骨髓基质细胞微环境诱导神经干细胞的分化

学术背景：骨髓基质细胞能够表达多种细胞表面分子及分泌多种细胞生长因子，其共同组成的微环境可以调节造血干细胞的分化和发育。造血干细胞和神经干细胞可表达很多共同基因．提示二者存在类似的发育和分化机制。 目的：深入认识骨髓基质细胞提供的微环境对神经干细胞的诱导分化作用。 检索策略：由该论文的研究人员应用计算机检索Pubmed数据库1992—01／2007—03的相关文献，检索词“bone marrow stem cells，microenvironment，neuml stem cells，differentiation”，并限定文章语言种类为English。同时计算机检索CHKD期刊全文数据库1994—01／2006—12的相关文献，检索词“骨髓基质细胞，神经干细胞，分化”，并限定文章语言种类为中文。共检索到125篇文献，对资料进行初审，纳入标准：①文章所述内容应与骨髓基质细胞密切相关。②同一领域选择近期发表或在权威杂志上发表的文章。排除标准：①重复性研究。②Meta分析。 文献评价：文献来源主要是通过对骨髓基质细胞所提供的微环境诱导神经干细胞分化方面的内容进行汇总分析。所选用的31篇文献中，7篇为综述，其余均为临床或基础实验研究。 资料综合：①骨髓基质细胞易于分离培养，其分泌的各种细胞因子、生长因子和细胞外基质成分所构成的微环境能够调节神经干细胞的发育和分化。②从mRNA水平看，骨髓基质细胞不仅有脑源性神经生长因子和神经生长因子的表达，还有胶质细胞源性神经营养因子的表达。从蛋白水平看，采用ELISA法在骨髓基质细胞的培养液和细胞蛋白中均可检测到胶质细胞源性神经营养因子。在生理状态下，骨髓基质细胞能分泌具有神经营养活性的物质，对损伤的运动神经元具有保护作用。此外研究证实骨髓基质细胞分泌至细胞外的可溶性物质具有直接调节神经干细胞分化的能力。 结论：骨髓基质细胞通过分泌各种细胞因子所构成的微环境能够调节神经干细胞的发育，并诱导其向神经元方向分化。神经干细胞增殖分化的调控是非常复杂的过程，各因子间可能存在协同或拮抗作用，具体作用机制以及细胞信号转导通路还需深入研究。     

猫骨髓源神经干细胞的诱导分化

目的观察家猫骨髓源巢蛋白阳性细胞诱导分化情况,为家猫骨髓基质细胞(BMSC)作为自体神经干细胞种子细胞提供理论依据。方法抽取家猫的骨髓,从中分离得到骨髓基质细胞,在体外给予神经干细胞培养基培养增殖,然后用分化诱导因子进行诱导分化,倒置相差显微镜下观察活细胞及免疫细胞化学染色情况。结果猫骨髓基质细胞在体外培养中体型增大,继之出芽,贴壁生长,可形成克隆团,同时可传代培养,这些具有克隆能力的骨髓基质细胞能表达神经干细胞特异性抗原巢蛋白,与5-溴尿嘧啶(BrdU)共培养3d后可见部分大圆形细胞BrdU染色阳性,这些细胞经进一步的诱导可分化为神经元样细胞,而且表达神经元特异性抗原NSE和NeuN。结论家猫骨髓基质细胞具有干细胞特征,可诱导为巢蛋白阳性细胞,并可进一步分化为表达烯醇化酶(NSE)和NeuN的神经元样细胞。证实家猫BMSC可作为移植修复神经系统损伤的神经干细胞种子细胞。     

Naloxone interferes with granulocytopoiesis in long-term cultures of mouse bone marrow; buffering by the stromal layer

Long-term cultures of mouse bone marrow cells were treated with naloxone, starting at the time of culture initiation or in the 2nd or 4th week of culture. Cell proliferation was suppressed and the ratio of immature and mature granulocytes to macrophages diminished by naloxone treatment. The effect depended on the timing of naloxone addition to the cultures and on its concentration, with a bell-shaped dose-response curve. High and low concentrations of naloxone (10⁻⁴, 10⁻⁶, 10⁻¹⁴ M) interfered with hematopoiesis more strongly than the intermediate concentrations (10⁻⁸ to 10⁻¹² M). Early cultures lacking the stromal layer were more sensitive to naloxone than the cultures with established stroma. The bell-shaped dose-response curve has been attributed to an interplay of specific (opioid-receptor-mediated) and nonspecific mechanisms. Opioidergic mechanisms apparently participate in the regulation of hematopoiesis.     

Osteoclast-like cells form in long-term human bone marrow but not in peripheral blood cultures.

Transplantation studies have suggested that peripheral blood mononuclear cells contain precursors for osteoclasts. Thus we tested the capacity of peripheral blood monocytes to form osteoclasts in long-term culture. We have reported previously that mononuclear cells from feline, baboon, and human marrow form osteoclast-like cells in long term cultures. Further, the formation of these cells is increased in response to bone resorption stimulatory agents such as PTH, interleukin 1, and transforming growth factor alpha. We now report that these cells show characteristic cytoplasmic contraction with calcitonin and form resorption lacunae when cultured on sperm whale dentine. Thus, these bone marrow-derived multinucleated cells fulfill the functional criteria for osteoclasts. Although cultured peripheral blood monocytes can be induced to form multinucleated cells with 1,25-dihydroxyvitamin D3, these cells did not show similar responses to the osteotropic factors as multinucleated cells formed in the bone marrow cultures multinucleated cells. These results indicate that osteoclasts or cells closely related to osteoclasts form in long-term human bone marrow cultures. In contrast, few mononuclear cells in the peripheral blood appear capable of forming osteoclasts under the culture conditions used in these experiments.     

IL-6基因转染的骨髓基质细胞系对骨髓移植小鼠造血功能恢复的促进作用

目的:探讨白细胞介素6(IL-6)基因转染的骨位基质细胞系QXMSC, IL6对骨髓移植后造血功能的重建作用。方法:将骨髓造血细胞和骨髓基质细胞系一起经尾静脉注射给同系小鼠,建立骨髓移植(BMT)模型。小鼠的造血功能用脾结节(CFU-S)、粒-单系祖细胞(CFU-GM)、红系祖细胞(CFU-E、BFU-E)测定及外周血各项血液学指标来确定。结果:QXMSC_1 IL-6转基因骨髓基质细胞可明显增强BMT小鼠形成的CFU-S、CFU-GM、CFU-E和BFU-E数,促进外周血象的恢复。结论:QXMSC_1 IL-6细胞可明显促进小鼠骨髓移植后早期造血功能重建。     

促血小板生成素基因转染小鼠骨髓基质细胞的实验研究

目的探讨促血小板生成素（TPO）基因修饰的小鼠骨髓基质细胞（BMSCs）对TPO基因的表达的影响。方法用脂质体lipofectamine^TM2000将TPO cDNA真核表达质粒转染至骨髓基质细胞（BMSCs），RT—PCR方法检测TPO mRNA的表达，免疫组化法测定TPO的表达。结果转染TPO基因的BMSCs TPO mRNA及TPO表达量明显高于空载体组（P〈0．01）及未转染组（P〈0．01）。结论阳离子脂质体能有效介导TPO cDNA真核表达质粒转染BMSCs，且转染后BMSCs中TPO mRNA及TPO的表达显著上调。     

骨髓基质细胞诱导为神经原及胶质细胞

目的:发现一种新的方法代替雪旺细胞用来修复周围神经缺损。方法:从大鼠抽取骨髓在体外诱导为胶质细胞并做免疫组化。结果:骨髓基质细胞在体外培养数代,显示出极强的扩增能力,并在条件培养液的作用下分化为神经原及神经胶质细胞。结论:经论证,本实验在治疗中枢及外周神经疾病方面有重要意义。     

Hematopoietic effects of benzene inhalation assessed by murine long-term bone marrow culture.

The strong and long-lasting hematotoxic effect after benzene exposure in vivo (300 ppm, 6 hours/day, 5 days/week for 2 weeks) was assessed in mice with bone marrow cells grown in long-term bone marrow culture (LTBMC). Bone marrow cultures initiated 1 day after the last benzene exposure did not produce adequate numbers of hematopoietic cells over 3 weeks and, in most cases, no erythroid or myeloid clonogenic cells could be recovered. The adherent cell layer of these cultures had a lower capacity for supporting in vitro hematopoiesis after the second seeding with normal bone marrow cells compared with control cultures. Two weeks after the last benzene exposure, body weight, hematocrit, bone marrow cellularity, and committed hematopoietic progenitor content (BFU-E and CFU-GM) were regenerated to normal or subnormal values, whereas hematopoiesis in LTBMC was very poor. Over 8 weeks, little or no significant committed progenitor production was observed. Treatment of mice exposed to benzene with hemin (three doses of 3 micrograms/gm body wt, iv, over 2-week for a total dose 9 micrograms/gm) partially overcame the toxic effect of benzene on the hematopoietic system as measured by the LTBMC method. Cultures from mice treated with hemin had modest recovery of BFU-E and CFU-GM clonogenic potential after 5 to 6 weeks in LTBMC. In contrast, little or no recovery was obtained for the adherent cell layer clonogenic capacity, even after hemin treatment. These results clearly indicate a strong, long-lasting toxic effect on the bone marrow stroma and a limited recovery of hematopoietic potential by clonogenic cells of the nonadherent population after in vivo hemin treatment.     

The generation of natural killer (NK) cells from NK precursor cells in rat long-term bone marrow cultures

In this report, we describe a novel long-term bone marrow culture (LTBMC) system to study the origin and generation of natural killer (NK) cells from NK precursors. Rat bone marrow was cultured for 4 wk in RPMI 1640 with 5% fetal calf serum and 2-mercaptoethanol to allow the formation of an adherent stromal cell layer containing NK precursor cells. After addition of interleukin 2 (IL-2), the LTBMC generated high numbers (up to 100-fold expansion in 7 d) of pure 3.2.3+ large granular lymphocytes with lytic activity against NK-sensitive and -resistant tumor targets, as well as antibody-dependent cellular cytotoxicity. NK activity in LTBMC could be detected 3 d after addition of as little as 1 U/ml rIL-2, whereas lymphokine-activated killer activity was found 5 d after addition of at least 10 U/ml rIL-2. In vivo depletion and in vitro complement lysis studies showed that the NK precursor cells in LTBMC did not express the NK-associated surface markers asialo GM1 or 3.2.3. We also found that LTBMC cells did not exhibit colony growth in granulocyte/macrophage or spleen colony-forming unit assays. The generation of NK cells from NK precursors required, in addition to IL-2, a second growth/maturation factor(s), which was present in the conditioned medium of the LTBMC. This LTBMC system provides a unique in vitro model to study the development of NK cells from precursor cells, the role of the bone marrow stromal microenvironment in this development, and the lineage relationship of NK cells to other hematopoietic cells.     

IL—6基因转染的骨髓基质细胞系对骨髓移植小鼠造血功能恢复？…

目的：探讨白细胞介素６（ＩＬ－６）基因转染的骨髓基质细胞系ＱＸＭＳＣ１ＩＬ－６对骨髓移植后造血功能的重建作用。方法：将骨髓造血细胞和骨髓基质细胞系一起经尾静脉注射给同系小鼠，建立骨髓移植（ＢＭＴ）模型。小鼠的造血功能用脾结节（ＣＦＵ－Ｓ）、粒－单系祖细胞（ＣＦＵ－ＧＭ）、红系祖细胞（ＣＦＵ－Ｅ、ＢＦＵ－Ｅ）测定及外周血各项血液学指标来确定。结果：ＷＸＭＳＣ１ＩＬ－６转基因骨髓基质细胞可明显增强ＢＭ     

Atypical multinucleated cells form in long-term marrow cultures from patients with Paget''s disease.

Although Paget's disease is the most flagrant example of a primary osteoclast disorder, little is known of osteoclast biology in this disease. In this report we have studied the formation of cells with the osteoclast phenotype in long-term cultures of marrow mononuclear cells derived from patients with Paget's disease, and compared these with similar cells formed in long-term marrow cultures from normal individuals, and with osteoclasts present in pagetic bone. Osteoclasts formed in pagetic marrow cultures resembled osteoclasts present in pagetic bone, but were distinctly different from osteoclasts formed in normal marrow cultures. Osteoclast formation was 10-20-fold greater in pagetic marrow cultures than in normal cultures. The multinucleated cells formed in cultures of pagetic marrow were much larger in size, were hyperresponsive to 1,25(OH)2 vitamin D, had more nuclei per cell, had increased levels of tartrate-resistant acid phosphatase activity and had ultrastructural features which were not seen in multinucleated cells formed from normal marrow mononuclear cells. These pagetic marrow-derived multinucleated cells formed large resorption lacunae on calcified matrices and cross-reacted with monoclonal antibodies which preferentially bind to osteoclasts. The multinucleated cells formed from marrow obtained from uninvolved sites in Paget's patients also displayed these abnormal features.     

颅骨来源骨髓基质干细胞的分离培养方法

背景：从小鼠颅骨提取的一类骨髓基质干细胞称为PA6细胞,研究者发现当PA6细胞与其他干细胞共培养时可向神经细胞分化,此特性可被应用于神经损伤部位的修复。所以,PA6细胞越来越受到研究者的关注,但目前国内对类似骨髓基质干细胞PA6的提取方法鲜见报道。目的：分离培养SD大鼠颅骨来源的骨髓基质干细胞,体外观察细胞形态并免疫荧光鉴定。方法：无菌条件下,取新生的SD大鼠颅骨将其剪碎,用含体积分数为10%胎牛血清的DMEM培养液冲洗颅骨碎片,反复吹打制成单细胞悬液,将其置于培养瓶中培养,多次换液培养纯化细胞。取第2代生长均一的细胞,倒置显微镜下观察细胞形态,并用免疫荧光染色法进行细胞表面标记鉴定。结果与结论：原代细胞培养24 h后,骨髓基质干细胞开始贴壁;3 d后细胞数量增多,形态呈不规则形、多角形、三角形和扁平形;细胞传代后,形态基本均一,细胞排列呈放射状和束状,贴壁能力较强,部分细胞呈聚集生长,增殖速度比原代有明显的加快。免疫荧光染色检测CD105、CD73、CD44、CD90表达呈阳性, CD45、CD34、CD14和HLA-DR表达呈阴性,证实了提取的细胞为骨髓基质干细胞。     

Matrix formation is enhanced in co‐cultures of human meniscus cells with bone marrow stromal cells

The ultimate aim of this study was to assess the feasibility of using human bone marrow stromal cells (BMSCs) to supplement meniscus cells for meniscus tissue engineering and regeneration. Human menisci were harvested from three patients undergoing total knee replacements. Meniscus cells were released from the menisci after collagenase treatment. BMSCs were harvested from the iliac crest of three patients and were expanded in culture until passage 2. Primary meniscus cells and BMSCs were co‐cultured in vitro in three‐dimensional (3D) pellet culture at three different cell–cell ratios for 3 weeks under normal (21% O₂) or low (3% O₂) oxygen tension in the presence of serum‐free chondrogenic medium. Pure BMSCs and pure meniscus cell pellets served as control groups. The tissue generated was assessed biochemically, histochemically and by quantitative RT–PCR. Co‐cultures of primary meniscus cells and BMSCs resulted in tissue with increased (1.3–1.7‐fold) deposition of proteoglycan (GAG) extracellular matrix (ECM) relative to tissues derived from BMSCs or meniscus cells alone under 21% O₂. GAG matrix formation was also enhanced (1.3–1.6‐fold) under 3% O₂ culture conditions. Alcian blue staining of generated tissue confirmed increased deposition of GAG‐rich matrix. mRNA expression of type I collagen (COL1A2), type II collagen (COL2A1) and aggrecan were upregulated in co‐cultured pellets. However, SOX9 and HIF‐1α mRNA expression were not significantly modulated by co‐culture. Co‐culture of primary meniscus cells with BMSCs resulted in increased ECM formation. Co‐delivery of meniscus cells and BMSCs can, in principle, be used in tissue engineering and regenerative medicine strategies to repair meniscus defects. Copyright © 2012 John Wiley & Sons, Ltd.     

骨髓基质细胞在骨组织工程中的研究与进展

大面积骨缺损与骨不连等疾病一直是困扰临床骨科的难题,骨组织工程的提出为其治疗提供了一种可行的方法。种子细胞是骨组织工程的核心与基础,一直是人们普遍关注的热点。目前的种子细胞包括成骨细胞,骨细胞,骨膜成骨细胞,骨髓基质细胞等。其中骨髓基质细胞被公认为是一较理想的种子细胞:获取方便,来源较广泛,对供体损伤较小,体外培养技术成熟,细胞增殖较快,而且可以作为一些成骨因子基因转染的靶细胞。骨髓基质细胞、基质支架和/或成骨因子构成的复合材料能够修复骨缺损;特别是将含有成骨因子基因(如骨形态发生蛋白-2基因)的骨髓基质细胞与基质支架复合能够加快骨缺损的修复,这具有重要的临床意义。目前骨髓基质细胞在骨组织工程中应用面临的主要问题是细胞纯化、基因转染后的安全等问题,特别是后者亟待解决。     

In vitro formation of osteoclasts from long-term cultures of bone marrow mononuclear phagocytes

The origin of osteoclasts was studied in an in vitro model using organ cultures of periosteum-free embryonic mouse long-bone primordia, which were co-cultured with various cell populations. The bone rudiments were freed of their periosteum-perichondrium by collagenase treatment in a stage before cartilage erosion and osteoclast formation, and co- cultured for 7 d with either embryonic liver or mononuclear phagocytes from various sources. Light and electron microscopic examination of the cultures showed that mineralized matrix-resorbing osteoclasts developed only in bones co-cultured with embryonic liver or with cultured bone marrow mononuclear phagocytes but not when co-cultured with blood monocytes or resident or exudate peritoneal macrophages. Osteoclasts developed from the weakly adherent, but not from the strongly adherent cells of bone marrow cultures, whereas 1,000 rad irradiation destroyed the capacity of such cultures to form osteoclasts. In bone cultures to which no other cells were added, osteoclasts were virtually absent. Bone-resorbing activity of in vitro formed osteoclasts was demonstrated by 45Ca release studies. These studies demonstrate that osteoclasts develop from cells present in cultures of proliferating mononuclear phagocytes and that, at least in our system, monocytes and macrophages are unable to form osteoclasts. The most likely candidates for osteoclast precursor cells seem to be monoblasts and promonocytes.     

体外培养骨髓基质细胞向成骨细胞的分化

背景:目前如何有效地诱导骨髓基质细胞向成骨细胞转化,建立稳定的体外培养诱导模式,研究诱导条件的作用机制是骨组织工程研究的重点.目的:建立一种兔骨髓基质细胞向成骨细胞分化的体外培养体系,观察其成骨过程.设计:对比观察.材料:4～8周龄新西兰大白兔,雌雄不拘,用于骨髓基质细胞的原代与传代培养.方法:将兔股骨骨髓基质细胞进行原代培养,待细胞铺满瓶底近80%后,加入2.5 g/L胰蛋白酶消化传代培养.取生长良好的对数期第2代细胞,以1×108 L-1的细胞浓度接种于预置盖玻片的6孔培养板内.细胞分为2组,实验组使用条件诱导培养基(RPMI1640标准培养基加入地塞米松10-8 mol/L,β甘汕磷酸钠10 mmol/L,维生素C 50 μg/L),对照组继续使用标准培养基.两组细胞持续培养(常规两三天换液1次)进行细胞成骨分化观察.主要观察指标:倒置相差显微镜观察细胞生长及形态变化:苏木精-伊红染色、碱性磷酸酶染色、Ⅰ型胶原免疫组织化学染色、钙结节染色及扫描电镜观察骨髓基质细胞形态变化.结果:经诱导培养的细胞,在体外逐渐转化,增殖、分泌细胞外基质、最终形成了矿化基质.苏木精-伊红染色显示实验组传代细胞在接近融合为单层时形态多为梭形,多角形等.胞浆内可见颗粒:碱性磷酸酶染色可见实验组传代细胞染色呈强阳性,阳性率达83.2%,并能大量分泌Ⅰ型胶原:对照组仅有个别细胞染色呈阳性.四环素染色观察实验组骨髓基质细胞形成金黄色钙结节,甚至是骨样组织,与典型成骨细胞的牛物学特性相似.扫描电镜观察实验组培养的传代细胞已接近融合为单层,细胞呈梭形或多角形表现,细胞表面及细胞间可见钙盐结晶沉积.结论:实验所建立的兔骨髓基质细胞体外诱导成骨转化体系,能够培养出生长活跃,增殖迅速,稳定、多量向成骨细胞转化的骨髓基质细胞培养模式,所培养的细胞可作为成骨细胞的一种来源,为构建组织工程化骨提供种子细胞.     

Co-transplantation of bone marrow stromal cells transduced with IL-7 gene enhances immune reconstitution after allogeneic bone marrow transplantation in mice

Allogeneic bone marrow transplantation (allo-BMT) is followed by a period of profound immune deficiency, which results in significant susceptibility to infections and limits the extensive application of this approach in clinic. Here, we transduced human interleukin-7 (IL-7) gene into donor-derived bone marrow stromal cells (MSCs) using adenovirus vector, and transplanted this gene-engineered MSCs (MSC-IL-7) into lethally irradiated C57BL/6 mice to investigate their effects on immune reconstitution following allo-BMT. Recipient mice receiving MSC-IL-7 cells plus T-cell-depleted bone marrow cells of BALB/c mice showed a significant increase in thymopoiesis and homeostatic expansion of peripheral T lymphocytes. Furthermore, injection of MSC-IL-7 cells following allo-BMT protected the host from the lethality caused by acute graft-versus-host disease (GVHD) and prevented the occurrence of GVHD induced by transplanted T cells. Thus, the use of MSC-IL-7 cells may be therapeutically useful for enhancing immune reconstitution without aggravating GVHD in allo-BMT mice.     

Effect of hydrocortisone and BCNU on long-term murine bone marrow cultures

Hydrocortisone (HC) has been observed to influence an in vitro Dexter culture system with maintenance of pluripotent hematopoietic stem cells (CFU-s) and progenitor cells (CFU-gm, BFU-e). In the present study, a two-phase murine Dexter culture system was established to study the mechanism of HC-mediated suppression of stem cells or progenitor cells. Furthermore, the effect of exposing 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) to an adherent cell layer for 1 h was investigated for maintenance of progenitor cells. Cultures supplemented with selenium and transferrin showed relatively good growth. The BFU-e number was significantly enhanced at 2 and 3 weeks of culture in the BCNU-treated group. This system is a good model for monitoring the influence of agents to a hematopoietic-inductive microenvironment.     

川芎嗪对BMT后小鼠骨髓基质细胞bFGF表达水平的影响

本研究探讨川芎嗪对骨髓移植（BMT）后小鼠骨髓基质细胞bFGF表达水平的影响及其促进骨髓造血重建的机制。取健康BALB／c小鼠，随机分为3组：正常组（不做处理），BMT＋生理盐水组（简称生理盐水组）和BMT＋川芎嗪治疗组（简称川芎嗪组）。生理盐水组和川芎嗪组分别胃饲生理盐水0．2毫升／只和川芎嗪注射液每次2毫克／只，2次／天，直至处死为止。在BMT后第7、14、21、28天处死小鼠，用RT—PCR和Western blot方法检测骨髓基质细胞bFGF mRNA及其蛋白表达水平。结果表明：BMT后第7、14、21天川芎嗪组和生理盐水组骨髓基质细胞bFGF mRNA及其蛋白的表达均明显低于正常组（P〈0．01或P〈0．05），但川芎嗪组明显高于生理盐水组（P〈0．01或P〈0．05），到第28天，川芎嗪组bFGF mRNA及其蛋白的表达已恢复正常，而生理盐水组仍未恢复正常，两组之间有显著性差异（P〈0．01）。结论：川芎嗪可能通过增加bFGF的表达水平而促进骨髓微环境的修复，加速BMT后骨髓的造血重建。     

Regeneration of large bone defects in sheep using bone marrow stromal cells

Bone repair was addressed in a critical-sized defect model in sheep, combining a ceramic biomaterial and mesenchymal progenitor cells. The defects in the tibial mid-diaphysis were treated with autologous bone or with a silicon-stabilized tricalcium phosphate biomaterial, implemented or not by the addition of expanded bone marrow stromal cells. An internal locking compression plate and an external fixator were applied for stabilization. Radiographies were taken during the 8 months follow-up: the pixel grey levels of the lesion areas were determined to evaluate the repair process radiologically. Microradiography, histology and vascular density tests were performed. The autologous bone-treated group performed best, as assessed radiologically, within 20-24 weeks after surgery. Very limited healing was detected in the other experimental group: a partial bone deposition occurred at the periphery of the bony stumps only in the cell-seeded scaffolds. Interestingly, this effect ended within 20-24 weeks, as for the autologous bone, suggesting similar kinetics of the repair processes involved. Moreover, bone deposition was located where a significant reduction of the ceramic scaffold was detected. Faxitron microradiography and histology data confirmed these results. Vascular density analysis evidenced that cell-seeded scaffolds supported an increased vascular ingrowth. Thus, the interactions with the proper microenvironment and the oxygen and nutrient supply in the inner part of the constructs seem fundamental to initiate scaffold substitution and to improve cell performance in tissue-engineered approaches to bone repair.