大鼠骨髓Thy-1.1+干细胞诱导分化为肝干细胞样细胞的体外实验研究

目的：分离培养大鼠骨髓Thy-1．1^＋干细胞，探讨其肝分化条件及潜能。方法：分离大鼠骨髓细胞，Percoil密度梯度离心获取骨髓单个核细胞后免疫磁珠分选Thy-1．1^＋干细胞群，流式细胞术测定分选后细胞的纯度与表型。不同条件下诱导分化，用形态学观察、逆转录聚合酶链反应和免疫细胞化学法判定其所处的肝系分化阶段。结果：免疫磁珠分选Thy-1．1^＋细胞纯度达94．2％。Thy-1．1^＋细胞不表达CD34和c-kit；高表达β2微球蛋白和CD45；部分表达Flt-3。Thy-1．1^＋细胞在HGF／FGF-1／FGF-2诱导下呈多角形上皮状克隆样生长，诱导前后持续表达细胞角质素18，诱导过程中白蛋白的表达逐渐减少，甲胎球蛋白的表达逐渐增强。结论：免疫磁珠可有效分选Thy-1．1^＋细胞。Thy-1．1^＋细胞在HGF／FGF．1／FGF-2诱导下呈肝干细胞样细胞表现。     

粒细胞集落刺激因子动员对CD34^＋细胞黏附分子表达的影响

目的探讨粒细胞集落刺激因子（G—CSF）动员对CD34^＋细胞黏附分子表达的影响。方法应用流式细胞仪检测健康供者稳态及G—CSF动员过程中骨髓、外周血CD34^＋细胞的黏附分子表达变化，并应用结晶紫染色测定CD34^＋细胞的黏附功能。结果G-CSF动员后CD34^＋CD49d^＋细胞比例无显著下降，外周血CD34^＋CD621．^＋、CD34^＋CD54^＋和CD34^＋CDlla^＋细胞比例增高。动员后CD34^＋细胞表面CD49d的平均荧光强度显著减弱，但CD49e、CD62L、CDlla、CD54的平均荧光强度虽呈减弱趋势，却无统计学差异。动员后CD34^＋细胞黏附纤连蛋白能力下降。结论G—CSF通过降低造血干细胞与骨髓基质的黏附而产生动员效应。     

肝或肾移植术后纤维化胆汁淤积性肝炎肝组织中CD4+CD25+调节性T淋巴细胞的表达

目的观察肝或肾移植术后纤维化胆汁淤积性肝炎（FCH）患者肝组织中CD4^＋CD25^＋调节性T淋巴细胞的表达及分布,并对其作用机制进行初步探讨。方法对5例FCH患者进行肝活体组织病理学检查;采用免疫组织化学法检测肝组织中CD4^＋CD25^＋调节性T淋巴细胞的特异性标记物叉状头/翅膀状螺旋回转录因子（FOXP3）;采用末端脱氧核苷酸转移酶介导的脱氧三磷酸尿苷缺口末端标记（TUNEL）检测试剂盒对肝组织内的肝细胞凋亡情况进行观察。结果5例FCH患者中,3例为原位肝移植患者,2例为肾移植患者。光学显微镜下,肝脏汇管区及汇管区周围出现纤维化,肝细胞及胆小管明显胆汁淤积,肝细胞气球样变及毛玻璃样变。免疫组织化学检测显示HBsAg、HBcAg及前S1抗原阳性。FOXP3阳性信号定位于淋巴细胞胞质内,阳性细胞主要聚集在汇管区,小叶肝窦内可见散在的单个淋巴细胞呈阳性表达。汇管区周围可见较多的凋亡细胞。正常肝组织HBsAg、HBcAg及前S1抗原均为阴性,汇管区内有少量的阳性CD4^＋CD25^＋调节性T淋巴细胞,小叶内偶见凋亡细胞。结论FCH具有独特的组织学特征,可能与肝组织中的FOXP3高表达有关。     

人羊水来源c-kit+与c-kit-间充质干细胞生物学特征及体外心肌诱导分化能力的比较

目的比较人羊水来源c-kit＋与c-kit-间充质干细胞的生物学特征及体外心肌分化能力。方法通过常规产前诊断或自愿引产,以羊膜腔穿刺术获取14份孕中期羊水（15～31周）,通过流式细胞仪分选c-kit＋间充质干细胞,比较c—kit＋干细胞与c-kit-干细胞的细胞形态及生长曲线。流式细胞仪及免疫细胞化学分析对两种细胞进行表型鉴定,比较其体外向成骨、成脂及心肌样细胞诱导分化能力。RT-PCR和Westonblot检测相关基因及蛋白的表达。结果14份羊水标本中,有5份标本可以分选到c—kit＋羊水干细胞,分布在孕16-22周,分选的细胞数量占贴壁细胞的（3．30±1．24）％。羊水干细胞以成纤维状为主,可以稳定传代,体外培养可增殖到50代以上。经过c—kit分选后,细胞呈均匀一致的长纤维状,排列规律。C-kit＋及c-kit-羊水干细胞具有相似的生长曲线。两种细胞均具有间充质干细胞特征,表达间充质干细胞标记（CD29,CD44,CD73,CD90,CDl05）,不表达造血细胞系标记（CD34,CD45）,表达I类人类组织相容性抗原（HLA—ABC）,不表达II类人类组织相容性抗原（HLA—DR）。Oct-4及SSEA-4胚胎干细胞标记在c-kit＋干细胞的表达比率明显高于c．kit-干细胞。两种细胞在体外均可以向成脂细胞、成骨细胞分化。成脂细胞诱导后,可以看到细胞内脂滴聚集,油红0染色阳性。成骨细胞诱导后,可以看到钙盐沉积,VonKossa染色阳性。两种细胞分别与新生乳鼠心肌共培养,体外向心肌细胞诱导,c—kit＋羊水干细胞可以检测到GATA-4,α-actin,Cx43和cTnT基因及蛋白的表达。而c-kit-干细胞在诱导后只表达GATA-4的基因及蛋白。结论羊水来源c—kit＋及c—kit-干细胞均表达间充质干细胞表型特征,体外可以向成脂、成骨细胞分化。然而,c—kit＋干细胞的心肌样细胞分化能力明显强于c—kit-干细胞,可能与c—kit＋干细胞具有某些胚胎干细胞特征有关。     

IL-5诱导CD34+造血干细胞嗜酸性粒细胞分化的信号传导机制研究

目的 应用体外实验探讨骨髓中CD34^＋细胞定向分化为EOS的信号传导机制。方法 应用 MiniMACS磁性分离仪从体外脐血干细胞中分离CD34^＋细胞并应用流式细胞仪进行鉴定，将分离出的细胞进行培养并分成4组，即阴性对照组，定向分化组，和两种信号抑制组。各组细胞培养28d后应用western blot检测JAK1，STAT2的表达强度，同时观察各组CD34^＋分化为EOS的能力及EOS体外的活性。结果 在IL-5诱导下CD34^＋细胞培养至第28d时，EOS细胞已经占总细胞的87．2％左右，与B组比较，C组细胞在JAK1、 STAT2表达强度、ES比例、EPO活性、Eos脱颗粒能力差异均有统计学意义（P〈0．05），与C组比较，D组细胞在STAT2表达强度、ES比例、EPO活性、Eos脱颗粒能力差异均有统计学意义（P〈0．05）。结论 体外实验CD34^＋细胞定向分化为EOS是通过JAK1-STAT2途径介导的。JAK1-STAT2途径不但介导了EOS细胞数量上的增加，而且介导了EOS细胞活性的增强。     

再生障碍性贫血患者骨髓CD34~+细胞及粒细胞集落刺激因子受体的表达及意义

目的：检测再生障碍性贫血（AA）患者骨髓CD34^＋细胞占单个核细胞（MNC）的比率及其表面的粒细胞集落刺激因子受体（G-CSFR）的表达率,以探讨AA可能的发病机制。方法：用流式细胞术（FCM）检测13例AA患者及12例非血液病患者骨髓CD34^＋细胞占MNC的比率及其表面的G-CSFR的表达率。结果：AA组与对照组骨髓MNC中CD34^＋细胞的比率的比较差异有统计学意义（P〈0．05）,但G-CSFR的表达率的比较差异无统计学意义（P〉O．05）。大多数重型再生障碍性贫血患者（3／4）及很少慢性再生障碍性贫血患者（1／9）的骨髓MNC中的CD34^＋细胞的比率小于0．1％。结论：骨髓CD34^＋细胞的检测有助于判断AA患者病情,而其表面的G-CSFR的表达率与AA的发病无关。     

HepG2细胞抗原对脐血CD34^＋造血干细胞诱导分化的树突细胞免疫功能的影响

背景：树突细胞（DC）是体内功能最强的抗原呈递细胞,可激活初始型T细胞,生成辅助性T细胞和杀伤性T细胞。DC具有特异性呈递肿瘤抗原的能力,在肿瘤免疫中发挥重要作用。目的：探讨HepG2细胞抗原对脐血CD34^＋造血干细胞诱导分化的DC免疫功能的影响。方法：分离培养脐血CD34^＋造血干细胞后,加入细胞因子组合诱导生成DC并将其分成HepG2细胞抗原负载组和对照组．以流式细胞仪测定DC生成率和免疫表型,以酶联免疫吸附测定（ELISA）检测干扰素-γ（IFN-γ）含量,以MTT法检测细胞毒性T淋巴细胞（CTL细胞）对HepG2细胞的杀伤作用。结果：DC生成率为60．2％±9．4％。与对照组相比,HepG2细胞抗原负载组DC免疫表型CD1a^＋／CD40^＋、CD83^＋／CD86^＋、CD14^＋／HLA-DR^＋比例显著增高（57．6％±5．4％对33．2％±6．0％、32．5％±3．9％对26．0％±2．8％、38．1％±2．6％对29．1％±2．1％,P〈0．01）;IFN-γ含量呈时间依赖性增高;CTL细胞对HepG2细胞的杀伤作用显著增强（43.3％±11.3％对13．9％±4．6％,P〈0．01）。结论：应用HepG2细胞抗原孵育脐血CD34^＋造血干细胞可诱导分化成熟DC,DC可促进异基因淋巴细胞活化分泌IFN-γ,并产生特异性CTL细胞,杀伤肝癌HepG2细胞。     

AA和MDS患者骨髓CD+34细胞表面GM-CSFR的表达及意义

目的 探讨再生障碍性贫血（AA）和骨髓增生异常综合征（MDS）患者骨髓CD34^＋细胞表面粒细胞-巨噬细胞集落刺激因子受体（GM—CSFR）的表达情况。方法 用流式细胞术（FCM）和单克隆抗体技术检测14例AA、23例MDS和8例非血液病患者骨髓CD34^＋细胞表面的GM—CSFR表达率。结果 MDS组骨髓CD34^＋细胞表面GM-CSFR的表达率明显高于对照组及AA组（P〈O．05），而AA组与对照组间差异无统计学意义（P〉0．05）。结论 AA患者造血干细胞没有质的缺陷；造血干细胞异常可能是MDS的主要发病机制之一。     

ImmuKnow^TM免疫细胞功能检测在肝移植术后排斥反应中的意义

目的探讨CD4T淋巴细胞内ATP检测在肝移植术后排斥反应中的作用及意义。方法选取肝移植术后患者101例,共采集外周血样本142例次。标本检测采用ImmuKnow^TM免疫细胞检测试剂盒进行。根据患者是否发生排斥反应及不同的免疫状态,分析CD4^＋T淋巴细胞内ATP与肝移植术后排斥反应的关系。结果本组患者共发生排斥反应5例次,排斥反应组与非排斥反应组的CD4^＋T淋巴细胞ATP值分别为（762．00±135．34）、（390．23±238．77）ng／ml,P=0．001。低免疫应答组、正常免疫应答组和高免疫应答组肝移植术后排斥反应发生率分别为0、1．7％（1／59）和9．3％（4／43）,P=0．044。他克莫司（FK506）低浓度组和高浓度组CD4^＋T淋巴细胞ATP值比较无统计学差异,FK506浓度与CD4^＋T淋巴细胞ATP无直线相关关系（r=-0．055,P=0．518）。结论外周血CD^＋T淋巴细胞ATP对肝移植术后排斥反应的防治有重要意义。     

肾癌干细胞的培养及鉴定

目的从肾癌细胞中获取肾癌干细胞。方法将肾癌细胞悬浮于含有表皮生长因子（EGF）和成纤维生长因子（bFGF）的无血清培养基中培养。用流式细胞仪检测其CD133及CD34。收集无血清培养7d后的细胞重新常规培养并观察其分化情况。结果悬浮培养2d后出现肾癌干细胞球,培养7d后表达CD133^＋,CD34^-的细胞占8．33％±1．26％,高于肾癌细胞中表达CD133^＋CD34^-的细胞（P〈0．05）,后者只占1．24％±0．36％。结论用无血清培养基和悬浮培养的方法可以从肾癌细胞中获取肾癌干细胞。     

Expression of murine CD38 defines a population of long-term reconstituting hematopoietic stem cells

Using a monoclonal antibody to murine CD38, we showed that a population of adult bone marrow cells that expressed the markers Sca-1 and c-kit but lacked the lineage markers Mac-1, GR-1, B220, IgM, CD3, CD4, CD8 and CD5 could be subdivided by the expression of CD38. We showed that CD38high c-kit+ Sca-1+, linlow/-cells sorted from adult bone marrow cultured with interleukin-3 (IL-3), IL-6, and kit-L produced much larger colonies in liquid culture at a greater frequency than their CD38low/- counterparts. In addition, we found that CD36low/ - cells contained most of the day-12 colony-forming units-spleen (CFU-S) but were not long-term reconstituting cells, whereas the population that expressed higher levels of CD38 contained few, but significant, day-12 CFU-S and virtually all the long-term reconstituting stem cells. Interestingly, the CD38high Sca-1+ c-kit+ linlow/- cells isolated from day-E14.5 fetal liver were also found to be long-term reconstituting stem cells. This is in striking contrast to human hematopoietic progenitors in which the most primitive hematopoietic cells from fetal tissues lack the expression of CD38. Furthermore, because antibodies to CD38 could functionally replace antibodies to Thy-1.1 in a stem cell purification procedure, the use of anti-CD38 may be more generally applicable to the purification of hematopoietic stem cells from mouse strains that do not express the Thy-1.1 allele.     

Expression of CD33, CD38, and HLA-DR on CD34+ human fetal liver progenitors with a high proliferative potential

High proliferative-potential colony-forming cells (HPP-CFC) have been identified in the bone marrow of mice and adult humans, and have been characterized as a compartment of primitive progenitors possibly including stem cells. In this report we describe the human fetal liver (FL) as a source of HPP-CFC. These FL HPP-CFC develop in clonal cultures in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-3 (IL-3) within 3 to 4 weeks. The median frequency of HPP-CFC in FL tissues between 16 and 21 weeks of gestational age was 1 in 3,000 total FL cells. After 4 weeks of growth, FL HPP-CFC grew to a median colony size of 8.3 x 10(4) cells/colony. Using cell-sorting techniques FL HPP-CFC were shown to be predominantly contained in the CD34+ CD33+ CD38- fraction of FL cells. FL HPP-CFC were heterogeneous for HLA-DR expression, and no differences in proliferative capacities were observed between HLA-DR+ and HLA-DR- HPP- CFC. The CD34+ CD33-HLA-DR- CD38- population, previously suggested to contain stem cells, was observed to be very rare in the FL, representing approximately 1 in 1.7 x 10(5) light-density FL cells and containing almost no CFC. Therefore, it is possible that stem cells are contained in the CD33+ fraction of FL cells. Phenotypic characterization of CD34+ CD33+ CD38- lin -LDFL cells showed that these cells are also CD13+, predominantly Thy-1+, CD45RA-, CD45RO-, CD71-, and heterogenoeous for c-kit expression. These data suggest that FL HPP- CFC represent a heterogeneous compartment of primitive myeloid progenitors that may include stem cells.     

Blood-derived autografts collected during granulocyte colony- stimulating factor-enhanced recovery are enriched with early Thy-1+ hematopoietic progenitor cells

It was the objective of the study to characterize CD34+ hematopoietic progenitor cells from peripheral blood (PB) and bone marrow (BM) in a group of 24 cancer patients. After cytotoxic chemotherapy, R-metHu granulocyte colony-stimulating factor (R-metHuG-CSF; filgrastim, 300 micrograms daily, subcutaneously) was given to shorten the time of neutropenia as well as to increase the rebound of peripheral blood progenitor cells (PBPC) for harvesting. The proportion of CD34+ cells in the leukapheresis products (LPs) was 1.4-fold greater than in BM samples that were obtained at the same day (LP: median, 1.4% v BM: median, 1.0%, P < .01). Two- and three-color immunofluorescence showed that blood-derived CD34+ cells comprised a greater proportion of a particular early progenitor cell than CD34+ cells of bone marrow. Blood- derived progenitor cells tended to have a higher mean fluorescence intensity of CD34 and expressed significantly lower levels of HLA-DR (mean fluorescence intensity of HLA-DR: 442.6 +/- 44.9 [LP] v 661.5 +/- 64.6 [BM], mean +/- SEM, P < .01). Furthermore, the blood-derived CD34+ cells comprised a 1.7-fold greater proportion of Thy-1+ cells (LP: median, 24.4% v BM: median, 14.4%, P < .001) and expressed significantly less c-kit (LP: median, 20.5% v BM: median, 31.0%, P < .01). Three-color analysis showed that high levels of Thy-1 expression were restricted to CD34+/HLA-DRdim or CD34+/HLA-DR- cells confirming the early developmental stage of this progenitor cell subset. The proportion of CD34+/CD45RA(bright) cells representing late colony- forming unit granulocyte-macrophage (CFU-GM) was smaller in LPs compared with BM (P < .05). For an examination of BM CD34+ cells before the mobilization chemotherapy, samples of 16 patients were available. The mean proportion of c-kit expressing CD34+ cells in the bone marrow during G-CSF-stimulated reconstitution decreased 1.8-fold compared with baseline values. There was no difference in the proportion of BM- derived CD34+/Thy-1+ cells and CD34+/CD45RA+ cells between steady-state hematopoiesis and G-CSF-supported recovery. Our data suggest that during G-CSF-enhanced recovery, CD34+ cells in the PB are enriched with more primitive progenitor cells to evenly replenish the BM after the chemotherapy-related cytotoxic damage.     

Antigenic analysis of human haemopoietic progenitor cells expressing the growth factor receptor c-kit

The cell surface molecule encoded by the protooncogene c-kit has recently been identified as the receptor for a growth factor variously termed stem cell factor (SCF), mast cell growth factor or steel factor. Using the c-kit antibody 17F11 we analysed, in triple staining experiments, the surface molecule profile and scatter characteristics of c-kit+CD34+ human haemopoietic progenitor cells. In 10 normal bone marrow samples we found 19-51% of CD34+ bone marrow progenitor cells to coexpress c-kit. These c-kit+CD34+ bone marrow cells turned out to represent a phenotypically heterogeneous population. A considerable proportion coexpressed CD33 (52 +/- 23%), and/or CD71 (62 +/- 26) antigens, marker molecules previously shown to be expressed by committed in vitro colony forming cells but not by their precursors. In line with a relatively differentiated phenotype c-kit+CD34+ cells also gave rise to on average higher forward and right-angle light scattering signals. The proportions of CD38 and/or HLA-D expressing cells were similar in the c-kit+ and in the c-kit- subsets of CD34+ progenitor cells. Coexpression of CD19 was found to be less frequent in the c-kit+ (4 +/- 5%) as compared to the c-kit- (17 +/- 14%) fraction of CD34+ cells. CD7+ CD34+ bone marrow cells were hardly detectable and their numbers too low to allow further subdivision in c-kit+ and c-kit- subsets.     

Hepatic gene induction in murine bone marrow after hepatectomy

BACKGROUND/AIMS: Bone marrow cells are highly plastic and differentiate into various cell types, including hepatocytes. To explore the mechanisms underlying these processes, we focused on the initial responses of bone marrow to hepatectomy, using a mouse model. METHODS: To evaluate hepatic differentiation in bone marrow cells we measured hepatocyte-related gene expression in mice undergoing partial hepatectomy with or without pretreatment for 1 week with 2-acetyl aminofluorene (AAF). RESULTS: Hepatectomy induced several genes related to early hepatic differentiation in bone marrow. Expression of these genes was enhanced by the administration of AAF, whereas genes specific for mature hepatocytes were not detected. We characterised the bone marrow cell population expressing hepatocyte differentiation genes. alpha-fetoprotein mRNA was induced in Lin- and either CD34+, c-kit+, Sca-1+, CD49f+ or CD45+ cells. The genes upregulated in the liver after AAF treatment and hepatectomy were identified using oligonucleotide microarrays. These included genes associated with the acute phase response. Dexamethasone inhibited the expression of early hepatic differentiation genes in the bone marrow of AAF/PHx mice. CONCLUSIONS: Early hepatic differentiation genes were induced in bone marrow in response to hepatectomy, especially when regeneration of the remnant liver was suppressed. Circulating signals generated in the AAF/PHx liver might activate this differentiation.     

Multiparameter analysis of murine bone marrow side population cells

We describe the multiparameter flow cytometric analysis of the relationship between side population (SP) formation and well-characterized, antigen-defined stem cell subsets. We also compared the competitive repopulation ability of subsets defined by the SP profile to those identified by antigenic markers. The vast majority of SP cells possessed a primitive cell phenotype (c-kit+, SCA-1+, Thy-1+, CD31+, CD135neg, lineage neg), but only a minority of antigen-defined subsets were SP cells. Hence, although SP cells are identified independently of antigenic markers, they are not distinct from established stem cell phenotypes, but are a small subset of them. Approximately half of SP cells expressed CD34 at readily detectable levels, and one-third of SP cells possessed the primitive c-kit+, SCA-1+, lineage neg, CD34neg cell phenotype. Since most SP cells are a subset of c-kit+, Thy-1+, lineage neg, SCA-1+ cells (KTLS), we determined whether the SP cell subset represents a further enrichment in long-term repopulating cell content. The SP+ subset of KTLS+ cells was more enriched for competitive repopulation units than the SP- fraction of KTLS+ cells. Hence, the SP cell fraction highlights a subset of the long-term repopulating cells found within the already highly purified KTLS fraction.     

Isolation of a candidate human hematopoietic stem-cell population.

We have identified a rare (0.05-0.1%) subset of human fetal bone marrow cells that contains multipotent hematopoietic precursors. The population of human precursor cells that express Thy-1 and CD34 but no known lineage markers is enriched for clonogenic activity that establishes long-term, multilineage (myelomonocytic and B lymphoid) cultures on mouse marrow stromal lines. Further, the Thy-1+CD34+ subset that takes up little of the fluorescent mitochondrial dye rhodamine 123 contains virtually all the cells that establish long-term cultures. In human fetal thymus transplanted into SCID (severe combined immunodeficiency) mice, Thy-1+CD34+ fetal bone marrow cells differentiate into T lymphocytes. In two of nine cases, allogeneic Thy-1+CD34+ cells could engraft intact human fetal bone marrow grown in SCID mice, resulting in donor-derived myeloid and B cells. By extrapolation, the rare human Thy-1+Lin-CD34+ cell population contains pluripotent hematopoietic progenitors; we propose that it is highly enriched for candidate hematopoietic stem cells.     

Correlation of c-kit expression and cell cycle regulation by transforming growth factor-beta in CD34+ CD38- human bone marrow cells

To investigate the relationship between c-kit expression and cell cycle regulation by endogenous transforming growth factor-beta (TGF-beta) in human bone marrow hematopoietic progenitor cells, CD34+ CD38- c-kit(low/-) and CD34+ CD38- c-kit(high) populations were cultured in stem cell factor, thrombopoietin, interleukin-3 (IL-3), IL-6, granulocyte colony-stimulating factor, granulocyte/macrophage colony-stimulating factor and anti-TGF-beta, and analyzed for cell cycle status. Arrest in G0/G1 was most prominent in the precultured CD34+ CD38- c-kit(low/-) subset (95.62 +/- 4.15%). While postcultured CD34+ CD38- c-kit(high) cells initiated from CD34+ CD38- c-kit(high) cells entered cell cycle within 36 hr, postcultured CD34+ CD38- c-kit(low/-) cells initiated from CD34+ CD38- c-kit(low/-) cells remained dormant until 36 hr and entered cell cycle within 90 hr. Anti-TGF-beta increased the percentage of S/G2M phase postcultured CD34+ CD38- c-kit(high) cells (from 19.08 +/- 11.95 to 47.04 +/- 2.93%), but no significant change was observed in postcultured CD34+ CD38- c-kit(low/-) cells. These results suggest that endogenous TGF-beta plays an important role in the cell cycle arrest of c-kit(high) but not c-kit(low/-) cells in CD34+ CD38- cells, which proliferate without undergoing differentiation. The different regulatory mechanism of cell cycle entry of the CD34+ CD38- c-kit(high) and CD34+ CD38- c-kit(low/-) subsets might be the result of differences in their sensitivity to endogenous TGF-beta.