Germinal vesicle position and meiotic maturation in mouse oocyte.

During meiotic maturation, mammalian oocytes undergo an asymmetric division which is crucial for the formation of a functional gamete. In various organisms, accurate positioning of the nucleus before M-phase plays a major role in asymmetric cell divisions. However, the role of the position of the nucleus (or germinal vesicle, GV) during the prophase I arrest has not been investigated in mammalian oocytes. Here, we show that incompetent mouse oocytes possess a peripheral GV, while competent oocytes mainly exhibit a central position of the GV. At that time, the position of the GV correlates with the ability of the oocyte to complete meiotic maturation. Moreover, a lower efficiency in GV centering and a reduced ability to progress through meiosis are observed in oocytes from old mice. Thus, the position of the GV could be used as a simple morphological marker of oocyte quality.