Visualizing neural tube zipping in the mouse revealed cell behaviors at the time of fusion, but what are the forces driving this attachment of the apposing neural folds? To better quantify the mechanisms of neural tube zipping, we will examine a simpler vertebrate system, that of Ciona intestinalis. Tunicates (also called ascidians) such as Ciona form a neural tube through primary neurulation, which includes a very similar zipper-like fusion event that proceeds in a posterior-to-anterior direction (Figure 1A; Nicol and Meinertzhagen 1988a,b; Hashimoto et al. 2015). Live-cell imaging of the membrane junction points between epidermal and neural cells during neural tube closure revealed a mechanism of sequential exchange of junctions at apical membrane junctions (Figure 1B). The driving force for zipper advancement in Ciona may be the localized activation of actomyosin contraction (i.e., myosin moving on filamentous actin) in the apical membranes of epidermal cells lying immediately ahead of the zipper point (Figure 1C). Junctional tension is highest between the apical membrane of these epidermal cells and their adjacent neural ectodermal neighbors. Moreover, inhibition of myosin prevents zipper advancement. These data suggest that a stepwise exchange of cell junctions is initiated by the apical activation of actomyosin contraction, which is then followed by a release in the attachments of posterior junctions to the zipper point and consequently a reduction in posterior resistance. As a result of these junctional exchanges, epidermal-to-neural attachments are replaced with epidermal-to-epidermal adhesion, and neural tube closure advances (Hashimoto et al. 2015).
Hashimoto, H., F. B. Robin, K. M. Sherrard, and E. M. Munro. 2015. Sequential contraction and exchange of apical junctions drives zippering and neural tube closure in a simple chordate. Dev. Cell. 32: 241–255.
Nicol, D. and I. A. Meinertzhagen. 1988b. Development of the central nervous system of the larva of the ascidian Ciona intestinalis L. II. Neural plate morphogenesis and cell lineages during neurulation. Dev. Biol. 130: 737–766.