Comparative morphology of the marrow sac

Electron microscopic techniques have been used to profile the morphologies of marrow sacs in different laboratory species. These structures all comprise a condensed layer of overlapping fibroblast-like stromal cells and apparently confine the medullary and endosteal osteoblast/lining cells to separate histiotypic compartments. There were some variations in the morphology of the sac cells in the different species. In rats, cats, and sheep, scanning electron microscopy (SEM) showed a seamless arrangement of marrow sac cells which resembled a thin, flat simple squamous epithelium; they displayed few intercellular cytoplasmic processes. In the rabbit and pigeon, the sac comprised a more woven, multilayered fabric of broadly elongate flat fibroblast-like cells which displayed numerous intercellular processes. Transmission electron microscopy (TEM) showed that all marrow sac cells were attenuated with elongated nuclei, a few small round mitochondria, and a sparse rough endoplasmic reticulum. In the majority of animals, the sac was one to two cell layers thick. The rabbit and pigeon sacs were multilayered, and never less than three to four cells deep. The cell layers were not closely apposed. Tight or gap junctions were absent at the points of intercellular contact. These morphological results suggest that marrow sacs are common elements of the vertebrate skeleton with species specific morphologies.     

大鼠骨髓细胞诱导分化神经细胞的形态学观察

本实验探讨了从骨髓诱导培养分化神经细胞的方法,并从形态学上观察了其演变规律。抽取SD大鼠骨髓做全骨髓细胞培养,用光镜、扫描电镜观察不同阶段的形态变化,用免疫细胞化学方法进行鉴定。观察到骨髓细胞由单细胞、细胞分裂、形成克隆团(球)、出芽,直至形成突起成为神经元样细胞的演变过程。可确定神经元样细胞的前体在骨髓阶段是一类"大圆细胞"。24h左右开始贴壁,前体细胞长出"足"样结构与细胞外基质相连接。培养25~35 d细胞分化为神经元样,神经元样细胞的形态多样,有球形、锥形、星形或梭形等,直径在15.3~36.5 μm之间。应用免疫细胞化学鉴定出神经干细胞、神经元和胶质细胞。结果表明,骨髓细胞可分化为神经细胞,扫描电镜从超微结构上给予佐证。为骨髓细胞向神经细胞分化的发育过程提供了形态学资料。     

Characteristic Morphology and Distribution of Bone Marrow Derived Cells in the Cornea

Enhanced green fluorescence protein (eGFP)‐labeled bone marrow (BM) cells were transplanted into syngeneic C57BL/6 (wild‐type) mice to investigate the distribution pattern, immunohistochemical characteristics, three‐dimensional structure, and ultrastructure of the BM‐derived cells in the mouse cornea using a fluorescence microscope, a confocal laser scanning microscope, and a transmission electron microscope. This study provided direct evidence that two morphologically distinct types of BM‐derived cells were distributed in the mouse cornea. The majority of the GFP(+) cells showed a flattened polygonal form with obtuse angles and these cells were distributed in the corneal stroma. The other type was the GFP(+) cells demonstrating slim cell bodies with long and extremely thin dendrites and which were distributed in the corneal epithelium. The immunohistochemical characteristics and ultrastructure of BM‐derived cells suggest that most of these cells have a macrophage lineage, whereas some cells in the corneal stroma do not. Interestingly, the direct intimate contact between GFP‐labeled BM derived cells and non‐GFP‐labeled resident cells within the corneal stroma were also clearly visualized at the fine structural level. These data provide new and more detailed insight into the nature of BM‐derived cells in the cornea. Anat Rec, 2009. © 2009 Wiley‐Liss, Inc.     

Ultrastructural study of cell-cell interaction between osteoclasts and osteoblasts/stroma cells in vitro

Many biochemical reports support cell-cell interaction between osteoclasts and osteoblasts/stroma cells in vitro, however there have been few morphological studies supporting this. Details of cell-cell interaction between osteoclasts and osteoblasts/stroma cells remain unclear. The present study examined cell-cell interaction between osteoclasts and osteoblasts/stroma cells by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Osteoclasts, osteoblasts/stroma cells, and bone marrow cells obtained from 10-day-old ddY mice were cultured on dentin slices for 72 hr. Specimens were fixed, and some were examined by SEM. Specimens were decalcified, embedded in Epon after determination of the tartrate-resistant acid phosphatase activity (TRAP), and TRAP-positive cells for investigation were serially sectioned by alternating semithin and ultrathin sections, and then examined by TEM. By SEM, many cellular contacts were seen between the cells cultured on the dentin, but by TEM there were few special structures on the cell membranes between osteoclasts and osteoblasts/stroma cells, or between osteoclasts and bone marrow cells. A special structure on the cell membranes of osteoclasts was observed between an osteoclast and a cytoplasmic process of osteoblast/stroma cells, and this cell membrane was coated with electron dense or bristle-like structures. These bristle-like structures were very similar to those of coated pits. The present results show that the coated pit-like structure plays an important role in cell-cell interaction between osteoclasts and osteoblasts/stroma cells in vitro, and suggest that macromolecules binding to the osteoclast-surface receptor via ligands, accumulate in the coated pits, and enter the osteoclast as receptor-macromolecule complexes in endocytic vesicles.     

干细胞移植预处理对骨髓基质细胞形态及生长能力的影响

目的：探讨外周血干细胞移植后患者骨髓造血微环境损伤的机制。 方法： 采用Dexter型方法, 观察21例外周血干细胞移植患者经单化或化放结合两种预处理方案前后不同时间骨髓基质细胞超微结构及集落形成能力（CFU-F）。 结果： 光镜和扫描电镜下单化组和放化组骨髓基质细胞出现不同特征的形态学变化。透射电镜观察：放化组骨髓基质细胞的内质网扩张，线粒体基质变淡，嵴消失；单化组基质细胞的线粒体重度退行性变，结构紊乱；放化组骨髓基质细胞集落形成能力于预处理后各时点均显著低于预处理前和单化组预处理后(P<0.01)；单化组于预处理后各时点的CFU-F显著低于预处理前(P<0.01)，至预处理后第90 d仍未恢复。 结论： 外周血干细胞移植患者预处理后骨髓基质细胞不同程度损伤，放化组基质细胞的损伤重于单化组，是导致造血功能恢复缓慢的原因。     

大鼠骨髓间充质干细胞成骨诱导分化的超微结构变化

背景：目前对骨髓间充质干细胞成骨诱导分化的超微结构观察的报道甚少。 目的：采用全骨髓贴壁法分离培养大鼠骨髓间充质干细胞,成骨诱导并染色鉴定,利用电镜观察诱导前后细胞超微结构变化特点。 方法：全骨髓贴壁法体外分离、培养、纯化大鼠骨髓间充质干细胞,流式细胞仪检测细胞表面标记物,成骨染色鉴定向成骨方向诱导分化,扫描电镜及透射电镜观察成骨诱导前后细胞超微结构变化。 结果与结论：培养的第3代骨髓间充质干细胞纯度高、活力强,成骨诱导后的碱性磷酸酶活性染色、钙化结节染色均呈阳性。扫描电镜及透射电镜观察显示,经向成骨细胞诱导分化后,细胞形态铺展,不规则,其线粒体、粗面内质网、空泡明显增多,表明细胞功能活跃。     

可注射藻酸盐支架与人骨髓基质干细胞的体外复合培养*☆

背景：目前可注射组织工程骨的研究主要限于动物实验,若人骨髓基质干细胞与藻酸盐生物相容性良好,可注射组织工程骨将是极具前途的临床治疗手段。 目的：体外观察人骨髓基质干细胞与可注射支架藻酸钙凝胶的生物相容性。 方法：实验组将第2代人骨髓基质干细胞与藻酸钙凝胶复合培养,对照组单纯接种骨髓基质干细胞。倒置相差显微镜、扫描电镜观察各组细胞形态及增殖情况,MTT法半定量检测细胞增殖情况。 结果与结论：倒置显微镜下见实验组细胞生长良好,与对照组无明显差异。扫描电镜见骨髓基质干细胞在藻酸钙表面贴附、增殖良好,第6天时细胞已跨越微孔表面或向孔内生长。MTT法显示与对照组相比,实验组细胞增殖能力不受影响。结果初步表明藻酸钙与人骨髓基质干细胞体外生物相容性较好。      

小鼠卵黄囊间充质干细胞的培养及鉴别

目的 研究小鼠卵黄囊间充质干细胞(YS-MSCs)的分离、培养和鉴定.方法 显微分离妊娠10d的小鼠胚胎卵黄囊,经0.1%的Ⅰ型胶原酶消化1h得到卵黄囊细胞,取贴壁细胞培养,并于接近汇合时进行传代培养,透射电镜下观察YS-MSCs的超微结构;流式细胞仪检测YS-MSCs表面标志;钙钴法测定YS-MSCs碱性磷酸酶(AKP)活性;细胞组织化学检测YS-MSCs化学变化;地塞米松、胰岛素定向诱导YS-MSCs分化为脂肪细胞,油红0检测中性脂肪.结果 体外可获得YS-MSCs,细胞大多数呈梭形;透射电镜下YS-MSCs表面有微绒毛,胞浆中有丰富的线粒体、粗面内质网、高尔基复合体;核大,形态不规则;流式细胞术分析YS-MSCs免疫表型显示,原代和第1代YS-MSCs均为CD44、CD105阳性,CD34也有少量表达;细胞化学YS-MSCs PAS-过碘酸雪夫染色阳性,苏丹黑-B (S8)及碱性磷酸酶(AKP)染色阴性;YS-MSGs经成脂诱导后,胞浆中有脂滴形成,经油红0染色脂滴呈鲜红色.结论 小鼠YS-MSCs的生物学特性与成体间充质干细胞相似,且更为原始,提示其可作为组织工程的种子细胞来源.     

Monoclonal Antibody Against Bone Marrow Stromal Cells Its Production and Characterization

Bone marrow stromal cells play an essential role in the proliferation and differentiation of hematopoietic stem cells (1,2). As a means of analyzing of the bone marrow microenvironment immunohistochemically, we attempted to produce a rat monoclonal antibody against the murine preadipocyte line HI derived from long-term bone marrow culture (LTBMC) of C57BL/6 mice (3,4). A newly established monoclonal antibody, designated R4-A9, was obtained from a hybridoma prepared by fusion of Y.B2/ 3.0Ag20(YO) rat myeloma cells with spleen cells of LEW rats immunized with HI cells. The immunofluorescence of live HI cells showed that the antigen reacting with this antibody was strongly expressed on the cell surface. The specificity of R4-A9 was assessed immunohistochemically on frozen sections of various tissues from normal adult mice. R4-A9 demonstrated specificity for hematopoietic stroma in bone marrow and spleen. No staining was observed in thymus, lymph nodes or other tissues examined, with the exception of Leydig cells in the testis and the endothelium of small arteries in several organs. Detailed immunohistochemical observations at both the light microscopy and electron microscopy level showed that R4-A9 selectively reacted with the sinusoidal endothelium, peri-sinusoidal adventitial cells (5) (adventitial reticular cells (6)) and intersinusoidal reticular cells (5) and the reticular cells of the splenic red pulp. These findings indicate that reticular cells and the endothelium of the bone marrow possess the common cell surface molecules recognized by R4 A9. SDS-PAGE analysis showed that R4-A9-immuno-precipitated proteins had a molecular mass of 100 kDa under reducing conditions. These data show that R4-A9 is highly specific for reticular cells and the endothelium of murine bone marrow and spleen, providing a new approach for detailed structural analysis of the constituents of bone marrow stroma. Acta Pathol Jpn 41: 499–506, 1991.     

Marrow-derived mesenchymal stem cells: Role in epithelial tumor cell determination

Marrow stroma represents an advantageous environment for development of micrometastatic cells. Within the cellular structure of marrow stroma, mesenchymal stem cells (MSC) have been postulated as an interacting target for disseminated cancer cells. The studies reported here were performed to gain more information on the interaction of the human breast cancer cell line MCF-7 with human bone marrow-derived MSC cells and to investigate whether this interaction affects tumor cell properties. The results showed that after co-culture with MSC, changes were detected in the morphology, proliferative capacity and aggregation pattern of MCF-7 cells, but these parameters were not affected after the co-culture of MSC cells with a non-tumorigenic breast epithelial cell line, MCF-10. Since the indirect culture of MCF-7 with MSC or its products also resulted in functional changes in the tumor cells, we evaluated whether these effects could be attributed to growth factors produced by MSC cells. It was found that VEGF and IL-6 mimic the effects produced by MSC or its products on the proliferation and aggregation properties of MCF-7, cells, respectively. Thus, it seems that after entry of disseminated tumor cells into the marrow space, their proliferative and morphogenetic organization patterns are modified after interaction with distinct stromal cells and/or with specific signals from the marrow microenvironment.     

骨形态蛋白2体外诱导大鼠骨髓间充质干细胞分化为心肌样细胞

目的 应用骨形态蛋白2(BMP-2)体外诱导骨髓间充质干细胞(MSCs)向心肌样细胞分化,探索MSCs向心肌细胞分化的诱导方法.方法 取SD大鼠四肢骨骨髓,分离培养MSCs,应用BMP-2定向诱导,相差显微镜观察细胞形态学变化,应用免疫细胞化学、激光扫描共焦显微镜技术检测结蛋白(desmin)、α-横纹肌肌动蛋白(α-sareomeric actin)、心肌特异性肌钙蛋白(C-TnT)的表达,透射电镜鉴定.在诱导后7d、21d和28d 3个时间点以半定量RT-PCR方法检测细胞心肌早期转录因子(GATA4)和心肌特异性α-肌凝蛋白重链(α-MHC)的表达.结果 BMP-2诱导后的MSCs细胞伸出伪足,排列方向渐趋一致.MSCs体外经BMP-2诱导后分化的细胞结蛋白、α-横纹肌肌动蛋白、C-TnT均表达阳性,结蛋白、α-横级肌肌动蛋白阳性率较高,分别为37.28%和63.94%,而C-TnT阳性率较低为34.66%.透射电镜下可见到平行排列的肌丝,大量的粗面内质网和线粒体,富含糖原和核糖体.RT-PCR结果显示,GATA4于诱导后7d弱表达,21d表达增强,28d表达减弱.α-MHC在诱导后7d不表达,21d弱表达,28d表达明显.结论骨髓间充质干细胞在BMP-2诱导下可定向分化为心肌样细胞,是自体心肌细胞的一种良好供体来源.     

Distribution and role of myofibroblasts in human normal seminal vesicle stroma

We studied the distribution of myofibroblasts in the stroma of normal seminal vesicles. Twelve normal seminal vesicles obtained by surgery on the diagnosis of some diseases were selected, and we evaluated the distribution of myofibroblasts in the seminal vesicles using immunohistochemical and electron and immunoelectron microscopic techniques. Immunohistochemically, myofibroblasts, which were positive for alpha-smooth muscle actin (ASMA) and negative for high molecular weight caldesmon (h-CD), were observed in the stroma just beneath the epithelium of normal seminal vesicles. Moreover, an electron microscope examination revealed the presence of spindle or stellate cells in the stroma of the lamina propria beneath the seminal vesicle epithelium, and an immunoelectron microscopic examination showed that these cells were positive for ASMA. Finally, myofibroblasts are distributed in the lamina propria of human normal seminal vesicles and may play an important role in the ejection of sperm plasm.     

Zur Histogenese der Angiomatosis uteri — sog. Stromatosis uteri

Summary On the basis of our light microscope findings and the additional information howest from the electron microscope, in two cases, the former assumption that the histogenesis is derived from the endometrial stroma is questioned. The electron microscope examination prouved that neither a resemblance to the endometrial stroma nor to the endometrial stromal sarcoma exists. Four types of cells can be differentiate by electronmicroscope, indifferent, endothelial-like, pericyte-like and vessel-muscle cell-like. All four posses common characteristics and are clearly differentiated from the endometrial stromal cells. The cells show a similarity in regard to the corresponding cell bodies and nuclei, hetero- and intercellular contacts, the identical membrane-specialisations and the general existing external-laminae. The latter are on the indifferent cells fragmentated or clustered to a conglomerate, whereas on the other external-laminae are complete. The individual cellforms are arranged in a three dimensional net, so that from an indiffertiated cell mass one is able to derive innummerable vessel-caricatures. The above described structure when compared to the angiomas electron microscopically woud prove the angiome nature of the endolymphatic stromal myosis. Until now this type of angioma has only been described in the uterus, further research is necessary to clarify whether formal genetic connections exist with the uterine vessels and if so to what degree.     

Granulocyte-colony stimulating factor-mobilized mesenchymal stem cells: A new resource for rapid engraftment in hematopoietic stem cell transplantation

The bone marrow (BM) microenvironment plays an important role in regulating hematopoietic stem cell self-renewal and differentiation. Mesenchymal stem cells (MSCs), which constitute approximately 0.01-0.0001% of the nucleated cells in the adult human BM, are an important component of the BM stroma that supports hematopoiesis. The BM stroma system is often damaged in patients who have undergone high-dose chemotherapy and/or radiation treatment. Thus, the BM stroma should be reconstructed during hematopoietic stem cell transplantation (HSCT). Granulocyte-colony stimulating factor (G-CSF) is a potent hematopoietic cytokine that regulates neutrophil generation within the BM by modulating the mobility, proliferation and maturation of neutrophil progenitor cells. The results from our study here show that G-CSF markedly increased the number of donor-derived MSCs in the BM and the peripheral blood. Engraftment was faster in HSCTs with bone marrow that was treated with G-CSF (G-BM) or with G-BM- and G-CSF-treated peripheral blood stem cells compared to stead-state bone marrow (SS-BM). Based on these findings, we hypothesize that G-CSF-mobilized treatment of MSCs may accelerate engraftment in HSCT.     

碱性成纤维细胞生长因子体外诱导大鼠骨髓间充质干细胞分化为心肌样细胞

Objective To study the feasibility of basic fibroblast growth factor （bFGF）in inducing bone marrow mesenchymal stem cells(BMSCs) to differentiate into cardiomyocyte-like cells in vitro. Methods SD rat BMSCs were isolated and cultured from rat bone marrow, and then induced by bFGF. The cultured cells were observed by a phase-contrast microscope. The immunohistochemical technique and laser scanning confocal microscope (LSCM) were used for examination of the expression of desmin, α-actin and C-TnT. The ultrastructure of induced cells was observed by a transmission electron microscope. GATA4 andα-MHC expressions were detected by relative quantitative RT-PCR after 7, 21, 28 days of induction respectively. Results After primary cells were cultured for 48 hours,most of them became fusiform fibroblast-like shape with two or three processes and a few of them were flat in shape. The morphology of BMSCs induced by bFGF changed obviously. After being induced by bFGF for one week, the cells became larger and most of them became myocyte-like short column in shape or long shuttle-shape while paralleled. After four weeks, most cells became short column-shape and touched with adjacent cells tightly to form myotube-like structure,with apparent directionality of cell arraying. BMSCs induced by bFGF were identified by the positive staining for desmin, α-sarcomeric actin and C-TnT. Of BMSCs, there were more desmin positive and α-actin-positive cells made up higher of all BMSCs than C-TnT-positive cells. Desmin and α-actin-positive cells were about 33.82% and 58.64%, and C-TnT-positive cells were about 28.94%. Desmin(α-actin ) appeared red, and C-TnT was green under a laser scaning confocal microscopy. Their co-expression appeared yellow. Transmission electron microscope showed that the induced cells were rod in shape and the ovoid nuclei were positioned in the center of the cell. lots of mitochondria, rough endoplasmic reticulum, ribosome and paralleled myofilaments were founded in plasm.RT-PCR assessment showed that the differentiated cells began to express GATA-4 from day 7 to day 28 of differentiation and began to express α-myosin heavy china(α-MHC) fro     

纳米级二氧化锆增韧羟基磷灰石生物复合陶瓷对兔骨髓基质干细胞增殖、分化的影响

背景：课题组拟对纳米级二氧化锆增韧的羟基磷灰石生物陶瓷进行一些初步研究,主要集中在生物力学匹配性,化学稳定性,以及生物相容性实验,其中体外细胞培养实验具有可控性,可重复性,能很好地反映材料的生物相容性。 目的：比较纳米级二氧化锆增韧的羟基磷灰石、纯羟基磷灰石两种材料对兔骨髓基质干细胞增殖、分化的影响。 方法：将骨髓基质干细胞置于含体积分数为20%胎牛血清的DMEM培养基中培养,传代后改用含β-甘油磷酸钠,地塞米松和维生素C的条件培养基培养。取传至第3代的成骨细胞,以1.0×108 L-1浓度接种于放有材料块的细胞培养板中,培养第1~10天倒置相差显微镜观察细胞生长情况,绘制细胞生长曲线,并进行碱性磷酸酶活性检测。培养第6天的细胞和材料复合物用多聚甲醛固定进行扫描电镜观察。 结果与结论：MTT法测得两种材料培养的细胞生长曲线无显著差异。复合培养的兔骨髓基质干细胞能够保持正常分泌碱性磷酸酶的功能。电镜照片也同样证实了两种材料表面均有细胞的附着。说明纳米级二氧化锆增韧的羟基磷灰石、纯羟基磷灰石均不影响成骨细胞增长分化,具有优良的成骨细胞相容性。     

诱导大鼠间充质干细胞形成的神经元样细胞的形态特征

目的 研究诱导大鼠骨髓间充质于细胞(MSCs)形成的神经元样细胞的形态特征.方法 通过贴壁法培养鉴定大鼠骨髓MSCs,体外培养扩增纯化后加入新生鼠脑匀浆上清诱导48 h.倒置显微镜和透射电镜下观察诱导前后细胞的形态结构变化,免疫细胞化学方法鉴定诱导后细胞的性质.结果 倒置显微镜下可见MSCs呈纺锤形和多角形,核居中,有1或2个核仁,诱导后细胞呈神经元样,细胞伸出较长的轴突样和树突样突起;免疫组织化学方法显示NSE、NF阳性,GFAP阴性.透射电镜下可见诱导前MSCs胞质较丰富,有较多的细胞器,细胞核呈圆形或椭圆形,也可见不规则形核,核染色质较疏松,有明显核仁,多为1个.有的MSCs表面可见较多的微绒毛.诱导后细胞胞质丰富,细胞核多为不规则形,染色质疏松,核仁较大,1～3个不等,细胞表面有发达的微绒毛.结论 MSCs是一种多分化潜能的细胞,诱导后分化成为具有成熟细胞器的神经元.     

犬骨髓间充质干细胞定向分化为内皮细胞的研究

应用密度梯度离心法分离狗骨髓间充质干细胞,建立诱导分化内皮细胞的方法及条件。密度梯度离心法分离狗骨髓间充质干细胞后,在体外经血管内皮细胞生长因子(Vascular endothelial growth factor,VEGF)、内皮细胞生长因子(Endothelial growth factor,EGF)等诱导后,分化形成内皮样细胞。结果表明:光镜下细胞单层融合生长,呈铺路石样形态,单个核位于中央。电镜下可见到Weibel-Palade小体,在细胞胞浆vWF染色阳性。骨髓间质干细胞,在体外可诱导分化成内皮细胞。     

IL-3对小鼠骨髓来源树突状细胞分化发育的影响

比较 GM- CSF IL- 4与 IL- 3 IL- 4两种方法培养制备的树突状细胞 (Dendritic cell,DC)在形态、产量、免疫表型和抗原摄取能力方面的差异。用 GM- CSF IL- 4和 IL- 3 IL- 4分别诱导小鼠骨髓来源的前体细胞分化为成熟树突状细胞 ,通过相差显微镜、扫描电镜、激光共聚焦扫描显微镜进行形态观察 ,流式细胞术分析细胞免疫表型和摄取抗原能力。结果表明 ,IL- 3 IL- 4诱导的 DC产量高 ,细胞纯度好 ,形态上与 GM- CSF IL- 4诱导的DC类似 ;细胞免疫表型显示高表达 MHC 类分子 ,但低表达或不表达 CD80、CD86 ;IL- 3 IL- 4诱导的 DC具有强大的摄取抗原能力。 IL- 3替代 GM- CSF可诱导低表达共刺激分子的耐受型 DC     

The epigenetic influences of bone marrow and fetal liver stroma cells on the developmental potential of Ly-1+ pro-B lymphocyte clones

The Ly-1+ Mac-1+ B-220+ CC11+ progenitor clones LyD9 and LyB9 were previously shown to give rise to Ly-1+IgM+ B lymphocytes either in vivo or in vitro by co-culture with nonlymphoid accessory cells from spleen and lipopolysaccharide. The clones did not generate T lymphocytes either in vivo or in vitro. We now find that both LyD9 and LyB9 progenitors are induced to differentiate in vitro by co-culture with the RP.0.10 bone marrow stroma clone or with heterogeneous marrow-adherent stroma cell populations obtained from adult mice into Ly-1-IgM+ B lymphocytes as well as myeloid GM1.2+ Mac-1+ cells. We could obtain evidence that a high proportion of LyD9 and LyB9 cells already switched off expression of Ly-1 and CC11 (interleukin 3 receptor) surface molecules 2 days after initiation of the cultures, and by days 8-10 of culture no detectable Ly-1+ cells and only about 20% CC11+ cells were observed. Ly-1 surface expression could not be re-induced on the progeny of LyD9 and LyB9 progenitors generated under the influence of marrow stroma cells. Remarkably, the LyD9 and LyB9 progenitors gave rise to both Ly-1+IgM+ and Ly-1-IgM+ B cells upon culture with heterogeneous stroma monolayers obtained from 18-day fetal liver. Finally, the differentiating property of the stroma cells for the LyD9 and LyB9 progenitors could not be replaced with soluble factors produced either spontaneously or after stimulation by the marrow stroma cells. Our results show the importance of epigenetic influences provided by a given microenvironment on the developmental potential of B cell progenitors. They provide direct evidence that the same pro-B lymphocyte can give rise to both Ly-1+ and Ly-1-IgM+ B cells depending on both the time of development and the tissue of origin of stroma cells with which the B cell progenitor interacts. Also, the results strongly suggest that cell contact between the stroma cell and the B cell progenitor is essential to induce rearrangement and expression of the Ig genes in pro-B lymphocytes.