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

The spindle is a key structure in cell division as it orchestrates the accurate segregation of genetic material. While its assembly and function are well-studied, the mechanisms regulating spindle architecture remain elusive. In this study, we focus on the differences in spindle organization between Xenopus laevis and Xenopus tropicalis, leveraging expansion microscopy (ExM) to overcome the limitations of conventional imaging techniques. We optimized an ExM protocol tailored for Xenopus egg extract spindles, improving upon fixation, denaturation and gelation methods to achieve higher resolution imaging of spindles. Our protocol preserves spindle integrity and allows effective pre-expansion immunofluorescence. This method enabled detailed analysis of the differences in microtubule organization between the two species. X. laevis spindles overall exhibit a broader range of bundle sizes, while X. tropicalis spindles are more limited to smaller bundles. Moreover, while both species favor larger bundle sizes near and at the spindle center, X. tropicalis spindles otherwise prefer very small bundles, and X. laevis spindles medium-sized bundles. By enhancing resolution and minimizing distortions and fixation artifacts, our optimized ExM approach offers new insights into spindle morphology and provides a robust tool for studying the structural intricacies of these large cellular assemblies. This work advances our understanding of spindle architecture and opens up new avenues for exploring spindle-related questions.