The researchers took advantage of the transparency of zebrafish to look at vessels emerging from the dorsal aorta. Initially, they saw highly dynamic vacuoles that appeared and disappeared. The vacuoles then fused together and enlarged to take up most of the cell volume, thus forming the lumen.
Quantum dots injected into the circulation got into the lumen formed by each cell in turn. Preliminary evidence suggests that vacuoles form independently in each cell, but enlargement may be a progressive process: the cells nearest to established vessels may help out their more distal neighbors.
The tubulating vacuoles formed in vitro are pinocytic in origin. Given the visual similarities, the same may be true in vivo. Weinstein suggests that these vacuoles may mature by establishing their identity as apical membranes, as the inside of a tubule is ultimately defined as apical. He now plans to track apical markers in transgenic zebrafish.