Summary The mechanisms that dictate nuclear shape are largely unknown. Here we screened the budding yeast deletion collection for mutants with abnormal nuclear shape. A common phenotype was the appearance of a nuclear extension, particularly in mutants in DNA repair and chromosome segregation genes. Our data suggest that these mutations led to the abnormal nuclear morphology indirectly, by causing a checkpoint-induced cell cycle delay. Indeed, delaying cells in mitosis by other means also led to the appearance of nuclear extensions, while inactivating the DNA damage checkpoint pathway in a DNA repair mutant reduced the fraction of cells with nuclear extensions. Formation of a nuclear extension was specific to a mitotic delay, as cells arrested in S or G2 had round nuclei. Moreover, the nuclear extension always coincided with the nucleolus, while the morphology of DNA mass remained largely unchanged. Finally, we found that phospholipid synthesis continues unperturbed when cells delay in mitosis, and inhibiting phospholipid synthesis abolished the formation of nuclear extensions. Our data suggest a mechanism that promotes nuclear envelope expansion during mitosis. When mitotic progression is delayed, cells sequester the added membrane to the nuclear envelope associated with the nucleolus, possibly to avoid disruption of intra-nuclear organization.

Take a look at a textbook illustration of a cell and you will immediately be able to locate the nucleus, which is often drawn as a spherical or ovoid shaped structure. But not all cells have such nuclei. In fact, some disease states are diagnosed by the presence of nuclei that have an abnormal shape or size. What defines nuclear shape and nuclear size, and how does nuclear geometry affect nuclear function? While the answer to the latter question remains largely unknown, significant progress has been made towards understanding the former. In this review, we provide an overview of the factors and forces that affect nuclear shape and size, discuss the relationship between ER structure and nuclear morphology, and speculate on the possible connection between nuclear size and its shape. We also note the many interesting questions that remain to be explored.

Each class of microscope is limited to imaging specific aspects of cell structure and/or molecular organization. However, imaging the specimen by complementary microscopies and correlating the data can overcome this limitation. Whilst not a new approach, the field of correlative imaging is currently benefitting from the emergence of new microscope techniques. Here we describe the correlation of cryogenic fluorescence tomography (CFT) with soft x-ray tomography (SXT). This amalgamation of techniques integrates 3-D molecular localization data (CFT) with a high-resolution, 3-D cell reconstruction of the cell (SXT). Cells are imaged in both modalities in a near-native, cryopreserved state. Here we describe the current state of the art in correlative CFT-SXT, and discuss the future outlook for this method.