Spindle Birefringence of Isolated Mitotic Apparatus Analysed by Treatments With Cold, Pressure, and Diluted Isolation Medium

Mitotic apparatus (MA) were isolated from sea-urchin zygotes using glycerol-dimethyl-sulphoxide. Cold treatment had no effect on MA birefringence when MA were in isolation medium, but caused a 10-15% reduction of MA birefringence when MA were in quarter-strength isolation medium. Pressure treatment also caused a reduction in MA birefringence, but the cold and pressure treatments were not additive, suggesting that both treatments affected the same MA component. MA were not stable in quarter-strength isolation medium, and birefringence gradually decayed, with a half-life of about 40 h. Electron microscopy after cold treatment, or after decay of 55% of the MA birefringence showed abundant, normal-looking microtubules, suggesting that alterations in non-microtubule components cause the reductions in birefringence. Addition of EGTA eliminates the effect of cold treatment, suggesting that Ca2+ has a role in maintenance of spindle structure. We discuss possible reasons why isolated MA do not respond to cold treatment like MA in vivo.