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Equipes de recherche - Biologie du Développement de Villefranche sur mer
Unité Mixte de Recherche CNRS 7009





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Images of organism : "Phallusia mammilata". Click image


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Cell cycle control during development

More than a century ago Conklin produced the fate map of all blastomeres in ascidian embryos up to gastrula stage (112 cell) that the community still uses today (Conklin, 1905).  We have been trying to understand how the cell cycle is remodeled to generate this fate map.  Since there is no major phase of blastomere migration or rearrangement in ascidian embryos every blastomere remains in the position it will come to occupy following cell division, indicating that the orientation of the cleavage plane is precisely controlled.  In addition to cleavage plane orientation, some blastomeres such as the germ cell precursors divide unequally three times in succession to generate different sizes of cells.  Finally, specific lineages display different cell cycle durations creating brief 24 and 76 cell stages during ascidian early embryonic development.  Thus oriented cell division (OCD), unequal cleavage and cell cycle asynchrony underlie Conklin's fate map yet it is largely unknown how these processes are controlled.  Remarkably, these cell cycle dependent phenomena are robust since they have been conserved in distantly-related ascidian species (e.g. Halocynthia and Phallusia).  Through a combination of 4D imaging of microtubule and chromosome dynamics coupled with morpholino knockdown and blastomere isolation experiments, we have so far partially uncovered the mechanism controlling unequal cleavage in the germ lineage (Prodon et al., 2010).  We are currently using similar techniques to explore OCD during early cleavage cycles.  Our aim is to provide a molecular description of how the cell cycle has been co-opted during development to generate the 112 cell gastrula.
A longer term aim is to extend this analysis to the tadpole larva.  Ascidians forms a tadpole larva composed of ~2600 cells containing precisely 36 muscle cells, 40 notochord cells, 330 central nervous system cells, about 500 endoderm cells and a large number of epithelial and mesenchyme cells.