Integrated Vascular Engineering: Vascularization of Reconstructed Tissue

Abstract

In this chapter, we describe culture methods to construct microvascular networks as well as approaches to integrating capillary networks with 3D epithelial tissueengineered constructs. First, culture models of microvascular networks such as in vitro angiogenesis and vasculogenesis models are introduced. Using these culture models, the roles of endothelial cells (ECs), such as endothelial tip, stalk, and phalanx cells, are demonstrated. Additionally, regulatory factors, including both biochemical and biophysical factors, are discussed in the context of 3D capillary formation, including the process of vascular development, growth, and maturation. Next, we focus on the use of microfluidics technologies for investigating capillary morphogenesis. Examples of 3D capillary formation assays with growth factor gradients and different extracellular matrix materials are described. Cocultures of ECs and the other cell types in microfluidic devices are also introduced to show the potential of microfluidic vascular formation models. The vascularization of constructed tissues is discussed from the viewpoints of horizontal and vertical approaches for combining capillary structures and epithelial tissues in vitro. Finally, the concept of integrated vascular engineering and future perspectives are discussed. General Introduction for In Vitro Capillary FormationBlood vessels are tubular structures through which blood passes to provide oxygen and nutrients to individual cells within tissues and organs. Because oxygen and nutrients are the most important fundamental factors for vital activity, blood vessels are essential tissues in the body. The luminal surface of blood vessels is covered by endothelial cells (ECs). Thus, we need to understand how ECs form blood vessels for the development of vascular tissue engineering. In particular, in the context of tissue engineering for three-dimensional (3D) tissues and organs, construction of microvascular networks, rather than large blood vessels, has a high priority.There are two processes of microvessel formation in vivo, referred to as "angiogenesis" and "vasculogenesis" (Risau 1997). Angiogenesis is the formation of new capillaries branching from preexisting blood vessels. To investigate this angiogenic process, an in vitro 3D angiogenesis model has been developed. In this model, hydrogels, such as collagen gel and fibrin gel, are formed in a culture dish, and ECs are seeded on the surface of the 3D hydrogel scaffold. ECs grow on the gel surface and finally form a confluent monolayer. When the cells are cultured with no growth factors, they maintain a confluent monolayer, which is similar to the ECs covering the luminal surface of blood vessels in vivo (step 1, Fig. 16.1a). However, when ECs are cultured and supplemented with angiogenic growth factors, such as vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF), some ECs penetrate into the underlying gel to form vascular sprouts, which is the beginning of vascular formation (step 2, Fig. 16.1a). T...

Construction of Continuous Capillary Networks Stabilized by Pericyte-like Perivascular Cells

“…T he construction of small and continuous capillary networks is a fundamental challenge in three-dimensional (3D) tissue engineering. 1 In addition, the construction of vital organs such as the liver, pancreas, and kidney by the use of 3D tissue engineering remains a big challenge. Cells of these organs, when cultured in a 3D configuration, require oxygen and nutrients to be delivered and require waste products to be removed through peripheral capillary networks within the thick tissue rather than through diffusion from the tissue surface.…”