神经生长因子对小鼠红系造血的影响及内在机制研究

本研究探讨神经生长因子（NGF）对红系造血的影响及内在机制。采用流式细胞术、祖细胞集落测定、血细胞计数、实时荧光定量PCR及酶联免疫吸附分析法（ELISA），对经大腿肌肉注射NGF一定时间后小鼠骨髓细胞的（BMC）增殖周期、BFU—E、CFU—E集落产率、外周血红细胞相关指标、肾脏EPO水平、骨髓细胞GM—CSF、脾脏EPO受体（EPOR）mRNA的表达及血清中EPO，GM—CSF和IL-1浓度进行检测，并观察培养体系中加入不同浓度的NGF时BFU—E、CFU—E集落产率的变化及与EPO、IL-3的关系。结果表明：注射NGF（7．5μg／kg）持续7天，骨髓细胞中S＋G2／M期细胞比例、CFU-E和BFU—E集落产率、脾EPOR mRNA量显著高于注射生理盐水的对照组。持续13到19天，外周血网织红细胞比例、红细胞数、血红蛋白含量也有升高；在体外CFU—E培养中，无论是否加入EPO，加入的NGF均能剂量依赖性地促进集落的形成，且只加入NGF组的集落产率显著高于只加入EPO组。在BFU—E培养中加入NGF和IL-3时，集落产率显著高于加入EPO和IL-3组。结论：NGF可能通过提高造血细胞对EPO的反应性并可激活造血细胞上与EPO作用后一样的信号通路，进而促进骨髓细胞进入有丝分裂，促进造血干细胞向红系方向分化，促进BFU—E和CFU—E的形成，增加外周血中网织红细胞、红细胞、血红蛋白含量。     

血管紧张素Ⅱ对脐血CD34+细胞体外扩增的作用

血管紧张素Ⅱ (AngⅡ )是肾素 血管紧张素系统的一种主要生物活性物质。为了研究它在造血系统中的影响 ,本实验通过体外细胞培养的方法 ,探讨AngⅡ与不同的细胞因子联合刺激脐血CD34+ 细胞生长和分化的作用。研究结果发现 ,悬浮培养体系中的AngⅡ可同时刺激BFU E和CFU GM的扩增 ,BFU E和CFU GM的数量在一定范围内随AngⅡ浓度 (0 0 1- 0 1μmol/L)的升高而增多 ;在半固体培养基中的AngⅡ则仅刺激CFU GM的形成 ,却不影响BFU E ;悬浮培养体系中AngⅡ与细胞因子SCF +G CSF +GM CSF +IL 3联合可使CFU GM的扩增倍数由 2 3± 0 8升高到 7 8± 1 9倍 ,与SCF +EPO +TPO +IL 3联合 ,BFU E的扩增倍数由 3 1± 1 8提高到 9 2± 2 3倍。结论 :AngⅡ与其他细胞因子联合可刺激脐血造血干 /祖细胞体外扩增。     

FL与SCF、IL-3、GM-CSF及EPO协同调控脐血造血干/祖细胞的体外扩增

目的　探讨造血细胞因子的不同组合对脐血造血干 /祖细胞的扩增作用。方法　应用免疫磁珠法分离纯化脐血CD3 4+ 造血干 /祖细胞 ,在体外液体培养体系中经各种不同细胞因子组合扩增 1周 ,用流式细胞仪检测CD3 4+ 造血干 /祖细胞并进行甲基纤维素法半固体培养 2周 ,在倒置显微镜下计数集落产率。结果　在FL、SCF、IL 3、GM CSF、EPO造血细胞因子的不同组合下脐血造血干 /祖细胞得以扩增 ,以IL 3+SCF +FL +EPO组的扩增效率最高 ,其有核细胞数、CD3 4+ 细胞及CFU GM、BFU E集落分别扩增 46 2± 175、3.47± 1.6 4、2 6 4± 10 5和 12 8± 6 7倍。结论　FL对扩增CD+ 3 4细胞具有较强的协同作用 ,合理的细胞因子组合扩增的脐血造血干 /祖细胞可成为异基因造血干细胞移植的主要来源。     

脐血和骨髓来源的CD34^＋细胞体外扩增巨核祖细胞差异的研究

本研究探讨脐血（CB）和骨髓（BM）来源的CD34^＋细胞体外扩增巨核祖细胞的差异。采用Ficoll—Hypaque分离法分离人CB及BM单个核细胞，免疫磁珠法制备CD34^＋细胞，在含血小板生成素（TPO）、TPO＋白介素11（IL—11）或TPO＋IL11＋肝素的无血清液体培养体系中培养14天。流式细胞术检测扩增产物（CD34^＋、CD41a^＋及CD34^＋CD41a^＋细胞）免疫表型、巨核细胞凋亡率及DNA含量，并以集落形成单位测定法进行粒巨-噬细胞集落形成单位（CFU—GM）、红系爆式集落形成单位（BFU—E）及巨核细胞集落形成单位（CFU—Mk）计数。结果表明：14天培养中，CB来源细胞在总细胞数、CD41a^＋及CD34^＋CD41a^＋细胞扩增倍数上均高于BM（P均〈0．05）。0天CB及BM来源CD34^＋细胞在CFU—GM、BFU—E及总的CFU—Mk的形成能力上无显著性差异（P均〉0．05），但CB来源CD34^＋细胞形成的CFU—Mk以大集落为主，其数量高于BM（P〈0．05）；在培养7、10和14天，CB及BM来源细胞CFU—GM扩增倍数无显著性差异（P均〉0．05），但CB来源细胞的BFU—E及总的CFU—Mk扩增倍数均高于BM（P均〈0．05）。14天培养中CB和BM来源巨核细胞的凋亡率无显著性差异（P均〉0．05）。DNA含量检测发现，14天培养中CB来源巨核细胞始终以2N细胞为主（比例〉90％），而BM来源巨核细胞随着培养时间延长，4N、8N及以上倍体巨核细胞比例逐渐增加。结论：CB来源CD34^＋细胞体外扩增巨核祖细胞能力高于BM，它可能是巨核祖细胞体外扩增较好的来源。     

低剂量辐射对造血系统兴奋效应的研究

目的 探讨低剂量辐射对造血系统的兴奋效应。方法 采用深部X线对小鼠进行低剂量辐射,甲基纤维素半固体培养造血祖细胞,酶联免疫吸附试验（ELISA）检测血清中GM－CSF、IL－3水平变化,脾组织切片原位杂交、脾细胞RNA狭缝杂交、Northern blot检测GM－CSF、G－CSF和IL－3的mRNA表达水平。结果 ①受照后小鼠造血细胞体外培养中CFU－GM和BFU－E数量明显增多;②血清中GM－CSF水平升高;③GM－CSF、G－CSF的转录升高。结论 低剂量辐射对造血系统有兴奋效应,这种兴奋效应可能与造血刺激因子升高有关。     

脐血造血干/祖细胞冷藏效果的比较研究

目的：探讨脐血细胞和1.064g/L ficoll分离的脐血造血干／祖细胞（简称1064脐血造血干／祖细胞）在不同冷藏条件下的效果。方法：21例脐血和12例1064例脐血造血干／祖细胞在4℃保存；16例1064脐血造血干／祖细胞分别冷藏在－80℃和－196℃，观察不同时间造血细胞活性；单个核细胞（MNC）采用自动血细胞分析仪测定，CD34^ 细胞采用流式细胞技术（FCM）测定，体外造血细胞培养技术分析粒－单系祖细胞（CFU－GM）和红系祖细胞（CFU－E)产率，台盼蓝着色法判断造血细胞活率。结果：脐血细胞在4℃保存第1、2d时，MNC、细胞活率、CFU－GM和CFU－E产率均无明显改变，第3d-5d CFU-GM和CFU－E产率明显下降；1064脐血造血干／祖细胞在4℃保存第3d出现CFU－GM和CFU－E的下降，至第5d下降更显著；1064脐血造血干／祖细胞冷藏在－80℃和－196℃，在半年内CFU－GM、CFU－E、CD34^ 细胞及细胞周期无显著变化，在－80℃冷藏一年时，CFU－GM、CFU－E和CD34^ 阳性细胞下降显著，而－196℃储藏一年时，上述指标及细胞周期无明显改变，98％以上的细胞处在G0－G1期。结论：脐血细胞4℃保存时间不宜超过3d；1064脐血造血干／祖细胞于－80℃和－196℃冷藏在半年内效果一致；－196℃长期冷藏脐血造血干／祖细胞是一种最可靠的方法，但－80℃冷藏脐血造血细胞是一种值得会尝试和实用、费用低的有效方法。     

成骨生长肽对红系造血祖细胞的体外促增殖作用

目的：通过研究人工合成成骨生长肽(sOGP)对红系造血祖细胞的体外促增殖作用，初步探索sOGP促造血活性的作用环节。方法：将正常小鼠和人骨髓有核细胞在标准红系祖细胞基础培养基中进行培养，设定sOGP的不同浓度梯度，于培养48h后计数小鼠CFU—E集落数，培养第4天与第14天计数人CFU—E、BFU—E集落数，将数据与空白对照组及EPO阳性对照组进行比较。结果：sOGP能够有效促进小鼠CFU—E及人CFU—E、BFU—E的体外增殖，量效关系明显，其有效作用浓度范围为10^-16～10^-10mol／L，高峰浓度约为10^-14～1011^-12 mol／L，呈低浓度、广范围及双相作用的特点。但是，sOGP对红系造血祖细胞的体外促增殖效果要弱于阳性对照药EPO。结论：sOGP能够在体外有效地促进红系造血祖细胞的增殖，可能是通过对骨髓基质细胞的作用而促进造血和造骨。     

黄芪注射液影响贫血小鼠粒单系、红系造血及作用机制的探讨

目的：研究黄芪注射液对贫血小鼠粒单系和红系血发生的影响,并探讨其作用机制。方法：制备贫血小鼠模型,随机分为给药组和对照组,采用造血祖细胞体外培养等实验血液学技术。结果：一定浓度的黄芪注射液体内作用可使贫血小鼠血清集落刺激因子（CSF）水平明显升高,刺激贫血小鼠粒单系、红系造血祖细胞增殖和分化以及骨髓基质细胞增殖,使降低的骨髓有核细胞（BMC）数以及外周血象明显回升,黄芪注射液和造血细胞直接一起孵育对粒单系祖细胞（CFU－GM）和红系祖细胞（CFU－E、BFU－E）的产率无明显影响。结论：一定浓度的黄芪注射液体内作用可促进贫血小鼠粒单系和红系造血功能恢复。     

Thl细胞在再生障碍性贫血发病机制中的作用

目的：探讨再生障碍性贫血（再障）患者Th1、Th2细胞的数量、比例、产生细胞因子的功能状态及其与造血衰竭的关系，判断Th1细胞在重再障（SAA）发病机制中的作用。方法：（1）采用甲基纤维素培养法扩增11例SAA患者骨髓单个核细胞（BMMNC）去除CD4^ 细胞前、后红系集落形成单位（CFU－E）、红系爆式集落形成单位（BFU－E）、粒－巨噬系集落形成单位（CFU－GM）；（2）用流式细胞术检测经体外刺激后的21例SAA患者及17名正常人外周血单个核细胞（PBMNC）中Th1、Th2细胞；（3）RT－PCR检测27例再障、26例非再障患者及11名正常人未经体外刺激的BMMNC中Th1细胞的代表性细胞因子IFN－γ和Th2细胞的代表性细胞因子IL－4的基因表达。结果：（1）去除CD4^ 细胞后，SAA患者BMMNC中的CFU－GM、CFU－E和BFU－E产率都有显著提高；（2）SAA患者外周血Th1与Th2细胞比例明显失衡，Th1细胞显著增多；（3）SAA和慢性再障患者骨髓中Th1细胞产生IFN-γ的功能异常增高，而Th2细胞产生IL－4的功能未发现明显异常。结论：再障患者的CD4^ T淋巴细胞亚群失衡、Th1细胞数量增多及功能亢进可能是导致SAA骨髓衰竭的重要环节，也是再障区别于其它全血细胞减少症的标志之一。     

rhIL-11联合rhG-CSF动员小鼠外周血造血干/祖细胞的研究

目的　研究rhIL 11对小鼠巨核系造血干 /祖细胞的动员作用。方法　rhIL 112 5 0μg·kg-1·d-1或联合rhG CSF 2 5 0 μg·kg-1·d-1给C5 7BL/ 6小鼠皮下注射 1～ 7d ,观察用药前和用药第 3,5 ,7,9天小鼠外周血白细胞、血小板计数 ,CD34 +细胞比例 ,CFU GM、CFU MK、CFU E的数量变化。结果　单用rhIL 11或与rhG CSF联合使用时 ,外周血白细胞、血小板、CD34 +细胞比例及各种造血细胞集落数明显高于对照组 (P <0 .0 1)。在含有IL 11的实验组中 ,CFU MK明显高于rhG CSF组 (P <0 .0 1)。结论　rhIL 11可升高外周血白细胞、血小板 ,同时增加外周血CD34 +细胞的比例 ,提高粒、红、巨核系造血细胞集落形成单位的数量 ,特别是对CFU MK作用较强 ;与rhG CSF联合使用对动员骨髓造血干 /祖细胞进入外周血有明显的协同作用。     

CD45 cell surface antigens are linked to stimulation of early human myeloid progenitor cells by interleukin 3 (IL-3), granulocyte/macrophage colony-stimulating factor (GM-CSF), a GM-CSF/IL- 3 fusion protein, and mast cell growth factor (a c-kit ligand)

CD45 antigens are protein tyrosine phosphatases. A possible link was evaluated between expression of CD45 antigens on human myeloid progenitor cells (MPC) (colony-forming unit-granulocyte/macrophage [CFU-GM], burst-forming unit-erythroid [BFU-E], and colony-forming unit-granulocyte/erythroid/macrophage/megakaryocyte [CFU-GEMM]) and regulation of MPC by colony-stimulating factors (CSF) (interleukin 3 [IL-3], GM-CSF, G-CSF, M-CSF, and erythropoietin [Epo]), a GM-CSF/IL-3 fusion protein, and mast cell growth factor (MGF; a c-kit ligand). Treatment of cells with antisense oligodeoxynucleotides (oligos) to exons 1 and 2, but not 4, 5, or 6, of the CD45 gene, or with monoclonal anti-CD45, significantly decreased CFU-GM colony formation stimulated with GM-CSF, IL-3, fusion protein, and GM-CSF + MGF, but not with G-CSF or M-CSF. It also decreased GM-CSF, IL-3, fusion protein, and MGF-enhanced Epo-dependent BFU-E and CFU-GEMM colony formation, but had little or no effect on BFU-E or CFU-GEMM colony formation stimulated by Epo alone. Similar results were obtained with unseparated or purified (greater than or equal to one of two cells being a MPC) bone marrow cells. Sorted populations of CD343+ HLA-DR+ marrow cells composed of 90% MPC were used to demonstrate capping of CD45 after crosslinking protocols. Also, a decreased percent of CD45+ cells and CD45 antigen density was noted after treatment of column-separated CD34+ cells with antisense oligos to exon 1 of the CD45 gene. These results demonstrate that CD45 cell surface antigens are linked to stimulation of early human MPC by IL-3, GM-CSF, a GM-CSF/IL-3 fusion protein, and MGF.     

Preservation of immature hematopoietic progenitor cells responding to interleukin 3 in marrow treated with 4-hydroperoxycyclophosphamide

The toxic effects of 4-hydroperoxycyclophosphamide (4HC) on different human hematopoietic progenitor cells were determined. Day 7 colonies supported by granulocyte colony stimulating factor (G-CSF), day 10 colonies supported by granulocyte-macrophage colony stimulating factor (GM-CSF) were counted. Approximately 2 X 10(2) granulocyte-macrophage colonies per 5 X 10(4) bone marrow (BM) mononuclear cells were formed in each assay system. A combination of interleukin 3 (IL3) and erythropoietin (EPO) induces 3 types of day 14 colonies; erythroid burst-forming units (BFU-E, 40 +/- 29/5 X 10(4) BM cells), granulocytes-macrophage colony forming units (CFU-GM, 143 +/- 29/5 X 10(4) cells and mixed colonies (CFU-GEMM, 24 +/- 13/5 X 10(4) cells). After treatment with 4HC, all the colonies were reduced. However, the recovery rate of day 14 colonies supported by the combination of IL3 and EPO (mean 12.1%) was significantly higher than the recovery rate of both day 7 colonies supported by G-CSF (mean 1.0%) and day 10 colonies supported by GM-CSF (mean 4.0%). The recovery rate of colonies supported by IL3 and EPO as also higher than that of day 14 colonies supported by GM-CSF and EPO. Immature pluripotent hematopoietic progenitor cells supported by IL3 and EPO appeared less sensitive to 4HC treatment than those supported by G-CSF and GM-CSF. Colony forming assays using IL3 may be useful in order to predict the hematological reconstitution after autologous bone marrow transplantation with 4HC treated graft.     

Optimal growth of human blood hematopoietic progenitor cells collected by apheresis for autografts

For safe autografts with peripheral blood hematopoietic cells (PBSCT), better methods for determining the kinetics of stem cell populations and predicting engraftment speed after PBSCT need to be established. Current methods include culture in semi-solid medium and measurement of CD34 cell surface antigen. In this study with only partially purified blood cells obtained from children with cancer in remission, we compared the effects of phytohemagglutinin-stimulated lymphocyte-conditioned medium (PHA-LCM) and recombinant human cytokines on the growth of progenitor cells in a methylcellulose culture system. Interleukin-3 (IL-3) alone supported more progenitor growth than standard PHA-LCM by a factor of 1.54 for colony-forming unit granulocyte/macrophages (CFU-GM) and by a factor of 1.84 for burst-forming unit/erythroids (BFU-E). No significant change, in terms of the number of growing colonies, was observed by adding granulocyte/macrophage colony-stimulating factor (GM-CSF), granulocyte-CSF (G-CSF), or IL-1 to IL-3. However, the addition of G-CSF resulted in increased colony size. A further increase in CFU-GM growth was observed by the addition of IFN-γ to the combination of cytokines. No significant effect was observed when stem cell factor (SCF) was added to the combination of cytokines containing IL-3, G-CSF, and IFN-γ. This analysis suggests that the combination of IL-3, G-CSF, and IFN-γ may provide sufficient stimulation for the growth of human blood cells. The effects of different oxygen tensions on progenitor growth in the presence of IL-3, G-CSF, and IFN-γ were also evaluated. Both CFU-GM and BFU-E formation were increased when the culture was grown at 5% O₂, as compared with an ambient 20% O₂ tension. A small number of infused cells were grown in culture incorporating either IL-3, G-CSF, and IFN-γ at 5% O₂ or PHA-LCM at 20% O₂, and the number of infused cells was correlated to the speed of hematopoietic recovery after PBSCT. Although a significant negative correlation was observed between the number of infused CFU-GM per kilogram of the patient's body weight and the recovery of hematopoiesis under both culture conditions, a better correlation was found when the former method was applied (P lt; .001 vs. P lt; .05). These findings suggest that a culture containing IL-3. G-CSF, and IFN-γ at low O₂ tension provides satisfactory conditions for the proliferation of blood progenitors, and that this mixture of recombinant cytokines may enable a standardized hematopoietic progenitor assay for PBSCT.     

四种重组造血生长因子对小鼠巨核细胞集落的影响

小鼠体外巨核系祖细胞培养表明,重组鼠白细胞介素-3(IL-3),粒-巨噬细胞集落刺激因子(GM-CSF)、重组人白细胞介素-6(IL-6)及红细胞生成素(EPO)均有不同程度刺激巨核系祖细胞生长的活性。其最适浓度分别为100U/ml,5ng/ml,20ng/ml与1U/ml。对巨核细胞集落刺激作用最强的是IL-3,IL-6与GM-CSF次之,EPO最弱。在种植2×10~5骨髓单个核细胞中分别获得45±3,25±2,20±3及10±2个巨核细胞集落。     

Thrombopoietin is synergistic with other cytokines for expansion of cord blood progenitor cells

We investigated the effects of recombinant human thrombopoietin (TPO) in combination with various cytokines including erythropoietin (EPO), interleukin-3 (IL-3), interleukin-6 (IL-6), and stem cell factor (SCF) on megakaryopoiesis, and the expansion of CD34+CD41a+ cells from human cord blood CD34+ cells with these cytokines under serum-free conditions. Human cord blood CD34+ cells were cultured in Megacult (Stem Cell Technologies Inc. Vancouver, Canada) in the presence of recombinant growth factors. Colony-forming unit-megakaryocyte (CFU-M) colonies were counted on day 14. CD34+CD41a+ and CD34-CD41a+ cell expansion was analyzed using a serum-free liquid culture system for 7 days with recombinant growth factors. TPO alone had a concentration-dependent effect on megakaryocyte colony growth. At concentrations above 1 ng/ml, TPO supported significant CFU-Meg colony formation in a concentration-dependent manner. The combination of TPO plus other cytokines, including EPO, IL-3, and SCF, resulted in a synergistic enhancement of the number of CFU-Meg colonies, but IL-6 failed to enhance the effect of TPO. The number of CD41a+ cells increased after 7 days in liquid culture of human cord blood CD34+ cells with various cytokines (EPO, IL-3, IL-6, SCF) combined with TPO, but SCF plus TPO only resulted in a significant synergistic increment of CD34+CD41a+ cells compared with TPO alone. The results of the present study indicate that EPO, IL-3, and SCF can be synergistic with TPO to stimulate proliferation of CFU-Meg and suggest that SCF plus TPO can expand CD34+CD41a+ cells to effect the rapid recovery of platelets in patients following stem cell transplantation.     

脐血来源树突状细胞的体外诱导及扩增

本研究的目的是分析脐血的细胞组成 ,研究加入细胞因子培养前后脐血树突状细胞的变化 ,探索体外诱导、扩增树突状细胞的方法并进行表型鉴定。选择正常成人外周血 9份 ,脐血 12份 ,分离单个核细胞。在脐血单个核细胞中加入细胞因子GM CSF、IL 3、SCF和EPO ,培养 4周。应用流式细胞仪和CD4、CD8、CD19、CD34、CD38、CD1a、CD11c及CDw12 3单克隆抗体测定正常成人外周血、培养前后 1,2 ,3,4周脐血细胞表面抗原及树突状细胞情况。结果表明 :正常成人外周血CD34 细胞 0 .0 2× 10 5 ml,CD1a 细胞 0 .0 1× 10 5 ml,CD11c 细胞 4 .32×10 5 ml,CD83 细胞 1.31× 10 5 ml,CDw12 3 细胞 1.4 1× 10 5 ml。新鲜脐血中CD34 细胞 0 .2 2× 10 5 ml,CD1a 细胞 0 .2 7× 10 5 ml,CD11c 细胞 5 .87× 10 5 ml,CD83 细胞 1.94× 10 5 ml,CDw12 3 细胞 2 .73× 10 5 ml。加入细胞因子GM CSF ,IL 3,SCF ,EPO后培养 1- 4周的脐血单个核细胞分化为CD1a ,CD11c ,CD83 ,CDw12 3 树突状细胞 ,在培养的 2 - 4周 ,脐血树突状细胞数量明显增多 ,此后逐渐减少。通过培养 ,树突状细胞数量增加 ,CD1a 细胞达 11.0 2× 10 5 ml,CD11c 细胞 2 8.2 4× 10 5 ml,CD83 细胞 10 .5 7× 10 5 ml,CDw12 3 细胞 18.7× 10 5     

Growth factor-dependent inhibition of normal hematopoiesis by N-ras antisense oligodeoxynucleotides

To determine whether N-ras expression is required at specific stages of the process of in vitro normal human hematopoiesis, adherent- and T lymphocyte-depleted mononuclear marrow cells (A-T-MNC) or highly purified progenitors (CD34+ cells) were cultured in semisolid medium, under conditions that favor the growth of specific progenitor cell types, after exposure to N-ras sense and antisense oligodeoxynucleotides. N-ras antisense, but not sense, oligodeoxynucleotide treatment of A-T-MNC and CD34+ cells resulted in a significantly decreased number of granulocyte/macrophage colony-forming units (CFU-GM) induced by interleukin 3 (IL-3) or granulocyte/macrophage colony-stimulating factor (GM-CSF) and of macrophage colonies (CFU-M) induced by M-CSF, but not of granulocytic colonies induced with G-CSF or IL-5. However, the same treatment significantly inhibited colony formation induced by each of the above factors in combination with IL-3. Megakaryocytic colony (CFU-Meg) formation from A-T-MNC or CD34+ cells in the presence of IL-6 + IL-3 + erythropoietin (Epo) was also markedly decreased after antisense oligodeoxynucleotide treatment. Erythroid colonies derived from A-T-MNC in the presence of Epo (CFU-E) were not inhibited upon antisense treatment, whereas those arising from A-T-MNC or CD34+ cells in the presence of IL-3 + Epo (BFU-E) were markedly affected. These results are consistent with the hypothesis that distinct signal transduction pathways, involving N-ras or not, are activated by different growth factors in different hematopoietic progenitor cells.