The effect of hypergravity on bipolar spindle formation of meiosis II in Xenopus oocytes

Abstract

Full-grown Xenopus oocytes arrest at the prophase of meiosis I. The hormone progesterone induces resumption of the meiotic cell cycle, leading to maturation of the oocytes into fertilizable eggs. We have been investigating the effects of hypergravity on Xenopus oocyte maturation. Recently we reported that hypergravity did not affect the initiation of maturation, whereas the progression of maturation was affected regarding the external appearance of the white spot. The present study examined changes in spindle morphology during Xenopus oocyte maturation. Cytological examination showed that bipolar spindle formation at meiosis II was largely inhibited and monopolar (or monoastral) spindles were observed, although oocytes formed apparently normal bipolar spindle upon the entry into meiosis I. These findings suggest that hypergravity inhibits the bipolar spindle formation of meiosis ll and hence adversely affects the process of oocyte maturation in Xenopus laevis.