylcellulose assays or in methylcellulose assays of 6-week LTC were harvested after 12 to 14 days of growth, pooled, and suspended in phosphate-buffered saline (PBS; StemCell). Because erythroid colonies from fetal liver and cord blood progenitors develop slightly faster, these were generally harvested on day 12, whereas those derived from adult marrow were usually harvested 2 days later. Cells obtained from engrafted NOD/SCID mice were washed 1 to 3 times in PBS and then incubated for 1 hour at room temperature in 1 mL of 4% formaldehyde in PBS (BDH Inc, Toronto, Ontario, Canada). Glutaraldehyde (EM grade) in PBS (Polyscience Inc, Warrington, PA) was then added (final concentration, 0.01%), and the cells were vortexed vigorously before centrifugation and resuspension in 250 μL of 5% nonfat dry milk in PBS for 10 minutes at room temperature. Cells were finally resuspended in 500 μL of 0.01% Triton X-100 (Bio-Rad, Richmond, CA) in PBS/0.1% bovine serum albumin (BSA; StemCell) and stained with FITC- and PE-labeled isotype control antibodies (or streptavidin-PE [SAv-PE; Pharmingen] alone in some cases) or antihuman Hb F-FITC (or HbF-biotin + SAv-PE in some cases; Isolab Inc, Akron, OH) plus anti-glycophorin A-PE (30 minutes at room temperature). The cells were then washed once, resuspended in PBS, and analyzed by FACS. To maximize specificity, gates were set first to exclude cell debris (very low FSC events) and then to select positive cells within the glycophorin A+ population.

In vitro progenitor assays.

Assays for colony-forming unit-erythroid (CFU-E), burst-forming unit-erythroid (BFU-E), colony-forming unit–granulocyte-macrophage (CFU-GM), and colony-forming unit granulocyte, erythroid, monocyte, megakaryocyte (CFU-GEMM) were performed in methylcellulose medium (H4330; StemCell) supplemented with 50 ng/mL of human SF and 20 ng/mL each of IL-3 and GM-CSF (Novartis), IL-6 (Cangene, Mississauga, Ontario, Canada), granulocyte colony-stimulating factor (G-CSF; StemCell), and 3 U/mL Epo as described.43 

Colony-forming units-megakaryocyte (CFU-Mk) were assayed separately in a serum-free agarose culture medium supplemented with 50 ng/mL of human thrombopoietin (TPO; Genentech, San Francisco, CA) and 10 ng/mL each of IL-3 and IL-6 to allow specific identification by APAAP staining of pure Mk (≥3 CD41+ cells/colony) as well as mixed Mk colonies (containing other lineages as well as CD41+ Mk), as previously described.44 

Cells were assayed for LTC-IC in 6-week cocultures that contained mixed feeders of M2-10B4 and Sl/Sl fibroblasts genetically engineered to produce human IL-3 (10 ng/mL), human G-CSF (130 ng/mL), and human SF (10 ng/mL), as previously described.43 LTC-IC frequencies were determined by limiting dilution analysis of the proportions of negative cultures (<1 CFC produced per culture) in groups of cultures seeded with different numbers of test cells (3 to 4 dilutions spanning a 30-fold range, 13 to 24 cultures per dilution). Otherwise, LTC-IC values were determined by dividing the total CFC output obtained from bulk culture assays by the CFC per LTC-IC value determined for the respective type of LTC-IC (see Table 3).

Proliferation bioassays of murine and human Epo.

Purified recombinant murine and human Epo were gifts from Dr E. Goldwasser (University of Chicago, Chicago, IL) and Stem Cell, respectively. Murine BAF/3 cells expressing the murine Epo receptor45 were obtained from Dr G. Krystal (Terry Fox Laboratory, Vancouver, British Columbia, Canada) and maintained in RPMI with 10% fetal calf serum (FCS; Stem Cell) supplemented with 0.5 ng/mL murine IL-3 (Terry Fox Laboratory). Human TF-1 cells46 were obtained from Dr R. Kay (Terry Fox Laboratory) and maintained in RPMI with 10% FCS and 2.5 ng/mL human GM-CSF. To assay the response of BAF/3 cells to murine and human Epo, 104 log phase cells were incubated for 24 hours at 37°C in 100 mL of RPMI with 5% FCS containing various concentrations of Epo. To assay the response of TF-1 cells, log phase cells were first incubated for 24 hours at 37°C in RPMI with 10% FCS and were then transferred at 104 per 100 mL RPMI plus 5% FCS plus Epo for another 48 hours. In both cases,3H-thymidine (8 μCi/mL, 2.0 Ci/mmol; NEN Life Science Product, Inc, Boston, MA) was added 4 hours before harvesting the cells for measurement of their radioactivity.

Statistical analysis.

All values shown are the mean ± SEM unless otherwise indicated. Significant differences (P < .05) were established using the Student’s t-test.

RESULTS

Frequencies of different types of CFC in human fetal liver by comparison with cord blood and adult marrow.

Table 1 shows a comparison of the frequencies of different types of CFC in the light density fraction of 9 independently assessed human fetal liver samples. For comparison, the same reagents and procedures were used to analyze the light density cells from 12 human cord bloods and 7 normal adult human marrow samples. The frequency of every progenitor type measured (relative to the total nucleated cell population) was found to be higher in fetal liver than in either cord blood or adult marrow, although none of the differences was statistically significant (P > .05) due to the extensive variability in CFC frequencies in individual samples. The increased frequency of fetal liver CFC was most pronounced for CFU-GEMM (60- and 250-fold higher, respectively, in fetal liver), intermediate for BFU-E/CFU-E and CFU-GM (∼50-fold and 100-fold higher, respectively, in fetal liver), and least for CFU Mk (∼3-fold higher in fetal liver).

Table 1.

Comparison of the Frequencies of Different Types of CFC Per 105 Light-Density Human Fetal Liver, Cord Blood, and Adult Marrow Cells and Their Representation in the Total CFC Compartment

Type of CFC Assayed No. of CFC Per 105 Cells Assayed % of Total CFC
Fetal Liver Cord Blood Adult Marrow Fetal LiverCord Blood Adult Marrow
CFU-GEMM  490 ± 320 8 ± 4  2 ± 1  8 ± 5  3 ± 1  4 ± 5 
CFU-GM  2,500 ± 1,400  120 ± 30  35 ± 6 35 ± 6  63 ± 3  53 ± 4  
BFU-E 2,900 ± 800  58 ± 10  27 ± 5  55 ± 5 32 ± 3  39 ± 5  
CFU-E  130 ± 50  4 ± 1 4 ± 1  2 ± 1  2 ± 1  4 ± 2  
CFU-Mk (>50 Mk/col)  21 ± 20  15 ± 8  14 ± 7 0.1 ± 0.1  5 ± 2  4 ± 2  
CFU-Mk (3-50 Mk/col) 120 ± 90  26 ± 11  35 ± 11  0.6 ± 0.5 11 ± 5  13 ± 3  
Total  6,045 ± 2,100 190 ± 40  66 ± 25 
Type of CFC Assayed No. of CFC Per 105 Cells Assayed % of Total CFC
Fetal Liver Cord Blood Adult Marrow Fetal LiverCord Blood Adult Marrow
CFU-GEMM  490 ± 320 8 ± 4  2 ± 1  8 ± 5  3 ± 1  4 ± 5 
CFU-GM  2,500 ± 1,400  120 ± 30  35 ± 6 35 ± 6  63 ± 3  53 ± 4  
BFU-E 2,900 ± 800  58 ± 10  27 ± 5  55 ± 5 32 ± 3  39 ± 5  
CFU-E  130 ± 50  4 ± 1 4 ± 1  2 ± 1  2 ± 1  4 ± 2  
CFU-Mk (>50 Mk/col)  21 ± 20  15 ± 8  14 ± 7 0.1 ± 0.1  5 ± 2  4 ± 2  
CFU-Mk (3-50 Mk/col) 120 ± 90  26 ± 11  35 ± 11  0.6 ± 0.5 11 ± 5  13 ± 3  
Total  6,045 ± 2,100 190 ± 40  66 ± 25 

Values for the first 4 types of CFC are from 9 fetal liver, 12 cord blood, and 7 adult marrow samples assessed individually. Values for the CFU-Mk are from 13 fetal liver, 6 cord blood, and 8 adult marrow samples. Total refers to CFU-GEMM plus CFU-GM + BFU-E + CFU-E only.

Assessment of the frequency and CFC output of human fetal liver LTC-IC.

Preliminary experiments demonstrated that LTC-IC assays could not be performed directly on low-density human fetal liver cells, because this resulted in the rapid production of large numbers of macrophages, which was usually followed within the first 2 weeks by a complete destruction of the feeder layer (data not shown). However, initial removal of lin+ cells from the input fetal liver population allowed the presence of LTC-IC to be shown. Limiting dilution assays were used to determine the frequency of LTC-IC in the lincells (62% ± 8% CD34+ cells) obtained from 4 different fetal livers and in the CD34+CD38 cells isolated by FACS from 2 of these (same samples as used to obtain CFC frequencies in Table 1). This showed that 1.0% ± 0.2% of the lin fetal liver cells were LTC-IC. Calculation of the LTC-IC content of the original low-density fetal liver cell population (assuming all LTC-IC were recovered in the lin fraction) gives a frequency (7.6 ± 1.7 LTC-IC per 104 light-density fetal liver cells) which is approximately twice that previously determined for low-density adult marrow cells.43 In the 2 experiments in which a direct comparison was made of the frequency of LTC-IC in the lin (70% CD34+) and CD34+CD38 cells obtained from the same fetal liver samples, isolation of the CD34+CD38 cells gave a further approximately 4-fold enrichment of the LTC-IC, at a final purity of 4%, and showed that approximately 79% of the LTC-IC were recovered in the CD34+CD38 cell fraction.

Assessment in 1 of these experiments of the frequency of fetal liver LTC-IC using parental M2-10B4 cells (not producing human growth factors) as feeders showed that the number of LTC-IC detected was 2.5-fold lower than the value obtained with the human growth-factor–producing feeders. This difference is similar to what we have previously found for adult marrow and cord blood LTC-IC.43 

The LTC-IC frequency data obtained from the limiting dilution analyses were also used to calculate the average 6-week output per LTC-IC of each type of CFC assessed. Table 2 shows the results obtained together with those we have previously reported for adult marrow and cord blood LTC-IC assayed using the same reagents and procedures.43 The results for linCD34+ and CD34+CD38 fetal liver LTC-IC were not different (data not shown) and have been combined. By comparison with cord blood and adult marrow, fetal liver LTC-IC were found to produce, on average, significantly more (P< .05) CFC of all types, as well as proportionately more CFU-E and BFU-E (P = .06 for fetal liver v cord blood LTC-IC andP = .05 for fetal liver v adult marrow LTC-IC). Examination of the CFC produced by single LTC-IC (cultures initiated with a cell number containing on average ≤1 LTC-IC) showed a wide range of values (from 0 to 464 CFC per well). A broad range of CFC outputs was previously also described for individual adult marrow LTC-IC detected using human marrow feeders.47 On average, 20% of the wells plated with a single fetal liver LTC-IC in the present experiments contained both CFU-GM and BFU-E (and/or CFU-GEMM) as compared with 11% and less than 4% for single cord blood and adult marrow LTC-IC, respectively, assayed under the same conditions.

Table 2.

Output of Different Types of CFC in LTC From LTC-IC Changes According to the Ontological Origin of the LTC-IC

CFC Analyzed No. of CFC Per LTC-IC % of Total CFC Per LTC-IC
Fetal LiverCord Blood Adult Marrow Fetal Liver Cord BloodAdult Marrow
CFU-E + BFU-E  14 ± 7  0.6 ± 0.4 0.2 ± 0.1  17 ± 7  2 ± 2  1 ± 1  
CFU-GM 57 ± 14  28 ± 1  18 ± 3  81 ± 9 98 ± 2  99 ± 1  
CFU-GEMM  3 ± 1 0.03 ± 0.03  0.003 ± 0.003  2 ± 1 0.09 ± 0.09  0.03 ± 0.03  
Total  72 ± 18 28 ± 1  18 ± 2  100  100 100 
CFC Analyzed No. of CFC Per LTC-IC % of Total CFC Per LTC-IC
Fetal LiverCord Blood Adult Marrow Fetal Liver Cord BloodAdult Marrow
CFU-E + BFU-E  14 ± 7  0.6 ± 0.4 0.2 ± 0.1  17 ± 7  2 ± 2  1 ± 1  
CFU-GM 57 ± 14  28 ± 1  18 ± 3  81 ± 9 98 ± 2  99 ± 1  
CFU-GEMM  3 ± 1 0.03 ± 0.03  0.003 ± 0.003  2 ± 1 0.09 ± 0.09  0.03 ± 0.03  
Total  72 ± 18 28 ± 1  18 ± 2  100  100 100 

Values for fetal liver LTC-IC were derived from limiting dilution experiments described in the text. In each experiment, the total number of CFC (of the type indicated) produced in all of the cultures was divided by the total number of LTC-IC placed into these cultures. Values for cord blood and adult marrow LTC-IC were similarly calculated from the limiting dilution experiments reported in Hogge et al.43 

Analysis of the hematopoietic cell types regenerated in sublethally irradiated NOD/SCID mice transplanted with human fetal liver cells.

Figure 1 and Table 3 show the time course of changes in the numbers of various types of human hematopoietic cells detectable in the marrow of sublethally irradiated NOD/SCID mice assessed up to 16 weeks after the intravenous injection of 107 low-density human fetal liver cells. For comparison, comparable data (where available) from previously published time course studies of the patterns of engraftment seen with transplants of 107 low-density human cord blood42 or 2 × 107 low-density normal adult human marrow cells48 are shown. As found for other sources of human hematopoietic stem cells, less than 0.1% of any of the types of cells in human fetal liver that were injected could be found in the marrow of the mice 2 to 3 days later. After this, there was a rapid (within 2 to 4 weeks) increase in the marrow of the mice of many primitive and mature human hematopoietic cell types. All of these reached peak levels approximately 6 to 8 weeks posttransplant and were then sustained for the duration of the experiment, ie, up to 16 weeks posttransplant. Human populations detected included CD45/71+ cells, CD34+ cells, CD41+(Mk-lineage) cells, BFU-E, CFU-E, CFU-GM, and LTC-IC.

Fig. 1.

Kinetics of appearance of different types of human hematopoietic cells in NOD/SCID mice transplanted with 107low-density human fetal liver cells. Values shown represent the numbers calculated to be present in each entire mouse, assuming 2 femurs and 2 tibias comprise 25% of this value.60 Results from 2 independent experiments have been pooled (2 to 6 mice per time point). Previously published data for grafts of 107 light-density human cord blood cells (dashed line)42 or 2 × 107 light-density human adult marrow cells (dotted line)48 are shown for comparison.

Fig. 1.

Kinetics of appearance of different types of human hematopoietic cells in NOD/SCID mice transplanted with 107low-density human fetal liver cells. Values shown represent the numbers calculated to be present in each entire mouse, assuming 2 femurs and 2 tibias comprise 25% of this value.60 Results from 2 independent experiments have been pooled (2 to 6 mice per time point). Previously published data for grafts of 107 light-density human cord blood cells (dashed line)42 or 2 × 107 light-density human adult marrow cells (dotted line)48 are shown for comparison.

Table 3.

Kinetics of LTC-IC Production in NOD/SCID Mice Transplanted With 107 Light-Density Human Fetal Liver or Cord Blood or 2 × 107 Light-Density Adult Marrow Cells Are Similar

Time Posttransplant (wks) Fetal Liver Cord BloodAdult Marrow
Input  11,500 ± 600  730 ± 360 2,500 ± 1,100  
2  10,600 ± 9,300  110 ± 50 50 ± 20  
6  80,600 ± 58,100  1,430 ± 250 1,030 ± 540  
8-10  52,000 ± 11,700 3,400 ± 1,150  720 ± 630 
Time Posttransplant (wks) Fetal Liver Cord BloodAdult Marrow
Input  11,500 ± 600  730 ± 360 2,500 ± 1,100  
2  10,600 ± 9,300  110 ± 50 50 ± 20  
6  80,600 ± 58,100  1,430 ± 250 1,030 ± 540  
8-10  52,000 ± 11,700 3,400 ± 1,150  720 ± 630 

Values shown are the absolute LTC-IC numbers calculated per mouse body, assuming 2 femurs and 2 tibias represent 25% of the total60 and that the CFC output per LTC-IC is the same as the LTC-IC in the suspension originally transplanted (ie, 72 for fetal liver-derived LTC-IC, from Table 2; 28 for cord blood-derived LTC-IC and 18 for adult marrow-derived LTC-IC from Hogge et al43). The fetal liver data are from the same 2 experiments shown in Fig 1. The cord blood and adult marrow data are derived from Cashman et al.42,48 

Analysis of the distribution of human CFC subtypes among those present at different times posttransplant (Table 4) indicates that there was a slight shift between week 2 and week 6 in favor of granulopoietic CFC, as seen previously in NOD/SCID mice engrafted with human cord blood and adult marrow cells. Also of note is the observation that fetal liver grafts regenerated relatively more progeny LTC-IC (8-fold over input) than was achieved previously with transplants of cord blood (5-fold over input) or adult marrow (decreased 2-fold below input). Assessment of the number of glycophorin A+ cells (erythroblasts) showed large numbers of these to be present in the fetal liver and cord blood-engrafted mice, but none was detected in the marrow-engrafted mice (Fig 2). A 2-week course of 6 injections of human Epo, SF, IL-3, and GM-CSF restored the ability of adult human erythroid progenitors to mature into erythroblasts in vivo (as previously reported for similar transplants in SCID mice49), but had no effect (P > .05) on terminal human erythropoiesis in the mice engrafted with fetal liver or cord blood cells (Fig 2; P = .4 and P = .2, respectively) or on any other type of human progenitor in any of the groups (data not shown).

Table 4.

Relative Frequencies of Different Types of CFC in Human Fetal Liver-Engrafted NOD/SCID Mice Assessed at Different Time Points Posttransplant

Time Posttransplant (wks) % of Total CFC
CFU-E BFU-E CFU-GEMM CFU-GM
Input 2 ± 2  40 ± 4  3 ± 3  55 ± 10  
Week 2 8 ± 1  43 ± 5  9 ± 5  40 ± 8  
Week 6 2.5 ± 0.2  20 ± 2  3 ± 1  75 ± 1 
Week 9  1 ± 0.3  28 ± 2  4 ± 1 67 ± 2 
Time Posttransplant (wks) % of Total CFC
CFU-E BFU-E CFU-GEMM CFU-GM
Input 2 ± 2  40 ± 4  3 ± 3  55 ± 10  
Week 2 8 ± 1  43 ± 5  9 ± 5  40 ± 8  
Week 6 2.5 ± 0.2  20 ± 2  3 ± 1  75 ± 1 
Week 9  1 ± 0.3  28 ± 2  4 ± 1 67 ± 2 

Values shown are from the same 2 experiments shown in Fig 1.

Fig. 2.

Total human (CD45/71+) and mature erythroid (glycophorin A+) cells in NOD/SCID mice transplanted with equivalent grafts of human fetal liver, cord blood, or adult marrow cells and then injected with recombinant human growth factors (▪) or not (□) as described in the text. The difference plus and minus growth factors for all pairs was not significant (P > .05) except for glycophorin A+ cells in the mice transplanted with adult marrow (P << .001).

Fig. 2.

Total human (CD45/71+) and mature erythroid (glycophorin A+) cells in NOD/SCID mice transplanted with equivalent grafts of human fetal liver, cord blood, or adult marrow cells and then injected with recombinant human growth factors (▪) or not (□) as described in the text. The difference plus and minus growth factors for all pairs was not significant (P > .05) except for glycophorin A+ cells in the mice transplanted with adult marrow (P << .001).

Because we had found that freshly isolated human fetal liver LTC-IC display an enhanced erythropoietic activity in vitro by comparison with those present in cord blood or adult marrow, it was of interest to also examine the types of CFC produced by the human LTC-IC regenerated in fetal liver-engrafted mice. As shown in Table 5, these were characterized by the same increased output of CFU-E and BFU-E as the LTC-IC present in freshly isolated fetal liver suspensions.

Table 5.

Comparison of Different Types of CFC Produced by the LTC-IC Regenerated in Human Fetal Liver-Engrafted NOD/SCID Mice at Different Time Points Posttransplant

Time Posttransplant (wks)Relative Proportions of Total CFC Produced in LTC-IC Assays
CFU-E BFU-E CFU-GM CFU-GEMM
Input 4 ± 4  13 ± 7  83 ± 9  0.2 ± 0.2 
Week 2  3 ± 2  21 ± 9  75 ± 11  1 ± 1 
Week 6  2 ± 1  13 ± 2  84 ± 4  2 ± 1 
Week 9  2 ± 1  27 ± 6  70 ± 6 1 ± 1 
Time Posttransplant (wks)Relative Proportions of Total CFC Produced in LTC-IC Assays
CFU-E BFU-E CFU-GM CFU-GEMM
Input 4 ± 4  13 ± 7  83 ± 9  0.2 ± 0.2 
Week 2  3 ± 2  21 ± 9  75 ± 11  1 ± 1 
Week 6  2 ± 1  13 ± 2  84 ± 4  2 ± 1 
Week 9  2 ± 1  27 ± 6  70 ± 6 1 ± 1 

Values shown are from the same 2 experiments shown in Fig 1.

Murine Epo has a reduced activity on human cells.

A reduced ability of adult versus fetal and neonatal human erythroid cells to undergo terminal maturation in NOD/SCID mice would be anticipated from a slight differential (decreased) responsiveness of adult cells to Epo if the level of murine Epo in the mice were limiting. We therefore compared the ability of murine and human Epo to stimulate murine and human Epo receptor-expressing cells. When murine targets were used (murine Epo receptor-expressing BAF/3 cells),45 both preparations gave superimposable dose-response curves (data not shown). The unitage of the human Epo preparation could thus be used to assign a unitage to the murine Epo preparation. The 2 preparations were then tested for their abilities to stimulate the proliferation of human TF-1 cells.46 As shown in Fig 3, concentrations of murine Epo that had an equivalent ability to human Epo to stimulate murine cells were approximately 10-fold less active on human cells. Thus, levels of murine Epo that are sufficient to sustain murine erythropoiesis might also be adequate to stimulate human erythropoietic cells of fetal (and neonatal) but not adult origin, which could thus explain the unique requirement of adult human erythroid precursors for additional (exogenously administered) human Epo to terminally differentiate in an in vivo murine environment.

Fig. 3.

Dose-response curves showing the differential mitogenic effects of murine and human Epo on human TF-1 cells. The specific activities of both preparations were standardized using murine Epo-expressing BAF/3 cells.

Fig. 3.

Dose-response curves showing the differential mitogenic effects of murine and human Epo on human TF-1 cells. The specific activities of both preparations were standardized using murine Epo-expressing BAF/3 cells.

Fidelity of globin gene expression in the human erythroid progeny of cells produced under different conditions from ontologically distinct sources.

Terminally differentiating human erythroblasts generated in vitro from progenitors produced in LTC or in NOD/SCID mice or in LTC initiated with cells from NOD/SCID mice were assessed for the type of Hb they contained by FACS analysis of intracytoplasmically stained fixed cell preparations (as described in Materials and Methods). For each sample, the preparation was counter-stained with antihuman glycophorin A antibody labeled with a different fluorochrome to enhance the specificity of the procedure. Representative profiles are shown in Fig 4. The data for all sources of erythroblasts are summarized in Table 6. Most of the erythroblasts within, or derived in vitro from progenitors from, suspensions of fetal liver or cord blood expressed high levels of HbA and HbF. As expected, most of the erythroblasts present in freshly obtained adult marrow contained HbA and very few contained HbF, although increased production of HbF+ cells was seen when marrow progenitors were stimulated to generate erythroblasts in vitro.

Fig. 4.

FACS profiles of glycophorin A+ cells obtained directly from fetal liver, cord blood, and adult marrow or from directly plated 12- to 14-day-old erythroid colonies generated in methylcellulose assays of human fetal liver, cord blood, or adult marrow cells or from human CFU-E/BFU-E produced in NOD/SCID mice engrafted for 6 weeks with human fetal liver, cord blood, or adult marrow cells. Harvested cells were stained with anti-glycophorin A and anti-Hb antibodies as described in the Materials and Methods. Note that the intensity of staining with the 2 different anti-Hb antibodies provide only a relative measure of the amount of HbA and HbF in each cell and therefore cannot be used to compare absolute HbA and HbF expression levels.

Fig. 4.

FACS profiles of glycophorin A+ cells obtained directly from fetal liver, cord blood, and adult marrow or from directly plated 12- to 14-day-old erythroid colonies generated in methylcellulose assays of human fetal liver, cord blood, or adult marrow cells or from human CFU-E/BFU-E produced in NOD/SCID mice engrafted for 6 weeks with human fetal liver, cord blood, or adult marrow cells. Harvested cells were stained with anti-glycophorin A and anti-Hb antibodies as described in the Materials and Methods. Note that the intensity of staining with the 2 different anti-Hb antibodies provide only a relative measure of the amount of HbA and HbF in each cell and therefore cannot be used to compare absolute HbA and HbF expression levels.

Table 6.

Proportion of Erythroid Cells Produced From Different Sources of Progenitors Containing Different Types of Hb

Origin of Erythroblasts % HbA+ Erythroid Cells % HbF+Erythroid Cells
Fetal Liver Cord Blood Adult MarrowFetal Liver Cord Blood Adult Marrow
Erythroblasts 82 ± 14  97 ± 3  94  84 ± 8  96 ± 4 4  
BFU-E/CFU-E  65 ± 12  62 ± 7  76 ± 5  96 ± 0.2  76 ± 13  36 ± 8  
LTC-IC–derived BFU-E/CFU-E  84 ± 14  76 ± 19  Not detectable 92 ± 4  88 ± 2  Not detectable  
BFU-E/CFU-E from NOD/SCID mice  63 ± 12  61 ± 11  55 ± 13 87 ± 6  38 ± 8  20 ± 1  
LTC-IC–derived BFU-E/CFU-E from NOD/SCID mice  66 ± 6  ND  Not detectable  71 ± 4  ND  Not detectable 
Origin of Erythroblasts % HbA+ Erythroid Cells % HbF+Erythroid Cells
Fetal Liver Cord Blood Adult MarrowFetal Liver Cord Blood Adult Marrow
Erythroblasts 82 ± 14  97 ± 3  94  84 ± 8  96 ± 4 4  
BFU-E/CFU-E  65 ± 12  62 ± 7  76 ± 5  96 ± 0.2  76 ± 13  36 ± 8  
LTC-IC–derived BFU-E/CFU-E  84 ± 14  76 ± 19  Not detectable 92 ± 4  88 ± 2  Not detectable  
BFU-E/CFU-E from NOD/SCID mice  63 ± 12  61 ± 11  55 ± 13 87 ± 6  38 ± 8  20 ± 1  
LTC-IC–derived BFU-E/CFU-E from NOD/SCID mice  66 ± 6  ND  Not detectable  71 ± 4  ND  Not detectable 

Values shown are the percent of the total number of human glycophorin A+ cells that stained positive for the type of Hb being analyzed. Human CFC and LTC-IC from engrafted NOD/SCID mice were assessed 6 weeks posttransplant.

Abbreviation: ND, not done.

Analysis of the erythroblast progeny of progenitors generated from human fetal liver cells within the microenvironment of the adult NOD/SCID mouse bone marrow, or in vitro from LTC-IC generated in human fetal liver-engrafted mice, showed most of these to continue to be HbF+. Similarly, the relatively low proportion of HbF+ erythroblasts produced in vitro from progenitors generated in adult human marrow-engrafted mice was the same as for those generated from freshly isolated adult human marrow progenitors. Interestingly, in the human cord blood-engrafted mice, the proportion of HbF+ erythroblasts obtained from progenitors produced 6 weeks posttransplant had declined to half the level seen in erythroblasts obtained from freshly isolated progenitors (P < .04).

To examine the dual expression of HbA and HbF in the same cells (which precluded the use of glycophorin A-staining), cells were harvested from maturing erythroid colonies generated in vitro from progenitors of different sources. These provide an enriched, although not pure, population of erythroblasts. As can be seen in Fig 5, most of the Hb+ cells obtained from colonies of fresh fetal liver origin contained both HbA and HbF. Similar results were obtained for colonies generated from human BFU-E isolated from fetal liver-engrafted mice. In contrast, many more of the Hb+ cells in colonies produced by adult marrow contained exclusively HbA.

Fig. 5.

Demonstration of cells containing both HbF and HbA in 12- to 14-day-old erythroid colonies generated in methylcellulose assays of human fetal liver or adult marrow CFC and CFC obtained from NOD/SCID mice transplanted 6 weeks previously with human fetal liver cells.

Fig. 5.

Demonstration of cells containing both HbF and HbA in 12- to 14-day-old erythroid colonies generated in methylcellulose assays of human fetal liver or adult marrow CFC and CFC obtained from NOD/SCID mice transplanted 6 weeks previously with human fetal liver cells.

DISCUSSION

The present study shows a number of important features of human fetal liver hematopoiesis that identify differences in how this process is regulated in the fetus and in the adult. At both stages of ontogeny, terminally differentiating erythroid and myeloid cells make up the majority of the light-density population in the hematopoietic tissues in which they are produced. However, erythroid cells predominate in the fetal liver, whereas myeloid cells predominate in the adult. In contrast, more primitive cells, detected in vitro as erythroid and granulopoietic CFC, although present at a higher overall frequency (relative to the total number of cells) in fetal liver, were also found to be present in both fetal liver and adult marrow at similar frequencies to each other (Table 1).5,33 The lack of a selective increase in the number of erythroid progenitors accumulated in vivo does not argue in favor of a model of increased commitment of fetal hematopoietic stem cells to the erythroid pathway to explain the predominance of erythroid cells seen in the terminally differentiating fetal liver populations. Nevertheless, a shift in that direction was suggested here by the increased output of erythroid progenitors from individual fetal liver (as compared with adult marrow) LTC-IC under LTC conditions. These conditions are nonpermissive for erythropoiesis and hence allow detection of the first progenitors to commit to that pathway.

We also found that the frequency of megakaryocyte progenitors was increased, although to a much smaller degree, whereas the frequency of multipotent CFC was increased to a greater extent than the granulopoietic and erythroid progenitors. As noted by others,7,50 we observed the maximum size of colonies produced in vitro by all types of CFC to be larger than those produced by their adult marrow counterparts. However, this increased CFC proliferative potential of fetal liver CFC is not realized in vivo, because the ratio of CFC to mature cells is decreased. This could occur either because the microenvironment of the fetal liver promotes the more rapid differentiation of fetal progenitors and/or because a higher rate of apoptosis occurs among their progeny. Because the composition of the mature cell compartment in adult marrow and fetal liver also differ, the mechanisms responsible for affecting these changes in cell output must be distinct for each lineage.

Given the large increases seen in the frequency of CFC in the human fetal liver (overall, ∼70-fold relative to adult marrow), the discovery that the frequency of LTC-IC is only 2-fold higher was unanticipated. This means that the ratio of CFC to LTC-IC in the fetal liver is much higher than in adult marrow. Interestingly, this feature was reproduced in the LTC system, in which we also noted an increased (4-fold higher) 6-week output of CFC by comparison with adult marrow LTC-IC (72 v 18 CFC per LTC-IC). A similarly enhanced output of CFC by fetal liver LTC-IC was noted by Roy et al7 using a shorter (5-week) assay on parental M2-10B4 cell feeders that may detect a less primitive LTC-IC subset.43 Similarly, a greater output of progeny from fetal versus adult sources of CD34+CD45RACD71−31 or CD34+CD38 cells51 in stroma-free cytokine-supplemented cultures has been reported. Such a result could be explained by a decreased self-renewal probability of primitive fetal cells resulting in an expanded yield of CFC or a reduced cell cycle time resulting in an accumulation of more CFC through more cell generations. It is also possible that the progeny of fetal LTC-IC may acquire an ability to form colonies in semisolid media without such a precipitous loss of proliferative potential, or they may be less susceptible to conditions that cause apoptosis of their adult counterparts. Current evidence argues against the first possibility, because fetal stem cells of both human52 and murine17,19 origin have been found to display increased rather than decreased self-renewal properties. In addition, evidence of a shorter cell cycle time and faster differentiation of fetal hematopoietic cells has been reported.3,16,53 

The present studies also provide evidence of the intrinsic determination of a number of other properties of fetal human hematopoietic cells that are linked to their embryonic status. These include an increased output of erythroid CFC by fetal LTC-IC, an increased sensitivity of fetal CFU-E to factors that support their terminal differentiation, and an increased production of HbF by their erythroblast progeny. All of these characteristics were shown to be stably transmitted to the progeny of fetal stem cells stimulated to proliferate and differentiate in the marrow of NOD/SCID mice; in some cases, even those generated after transfer of in vivo-derived LTC-IC to cultures containing murine stromal feeder layers engineered to produce human G-CSF, IL-3, and SF. Less stability of HbF production was seen previously when fetal sheep cells were transplanted into adult sheep54 or when human fetal cells were cultured for prolonged periods in the absence of stroma.55 From such results it has been suggested that the type of hemoglobin produced can be influenced by the environment in which the cells are generated. On the other hand, a fetal human erythroid program was seen in the erythroid cells produced in both patients56,57 and sheep58 transplanted with fetal cells, consistent with the results reported here. The continued production in the presence of murine stroma of human progenitors that remain committed to a high level of HbF and low HbA synthesis appears to represent only one of several manifestations of a preserved fetal hematopoietic program that affects multiple stages of differentiation. It seems unlikely that the different behaviors of corresponding stages of differentiation of fetal and adult cells will reflect a single common underlying intrinsic molecular alteration. However, it is interesting to note that, even in adult erythroid precursors, HbF expression is enhanced by strong growth factor stimulation34,59 and that a feature of fetal erythroid precursors appears to be their enhanced sensitivity to growth factor stimulation. Such findings thus invite speculation as to whether some basic differences in the intracellular signaling machinery of fetal hematopoietic cells may account for their distinct biology. Investigations of this possibility are now underway.

ACKNOWLEDGMENT

The authors thank the staff of the Stem Cell Assay Service for their assistance in the initial preparation of primary human cell samples, Jessyca Maltman and Maya Sinclaire for their assistance in the animal studies, Gayle Thornbury and Giovanna Cameron for operating the FACS, and Tara Palmater for secretarial assistance. We are also grateful to E. Goldwasser (University of Chicago), P. Lansdorp, G. Krystal, and R. Kay (Terry Fox Laboratory), and Amgen, Cangene, Isolab, Novartis, and Stem Cell for valuable gifts of cells or reagents.

Supported by grants from the National Institutes of Health (PO1 HL55435), Novartis (Basel, Switzerland), and the National Cancer Institute of Canada (NCIC) with funds from the Terry Fox Run. T.L.H. holds a United Kingdom Leukemia Research Fund Senior Lectureship and P.P.Y.C. holds a Roman Babicki Graduate Studentship from the University of British Columbia. C.J.E. is a Terry Fox Cancer Research Scientist of the NCIC.

The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. section 1734 solely to indicate this fact.

REFERENCES

REFERENCES
1
Kelemen
E
Janossa
M
Macrophages are the first differentiated blood cells formed in human embryonic liver.
Exp Hematol
8
1980
996
2
Rowley
PT
Ohlsson-Wilhelm
BM
Farley
BA
Erythroid colony formation from human fetal liver.
Proc Natl Acad Sci USA
75
1978
984
3
Hassan
MW
Lutton
JD
Levere
RD
Rieder
RF
Cederqvist
LL
In vitro culture of erythroid colonies from human fetal liver and umbilical cord blood.
Br J Haematol
41
1979
477
4
Peschle
C
Migliaccio
AR
Migliaccio
G
Ciccariello
R
Lettieri
F
Quattrin
S
Russo
G
Mastroberardino
G
Identification and characterization of three classes of erythroid progenitors in human fetal liver.
Blood
58
1981
565
5
Hann
IM
Bodger
MP
Hoffbrand
AV
Development of pluripotent hematopoietic progenitor cells in the human fetus.
Blood
62
1983
118
6
Muench
MO
Cupp
J
Polakoff
J
Roncarolo
MG
Expression of CD33, CD38, and HLA-DR on CD34+ human fetal liver progenitors with a high proliferative potential.
Blood
83
1994
3170
7
Roy
V
Miller
JS
Verfaillie
CM
Phenotypic and functional characterization of committed and primitive myeloid and lymphoid hematopoietic precursors in human fetal liver.
Exp Hematol
25
1997
387
8
Zanjani
ED
Flake
AW
Rice
H
Hedrick
M
Tavassoli
M
Long-term repopulating ability of xenogeneic transplanted human fetal liver hematopoietic stem cells in sheep.
J Clin Invest
93
1994
1051
9
Zanjani
ED
Pallavicini
MG
Ascensao
JL
Flake
AW
Langlois
RG
Reitsma
M
MacKintosh
FR
Stutes
D
Harrison
MR
Tavassoli
M
Engraftment and long-term expression of human fetal hemopoietic stem cells in sheep following transplantation in utero.
J Clin Invest
89
1992
1178
10
Yurasov
S
Kollmann
TR
Kim
A
Raker
CA
Hachamovitch
M
Wong-Staal
F
Goldstein
H
Severe combined immunodeficiency mice engrafted with human T cells, B cells, and myeloid cells after transplantation with human fetal bone marrow or liver cells and implanted with human fetal thymus: A model for studying human gene therapy.
Blood
89
1997
1800
11
Humeau
L
Chabannon
C
Firpo
MT
Mannoni
P
Bagnis
C
Roncarolo
M-G
Namikawa
R
Successful reconstitution of human hematopoiesis in the SCID-hu mouse by genetically modified, highly enriched progenitors isolated from fetal liver.
Blood
90
1997
3496
12
Touraine
JL
Raudrant
D
Laplace
S
Transplantation of hemopoietic cells from the fetal liver to treat patients with congenital diseases postnatally or prenatally.
Transplant Proc
29
1997
712
13
Fleming
WH
Alpern
EJ
Uchida
N
Ikuta
K
Spangrude
GJ
Weissman
IL
Functional heterogeneity is associated with the cell cycle status of murine hematopoietic stem cells.
J Cell Biol
122
1993
897
14
Rebel
VI
Miller
CL
Thornbury
GR
Dragowska
WH
Eaves
CJ
Lansdorp
PM
A comparison of long-term repopulating hematopoietic stem cells in fetal liver and adult bone marrow from the mouse.
Exp Hematol
24
1996
638
15
Morrison
SJ
Hemmati
HD
Wandycz
AM
Weissman
IL
The purification and characterization of fetal liver hematopoietic stem cells.
Proc Natl Acad Sci USA
92
1995
10302
16
Kubanek
B
Rencricca
N
Porcellini
A
Howard
D
Stohlman
F
The pattern of stem cell repopulation in heavily irradiated mice receiving transplants of fetal liver.
Blood
35
1970
64
17
Micklem
HS
Ford
CE
Evans
EP
Ogden
DA
Papworth
DS
Competitive in vivo proliferation of foetal and adult haematopoietic cells in lethally irradiated mice.
J Cell Physiol
79
1972
293
18
Harrison
DE
Astle
CM
Loss of stem cell repopulating ability upon transplantation. Effects of donor age, cell number, and transplantation procedure.
J Exp Med
156
1982
1767
19
Rebel
VI
Miller
CL
Eaves
CJ
Lansdorp
PM
The repopulation potential of fetal liver hematopoietic stem cells in mice exceeds that of their adult bone marrow counterparts.
Blood
87
1996
3500
20
Pawliuk
R
Eaves
C
Humphries
RK
Evidence of both ontogeny and transplant dose-regulated expansion of hematopoietic stem cells in vivo.
Blood
88
1996
2852
21
Harrison
DE
Zhong
RK
Jordan
CT
Lemischka
IR
Astle
CM
Relative to adult marrow, fetal liver repopulates nearly five times more effectively long-term than short-term.
Exp Hematol
25
1997
293
22
Rich
IN
Kubanek
B
Erythroid colony formation (CFU-e) in fetal liver and adult bone marrow and spleen from the mouse.
Blut
33
1976
171
23
Ikuta
K
Weissman
IL
Evidence that hematopoietic stem cells express mouse c-kit but do not depend on steel factor for their generation.
Proc Natl Acad Sci USA
89
1992
1502
24
Miller
CL
Rebel
VI
Helgason
CD
Lansdorp
PM
Eaves
CJ
Impaired steel factor responsiveness differentially affects the detection and longterm maintenance of fetal liver hematopoietic stem cells in vivo.
Blood
89
1997
1214
25
Rebel
VI
Lansdorp
PM
Culture of purified stem cells from fetal liver results in loss of in vivo repopulating potential.
J Hematother
5
1996
25
26
Miller
CL
Eaves
CJ
Expansion in vitro of adult murine hematopoietic stem cells with transplantable lympho-myeloid reconstituting ability.
Proc Natl Acad Sci USA
94
1997
13648
27
Kurata
H
Mancini
GC
Alespeiti
G
Migliaccio
AR
Migliaccio
G
Stem cell factor induces proliferation and differentiation of fetal progenitor cells in the mouse.
Br J Haematol
101
1998
676
28
Barker
JE
Development of the mouse hematopoietic system.
Dev Biol
18
1968
14
29
Hardy
RR
Hayakawa
K
A developmental switch in B lymphopoiesis.
Proc Natl Acad Sci USA
88
1991
11550
30
Weissman
IL
Developmental switches in the immune system.
Cell
76
1994
207
31
Lansdorp
PM
Dragowska
W
Mayani
H
Ontogeny-related changes in proliferative potential of human hematopoietic cells.
J Exp Med
178
1993
787
32
Rich
IN
Kubanek
B
The ontogeny of erythropoiesis in the mouse detected by the erythroid colony-forming technique.
J Embryol Exp Morphol
58
1980
143
33
Migliaccio
AR
Migliaccio
G
Human embryonic hemopoiesis: Control mechanisms underlying progenitor differentiation in vitro.
Dev Biol
125
1988
127
34
Papayannopoulou
TH
Brice
M
Stamatoyannopoulos
G
Hemoglobin F synthesis in vitro: Evidence for control at the level of primitive erythroid stem cells.
Proc Natl Acad Sci USA
74
1977
2923
35
Papayannopoulou
T
Chen
P
Maniatis
A
Stamatoyannopoulos
G
Simultaneous assessment of i-antigenic expression and fetal hemoglobin in single red cells by immunofluorescence.
Blood
55
1980
221
36
Zandstra
PW
Conneally
E
Piret
JM
Eaves
CJ
Ontogeny-associated changes in the cytokine responses of primitive human haematopoietic cells.
Br J Haematol
101
1998
770
37
Weekx
SFA
van Bockstaele
DR
Plum
J
Moulijn
A
Rodrigus
I
Lardon
F
De Smedt
M
Nijs
G
Lenjou
M
Loquet
P
Berneman
ZN
Snoeck
H-W
CD34++CD38− and CD34+CD38+ human hematopoietic progenitors from fetal liver, cord blood, and adult bone marrow respond differently to hematopoietic cytokines depending on the ontogenic source.
Exp Hematol
26
1998
1034
38
Shultz
LD
Schweitzer
PA
Christianson
SW
Gott
B
Schweitzer
IB
Tennent
B
McKenna
S
Mobraaten
L
Rajan
TV
Greiner
DL
Leiter
EH
Multiple defects in innate and adaptive immunologic function in NOD/LtSz-scid mice.
J Immunol
154
1995
180
39
Unkeless
JC
Characterization of a monoclonal antibody directed against mouse macrophage and lymphocyte Fc receptors.
J Exp Med
150
1979
580
40
Schmitt
C
Eaves
CJ
Lansdorp
PM
Expression of CD34 on human B cell precursors.
Clin Exp Immunol
85
1991
168
41
Conneally
E
Cashman
J
Petzer
A
Eaves
C
Expansion in vitro of transplantable human cord blood stem cells demonstrated using a quantitative assay of their lympho-myeloid repopulating activity in nonobese diabetic-scid/scid mice.
Proc Natl Acad Sci USA
94
1997
9836
42
Cashman
J
Bockhold
K
Hogge
DE
Eaves
AC
Eaves
CJ
Sustained proliferation, multi-lineage differentiation and maintenance of primitive human haematopoietic cells in NOD/SCID mice transplanted with human cord blood.
Br J Haematol
98
1997
1026
43
Hogge
DE
Lansdorp
PM
Reid
D
Gerhard
B
Eaves
CJ
Enhanced detection, maintenance and differentiation of primitive human hematopoietic cells in cultures containing murine fibroblasts engineered to produce human Steel factor, interleukin-3 and granulocyte colony-stimulating factor.
Blood
88
1996
3765
44
Hogge
D
Fanning
S
Bockhold
K
Petzer
A
Lambie
K
Lansdorp
P
Eaves
A
Eaves
C
Quantitation and characterization of human megakaryocyte colony-forming cells using a standardized serum-free agarose assay.
Br J Haematol
96
1997
790
45
Damen
J
Mui
AL-F
Hughes
P
Humphries
K
Krystal
G
Erythropoietin-induced tyrosine phosphorylations in a high erythropoietin receptor-expressing lymphoid cell line.
Blood
80
1992
1923
46
Kitamura
T
Tange
T
Terasawa
T
Chiba
S
Kuwaki
T
Miyagawa
K
Piao
YF
Miyazona
K
Urabe
A
Takaku
F
Establishment and characterization of a unique human cell line that proliferates dependently on GM-CSF, IL-3, or erythropoietin.
J Cell Physiol
140
1989
323
47
Sutherland
HJ
Lansdorp
PM
Henkelman
DH
Eaves
AC
Eaves
CJ
Functional characterization of individual human hematopoietic stem cells cultured at limiting dilution on supportive marrow stromal layers.
Proc Natl Acad Sci USA
87
1990
3584
48
Cashman
JD
Lapidot
T
Wang
JCY
Doedens
M
Shultz
LD
Lansdorp
P
Dick
JE
Eaves
CJ
Kinetic evidence of the regeneration of multilineage hematopoiesis from primitive cells in normal human bone marrow transplanted into immunodeficient mice.
Blood
89
1997
4307
49
Lapidot
T
Pflumio
F
Doedens
M
Murdoch
B
Williams
DE
Dick
JE
Cytokine stimulation of multilineage hematopoiesis from immature human cells engrafted in SCID mice.
Science
255
1992
1137
50
Stamatoyannopoulos
G
Rosenblum
BB
Papayannopoulou
T
Brice
M
Nakamoto
B
Shepard
TH
Hb F and Hb A production in erythroid cultures from human fetuses and neonates.
Blood
54
1979
440
51
Huang
S
Law
P
Young
D
Ho
AD
Candidate hematopoietic stem cells from fetal tissues, umbilical cord blood vs. adult bone marrow and mobilized peripheral blood.
Exp Hematol
26
1998
1162
52
Holyoake TL, Nicolini FE, Eaves CJ: Functional differences between transplantable human hematopoietic stem cells from fetal liver, cord blood, and adult marrow. Exp Hematol (in press)
53
Moore
MAS
Williams
N
Analysis of proliferation and differentiation of foetal granulocyte-macrophage progenitor cells in haemopoietic tissue.
Cell Tissue Kinet
6
1973
461
54
Zanjani
ED
McGlave
PB
Bhakthavathsalan
A
Stamatoyannopoulos
G
Sheep fetal haematopoietic cells produce adult haemoglobin when transplanted in the adult animal.
Nature
280
1979
495
55
Stamatoyannopoulos
G
Papayannopoulou
T
Brice
M
Kurachi
S
Nakamoto
B
Lim
G
Farquhar
M
Cell biology of hemoglobin switching I. The switch from fetal to adult hemoglobin formation during ontogeny
Hemoglobins in Development and Differentiation.
Stamatoyannopoulos
G
Nienhuis
A
1981
287
Liss
New York, NY
56
Papayannopoulou
T
Nakamoto
B
Agostinelli
F
Manna
M
Lucarelli
G
Stamatoyannopoulos
G
Fetal to adult hemopoietic cell transplantation in humans: Insights into hemoglobin switching.
Blood
67
1986
99
57
Delfini
C
Saglio
G
Mazza
U
Muretto
P
Filippetti
A
Lucarelli
G
Fetal haemoglobin synthesis following fetal liver transplantation in man.
Br J Haematol
55
1983
609
58
Wood
WG
Bunch
C
Kelly
S
Gunn
Y
Breckton
G
Control of haemoglobin switching by a developmental clock?
Nature
313
1985
320
59
Terasawa
T
Ogawa
M
Porter
PN
Golde
DW
Goldwasser
E
Effect of burst-promoting activity (BPA) and erythropoietin on hemoglobin biosynthesis in culture.
Blood
56
1980
1106
60
Boggs
DR
The total marrow mass of the mouse: A simplified method of measurement.
Am J Hematol
16
1984
277

Author notes

Address reprint requests to Connie J. Eaves, PhD, Terry Fox Laboratory, 601 W 10th Ave, Vancouver, BC, V5Z 1L3 Canada; e-mail:connie@terryfox.ubc.ca.