Figure 2.

States of arrest and checkpoints during mammalian oocyte maturation. The mammalian oocyte enters prophase arrest in embryonic life. Resumption of meiosis can occur after the onset of puberty. Following ovarian exposure to LH, the oocyte resumes meiosis, undergoes germinal vesicle breakdown (GVBD) and enters the first meiotic M-phase. After a lengthy prometahase I during which the first meiotic spindle is formed, homologous chromosome pairs align at the spindle equator. The Spindle Assembly Checkpoint (SAC) inhibits chromosome disjunction until all the chromosomes are properly attached to spindle microtubules from opposite spindle poles and under tension from pulling forces directed towards the poles. Chromosome segregation leads to an asymmetric meiotic division and immediate entry into meiosis II, where the oocyte arrests at metaphase II (MII) awaiting fertilization. Severe DNA damage imposed during prophase arrest launches the G2/prophase DNA damage checkpoint (DDC) which inhibits the resumption of meiosis. A damaged oocytes that slips through the DDC may become arrested during the first meiotic M-phase due to the activation of the SAC.