We made use of the laacz procedure of single-cell labelling to visualize clones labelled before neuromere formation, in 12.5-day mouse embryos. This allowed us to deduce two successive phases of cell dispersion in the formation of the rhombencephalon: an initial anterior-posterior (AP) cell dispersion, followed by an asymmetrical dorsoventral (DV) cell distribution during which AP cell dispersion occurs in territories smaller than one rhombomere. We conclude that the general arrest of AP cell dispersion precedes the onset of morphological segmentation and is not imposed by the interface between adjacent rhombomeres. This demonstrates a major change in the mode of epithelial growth that precedes or accompanies the formation of neuromeres. We also deduced that the period of DV cell dispersion in the neuroepithelium is followed by a coherent growth phase. These results suggest a cell organization on a Cartesian grid, the coordinates of which correspond to the AP and DV axis of the neural tube. A similar sequence of AP cell dispersion followed by an arrest of AP cell dispersion, a preferential DV cell dispersion and then by a coherent neuroepithelial growth, is also observed in the spinal cord and mesencephalon. This demonstrates that a similar cascade of cell events occurs in these different domains of the CNS. In the prosencephalon, differences in spatial constraints may explain the variability in the orientation of cell clusters. Genetic and clonal patterning in the AP and DV dimensions follow the same spatial sequence. An interesting possibility is that these successive patterns of cell growth facilitate the acquisition of positional information.

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