A phase of chromosome aggregation during meiosis in human oocytes

A higher rate of chromosomal abnormality occurs in human oocytes compared with other animal oocytes. In this study, chromosome movement has been successfully observed during first and second meiosis using a time-lapse culture system and a differential interference contrast inverted microscope. In human oocytes, a specific sequence of early maturation changes was observed. Following the completion of nucleolar breakdown, chromosomes were assembled into a single aggregation that heralded the start of nuclear membrane breakdown. The chromosome aggregation phase (gere phase) persisted after germinal vesicle (GV) breakdown, lasting several hours, and a similar gere phase (chromosome gathering) occurred after the first polar body extrusion, lasting 1–4 h. In contrast, in mouse GV oocytes, nucleolar and nuclear membranes started to break down almost at the same time. A chromosome aggregation phase was not observed in mouse oocytes. The discovery of a gere phase during human oocyte maturation may provide important information related to the mechanism of abnormal chromosomal segregation, which often occurs during meiosis.