J. Fulka et al., Checkpoint control of the G2/M phase transition during the first mitotic cycle in mammalian eggs, HUM REPR, 14(6), 1999, pp. 1582-1587
The high incidence of chromosomally abnormal human embryos is frequently as
sumed to be due to a lack of checkpoint controls operating during early emb
ryogenesis. In our study we have analysed when these mechanisms first becom
e functional. Mouse oocytes treated in late metaphase I with either of two
different cyclin-dependent kinase inhibitors [butyrolactone 1 (BL1) or 6-di
methylaminopurine (6-DMAP)] form nuclei in the cytoplasm, BL1-treated eggs
enter S-phase at 16-18 h post-treatment and, after completion of DNA synthe
sis, cleave to 2-cell stage embryos. 6-DMAP treatment results in the rapid
initiation of DNA synthesis, its completion by 12 h and then arrest in the
G2 phase. Thus, two different cell cycle stages can be obtained at the same
time point after the initiation of treatment: G1- after BL1 and G2-staged
nuclei after 6-DMAP treatment, That this approach greatly facilitates cell
cycle studies has been shown by analysing checkpoint function during the fi
rst division. Whilst G2-staged eggs enter M phase within 2-3 h when 6-DMAP
is washed out, the onset of M phase is delayed after their fusion to G1 (BL
1) cells. Here M phase occurs only after the less advanced nucleus complete
s DNA replication. Our results indicate that checkpoints in mammalian eggs
are functional during the first mitotic cycle.