ACQUISITION OF MEIOTIC COMPETENCE IS RELATED TO THE FUNCTIONALITY OF THE PHOSPHOINOSITIDE CALCIUM SIGNALING PATHWAY IN THE MOUSE OOCYTE/

The meiosis resumption process has been related to spontaneous cytopla smic InsP3-dependent calcium oscillations in fully grown mouse oocytes . Our purpose was to determine whether the acquisition of meiotic comp etence during the growth phase of oogenesis was associated with that o f Ca2+ oscillations and whether these oscillations were dependent on t he 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 exp ected, oocytes isolated from 12-day-old mice were totally incompetent to undergo GVB in vitro, whereas the GVB rate increased progressively with mouse age and oocyte diameter. The percentage of oocytes exhibiti ng spontaneous calcium oscillations and that of oocytes resuming meios is were similarly correlated with the female age, with incompetent ooc ytes failing to exhibit spontaneous Ca2+ oscillations. It is noteworth y that regardless of the stage of growth, thapsigargin induced an oopl asmic calcium release from the InsP3-sensitive stores when it was adde d to the culture medium. However, intracytoplasmic microinjection of I nsP3 induced a shorter sequence of Ca2+ oscillations in 12-day-old mou se oocytes than in 15-day-old mouse oocytes and, whereas InsP3 increas ed the GVB rate at 15 days, it was unable to induce GVB at 12 days, Th ese data lead us to conclude that the acquisition of meiotic competenc e is related to the functionality of the InsP3 pathway and, correspond ingly, to the oocyte's ability to generate spontaneous cytoplasmic Ins P3-dependent calcium oscillations. (C) 1997 Academic Press.