Fertilised vertebrate oocytes receive the male chromosomes and one centrosome. Therefore, the spindle, which equally partitions the chromosomes into the two daughter cells, must – at the oocyte-to-embryo transition – undergo major modifications in most species to account for the conversion from an acentrosomal chromosome-dependent pathway during meiosis to a mechanism that must integrate the duplicated male centrosomes. Here (p. ), Isabelle Vernos and colleagues use the Xenopus egg extract to investigate how this integration is achieved. By using this system, the authors recapitulate in vitro the assembly of the first embryonic spindle, and dissect the relative contributions of the centrosomes and the chromosome-dependent pathway up to the establishment of bipolarity in the spindle. The authors' data show that centrosomes dictate the kinetics of spindle bipolarisation, imposing a constraint on the self-organisation potential of the chromosome-dependent pathway. This mechanism is tightly associated with the correct positioning of the two centrosomes at each spindle pole, thereby ensuring the faithful segregation of the duplicated male centrosomes to the embryonic cells. These results indicate that centrosomes have a key role in favouring the transition from meiotic to mitotic spindle assembly.
