γ-Tubulin Is Asymmetrically Distributed in the Cortex of Xenopus Oocytes

Abstract

Stage VI Xenopus oocytes contain an extensive network of cytoplasmic microtubules (MTs), with no evidence of a functional centrosome. Recently, Stearns et al. (1991) demonstrated that Xenopus eggs contain a substantial pool of the centrosomal protein γ-tublin (γ-Tb). For this report, I have used confocal immunofluorescence microscopy to examine the distribution of γ-Tb during the later stages of oogenesis in Xenopus laevis. γ-Tb was apparent surrounding the germinal vesicle (GV) of stage VI oocytes, consistent with previous results suggesting that the GV serves as an microtubule organizing center in later oogenesis. Surprisingly, γ-Tb was also concentrated in the cortex of stage VI oocytes, and the distribution of cortical γ-Tb was polarized along the animal-vegetal (A-V) axis. In the vegetal cortex, γ-Tb was observed in brightly stained foci, often organized into short linear arrays. In the animal hemisphere, γ-Tb was more evenly distributed as small cortical foci. Dual immunofluorescence microscopy revealed that γ-Tb in the vegetal hemisphere was associated with MTs in the cortical cytoplasm. The distribution of γ-Tb was not significantly affected by either cold or nocodazole, but was partially disrupted by cytochalasin B. γ-Tb thus may serve as a link between the oocyte MT network and cortical actin. Finally, polarization of the distribution of cortical γ-Tb temporally coincides with formation of the A-V axis and polarization of the oocyte MT cytoskeleton during stage IV of oogenesis. These observations raise a number of questions regarding the organization and orientation of MTs during Xenopus oogenesis and the role of γ-Tb in the polarization of the oocyte cytoskeleton during A-V axis formation.