Steps of nuclear pore complex disassembly and reassembly during mitosis inearly Drosophila embryos

The mechanisms of nuclear pore complex (NPC) assembly and disassembly durin g mitosis in vivo are not well defined. To address this and to identify the steps of the NPC disassembly and assembly, we investigated Drosophila embr yo nuclear structure at the syncytial stage of early development using fiel d emission scanning electron microscopy (FESEM), a high resolution surface imaging technique, and transmission electron microscopy. Nuclear division i n syncytial embryos is characterized by semiclosed mitosis, during which th e nuclear membranes are ruptured only at the polar regions and are arranged into an inner double membrane surrounded by an additional 'spindle envelop e'. FESEM analysis of the steps of this process as viewed on the surface of the dividing nucleus confirm our previous in vitro model for the assembly of the NPCs via a series of structural intermediates, showing for the first time a temporal progression from one intermediate to the next. Nascent NPC s initially appear to form at the site of fusion between the mitotic nuclea r envelope and the overlying spindle membrane. A model for NPC disassembly is offered that starts with the release of the central transporter and the removal of the cytoplasmic ring subunits before the star ring.