Cytokinetic furrowing in toroidal, binucleate and anucleate cells in C-elegans embryos

Classical experimental studies on echinoderm zygotes concluded that the juxtaposition of two astral microtubule arrays localizes the stimulus for cytokinetic furrowing. However, recent experimental and genetic studies in Caenorhabditis elegans, Drosophila and mammalian cultured cells implicate microtubules of the central spindle, and regulatory proteins associated with this structure, suggesting that the essential conditions for furrow induction may differ from one animal cell to another. We used micromanipulation and laser microsurgery to create, in three ways, the juxtaposition of astral microtubules in C. elegans embryonic cells. In toroidal cells we observe that furrows initiate both where astral microtubule arrays are juxtaposed, and where the cortex most closely approaches the central spindle. We find that binucleate cells successfully furrow not only across the spindles, but also between unconnected spindle poles. Finally, we find that anucleate cells containing only a pair of centrosomes nevertheless attempt to cleave. Therefore, in C. elegans embryonic cells, as in echinoderms, juxtaposition of two asters suffices to induce furrowing, and neither the chromatin nor the physical structure of the central spindle are indispensable for furrow initiation. However, furrows that cross a central spindle are more likely to complete than those that do not.