Local fluidization of an active cytoplasmic gel partitions large cells

During embryonic development, animal embryos rapidly divide to partition cytoplasmic volumes that are many times larger than those of individual somatic cells. This process requires the positioning of nuclei, centrosomes, and organelles within a short timeframe and over significant distances. Microtubule asters play a crucial role in positioning the cleavage furrow, but the mechanism behind the long-range transport of cytoplasmic components prior to cell division has remained unclear.

In this study, researchers demonstrate that cytoplasm functions as a locally switchable active material. By employing actin-intact cycling Xenopus egg extract and Xenopus and medaka embryos, the team finds that F-actin links microtubule asters, organelles, nuclei, and centrosomes into a gel-like composite. This composite material facilitates the propagation of forces across hundreds of micrometers.

Post-mitosis, Aurora B kinase patterns a midplane that is locally fluidized, with myosin-II contractility driving coherent cytoplasmic flows.

The researchers developed a fluid dynamics model that aligns with the observed flow geometry and rates, providing a comprehensive understanding of the observed phenomena. These findings reveal how the control of cytoplasm's material state at the local level transforms mitotic symmetry breaking into long-range intracellular transport. The study also highlights bulk actomyosin as the active stress generator responsible for partitioning embryonic cytoplasm into a composite gel.