CORTICAL AND CYTOPLASMIC FLOW POLARITY IN EARLY EMBRYONIC-CELLS OF CAENORHABDITIS-ELEGANS

We have examined the cortex of Caenorhabditis elegans eggs during pseu docleavage (PC), a period of the first cell cycle which is important f or the generation of asymmetry at first cleavage (Strome, S. 1989. Int . Rev. Cytol. 114: 81-123). We have found that directed, actin depende nt, cytoplasmic, and cortical flow occurs during this period coinciden t with a rearrangement of the cortical actin cytoskeleton (Strome, S. 1986. J. Cell Biol. 103: 2241-2252). The flow velocity (4-7 mum/min) i s similar to previously determined particle movements driven by cortic al actin flows in motile cells. We show that directed flows occur in o ne of the daughters of the first division that itself divides asymmetr ically, but not in its sister that divides symmetrically. The cortical and cytoplasmic events of PC can be mimicked in other cells during cy tokinesis by displacing the mitotic apparatus with the microtubule pol ymerization inhibitor nocodazole. In all cases, the polarity of the re sulting cortical and cytoplasmic flows correlates with the position of the attenuated mitotic spindle formed. These cortical flows are also accompanied by a change in the distribution of the cortical actin netw ork. The polarity of this redistribution is similarly correlated with the location of the attenuated spindle. These observations suggest a m echanism for generating polarized flows of cytoplasmic and cortical ma terial during embryonic cleavages. We present a model for the events o f PC and suggest how the poles of the mitotic spindle mediate the form ation of the contractile ring during cytokinesis in C. elegans.