Human Reproduction, Vol. 14, No. 6, 1582-1587,
June 1999
© 1999 European Society of Human Reproduction and Embryology
Checkpoint control of the G2/M phase transition during the first mitotic cycle in mammalian eggs
1 The Institute of Animal Production, CS-104 01 Prague 10, Czech Republic, 2 The Babraham Institute, Development and Genetics Programme, Babraham, Cambridge CB2 4AT, UK, and 3 3University of Wisconsin, Department of Meat and Animal Science, 1675 Observatory Drive, Madison, WI 53706, USA
The high incidence of chromosomally abnormal human embryos is frequently assumed to be due to a lack of checkpoint controls operating during early embryogenesis. In our study we have analysed when these mechanisms first become functional. Mouse oocytes treated in late metaphase I with either of two different cyclin-dependent kinase inhibitors [butyrolactone 1 (BL1) or 6-dimethylaminopurine (6-DMAP)] form nuclei in the cytoplasm. BL1-treated eggs enter S-phase at 16–18 h post-treatment and, after completion of DNA synthesis, 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 first 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 (BL1) cells. Here M phase occurs only after the less advanced nucleus completes DNA replication. Our results indicate that checkpoints in mammalian eggs are functional during the first mitotic cycle.
Key words: checkpoints/cyclin-dependent kinase inhibitors/DNA replication/mitosis/oocytes
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