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Vascularization and Angiogenesis in Tissue Engineering: Beyond Creating Static Networks

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Vascularization and Angiogenesis in Tissue Engineering: Beyond Creating Static Networks Special Issue: Biofabrication Review Vascularization and Angiogenesis in Tissue Engineering: Beyond Creating Static Networks 1,2,3, 1,2,4,5,6, Jeroen Rouwkema * and Ali Khademhosseini * Engineered tissues need a vascular network to supply cells with nutrients and Trends oxygen after implantation. A network that can connect to the vasculature of the Engineered tissues of a clinically rele- in vitro patient after implantation can be included during culture. For optimal vant size need a vascular network to supply the cells with nutrients and oxy- integration, this network needs to be highly organized, including venules, gen. Including a vascular network capillaries, and arterioles, to supply all of the cells with sufficient nutrients. before implantation can aid in this Owing to the importance of vascularization for the clinical applicability of tissue need, by connecting to the vasculature of the patient. engineering, many approaches have been investigated to include an organized vascular network in tissue constructs. This review will give an overview of recent To supply all cells with sufficient nutri- ents, and to successfully connect to efforts, and will propose future perspectives to engineer the optimal, functional the patient vasculature, the engineered vascular network. vascular network needs to be highly organized. Using microfabrication technology such as photo patterning Importance of Vascularization in Tissue Engineering and bioprinting, the initial organization One of the goals of tissue engineering is to generate tissues that can be used as alternatives for of vascular networks can be designed donor material to repair or replace damaged tissues or organs [1]. Tissues generated for this and controlled. purpose will generally be of a size larger than the diffusional limit for nutrients and oxygen [2]. The geometry of vascular networks can Therefore, a need for a system to distribute nutrients within the tissue is apparent. During culture also be controlled by adapting local in the lab this distribution can be facilitated by using, for instance, perfusion bioreactors, but after microenvironments. The patterning of fl fl implantation the tissue will have to rely on a vascular network to supply nutrients to all cells within mechanical signals, uid ows, or the availability of growth factors leads to the tissue. As part of the foreign body response, a vascular network will generally invade an directed vascular organization. implant. However, this is a process that takes days or weeks, meaning that cells in the middle of the tissue will be starved of nutrients for a considerable amount of time [3], resulting in suboptimal tissue integration or cell death. 1 Department of Medicine, Biomaterials Innovation Research Center, Brigham and Women's Hospital, Harvard To decrease the time that is needed to vascularize an engineered tissue, researchers have been Medical School, Cambridge, MA, exploring the possibility of adding a vascular network before implantation. This network has 02115, USA 2 the potential to connect to the vasculature of the patient, resulting in a much faster perfusion of Harvard-MIT Division of Health Sciences and Technology, the implant [4]. Theoretically, the perfusion will be instantaneous if the pre-engineered network in Massachusetts Institute of Technology, the implant is sufficiently organized, and can be microsurgically connected to the patient during Cambridge, MA, 02319, USA 3 the implantation procedure. Department of Biomechanical Engineering, MIRA Institute for Biomedical Technology and Technical An optimal vascular network in an engineered tissue needs to possess several characteristics. Medicine, University of Twente, One of the key tasks of a vascular network is to supply all cells in a tissue with sufficient nutrients. Enschede, The Netherlands 4 Wyss Institute for Biologically This means that all of the cells need to be within a distance of 200 mm from a vessel, which is Inspired Engineering, Harvard Medical generally regarded as the diffusion limit of oxygen and nutrients within a tissue [5]. To reach this School, Boston, MA, 02155, USA Trends in Biotechnology, September 2016, Vol. 34, No. 9 http://dx.doi.org/10.1016/j.tibtech.2016.03.002 733 © 2016 Elsevier Ltd. All rights reserved. 5 College of Animal Bioscience and fine distribution while minimizing the pressure that is needed for the blood flow, the vascular Technology, Department of network should be organized as a vascular tree, where larger vessels branch into smaller Bioindustrial Technologies, Konkuk vessels, which ultimately branch into capillaries that are distributed throughout the tissue University, Hwayang-dong, Gwangjin- gu, Seoul 143-701, Republic of Korea volume. Apart from that, the vascular network should act as a barrier that selectively controls 6 Department of Physics, King the passage of materials from the vessels to the surrounding tissue to prevent excessive outflow Abdulaziz University, Jeddah 21569, of fluid leading to tissue edema. Finally, to supply the tissue with nutrients shortly after Saudi Arabia implantation, it should be possible for the network to easily connect to the vasculature of the patient, or to be microsurgically connected to it. For the microsurgical approach, vascular *Correspondence: structures with a diameter of several hundred micrometers are needed. [email protected] (J. Rouwkema) and [email protected] (A. Khademhosseini). Vascularization is currently regarded as one of the main hurdles that need to be taken to translate tissue engineering to clinical applications at a large scale [6]. As such, it is one of the main research topics in the tissue engineering community. This review will give an overview of current strategies that have been explored in the past to prevascularize engineered tissues, and will propose future perspectives to engineer the optimal vascular network within an engineered tissue. Angiogenesis and Vascular Remodeling To include an organized vascular network in an engineered tissue, it is important to understand the process of vascular formation and remodeling. A starting point for this process is to look at the formation of the vascular network during embryonic development and growth. Two pro- cesses can be distinguished during the formation of a natural vascular network: vasculogenesis and angiogenesis. Vasculogenesis is the process that takes place during early embryonic development where angioblasts (see Glossary) differentiate into endothelial cells, and prolifer- ate within a previously avascular tissue to form a primitive capillary network [7]. Vasculogenesis can also occur in adults to revascularize a tissue following extensive damage or during tumor growth [8,9]. During these so-called postnatal vasculogenesis processes, bone marrow-derived endothelial progenitor cells are mobilized into the circulation, home to the tissue repair site, and differentiate into mature endothelial cells to form a primitive vascular network [8]. Most of the processes involved in vascular organization and remodeling, both in embryonic development and afterwards, are angiogenic processes (Box 1). Angiogenesis is defined as the formation of new vessels from an existing vascular network. These vessels can be formed by sprouting angiogenesis, where endothelial cells form sprouts starting from pre-existing vessels, or intussusceptive angiogenesis, where tissue pillars are inserted within existing capillaries to split the vessels [10]. Angiogenesis is mainly driven by the need to supply tissues with sufficient nutrients and oxygen. As such, angiogenesis is regulated to a large extent by oxygen levels within tissues. Hypoxic tissues secrete growth factors and chemokines that activate vascular growth and remodeling [11]. Endothelial cells are stimulated to break out of their stable position in the vessel wall and jointly coordinate sprouting, branching, and new lumenized network formation [12]. When perfusion of the tissue has increased and the supply of oxygen meets the demand, quiescence can be re-established resulting in a stable vascular network. The process of angiogenesis initially leads to the development of capillaries. This can be followed by a process called arteriogenesis, where the vascular structures further mature and increase in diameter and the vascular wall thickness [13]. Mural cells proliferate, and acquire specialized characteristics, including contractile components [14]. The mechanisms governing arteriogen- esis are not yet completely understood, but the remodeling process is mainly mediated by fluid flow shear stresses [15]. Natural Organization of Endothelial Cells in Engineered Tissues Many research efforts in prevascularized tissue engineering have relied on the spontaneous organization of endothelial cells to form vascular networks on scaffolds [16–18], in extracellular 734 Trends in Biotechnology, September 2016, Vol. 34, No. 9 Box 1. Growth Factor Regulation of Angiogenesis Glossary Angiogenesis, both during growth and in disease states such as cancer, is a complex process regulated by multiple Anastomosis: in the context of factors. Understanding these processes is important to tune vascular organization in engineered tissues. VEGF is one of angiogenesis, the connection of two the key players, being involved in most morphogenic events [82]. Stimulation with VEGF results in various distinct vascular structures. responses in endothelial cells, including proliferation, migration, differentiation, and survival [83]. In vascular patterning Angioblast: a precursor
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