Regional Response to Cytochalasin B of the Equatorial Cell Cortex in Sea-Urchin Eggs during the First Mitosis. (surface architecture/cortical microfilaments/cytochalasin B/sea-urchin egg/first mitosis)

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Abstract
Sea-urchin eggs treated with cytochalasin B (CB) during the first mitosis were examined for topography of the cell surface and the cortical microfilaments. Scanning electron microscopy revealed microvillar elongation and clumping on the whole cell surface of interphase and prophase eggs, but such changes were restricted to the equatorial surface in eggs treated with CB after metaphase up to the end of telophase. The clumping occurred also all over the cell surface of binuclear eggs after prolonged treatment up to the following interphase. These indicate localization of a target structure(s) of CB during mid- and late mitotic stages. Extraction with Triton X-100 allowed detection of CB-induced alterations in cortical microfilaments: The microvillar microfilaments were transformed into short filaments with random orientation. The submembranous meshwork and the contractile ring microfilaments were transformed into a network of shorter filaments. The prolonged CB treatment led to incorporation of the network microfilaments into the basal portions of the microvillar clumps. Thus, CB application during mid- and late mitotic stages induced (a) local microvillar changes and (b) alterations in the meshwork and the contractile ring. The present results suggest that the microvillar changes are caused by alterations of underlying cortical microfilamentous structures.