Coupling of osteogenesis and angiogenesis in bones

doi:10.3969/j.issn.2095-4344.2017.36.020

Abstract

Abstract:

BACKGROUND: In addition to a conduit system for oxygen, nutrients and waste products, blood vessels in bones manipulate multiple aspects of bone formation and provide niches for bone marrow hematopoietic stem cells. Coupling of osteogenesis and angiogenesis plays a crucial role in bone development, maturation, ageing and diseases.
OBJECTIVE: To review the architecture of bone vasculature and process of angiogenesis, and then discuss the factors regulating coupling of osteogenesis and angiogenesis.
METHODS: CBM and PubMed databases were retrieved using the keywords of “osteogenesis, blood vasculature, angiogenesis” in Chinese and English, respectively. High-quality articles related to the blood vasculature and angiogenesis in bones were included, and the repetitive studies were excluded. Finally 65 eligible articles were included to explore the coupling of osteogenesis and angiogenesis.
RESULTS AND CONCLUSION: The formation and stabilization of the skeletal system rely on the coordination of several different types of cellular activities, especially endothelial cells as an important source of signals that regulate chondrocytes and osteogenesis-related cells, thus allowing the coupling of angiogenesis and osteogenesis during development and regeneration. Like other organs, the vascular system in the bones exhibits a typical hierarchy; the arterial trunk branches flow into the capillary network and then converge into the large veins in the center of the bone. Vascular endothelial growth factor, fibroblast growth factor 2, platelet-derived growth factor, matrix metalloproteinases, hypoxia, and Notch signaling pathway are involved in the angiogenesis in bones. The signaling pathways and related cytokines regulating the coupling of osteogenesis and angiogenesis will provide a new treatment strategy to battle bone diseases such as osteoporosis, bone tumor, bone fracture and bone repair.

中国组织工程研究杂志出版内容重点：组织构建；骨细胞；软骨细胞；细胞培养；成纤维细胞；血管内皮细胞；骨质疏松；组织工程

Key words: Osteogenesis, Blood Vessels, Endothelial Cells, Chondrycytes, Tissue

CLC Number:

R318

Yang Jin-ting, Han Xiang-long. Coupling of osteogenesis and angiogenesis in bones[J]. Chinese Journal of Tissue Engineering Research, 2017, 21(36): 5855-5861.

Figures/Tables 2

2.1 血管及血管形成血管是生物运送血液、营养、氧、代谢产物、细胞因子以及细胞源等的管道，由几种不同的细胞构成，内层为内皮细胞，周围覆盖管周细胞(或称壁细胞)，这些壁细胞又根据其形态及表面标志物的不同，被分为周细胞和血管平滑肌细胞。周细胞嵌入内皮细胞基底膜下，与毛细血管内皮细胞发生直接接触；血管平滑肌细胞主要覆盖较大的血管，静脉和动脉，与内皮细胞没有直接接触[2]。血管的新生包括血管发生和血管形成两个过程。早期胚胎发育中，中胚层细胞祖细胞分化出的血管内皮祖细胞经募集、增殖、分化和迁移，形成功能尚不成熟的初级血管丛，此过程称为血管发生。而血管形成是指已存在的血管网的扩张延展，微血管以出芽或分裂的方式从已存在的血管床长出，并形成新的血管分支及毛细血管丛，这一系列过程包括内皮细胞出芽、迁移、增殖、管腔吻合和血管修整[3-6]。血管形成的实现要求不同血管细胞间的广泛性协调，保证新生血管具备完全的功能和稳定。例如，毛细血管床的扩张过程涉及内皮细胞向动、静脉方向特异性分化，使其形成动脉或静脉。周细胞和平滑肌细胞在血管重塑、稳定、成熟中也是必须的[6-7]。越来越多的证据表明血管形成和器官特异性分化，受到局部微环境的调控，形成特异性的内皮细胞[8-10]。这种特异性分化的机制也体现在骨组织的血管结构中，下文将进一步阐述。 2.2 骨组织的血管结构骨骼系统是一个适应性反馈调控系统，与机械信号、生化信号、神经信号持续性整合[11]。高度钙化，基质丰富的骨组织，骨髓中含有造血细胞，其功能不仅涉及钙磷代谢和转移，还与能量代谢和造血生成相关，并且各功能间存在相互联系。对一些功能来说，这些联系的基本要素就是骨血管化。骨作为一个高度血管化的组织，又存在一些与其他器官血管不一致的地方如生长板和关节软骨。和其他器官一样，骨组织中的血管系统表现出典型的层次结构，动脉干分支流入广泛的毛细血管网，然后汇入骨干中心的大静脉。骨干是骨含骨髓的主要部位[12]。基于标志物表达和功能特征，骨组织的毛细血管可分为H和L两个亚类[12]，而H型和L型毛细血管相互交联。H型毛细血管在骨骺端，即无血管生长板区域。骨骺端H型毛细血管呈管束状排列，在远端近生长板的区域相互交联。此外，H型毛细血管还分布在骨干段靠近密质骨的骨内膜区域。H型毛细血管与血管周围表达Osterix的骨祖细胞相联系，且高表达连接蛋白CD31(或PECAM1)和唾液酸糖蛋白endomucin(EMCN)。从长骨远端横截面上观察，H型血管分布致密，且倾向于在生长板附近发生交联。相反地，L型血管在骨干骨髓腔内形成致密且高度分支的毛细血管网，类似窦状毛细血管网[13-14]。与H型毛细血管相反，L型血管表达低水平的CD31和EMCN。窦状L型毛细血管周围包绕着致密的造血细胞，与中心静脉相连。在这种组织结构下，动脉和远端小动脉不直接将血液运送至窦状L型毛细血管，而是动脉仅与骨骺和骨内膜的H型血管相连。由于骨组织中这种特殊结构的血管系统，血液流经动脉分布至H型毛细血管，再经骨干和骨骺端交界面的窦状L型毛细血管网，最终汇入大的中央静脉。因此，在出生后的小鼠长骨中可检测到不同的代谢环境：骨干由于缺乏动脉的直接供氧和缺乏造血细胞而处于高度缺氧状态，而骨骺相对来说含氧丰富[15]。此外，骨组织的血管系统含有各类壁细胞。骨髓中，两种管周细胞包绕着窦状L型毛细血管——即瘦素受体阳性细胞(leptin receptor+ cell，LEPR+ cell)和CXCL12丰富的网状细胞(CXCL12-abundant reticular cell，CAR cell)[16-17]。大量证据显示，这些管周细胞在调节造血作用中起着重要作用，它们分泌分子信号如干细胞因子，趋化因子CXCL12和促血管生成素。LEPR+细胞还表达血小板衍生生长受体α(platelet-derived growth factor receptor α，PDGFRα)，但不表达血小板衍生生长受体β(platelet-derived growth factor receptor β，PDGFRβ)和神经胶质抗原2(neural/glial antigen2，NG2)[17-18]。这些细胞低表达巢蛋白绿色免疫荧光(nestin-GFP)，能分化为不同的间充质细胞系如骨、软骨、脂肪细胞[19]。同软组织一样，骨组织中的动脉被α-平滑肌肌动蛋白阳性(alpha smooth muscle actin-positive，αSMA+ )平滑肌细胞覆盖，并且这些平滑肌细胞也表达NG2[20]。小动脉管周细胞表达NG2和nestin-GFPhigh，也具有能分化为不同的间充质细胞系的潜力[21-22]。骨骺H型毛细血管束也被血小板衍化生长因子Rβ+和NG2+管周细胞包绕，由H型毛细血管的内皮细胞分泌的血小板衍化生长因子B(platelet derived growth factor-B，PDGFB)调控[20]。 2.3 骨组织中的血管形成骨形成有两种方式即软骨内成骨和膜内成骨。在颅骨和面骨发育过程或骨修复过程中，膜内成骨骨化部位的间充质细胞先分化为成骨细胞，形成骨化点，产生骨组织的纤维和有机基质。钙盐渐次与基质结合分化成骨质，其外围的间充质分化为骨膜。在骨膜下的成骨细胞和破骨细胞的相互作用下，骨即从骨化点逐渐扩展，这一过程称为膜内成骨。这种成骨形式主要发生在扁骨如颅顶骨和面颅诸骨以及部分锁骨。另一种方式，软骨内成骨是由矿化的骨取代软骨，这是一个复杂的过程，由软骨间叶原基增殖的软骨细胞向非增殖的肥大状态分化而启动。在此过程中，间充质细胞分化为软骨细胞。血管入侵和软骨细胞增殖相互协调，进而延长骨。软骨内成骨主要发生在长骨[23-24]。骨组织中的血管系统似乎大部分，甚至可能全部由血管形成生成[25]。 2.3.1 软骨内成血管脊椎动物骨骼系统的大部分骨以软骨内成骨的方式形成，胚胎期间充质细胞凝聚引发软骨内成骨，生成软骨单元，血管随之入侵软骨，并带来血管周围的骨祖细胞、募集破骨细胞，最终导致软骨单元被吸收并逐渐被骨组织所替代。在鼠科动物的长骨中，在胚胎13.5-14.5 d时血管开始入侵软骨板，血管形成大部分在青春期或发育时期完成。软骨内成血管的过程涉及一系列的事件。首先，未来初级骨化中心(primary ossification center，POC)位点的软骨细胞停止增殖，变肥大，分泌促血管生长因子，刺激血管形成；位于初级骨化中心的骨祖细胞也是促血管生长因子的来源。接着，血管入侵肥大的软骨细胞，形成最初的血管网，并伴随骨化过程。长骨两端的成熟和肥大生长板软骨细胞释放信号，进一步促进血管形成和沿长轴的骨化，最终导致骨单位的扩张延伸。这一过程同时涉及不同的骨骺和骨干毛细血管网的形成。在随后的发育中，血管入侵长骨两端骨骺的软骨细胞，进而启动次级骨化中心(secondary ossification center，SOC)形成[25]，见图1。 2.3.2 膜内成血管通过膜内成骨形成的骨不具有软骨板。但是，和长骨一样，扁骨也是高度血管化的。相较于软骨内成骨和软骨内成血管的研究，膜内成血管却鲜有研究，因而膜内成骨和血管形成的相互作用目前尚不清楚[26-27]。 2.4 影响骨血管形成的因素提炼简表见表1。 2.4.1 血管内皮生长因子(vascular endothelial growth factor，VEGF) 近30年来，血管内皮生长因子已成为众多研究的焦点。科学家们发现了内皮细胞上的血管内皮生长因子受体——生理和病理性血管生成的关键因子[28-29]。血管内皮生长因子A是调控成血管的主要因子，在肥大软骨细胞中高表达和分泌[30-31]。血管内皮生长因子R2是血管内皮生长因子A的主要受体，二者结合后产"

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