Endothelial Cell Dynamics during Blood Vessel Morphogenesis

Abstract

Blood vessels, together with the heart, have a fundamental role in supporting the metabolic demands of tissues not only during development but also in adults. New blood vessels are frequently generated through angiogenesis when new vessels emerge from pre-existing ones (Fig. 2.1a). Initially, endothelial cells (ECs) lining an existing vessel are selected to become tip cells to spearhead the formation of new vascular sprouts. New sprouts grow through EC proliferation and the polarized collective migration of both tip and trailing stalk cells into the avascular tissue. In order to generate a network of interconnecting vessel segments, tip cells anastomose with neighboring tip cells to establish new vascular loops. Importantly, vascular sprouts develop into tubes through which oxygen, metabolites, cells, and waste products can circulate around the body. Finally, the tubular network of blood vessels are either maintained or, depending on the tissue requirements in which the vessels pervade, remodeled through pruning into a more refined vascular network that carries blood flow optimally to tissues (Fig. 2.1b).

Over the past few decades, many key signaling pathways that regulate blood vessel development have been identified using primarily the mouse as the model organism. These include the Neuropilin (NRP)/Vascular Endothelial Growth Factor (VEGF)/Vascular Endothelial Growth Factor Receptor (VEGFR), Jagged/Delta-like/Notch, Transforming Growth Factor β (TGFβ)/Bone Morphogenic Protein (BMP) and EphrinB/EphB signaling cascades (Adams RH, Alitalo K. Nat Rev Mol Cell Biol 8:464–478, 2007; Potente M, Makinen T. Nat Rev Mol Cell Biol 18:477, 2017). Although these studies have uncovered the fundamental principles of angiogenesis, temporal information on the cellular dynamics of angiogenesis has been lacking due to difficulties in performing live imaging in mouse embryos and tissues. These challenges are alleviated by the use of zebrafish, whose embryos develop ex utero, are optically transparent and are therefore highly suited for live imaging. Combined with recent advances in imaging techniques and the development of fluorescent biosensors or reporters, it is now possible to observe the dynamics of ECs at cellular and subcellular resolution as blood vessel morphogenesis takes place. Imaging vascular morphogenesis in the zebrafish embryo has been indispensable in the identification of morphogenetic events such as apical membrane invagination and the elucidation of the cellular mechanisms of anastomosis and vessel pruning, which are dynamic processes that are difficult to visualize and investigate in mouse models.

In this chapter, I will summarize recent findings from zebrafish studies that highlight the dynamic nature of ECs during angiogenesis and vessel remodeling and focus on how the actin cytoskeleton regulates EC morphogenesis and behavior.