Complementary volume electron microscopy-based approaches reveal ultrastructural changes in germline intercellular bridges

The Drosophila melanogaster egg chamber is a powerful model system to study germline intercellular bridges, or ring canals, which connect the developing oocyte to supporting nurse cells. Despite their importance, it is technically difficult to use electron microscopy-based approaches to monitor changes in ring canal structure. Here, we utilize a complementary set of volume electron microscopy-based approaches to visualize ultrastructural changes in the germline ring canals. The combination of array tomography and focused ion beam scanning electron microscopy provided insight into previously unappreciated aspects of ring canal structure. We quantified differences in ring canal size and thickness within and between germline cell clusters and visualized the formation of membrane interdigitations near the ring canals much earlier than previously reported. Reconstruction of multiple egg chambers provided insight into the three-dimensional orientation of these extensive cell-cell contacts. Finally, we identified a novel membrane structure that appeared to line the interior of the ring canal lumen. This imaging framework could be applied to other tissues with technical challenges, in which the small structure of interest is located within a large sample volume.