We have used whole-mount immunofluorescence mi-croscopy of late-stage Xenopus laevis oocytes and early embryos to examine the organization of their cortical cytokeratin systems. In both mature oocytes and early embryos, there is a distinct animal-vegetal polarity in cytokeratin organization. In mature (stage-VI) oocytes, the cytokeratin filaments of the vegetal region form a unique, almost geodesic net-work; in the animal region, cytokeratin organization appears much more variable and irregular. In unfer-tilized, postgerminal vesicle breakdown eggs, the cor-tical cytokeratin system is disorganized throughout both animal and vegetal hemispheres. After fertiliz-ation, cytokeratin organization reappears first in a punctate pattern that is transformed into an array of oriented filaments. These cytokeratin filaments ap-pear first in the vegetal hemisphere and are initially thin. Subsequently, they form bundles that grow thicker through the period of first to second cleavage, at which point large cytokeratin filament bundles form a loose, fishnet-like system that encompasses the vegetal portion of each blastomere. In the animal region, cytokeratin filaments do not appear to form large fibre networks, but rather appear to be organ-ized into a system of fine filaments. The animal-vegetal polarity in cytokeratin organization persists until early blastula (stage 5); in later-stage embryos, both animal and vegetal blastomeres possess qualitat-ively similar cytokeratin filament systems. The entire process of cytokeratin reorganization in the egg is initiated by prick activation. These observations indi-cate that the cortical cytoskeleton of Xenopus oocytes and early embryos is both dynamic and asymmetric.

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