Mol Biol Cell 2025 May 06;:mbcE24090421. doi: 10.1091/mbc.E24-09-0421.

Optimized expansion microscopy reveals species-specific spindle microtubule organization in Xenopus egg extracts.

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The spindle is key to cell division, ensuring accurate chromosome segregation. While its assembly and function are well studied, the mechanisms regulating spindle architecture remain elusive. Here, we investigate spindle organization differences between Xenopus laevis and tropicalis, leveraging expansion microscopy (ExM) to overcome conventional imaging limitations. We optimized an ExM protocol tailored for Xenopus egg extract spindles, refining fixation, denaturation, and gelation to achieve higher resolution while preserving spindle integrity. Our protocol enables pre-expansion immunofluorescence and is seamlessly compatible with both species. To quantitatively compare microtubule organization in expanded spindles between the two species, we developed an analysis pipeline able to characterize microtubule bundles throughout spindles. We show that X. laevis spindles exhibit overall a broader range of bundle sizes, while X. tropicalis spindles contain mostly smaller bundles. While both species show larger bundles near the spindle center, X. tropicalis spindles otherwise consist of very small bundles, whereas X. laevis spindles contain more medium-sized bundles. Altogether, our work reveals species-specific spindle architectures and suggests their adaptation to the different spindle size and chromatin amount. By enhancing resolution and minimizing artifacts, our ExM approach provides new insights into spindle morphology and a robust tool for further studying these large cellular assemblies. [Media: see text] [Media: see text].

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Species referenced: Xenopus tropicalis Xenopus laevis
GO keywords: microtubule bundle formation [+]