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Abstract

Polarity establishment, asymmetric division, and acquisition of cell fates are critical steps during earlydevelopment. In this review, we discuss processes that set up the embryonic axes, with an emphasis onpolarity establishment and asymmetric division. We begin with the first asymmetric division in theC. elegansembryo, where symmetry is broken by the local inactivation of actomyosin cortical contractility. Thiscontributes to establishing a polarized distribution of PAR proteins and associated components on the cellcortex along the longitudinal embryonic axis, which becomes the anterior-posterior (AP) axis. Thereafter, APpolarity is maintained through reciprocal negative interactions between the anterior and posterior corticaldomains. We then review the mechanisms that ensure proper positioning of the centrosomes and the mitoticspindle in the one-cell embryo by exerting pulling forces on astral microtubules. We explain how a ternarycomplex comprised of Gα(GOA-1/GPA-16), GPR-1/GPR-2, andLIN-5is essential for anchoring the motorprotein dynein to the cell cortex, where it is thought to exert pulling forces on depolymerizing astralmicrotubules. We proceed by providing an overview of cell cycle asynchrony in two-cell embryos, as well asthe cell signaling and spindle positioning events that underly the subsequent asymmetric divisions, whichestablish the dorsal-ventral and left-right axes. We then discuss how AP polarity ensures the unequalsegregation of cell fate regulators via the cytoplasmic proteinsMEX-5/MEX-6and other polarity mediators,before ending with an overview of how the fates of the early blastomeres are specified by these processes.

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