Mapping the molecular steps of secretory-lysosome-driven tracheal tube fusion.

During development, tubular networks form through the joining of lumenized branches. Further insights into tracheal tube fusion in Drosophila melanogaster now reveal the molecular steps that promote the connection of two apical membrane compartments within a single cell through secretory lysosomes.

Endothelial cell division in angiogenic sprouts of differing cellular architecture.

The vasculature of the zebrafish trunk is composed of tubes with different cellular architectures. Unicellular tubes form their lumen through membrane invagination and transcellular cell hollowing, whereas multicellular vessels become lumenized through a chord hollowing process. Endothelial cell proliferation is essential for the subsequent growth and maturation of the blood vessels. However, how cell division, lumen formation and cell rearrangement are coordinated during angiogenic sprouting has so far not been investigated at detailed cellular level. Reasoning that different tubular architectures may impose discrete mechanistic constraints on endothelial cell division, we analyzed and compared the sequential steps of cell division, namely mitotic rounding, cytokinesis, actin re-distribution and adherence junction formation, in different blood vessels. In particular, we characterized the interplay between cell rearrangement, mitosis and lumen dynamics within unicellular and multicellular tubes. The lumen of unicellular tubes becomes constricted and is ultimately displaced from the plane of cell division, where a de novo junction forms through the recruitment of junctional proteins at the site of abscission. By contrast, the new junctions separating the daughter cells within multicellular tubes form through the alteration of pre-existing junctions, and the lumen is retained throughout mitosis. We also describe variations in the progression of cytokinesis: while membrane furrowing between daughter cells is symmetric in unicellular tubes, we found that it is asymmetric in those multicellular tubes that contained a taut intercellular junction close to the plane of division. Our findings illustrate that during the course of normal development, the cell division machinery can accommodate multiple tube architectures, thereby avoiding disruptions to the vascular network.