Mitotic Asters Separate, although Chromosomes Do Not Separate at Slightly Acidic pHi in the Fertilized Egg of the Sea Urchin, Hemicentrotus pulcherrimus

The effect of slightly acidic intracellular pH (pHi) on the development of the sea urchin, Hemicentrotus pulcherrimus was investigated. At first cleavage, the fertilized eggs were treated with artificial sea water containing sodium acetate (Ac-pHSW) at pH 6.8 or 7.0 at the onset of nuclear envelope breakdown, and their pHi decreased from 7.30 to 6.68 or 6.78, respectively. When the eggs were observed after fixation by indirect immunofluorescence and differential interference contrast microscopy, the mitotic stage of the treated eggs was arrested at metaphase and the mitotic apparatus was maintained until more than 50 min after the treatment, although it was smaller in size than that of non-treated eggs. On the other hand, the number of the mitotic asters increased from 2 to 3-4, and further to 6-8 following prolonged exposure, suggesting that the centrosomes had divided and replicated. These results suggest that the centrosome cycle advanced at slightly acidic pHi, even when the mitotic cycle did not advance beyond metaphase.