To each nucleus its own ER

The embryonic fly genome divides 13 times before each of the resulting 6,000 nuclei acquires its own plasma membrane. Partitioning the various organelles in this cytoplasmic soup at cellularization seems like a daunting task. The new results show that microtubules divvy up the ER several nuclear divisions before this point.

As in other cell types, the embryonic fly ER started out as a continuous interconnected membrane, in which fluorescent proteins diffused freely. But during the last few common divisions, after the nuclei have migrated from the embryo interior to the cortex, this diffusion was greatly reduced. At this point, the ER was compartmentalized...