Mouse oocyte differentiation during antral follicle development.

During antral follicle development mouse oocytes undergo rearrangement of granulosa cell interactions and the oocytes released from follicles at the beginning or at the end of antral development are either devoid of denuded oocytes (DO) or strictly associated with cumulus-intact (CI) cumulus cells. In this study, these two oocyte classes were analyzed before germinal vesicle (GV) and after in vitro maturation (IVM) to evaluate (a) the ultrastructural aspect of oolemma microvilli by scanning electron microscopy analysis and (b) specific morphological markers of differentiation (chromatin organization, mitochondria, cortical granules, microfilaments, and spindle of metaphase II- MII-). At GV-stage, CI oocytes exhibited remarkable differences (a) in the oolemma microvillar ultrastructure and distribution with respect to DO and (b) in the chromatin organization that was typical of meiotically competent germ cells. By contrast, homogeneous patterns of distribution of mitochondria, cortical granules, and microfilaments characterized both the oocyte classes. At the end of culture, CI oocytes, even when matured without cumulus cells, reached more efficiently the MII stage and acquired an ultrastructural microvillous configuration different from DO. In addition, MII-arrested DO had a higher percentage of meiotic spindles with abnormal morphology in comparison with preovulatory oocytes, while cortical granule and microfilament patterns revealed no appreciable differences between the groups. With regard to mitochondria, a polarized distribution of these organelles was found in 82% of DO and in 97% of CI oocytes. These observations suggested that the achievement of the full antral follicle development is a condition for the acquisition of specific qualitative properties that are essential for the production of fertilizable oocytes, both in in vivo and in vitro models as well.