Early embryonic divisions in Drosophila are syncytial. Nuclei undergo mitosis synchronously for the first 14 nuclear cycles, followed by cellularization (Foe and Alberts, 1983). At nuclear cycle 10, nuclei are at the embryo cortex and can be easily visualized. Silverman's first movie (an ASCB 2005 Celldance contest winner) shows a Drosophila embryo of a line expressing a fusion protein of GFP and tubulin (Grieder et al., 2000). This fusion protein enables the visualization of spindle assembly and disassembly during 4 consecutive divisions. Spindle assembly starts when the nuclear membrane breaks down and microtubules are produced at centrosomes that are at opposite positions of nuclei. Next, microtubules attach to chromosomes and assemble onto a bipolar spindle. When the chromosomes segregate, the spindles elongate. Chromosome movement to poles is favored by disassembly of the microtubules to which they are connected. The other microtubules that are not attached to chromosomes form the midbody. In late telophase, midbodies disassemble. Next, the centrosomes duplicate and migrate to opposite positions of the nuclei and the cycle of spindle assembly starts again. With every cycle, the spindles are smaller and closer.
Embryos were collected 2 hours after they were laid, dechorionated with 50% bleach and visualized by confocal microscopy. Images were captured with a Nikon Eclipse TE2000-E inverted microscope (Nikon Mississauga, ON, Canada) using a Plan Apochromat 40 x 1.5/NA1.0 oil immersion lens. The visualization setting included a Perkin Elmer Life Sciences UltraVIEW confocal spinning disk (Perkin Elmer Life Sciences, Missisauga, ON, Canada) and a Hamamatsu Orca-ER camera (Hamamatsu Photonics, Hamamatsu, Japan). Images were acquired at room temperature, every 8 seconds at 3-5 Z series of 40 m steps using the MetaMorph software (Universal Imaging, West Chester, PA).
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