Dynamics of the endoplasmic reticulum and Golgi apparatus during early seaurchin development

The endoplasmic reticulum (ER) and Golgi were labeled by green fluorescent protein chimeras and observed by time-lapse confocal microscopy during the rapid cell cycles of sea urchin embryos. The ER undergoes a cyclical microt ubule-dependent accumulation at the mitotic poles and by photobleaching exp eriments remains continuous through the cell cycle. Finger-like indentation s of the nuclear envelope near the mitotic poles appear 2-3 min before the permeability barrier of the nuclear envelope begins to change. This permeab ility change in turn is similar to 30 s before nuclear envelope breakdown. During interphase, there are many scattered, disconnected Golgi stacks thro ughout the cytoplasm, which appear as 1- to 2-mu m fluorescent spots. The n umber of Golgi spots begins to decline soon after nuclear envelope breakdow n, reaches a minimum soon after cytokinesis, and then rapidly increases. At higher magnification, smaller spots are seen, along with increased fluores cence in the ER. Quantitative measurements, along with nocodazole and photo bleaching experiments, are consistent with a redistribution of some of the Golgi to the ER during mitosis. The scattered Golgi coalesce into a single large aggregate during the interphase after the ninth embryonic cleavage; t his is likely to be preparatory for secretion of the hatching enzyme during the following cleavage cycle.