Successive patterns of clonal cell dispersion in relation to neuromeric subdivision in the mouse neuroepithelium

We made use of the laacz procedure of single-cell labelling to visualize cl ones labelled before neuromere formation, in 12.5-day mouse embryos. This a llowed us to deduce two successive phases of cell dispersion in the formati on of the rhombencephalon: an initial anterior-posterior (AP) cell dispersi on, 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 ons et of morphological segmentation and is not imposed by the interface betwee n adjacent rhombomeres. This demonstrates a major change in the mode of epi thelial 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 organi zation on a Cartesian grid, the coordinates of which correspond to the AP a nd DV axis of the neural tube, A similar sequence of AP cell dispersion fol lowed by an arrest of AP cell dispersion, a preferential DV cell dispersion and then by a coherent neuroepithelial growth, is also observed in the spi nal cord and mesencephalon. This demonstrates that a similar cascade of cel l events occurs in these different domains of the CNS, In the prosencephalo n, differences in spatial constraints may explain the variability in the or ientation 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.