A function for the midbody remnant in embryonic patterning.

Asymmetric cell divisions combine cell division with fate specification and one general model of how this is achieved was proposed already decades ago(1,2): During interphase, the cell polarity axis is specified, followed by orientation of the spindle along the polarity axis and segregation of fate determinants along the polarity axis during mitosis. In most cells, the polarity axis and the spindle will usually align with the long axis that the cell had before division, also called Hertwig's rule(3-6). In the C. elegans embryo, the first polarity axis also forms along the long axis of the embryo by enrichment of myosin in the anterior(7) and formation of mutually exclusive anterior and posterior cortical polarity domains, mediated through directional cortical contractile flow(8-10). The directionality of this flow is determined by an extrinsic cue, the entry of the sperm, which inhibits Rho-dependent myosin activation at the future posterior pole by bringing with it the Rho GTPase activating protein CYK-4(11,12). Moreover, since there is no previous division 'history' before the first cleavage, mechanisms have to ensure that the nucleus-centrosome complex undergoes a 90 degree rotation so that the spindle can subsequently elongate along the long axis(13-15). Additional mechanisms ensure that the site of cleavage is perpendicular to the long axis(16,17). Hence, tight coupling of an extrinsic cue to intrinsic polarity formation and spindle elongation enables alignment of the division orientation with the long axis of the organism and successful segregation of fate determinants.