Multiple endothelial cells constitute the tip of developing blood vessels and polarize to promote lumen formation

Abstract

Blood vessel polarization in the apical-basal axis is important for directed secretion of proteins and lumen formation; yet, when and how polarization occurs in the context of angiogenic sprouting is not well understood. Here, we describe a novel topology for endothelial cells at the tip of angiogenic sprouts in several mammalian vascular beds. Two cells that extend filopodia and have significant overlap in space and time were present at vessel tips, both in vitro and in vivo. The cell overlap is more extensive than predicted for tip cell switching, and it sets up a longitudinal cell-cell border that is a site of apical polarization and lumen formation, presumably via a cord-hollowing mechanism. The extent of cell overlap at the tip is reduced in mice lacking aPKCζ, and this is accompanied by reduced distal extension of both the apical border and patent lumens. Thus, at least two polarized cells occupy the distal tip of blood vessel sprouts, and topology, polarization and lumenization along the longitudinal border of these cells are influenced by aPKCζ.

Venous-derived angioblasts generate organ-specific vessels during embryonic development

“…These cells were found to overlap during most of the process of ventral expansion of the mature SIV. Similar results were reported for different sprouting assays, including human umbilical endothelial cells, mouse retina and mouse embryonic back vessels ( Pelton et al, 2014 ). In all these cases, the majority of the angiogenic sprouts were shown to consist of two overlapping, parallel cells, a phenomenon that could not be accounted for by a transient overlap resulting from a switch in position between the cells.…”

Cell behaviors and dynamics during angiogenesis

“…Sprouts developing at later time points are often already lumenized during the very early sprouting phase and the lumen extends concomitantly with the outgrowing sprout (see Lenard et al, 2013). A similar phenomenon is also seen in the developing retinal vasculature in mice, where the lumen extends close to the front of tip cells (Gerhardt et al, 2003;Pelton et al, 2014).…”

A Rac/Cdc42 exchange factor complex promotes formation of lateral filopodia and blood vessel lumen morphogenesis

“…Both global genetic deletion of a single Dock4 allele in tumour-bearing mice and targeted knockdown in the vasculature of xenograft tumours perturbed blood vessel lumen formation, supporting the findings in the tissue culture angiogenesis model. Vascular lumens in vivo arise through different mechanisms 45 : extension of a transcellular lumen through coalescing intracellular vacuoles 46 47 ; or intercellular hollowing, through separation of apposing endothelial cells and redistribution of pre-formed junctions to the periphery of the developing tube 21 48 . The latter requires polarization in the apical-basal axis and lateral cell–cell contacts 21 48 , a feature of sprouting tubules in tissue culture and angiogenic blood vessels in vivo including intersegmental vessels in zebrafish embryos and retinal vessels in mice 48 49 .…”