Tube formation by complex cellular processes in Ciona intestinalis notochord

Abstract

In the course of embryogenesis multicellular structures and organs are assembled from constituent cells. One structural component common to many organs is the tube, which consists most simply of a luminal space surrounded by a single layer of epithelial cells. The notochord of ascidian Ciona forms a tube consisting of only 40 cells, and serves as a hydrostatic “skeleton” essential for swimming. While the early processes of convergent extension in ascidian notochord development have been extensively studied, the later phases of development, which include lumen formation, have not been well characterized. Here we used molecular markers and confocal imaging to describe tubulogenesis in the developing Ciona notochord. We found that during tubulogenesis each notochord cell established de novo apical domains, and underwent a mesenchymal-epithelial transition to become an unusual epithelial cell with two opposing apical domains. Concomitantly, extracellular luminal matrix was produced and deposited between notochord cells. Subsequently, each notochord cell simultaneously executed two types of crawling movements bi-directionally along the anterior/posterior axis on the inner surface of notochordal sheath. Lamellipodia-like protrusions resulted in cell lengthening along the anterior/posterior axis, while the retraction of trailing edges of the same cell led to the merging of the two apical domains. As a result, the notochord cells acquired endothelial-like shape and formed the wall of the central lumen. Inhibition of actin polymerization prevented the cell movement and tube formation. Ciona notochord tube formation utilized an assortment of common and fundamental cellular processes including cell shape change, apical membrane biogenesis, cell/cell adhesion remodeling, dynamic cell crawling, and lumen matrix secretion.

Tubulogenesis in a simple cell cord requires the formation of bi-apical cells through two discrete Par domains

“…Lumen connection does not involve membrane fission and fusion, but is accomplished through shape change of individual cells, leading to an overall tissue reorganization (Dong et al, 2009;Jiang and Smith, 2007). Each cell undergoes a bidirectional crawling movement that leads to the retraction of the diametrically opposite cell tips and drives a reverse intercalation process (Dong et al, 2009). Recently, an almost identical tubulogenesis mechanism has been described during the anastomosis of zebrafish intersomitic vessels (Herwig et al, 2011), confirming the potential relevance of the notochord system.…”

“…These entry clones were recombined using the Gateway LR reaction (Invitrogen) into previously described destination vectors (Dong et al, 2009) to obtain N-or C-terminal turboGFP or mCherry fusion constructs (see supplementary material Table S1 for more details). Lgl and zo1 entry clones from François Robin (University of Chicago, IL, USA), and rat myr-apkc plasmid from Brad Davidson (University of Arizona, Tucson, AZ, USA) were used to generate Cterminal fusion constructs.…”

“…We recently developed a new model for in vivo studies of tubulogenesis, the notochord of Ciona intestinalis, which provides a simple system to explore this process. Ciona notochord is composed of 40 large and geometrically identical cells that no longer divide, so that tubulogenesis involves only the remodeling of cell shape and tissue configuration (Denker and Jiang, 2012;Dong et al, 2009;Jiang and Smith, 2007). During development, notochord cells are also initially aligned in a single file.…”

“…Tubulogenesis begins with the de novo formation of central and extracellular lumen pockets between adjacent cylindrically shaped cells. Lumen connection does not involve membrane fission and fusion, but is accomplished through shape change of individual cells, leading to an overall tissue reorganization (Dong et al, 2009;Jiang and Smith, 2007). Each cell undergoes a bidirectional crawling movement that leads to the retraction of the diametrically opposite cell tips and drives a reverse intercalation process (Dong et al, 2009).…”

Tube fusion: Making connections in branched tubular networks

“…In contrast to tracheal FCs, endothelial tip cells are able to connect luminal spaces between fusing tubes also by undergoing extensive junctional remodelling, thus rearranging into a multicellular configuration [7,9]. A remarkably similar mechanism involving the connection of separate luminal pockets through extensive cell rearrangements was described to mediate lumen formation in the notochord of the Ascidian Ciona intestinalis [81].…”