Acquisition of Meiotic Competence Is Related to the Functionality of the Phosphoinositide/Calcium Signaling Pathway in the Mouse Oocyte

Abstract

The meiosis resumption process has been related to spontaneous cytoplasmic InsP3-dependent calcium oscillations in fully grown mouse oocytes. Our purpose was to determine whether the acquisition of meiotic competence during the growth phase of oogenesis was associated with that of Ca2+oscillations and whether these oscillations were dependent on the phosphoinositide cycle. We used confocal laser scanning microscopy to image free calcium ions in fluo-3/AM-loaded oocytes recovered from 12- to 26-day-old mice for 15 min following follicular release. As expected, oocytes isolated from 12-day-old mice were totally incompetent to undergo GVBin vitro,whereas the GVB rate increased progressively with mouse age and oocyte diameter. The percentage of oocytes exhibiting spontaneous calcium oscillations and that of oocytes resuming meiosis were similarly correlated with the female age, with incompetent oocytes failing to exhibit spontaneous Ca2+oscillations. It is noteworthy that regardless of the stage of growth, thapsigargin induced an ooplasmic calcium release from the InsP3-sensitive stores when it was added to the culture medium. However, intracytoplasmic microinjection of InsP3 induced a shorter sequence of Ca2+oscillations in 12-day-old mouse oocytes than in 15-day-old mouse oocytes and, whereas InsP3 increased the GVB rate at 15 days, it was unable to induce GVB at 12 days. These data lead us to conclude that the acquisition of meiotic competence is related to the functionality of the InsP3 pathway and, correspondingly, to the oocyte's ability to generate spontaneous cytoplasmic InsP3-dependent calcium oscillations.