INTRODUCTIONVertebrate neurulation (Fig. 1) involves a preciselyorchestrated set of morphogenetic movements within theneural plate itself (intrinsic processes) and also withinneighboring tissues (extrinsic processes) (Smith andSchoenwolf, 1997). Intrinsic processes include elongation andshaping of the neural plate by cell rearrangement, cell shape-change, and cell division; elevation and apposition of the neuralfolds; and fusion of the folds at the dorsal midline (Davidsonand Keller, 1999; Jacobson and Gordon, 1976; Schoenwolf andAlvarez, 1989; Schroeder, 1970; van Straaten et al., 1993).Extrinsic processes include medial pushing on the neural foldsby the adjacent epidermis, as well as inductive and mechanicalcontributions from the underlying mesoderm (Alvarez andSchoenwolf, 1992; Brun and Garson, 1983; Jacobson andJacobson, 1973; Moury and Schoenwolf, 1995; Poznanski etal., 1997; Schroeder, 1970; van Straaten et al., 1996). Mammalian neural tube defects (NTDs) stem from a failureof one or more of these morphogenetic processes (Harris andJuriloff, 1999), and recent genetic experiments in mice haveidentified individual genes that control specifically the manyindividual behaviors that comprise neurulation. For example,loss of p190RhoGAP inhibits neural fold closure by disruptingapical constriction of neuroepithelial cells (Brouns et al.,2000). Mutation of ephrin A5 does not affect the elevation orapposition of neural folds, but instead precludes their normalfusion at the dorsal midline (Holmberg et al., 2000). Genescontrolling extrinsic factors in neurulation have also beenidentified, such as AP-2, which is expressed in the non-neuralectoderm but is essential for tube closure (Zhang et al., 1996). One morphogenetic process that has not beenmechanistically related to NTDs is convergent extension, inwhich a tissue narrows in one axis and elongates in aperpendicular axis (reviewed by Wallingford et al., 2002).Though it has been described during neurulation in a varietyof vertebrates, the contribution of convergent extension to tubeclosure has been difficult to assess without tools for uncouplingit from the many other morphogenetic processes involved(Burnside and Jacobson, 1968; Elul and Keller, 2000; Jacobsonand Gordon, 1976; Keller et al., 1992; Lawson et al., 2001;Schoenwolf and Alvarez, 1989; van Straaten et al., 1996).Indeed, defects in axial elongation have been postulated tocontribute to NTD in the classical mouse mutant looptail(Kibar et al., 2001; Smith and Stein, 1962; Wilson and Wyatt,1992), though other studies have challenged that view (Gerrelliand Copp, 1997; Murdoch et al., 2001). Convergent extension is driven by the polarizedrearrangement of cells within the tissue (Elul and Keller, 2000;Keller et al., 2000; Shih and Keller, 1992), and Xenopus5815Development 129, 5815-5825 (2002)© 2002 The Company of Biologists Ltddoi:10.1242/dev.00123In Xenopus, Dishevelled (Xdsh) signaling is required forboth neural tube closure and neural convergent extension,but the connection between these two morphogeneticprocesses remains unclear. Indeed normal neurulationrequires several different cell polarity decisions, any ofwhich may require Xdsh signaling. In this paper we addresstwo issues: (1) which aspects of normal neurulation requireXdsh function; and (2) what role convergent extensionplays in the closure of the neural tube. We show that Xdshsignaling is not required for neural fold elevation, medialmovement or fusion. Disruption of Xdsh signaling thereforeprovides a specific tool for uncoupling convergent extensionfrom other processes of neurulation. Using disruption ofXdsh signaling, we demonstrate that convergent extensionis crucial to tube closure. Targeted injection revealed thatXdsh function was required specifically in the midline fornormal neural tube closure. We suggest that the inherentmovement of the neural folds can accomplish only a finiteamount of medial progress and that convergent extensionof the midline is necessary to reduce the distance betweenthe nascent neural folds, allowing them to meet and fuse.Similar results with Xenopus strabismus implicate theplanar cell polarity (PCP) signaling cascade in neuralconvergent extension and tube closure. Together, these datademonstrate that PCP-mediated convergent extensionmovements are crucial to proper vertebrate neurulation.Movies available on-lineKey words: Dishevelled, Strabismus, Planar cell polarity, Convergentextension, Neural tube defect, Neurulation, Midline, Floorplate,Craniorachischisis, Looptail, XenopusSUMMARYNeural tube closure requires Dishevelled-dependent convergent extension ofthe midlineJohn B. Wallingford* and Richard M. HarlandDepartment of Molecular and Cell Biology, 401 Barker Hall, University of California, Berkeley, CA 94720, USA*Author for correspondence (e-mail: [email protected])Accepted 22 August 20025816Dishevelled (Xdsh) controls this polarity via a vertebratecognate of the planar cell polarity (PCP) cascade (Tada andSmith, 2000; Wallingford et al., 2000). In our previous work,we showed that disruption of Xdsh signaling results in a failureof both neural convergent extension and neural tube closure(Wallingford and Harland, 2001). These results suggest the possibility that PCP-mediatedconvergent extension is directly required for neural tubeclosure. But without a mechanistic analysis, it remains equallypossible that Xdsh controls other important movements duringneurulation. Indeed, each of the many processes comprisingneurulation requires coordinated cell polarity (Fig. 1), and PCPsignaling components have been implicated in establishingboth planar and apicobasal cell polarity in a wide variety oftissues and animals (Bellaiche et al., 2001; Marsden andDeSimone, 2001; Sun et al., 2001; Theisen et al., 1994; Wigganand Hamel, 2002). In light of the previous work, two questions need to beaddressed. First, which processes of neurulation require Xdshsignaling? Second, does convergent extension directlycontribute to neural tube closure? In this paper, we show thatXdsh signaling is dispensable for the elevation, medialmovement and fusion of the neural folds, but time-lapseanalysis revealed a strict correlation between convergentextension and neural tube closure. The results heredemonstrate that midline convergent extension is a crucialcomponent of normal vertebrate neurulation, narrowing thedistance between the neural folds, allowing them to meet andfuse. In light of the severe NTDs of