Centrosomes and the cell cycle

Centrosomes are the ensemble of organelles that form the poles of the mitotic spindle. We have examined the properties of the mechanisms that control the precise doubling, or reproduction, of centrosomes during the cell cycle. A functional analysis of this event in sea urchin eggs indicates that it is limited by the reproduction of determinants that we call polar organizers, around which the centrosome is organized. Each centrosome contains two polar organizers whose splitting, physical separation, and duplication control the reproduction of the centrosome. The splitting and duplication events are distinct processes that can be experimentally put out of phase with each other for several cell cycles. A serial section ultrastructural analysis of centrosomes with altered reproduction shows that the reproductive capacity of a centrosome is correlated with the number of centrioles it contains. In other experiments we show that centrosome reproduction is cytoplasmically controlled; centrosomes repeatedly double in a normal fashion in physically enucleated sea urchin eggs and eggs in which DNA synthesis has been inhibited by aphidicolin. In addition, we show that centrosomes, through the spindle microtubules they nucleate, play an important role in the mechanisms that control the timing of mitotic events and the overall duration of the cell cycle. Taken together, our observations in concert with those of others suggest that the cell cycle is a cytoplasmic phenomenon that does not require nuclear activities for cells that are not growth-limited.