Neural tube closure in Xenopus laevis involves medial migration, directed protrusive activity, cell intercalation and convergent extension

We have characterized the cell movements and prospective cell identities as neural folds fuse during neural tube formation in Xenopus laevis, A newly developed wholemount, two-color fluorescent RNA in situ hybridization metho d, visualized with confocal microscopy, shows that the dorsal neural tube g ene xpax3 and the neural-crest-specific gene xslug are expressed far latera l to the medial site of neural fold fusion and that expression moves medial ly after fusion, To determine whether cell movements or dynamic changes in gene expression are responsible, we used low-light videomicroscopy followed by fluorescent in situ and confocal microscopy, These methods revealed tha t populations of prospective neural crest and dorsal neural tube cells near the lateral margin of the neural plate at the start of neurulation move to the dorsal midline using distinctive forms of motility, Before fold fusion , superficial neural cells apically contract, roll the neural plate into a trough and appear to pull the superficial epidermal cell sheet medially. Af ter neural fold fusion, lateral deep neural cells move medially by radially intercalating between other neural cells using two types of motility. The neural crest cells migrate as individual cells toward the dorsal midline us ing medially directed monopolar protrusions. These movements combine the tw o lateral populations of neural crest into a single medial population that form the roof of the neural tube. The remaining cells of the dorsal neural tube extend protrusions both medially and laterally bringing about radial i ntercalation of deep and superficial cells to form a single-cell-layered, p seudostratified neural tube, While ours is the first description of mediall y directed cell migration during neural fold fusion and re-establishment of the neural tube, these complex cell behaviors may be involved during cavit ation of the zebrafish neural keel and secondary neurulation in the posteri or axis of chicken and mouse.