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.