Age and gonadotropins control Ca2+-spike acquisition in mouse oocytes isolated from early preantral follicles

The action of gonadotropins upon the oocyte is known to be crucial at later stages of follicular development in mammals. However, its influence on oocytes at early preantral stages is still a matter of debate. In the present study we evaluated the onset of mouse oocyte's capacity to exhibit calcium spikes during preantral stages of follicular development, prior to meiotic competence acquisition. In particular, through the use of confocal microscopy, we probed for the specific effects of age and gonadotropin stimulation upon the calcium dynamics of preantral follicle oocytes. We found that important developmental changes on the Ca2+ signalling mechanisms take place early during follicular development. Specifically we demonstrate that both age and gonadotropin stimulation increase the capacity of oocytes recovered from preantral follicles to exhibit calcium spikes. We propose that a strictly morphological staging of follicular development is insufficient to predict oocyte behaviour and must take in consideration animal age and gonadotropin environment.