MOUSE OOCYTE MATURATION - MEIOTIC CHECKPOINTS

Mouse oocytes at different stages of maturation were fused together an d the ensuing cell cycle events were analyzed with the objective of id entifying checkpoints in meiosis. Fusion of maturing oocytes just unde rgoing germinal vesicle breakdown (GVBD) induces PCC (premature chromo some condensation) but no spindle formation in immature (GV) partner o ocytes. On the other hand, fusion of metaphase I (MI) oocytes containi ng spindles to GV oocytes induces both PCC and spindle formation in th e immature partner. Thus, while molecules required for condensation ar e present throughout metaphase, those involved in spindle formation ar e absent in early M-phase. Oocytes cultured for 6 h-early metaphase I (i.e., similar to 2 h before the onset of anaphase I) - and then fused to anaphase - telophase I (A-TI) fusion partners block meiotic progre ssion in the more advanced oocytes and induce chromatin dispersal on t he spindle. By contrast, oocytes cultured for 8 h (late MI) before fus ion to A-TI partners are driven into anaphase by signals from the more advanced oocytes and thereafter advance in synchrony to telophase I. When early (10 h) or late (12 h) metaphase II oocytes were fused to A- TI partners the signals generated from early MII oocytes block the ana phase to telophase I transition and induce a dispersal of A-TI chromos omes along the spindle, On the other hand, late MII oocytes respond to A-TI signals by exiting from the MII block and undergoing the A-TII t ransition. Moreover, the oocytes in late MI are not arrested in this s tage and progress without any delay through ATI to MII when fused to m etaphase II partners. The signals from the less-developed partner forc e the MII oocyte through A-TII to MIII. In total, these studies demons trate that the metaphase period is divided into at least three distinc t phases and that a checkpoint in late metaphase controls the progress of meiosis in mammalian oocytes. (C) 1995 Academic Press, Inc.