Oocyte maturation is the final phase of oogenesis during which a number of complex changes take place that involve progression of the oocyte from prophase-I to metaphase-II [
1,
2]. This process involves coordination of nuclear and cytoplasmic events necessary to produce quality oocytes capable of being fertilized and support preimplantation development [
3]. In vivo, this process depends on the close interaction between the cumulus cells and the oocyte involving a cascade of signaling pathways, particularly EGF-like proteins produced from the cumulus cells in response to LH [
4]. Since the original experiments of Pincus & Enzman (1935) and Edwards (1965) it became clear that oocytes undergo immediate and spontaneous maturation once removed from their follicular environment [
5,
6]. However, while IVM supports nuclear maturation, cytoplasmic maturation, a process required for the expression of developmental competence, is not assured [
3]. Overcoming the obstacles limiting cytoplasmic maturation during IVM remains a challenge for the production of quality oocytes of clinical value to the practice of human assisted reproductive technologies (ARTs) [
7]. Several features have been described that might contribute to the high developmental competence of IVO oocytes when compared to their IVM counterparts, such as ATP concentration [
8], number and distribution of active mitochondria [
9], glutathione content [
10], transcriptional and translational activity [
11,
12], and imprinting/methylation patterns in embryos derived from IVO and IVM [
13]. Our group has been especially interested in analyzing patterns of cytoskeletal organization in IVO and IVM matured oocytes to better define properties indicative of developmental competence. Specifically, when spindle size / shape and distribution of cytoplasmic Microtubule Organizing Centers (MTOCs) were analyzed, IVO oocytes exhibited compact and pointed-shaped spindles with increased number of cytoplasmic MTOCs. In contrast, IVM oocytes displayed barrel-shaped spindles, which exhibited much more γ-tubulin aggregates at the spindle poles, associated with a low number of cytoplasmic MTOCs [
14]. These features were found to result from the distinct cell cycle regulation with differential allocation of γ-tubulin stores during the first 5 h of oocyte maturation [
15]. Thus, spindle morphogenesis in IVM oocytes favors recruitment of γ-tubulin to the spindle, while retention of γ-tubulin foci in the cytoplasm with reduced γ-tubulin recruitment to the spindle is observed in IVO oocytes. Consequently, depletion of maternal factors, like γ-tubulin, from the cytoplasm is observed upon IVM and expected to increase with emission of the second polar body after fertilization [
16‐
18]. These critical differences in cytoskeleton organization were found to be a result of the distinct cell cycle progression between IVO and IVM oocytes. In fact, in IVO oocytes the follicular environment maintains the G2/M cell cycle delay necessary to keep nuclear lamina integrity and proper localization of cytoplasmic MTOCs which is not observed under IVM conditions [
15].
The communication established between the oocyte and its surrounding cumulus cells is crucial to promote the oocyte nuclear and cytoplasmic maturation needed to achieve developmental competence [
17‐
20]. When a G2/M phase delay was imposed in culture by treating oocytes with the specific MPF inhibitor, roscovitine, IVO-like cytoskeletal characteristics were partially achieved in IVM oocytes (IVM-Rosco), namely pointed-shape and cortically localized spindles associated with an increased number of cytoplasmic MTOCs [
15] suggesting that an imposed delay in cell cycle progression may be necessary to allow nuclear changes to wait for the cytoplasmic maturation events.
In the present study, additional parameters of oocyte quality, in particular fertilization and developmental competencies are evaluated after careful comparison of the pre-implantation development characteristics between embryo-derived IVO, IVM and IVM-Rosco oocytes. Special emphasis is placed on the expression of cytoskeletal proteins involved in cell polarity determination and stability of the nucleus.