Effects of Ca+2 and Mg+2 deprivation on cell shape in cultured ovarian granulosa cells

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Abstract
After 3 days in culture, rat ovarian granulosa cells assume a flattened epitheloid organization. Ca+2 and Mg+2 deprivation results in cellular rounding which is reversible and was monitored by phase‐contrast time‐lapse cinematography. Concomitant with the shape change is a dispersion of the structural proteins actin and α‐actinin. The arrays of large actin‐containing bundles (stress fibers) are converted to a diffuse network as observed by electron microscopy. Alpha‐actinin, which was observed by immunocytochemistry to be in a periodic array along the actin bundles, is disrupted also and redistributed in the periphery of the cell upon rounding. Measurements made of the culture medium during the rounding process indicate that there is a loss of Ca+2 and Mg+2 from the cell interior. These data led us to speculate that Ca+2 and/or Mg+2 are necessary in order to maintain the integrity of stress fibers and/or restrict the movement of α‐actinin anchoring sites within the membrane.