ER (green) is partitioned around each syncytial nucleus in the fly embryo.

Thousands of nuclei share a common cytoplasm in the early fly embryo. But they do not share a common ER, as revealed by Frescas et al. on page 219.

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 around individual nuclei. Proteins did not exchange between compartments.

This ER isolation was weakened by microtubule depolymerization. Microtubule reorganization, resulting in ER isolation, is probably coordinated by centrosomes, which arrive at the cortex along with the nuclei.

During these final divisions, each nucleus also had its own devoted Golgi stacks. The segregation of this membrane system seems to allow the cell to target secreted proteins to restricted areas of the plasma membrane near an individual nucleus. Indeed, a block in the secretion of membrane proteins in one spot of the embryo was not rescued by the ER/Golgi units of neighboring nuclei. Perhaps this targeting ability helps to maintain protein gradients essential for fly development.

Many other organelles, including endosomes and mitochondria, are linked to the microtubule network. If they are all similarly sectioned off, each nucleus might be essentially the equivalent of a cell, even without a plasma membrane barrier. Whether microtubules form a physical barrier, and whether at least some cytoplasmic proteins are also compartmentalized, is under investigation.