Two cellular models of vascular pruning.

<p>A pruning branch is initially a multicellular tube (A). The cellular rearrangements to follow depend on collapse or maintenance of lumen at this stage (pruning type I or II, respectively). If the lumen collapses before cell rearrangements (type I pruning, B’), cell rearrangements lead to formation of a unicellular connection (C’–D’). The last linking cell regresses (E’) and completely resolves the last connection (F’) to complete the pruning process (G). If the lumen is maintained, cell rearrangements lead most cells out of the branch (B‘‘, arrows) and force the remaining cell to undergo self-fusion and form a unicellular tube (C”, arrow). Transcellular lumen collapses in the unicellular tube, forming two separate luminal compartments (D”, arrows). The last cell reduces its contact to one of the major branches (E”) and eventually the last contact (F”) is resolved and pruning is complete (G). See also <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002126#pbio.1002126.s007" target="_blank">S6 Fig</a>.