Mouse oocyte differentiation during antral follicle development

Biased inheritance of mitochondria during asymmetric cell division in the mouse oocyte

A fundamental rule of cell division is that daughter cells inherit half the DNA complement and an appropriate proportion of cellular organelles. The highly asymmetric cell divisions of female meiosis present a different challenge because one of the daughters, the polar body, is destined to degenerate, putting at risk essential maternally inherited organelles such as mitochondria. We have therefore investigated mitochondrial inheritance during the meiotic divisions of the mouse oocyte. We find that mitochondria are aggregated around the spindle by a dynein-mediated mechanism during meiosis I, and migrate together with the spindle towards the oocyte cortex. However, at cell division they are not equally segregated and move instead towards the oocyte-directed spindle pole and are excluded from the polar body. We show that this asymmetrical inheritance in favour of the oocyte is not caused by bias in the spindle itself but is dependent on an intact actin cytoskeleton, spindle-cortex proximity, and cell cycle progression. Thus, oocyte-biased inheritance of mitochondria is a variation on rules that normally govern organelle segregation at cell division, and ensures that essential maternally inherited mitochondria are retained to provide ATP for early mammalian development.

Changes in the distribution of mitochondria before and after in vitro maturation of human oocytes and the effect of in vitro maturation on mitochondria distribution

OBJECTIVE

To clarify the relationship between oocyte maturation and mitochondria distribution and assess the effects of in vitro maturation (IVM) on the distribution of mitochondria in human oocytes.

DESIGN

Prospective randomized trial.

SETTING

Hospital-based IVF center.

PATIENT(S)

One hundred fifty-eight patients undergoing intracytoplasmic sperm injection (ICSI) treatment for male factors or combined with oviduct infertility and fifteen patients undergoing controlled ovarian hyperstimulation followed by coitus or IUI.

INTERVENTION(S)

Of all the 284 immature oocytes, 140 were fixed directly. The others were prepared for IVM before they were fixed. All the 21 oocytes matured in vivo were fixed directly and stained for mitochondria. Both immature and mature oocytes were stained by Mito Tracker Green FM. The distribution of mitochondria was observed using a confocal laser scanning microscope.

MAIN OUTCOME MEASURE(S)

Mitochondrial distribution.

RESULT(S)

Three mitochondria distribution patterns were identified: peripheral, semiperipheral, and evenly diffused. A peripheral distribution of mitochondria was presented by 64.1% (50/78) of the germinal vesicle (GV) oocytes; 45.2% (28/62) of the meiosis I oocytes maintained the peripheral distribution; and 38.7% (24/62) presented a diffused status. After IVM, 75.5% (80/106) of the oocytes displayed an evenly diffused type of distribution. The mitochondria were more abundant in the inner cytoplasm than in the peripheral region in most of the oocytes matured in vivo.

CONCLUSION(S)

There are obvious changes in the distribution of mitochondria in human oocytes before and after maturation. Distribution of mitochondria in oocytes matured in vitro is slightly different from that of oocytes matured in vivo. The results may partially explain the reduced developmental potential of oocytes matured in vitro compared with those matured in vivo.

Chromatin configurations in the ferret germinal vesicle that reflect developmental competence for in vitro maturation

In several mammalian species, the configuration of germinal vesicle (GV) chromatin correlates with the developmental competence of oocytes. Yet, no study has been published on the configuration of GV chromatin in ferret, nor is it known whether a specific configuration predicts meiotic competence in this species, in spite of the potential importance of ferret cloning to the study of human disease and to species conservation efforts. Here, we report on an analysis of the chromatin configuration in ferret GV oocytes and on how they correlate with meiotic development. Three distinct configurations were identified based on the degree of chromatin condensation: (1) fibrillar chromatin (FC), featuring strands of intertwined chromatin occupying most of the visible GV region; (2) intermediate condensed chromatin (ICC), characterized by dense, irregular chromatin masses throughout the GV; and (3) condensed chromatin (CC), which is highly compact and centered around the nucleolus. We also found that chromatin configuration was related to the extent of association with cumulus cells in cumulus-oocyte complexes; CC-configured oocytes were most often surrounded by a compact cumulus layer and also a compact corona but FC-configured oocytes were associated with neither. In addition, increasing chromatin condensation corresponded to an increase in oocyte diameter. Finally, following in vitro culture, significantly more CC-configured oocytes underwent maturation to meiotic metaphase II than did FC- or ICC-configured oocytes. We conclude that, in ferret, chromatin condensation is related to the sequential achievement of meiotic competencies during oocyte growth and differentiation, and thus can be used as a predictor of competence.

The Perivitelline Space-Forming Capacity of Mouse Oocytes is Associated with Meiotic Competence

Although mouse oocytes progressively acquire meiotic competence during their growth in the ovaries, only half of full-grown oocytes can accomplish meiosis. Two types of full-grown oocytes have been reported on the basis of their chromatin configuration, the surrounded-nucleolus (SN) type and the non-surrounded-nucleolus (NSN) type. Therefore, full-grown oocytes collected from the ovaries of adult animals comprise a heterogeneous population; some oocytes are meiotically incompetent (NSN-type), and some are competent (SN-type). In the present study, we found that full-grown oocytes could be classified into two groups using the criterion of formation of the perivitelline space (PVS) after culture with 3-isobutyl-1-methylxanthine (IBMX) for 1 h. In oocytes with a PVS, actin-filled processes within zona pellucidae originating from cumulus cells were reduced, while they were rich in oocytes without a PVS, suggesting that a reduction in these processes contributes to PVS formation. PVS formation was highly correlated with meiotic competence and SN-type configuration. The results of this study demonstrate that PVS formation is a useful criterion for easily distinguishing between SN- and NSN-type oocytes, without injury to the cells.