Ectopic spindle assembly during maturation of Xenopus oocytes: evidence for functional polarization of the oocyte cortex.

Inversion of stage VI Xenopus oocytes, when combined with cold-induced microtubule depolymerization, resulted in the displacement of the germinal vesicle (the oocyte nucleus) from its normal position in the animal hemisphere to a more equatorial or vegetal position. Confocal immunofluorescence microscopy with anti-tubulin antibodies revealed that the breakdown of ectopic germinal vesicles during progesterone-induced maturation of inverted oocytes was accompanied by the assembly of a transient MTOC-microtubule complex (MTOC-TMA complex) that migrated to the region of cortex nearest the site of GV breakdown. Formation of ectopic meiotic spindles, including spindle rotation and polar body formation, was observed with nearly equal frequency throughout the animal hemisphere. In contrast, the frequency of spindle formation in the vegetal hemisphere was substantially reduced. Many spindles that did form in more vegetal regions of the oocyte appeared unable to rotate and did not result in polar body formation. The results presented suggest that the assembly of meiotic spindles at the animal pole during maturation of Xenopus oocytes does not result from a specific targeting of the MTOC-TMA complex during oocyte maturation. However, the oocyte cortex does exhibit A-V polarization of its ability to support spindle rotation and polar body formation. Finally, the observed increase in the incidence of monaster formation in the vegetal hemisphere suggests that interactions between the nascent spindle and cortex may regulate spindle assembly and elongation.