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Assembly of endothelial tubes

The formation of epithelial tubes is critical for proper development of many different tissues and organs, and occurs via a variety of different mechanisms. Morphogenesis of seamless, properly patterned endothelial tubes is essential for development of a functional vertebrate circulatory system, but the mechanism of vascular lumenization in vivo remains unclear. Evidence dating back more than a hundred years has hinted at an important role for endothelial vacuoles in lumen formation. More than 25 years ago, in some of the first in vitro endothelial cell culture experiments, Folkman and Haudenschild described “longitudinal vacuoles” that “appeared to be extruded and connected from one cell to the next,” observations confirmed and extended by later in vitro studies showing that intracellular vacuoles arise from integrin‐dependent and cdc42/Rac1‐dependent pinocytic events downstream of integrin‐extracellular matrix signaling interactions. Despite compelling data supporting a model for assembly of endothelial tubes in vitro via formation and fusion of vacuoles, conclusive in vivo evidence has been lacking, primarily due to difficulties associated with imaging the dynamics of sub‐cellular endothelial vacuoles deep within living animals. We describe the results of studies using high‐resolution time‐lapse two‐photon imaging of transgenic zebrafish to examine how endothelial tubes assemble in vivo, comparing our results to time‐lapse imaging of human EC tube formation in three‐dimensional collagen matrices in vitro. Our observations provide strong support for a model in which the formation and intra‐ and inter‐cellular fusion of endothelial vacuoles drives vascular lumen formation.