Bovine oocyte diameter in relation to developmental competence

This study was conducted to determine the diameter of bovine oocytes that were able to attain their full developmental competence to blastocysts. Oocytes were recovered by aspiration of surface-visible follicles (1 to 7 mm in diameter) from slaughterhouse ovaries. Only healthy-looking cumulus-oocyte complexes were used for in vitro maturation, and they were divided into six groups based on diameter: < 110 microm, 110 to < 115 microm, 115 to < 120 microm, 120 to < 125 microm, 125 to < 130 microm and >/= 130 microm. Oocytes were processed through standard procedures for in vitro maturation, fertilization and culture. Following in vitro maturation or fertilization, some oocytes were stained to assess nuclear maturation and penetration rates. The numbers of embryos that cleaved at 42 h post insemination and developed to blastocysts and hatched blastocysts after 8 days of culture were recorded. The mean oocyte diameters were 114.0 +/- 4.8 microm. The oocytes displayed size-related ability to undergo meiotic maturation. The rates of nuclear maturation of oocytes in the greater than 115-microm size range were significantly higher than those of oocytes with diameters < 115 microm. In the < 120 microm diameter groups, the polyspermic fertilization rates of oocytes < 115 microm were significantly higher than those of oocytes 115 to < 120 microm in diameter. The rates of cleavage and development to blastocysts and hatched blastocysts rose as oocyte diameter increased. Among oocytes with a diameter >110 microm, oocytes < 120 microm were found to have significantly lower developmental competence than oocytes 120 to < 130 microm in diameter. These results suggest that bovine oocytes have acquired full meiotic competence at a diameter of 115 microm but not yet attained full developmental competence to blastocysts, and that oocytes have acquired full developmental competence at a diameter of 120 microm.     

Acquisition of meiotic competence in the rat: Role of gonadotropin and estrogen

Oocytes were removed from the follicles of rats at 15 to 31 days of age, and their ability to resume meiosis (“meiotic competence”) in vitro was correlated with their diameter and the stage of follicular development. The majority of oocytes explanted on day 15 did not resume meiosis when placed in culture, but the percentage of competent oocytes increased from 14.1% ± 3.0% on day 20 to 67.6% ± 3.3% on day 26 of age. This ability to resume maturation correlated well (r = 0.98) with the increase in diameter of oocytes and coincided with the development of antral follicles. Hypophysectomy on day 15 of age, but not on day 20, reduced the percentage (P < 0.001) and number (P < 0.001) of competent oocytes and was accompanied by a reduction in diameter of oocytes. Treatment with PMSG or E₂ increased the number (P < 0.001) and percentage (P < 0.001) of competent oocytes. These results suggest that the ability of oocytes to mature in vitro is dependent upon stimulation by gonadotropins and that this action of gonadotropin may be mediated by production of estrogen within the follicles.     

Influence of follicular components on oocyte meiosis in vitro

Oocytes and follicular components obtained from ovaries recovered from mature Hereford cows at slaughter were used to determine follicular influence on oocyte maturation. Some oocytes were fixed immediately to determine the stage of maturation. The remaining oocytes were cultured for 32 to 34 hr in various environments to determine the influences of the granulosum and follicular fluids on meiotic changes. All noncultured oocytes had dictyate nuclei except one in premetaphase. Oocytes cultured in 50 or 100% follicular fluid or in contact with stratum granulosum cells showed some meiotic inhibition both before and after germinal vesicle breakdown (GVB). The least resumption of meiosis occurred in oocytes cultured in their intact follicles.     

Cultivation of mouse oocytes in vitro: the ability to resume meiosis

It is well established that fully grown oocytes from sexually mature mice are capable of resuming meiosis when cultured in vitro. However it has not been established at what stage during the growth the oocyte acquires the ability to mature. The present study was undertaken to determine the precise stage of growth at which the ability to resume meiosis is acquired in ICR strain mice. Oocytes of various sizes were isolated from the ovary by mechanical dissection. This method yielded about 30% growing and fully-grown oocytes from the ovary. Cumulus-free oocytes dissociated from the ovaries of 5-week-old mice were cultured in vitro in a chemically defined medium for 3 hr. None of the 61- to 65-microns-diameter oocytes resumed meiosis; however 81.8% of the 66- to 70-microns-diameter oocytes were able to resume meiosis. Fifty- to 60-microns-diameter oocytes recovered by enzymatic dispersal of the basement membrane of isolated growing follicles were unable to resume meiosis. Follicle-enclosed 50- to 60-microns-diameter oocytes did not resume meiosis even when cultured in a medium containing Ca-ionophore A 23187 or 8-bromoadenosine 3', 5'-cyclic monophosphate which are known to induce resumption of meiosis of oocytes in Graafian follicles. These results indicate that the ability to resume meiosis is acquired dramatically at a specific stage of oocyte growth.     

Oocyte diameter in relation to meiotic competence and sperm penetration

This study was conducted to determine the diameter of canine oocytes that are able to attain full meiotic competence and sperm penetration. Oocytes were collected from ovaries of bitches at various stages of the estrous cycle. Only healthy-looking cumulus-oocyte complexes were used for in vitro maturation, and were divided into four groups based on diameter: <100, 100 to <110, 110 to <120 and >120 microm. Following in vitro maturation or fertilization, oocytes were stained to assess nuclear maturation and penetration rates. The mean oocyte diameter was 108.5 +/- 0.4 microm. The oocytes displayed size-related ability to undergo meiotic maturation. After culture for 72 h, the rates of oocytes that remained at the germinal vesicle stage in the <110 microm groups were significantly higher (P<0.01) than in the > or = 110 microm groups. None of the oocytes <110 microm reached metaphase II (MU), but 4.9 and 21.5% of the oocytes that were greater than 110 and 120 microm, respectively, progressed to MII. After in vitro fertilization for 20 h, 10 to 25% of oocytes were penetrated by spermatozoa, but there were no clear relationships between oocyte diameter and penetration rates of the oocyte by sperm. In the <120 microm groups, sperm penetration was mostly found in oocytes arrested at the germinal vesicle stage. However, a total of eight oocytes > or = 120 microm in diameter were penetrated by spermatozoa, of which five oocytes reached MII. These results suggest that there is a clear relationship between oocyte diameter and meiotic competence, but no relationship between oocyte diameter and sperm penetration. Canine oocytes may have acquired meiotic competence once they reach at a diameter of 120 microm, but the oocytes may allow the entry of spermatozoa into the ooplasm irrespective of oocyte diameter.     

Defective calcium release during in vitro fertilization of maturing oocytes of LT/Sv mice

Oocytes of LT/Sv mice have anomalous cytoplasmic and nuclear maturation. Here, we show that in contrast to the oocytes of wild-type mice, a significant fraction of LT/Sv oocytes remains arrested at the metaphase of the first meiotic division and is unable to undergo sperm-induced activation when fertilized 15 hours after the resumption of meiosis. We also show that LT/Sv oocytes experimentally induced to resume meiosis and to reach metaphase II are unable to undergo activation in response to sperm penetration. However, the ability for sperm-induced activation developed during prolonged in vitro culture. Both types of LT/Sv oocytes, i.e. metaphase I and those that were experimentally induced to reach metaphase II, underwent activation when they were fertilized 21 hours after germinal vesicle breakdown (GVBD). Thus, the ability of LT/Sv oocytes to become activated by sperm depends on cytoplasmic maturation rather than on nuclear maturation i.e. on the progression of meiotic division. We also show that sperm penetration induces fewer Ca(2+) transients in LT/Sv oocytes than in control wild-type oocytes. In addition, we found that the levels of mRNA encoding different isoforms of protein kinase C (alpha, delta and zeta), that are involved in meiotic maturation and signal transduction during fertilization, differed between metaphase I LT/Sv oocytes which cannot be activated by sperm, and those which are able to undergo activation after fertilization. However, no significant differences between these oocytes were found at the level of mRNA encoding IP(3) receptors which participate in calcium release during oocyte fertilization.     

Oocyte-granulosa cell heterologous gap junctions are required for the coordination of nuclear and cytoplasmic meiotic competence

Homologous gap junctions are generally recognized as a means of coordinating cellular behavior under developmental and homeostatic conditions. In the mammalian ovary, heterologous gap junctions between the oocyte and the granulosa cells have been widely implicated in the regulation of meiotic maturation late in oogenesis. However, the role of oocyte-granulosa cell gap junctions at earlier stages of oogenesis is poorly understood. Stage-specific defects in both oocyte and follicle development have been identified in juvenile mice deficient in heterologous oocyte-granulosa cell gap junctions due to targeted deletion of Gja4, the gene encoding connexin-37. Follicle development arrests at the type 4 preantral stage and although oocytes commence growth, oocyte growth ceases at a diameter of 52 microm (74.3% of control size). Analysis of cell cycle and cytoskeletal markers indicates that oocytes arrest in a G(2) state based on uniform decondensed GV chromatin, interphase microtubule arrays, and nonphosphorylated cytoplasmic centrosomes. Functional assays of meiotic competence confirm that oocytes from connexin-37-deficient mice are unable to enter M phase (initiate meiotic maturation) unless treated with the phosphatase inhibitor okadaic acid (OA). Unlike growing oocytes from heterozygous control animals, OA-treated oocytes from connexin-37-deficient mice respond acutely and progress rapidly to the circular bivalent stage of meiosis I and upon removal from OA rapidly revert to an interphase state. In contrast, OA-treated control incompetent oocytes are slow to respond, exhibit a lower proportion of chromosomal bivalent stage oocytes, but remain in and progress into meiotic M phase upon removal from OA. This study demonstrates that heterologous gap-junctional communication is required for the completion of oocyte growth and the acquisition of cytoplasmic meiotic competence.     

Meiotically incompetent and competent goat oocytes: timing of nuclear events and protein phosphorylation

The ability of mammalian oocytes to resume meiosis and to complete the first meiotic division is acquired sequentially during their growth phase. The acquisition of meiotic competence in goat oocytes has been previously correlated with follicular size (9). Since protein phosphorylation/dephosphorylation play a key role in oocyte maturation, it could be that in meiotically incompetent oocytes, such post-translational modifications are inadequate. The aim of this study was to analyze whether changes in oocyte proteins phosphorylation occurred during the acquisition of meiotic competence. For this propose, goat oocytes were divided into 4 classes according to follicular size and meiotic competence: Class A oocytes from follicles < 0.5 mm in diameter: Class B oocytes from follicles 0.5-0.8 mm; Class C oocytes from follicles 1-1.8 mm and class D oocytes from follicles > 3 mm. The protein phosphorylation patterns of these classes of oocytes were studied at different times of in vitro maturation. After 4h of culture, when all oocytes were in the germinal vesicle stage, only the oocytes from Class D displayed the phosphoproteins at 110 kD, 31 kD and around 63 kD. In contrast to Class D oocytes Classes B and C oocytes were partially competent to mature, they underwent germinal vesicle breakdown later than fully competent Class D oocytes and remained in early prometaphase I or in metaphase I, respectively. They exhibited the phosphoprotein changes that are associated with commitment to resume meiosis; but the changes occurred later than in Class D oocytes, which were fully competent to reach metaphase II. After 27 h of culture, the phosphorylation patterns of Class B, C and D oocytes were identical, whereas the meiotic stages reached were quite different. The phosphoprotein changes associated with oocyte maturation did not occur in meiotically incompetent Class A oocytes, which were blocked at the germinal vesicle stage. From these results it can be concluded that, at the GV stage, meiotically incompetent and competent goat oocytes display different patterns of protein phosphorylation. Once oocytes are able to resume meiosis they undergo specific phosphorylation changes, but whether these changes are markers or regulators of maturation events remains to be determined.     

Aspects of follicle and oocyte stage that affect in vitro maturation and development of bovine oocytes

Success of in vitro maturation (IVM) and production of bovine embryos as related to aspects of follicle source and oocyte size were evaluated. First, it was determined that bovine oocytes continue growing in all follicular sizes studied, including >1- to 15-mm follicles. Populations of oocytes were collected from surface visible (peripheral) and cortical follicles from the same ovaries. When the number of oocytes from both peripheral and cortical follicles was combined, the yield of oocytes was approximately double that collected from 1 ovarian site alone. Oocytes from cortical follicles were smaller than those from the surface population, and the smaller cortical oocytes had a lower potential for both meiotic maturation and embryo development Only cortical oocytes with the largest diameters underwent IVM and subsequently developed to blastocysts at rates comparable to oocytes from peripheral follicles. As the diameter of the oocytes recovered from peripheral follicles increased, so did their developmental potential. When the stage of the estrous cycle was observed, it was found to have no effect on developmental potential. Finally, oocytes which extruded polar bodies at an earlier time during maturation were, on average, larger than those which extruded polar bodies later. The results serve a practical purpose in assisting selection of oocytes capable of developing into blastocysts and they give useful correlates of oocyte competencies based on knowledge of follicle source and oocyte stage.     

Ion currents involved in oocyte maturation, fertilization and early developmental stages of the ascidian Ciona intestinalis

Electrophysiological techniques were used to study the role of ion currents in the ascidian Ciona intestinalis oocyte plasma membrane during different stages of growth, meiosis, fertilization and early development. Three stages of immature oocytes were discriminated in the ovary, with the germinal vesicle showing specific different features of growth and maturation. Stage-A (pre-vitellogenic) oocytes exhibited the highest L-type calcium current activity and were incompetent for meiosis resumption. Stage-B (vitellogenic) oocytes showed a progressive disappearance of calcium currents and the first appearance of sodium currents that remained high during the maturation process, up to the post-vitellogenic stage-C oocytes. The latter had acquired meiotic competence, undergoing spontaneous in vitro maturation and interacting with the spermatozoon. However, fertilized oocytes did not produce normal larvae, suggesting that cytoplasmic maturation may affect embryo development. In mature oocytes at the metaphase I stage, sodium currents were present and remained high up to the zygote stage. Oocytes fertilized in the absence of sodium showed significant reduction of the fertilization current amplitude and high development of anomalous "rosette" embryos. Current amplitudes became negligible in embryos at the 2- and 4-cell stage, whereas resumption of all the current activities occurred at the 8-cell embryo. Taken together, these results suggest: (i) an involvement of L-type calcium currents in initial oocyte meiotic progression and growth; (ii) a role of sodium currents at fertilization; (iii) a role of the fertilization current in ensuring normal embryo development.     

小鼠年龄及卵丘—卵母细胞间联接的解离对卵母细胞体外成熟的影响

实验研究了小鼠卵母细胞体外过程中卵丘－卵母细胞间的相互作用。实验小鼠为雌性Ｂ６Ｄ２杂交一代。激素处理４８小时后分离出卵后天和卵母细胞复合体,并培养在含有次黄嘌呤的培养液中。２４小时后检查卵母细胞核成熟情况。     

Mitogen-activated protein kinase activity during goat oocyte maturation and the acquisition of meiotic competence

Changes in MPF and MAPK activities during meiotic maturation of goat oocytes were investigated. Detection of MPF activity occurred concomitantly with GVBD, increased at MI, decreased during anaphase-telophase I transition, and increased thereafter in MII oocytes. The appearance of MAPK activity was delayed compared to MPF activity. MAPK activity increased after GVBD and persisted during the MI-MII transition. Whether MAPK was implicated in goat oocyte meiotic competence was also investigated by using oocytes from different follicle size categories that arrest at specific stages of the maturation process (GV, GVBD, MI, and MII). Results indicate that the ability of goat oocytes to resume meiosis is not directly related to the presence of Erk2. The ability to phosphorylate MAPK is acquired by the oocyte during follicular growth after the ability to resume meiosis. GVBD-arrested oocytes exhibited a high level of MPF activity after 27 hr of culture. However, 28% of oocytes from this group contained inactive MAPK, and 72% exhibited high MAPK activity. In addition, 29% of GVBD-arrested oocytes contained a residual interphasic network without recruitment of microtubules around the condensed chromosomes; 71% of GVBD-arrested oocytes displayed recruitment of microtubules near the condensed chromosomes and contained asters of microtubules distributed throughout the cytoplasm. These results indicate that oocytes arrested at GVBD were not exactly at the same point in the meiotic cell cycle progression, and suggest that MAPK could be implicated in the regulation of microtubule organization. The data presented here suggest that in goat oocytes, MAPK is not implicated in the early events of meiosis resumption, but rather in post-GVBD events such as spindle formation and MII arrest. © 1996 Wiley-Liss Inc.     

The effect of hypoxanthine on mouse oocyte growth and development in vitro: maintenance of meiotic arrest and gonadotropin-induced oocyte maturation

The concentration of hypoxanthine in mouse follicular fluid has been estimated to be 2-4 mM, and although this concentration maintains meiotic arrest in fully grown mouse oocytes in vitro, oocyte maturation in vivo is not induced by a decrease in the concentration of this purine in follicular fluid (J. J. Eppig, P. F. Ward-Bailey, and D. L. Coleman, Biol. Reprod. 33, 1041-1049, 1985). In the present study, the effect of 2 mM hypoxanthine on oocyte growth and development in vitro was assessed and the ability of gonadotropins to stimulate oocyte maturation in the continued presence of hypoxanthine was determined. Oocyte-granulosa cell complexes were isolated from 10- to 11-day-old mice and cultured in the presence or absence of 2 mM hypoxanthine. Oocytes from 10- to 11-day-old mice are in mid-growth phase and, without further development, are incompetent of undergoing meiotic maturation. During a 12-day culture period the granulosa cell-enclosed oocytes approximately doubled in size and, regardless of the presence or absence of hypoxanthine, 50-70% developed competence to undergo germinal vesicle breakdown (GVBD). Hypoxanthine promoted the continued association of oocytes with their companion granulosa cells during the 12-day culture period, and therefore had a beneficial effect on oocyte development. Most of the oocytes that acquired GVBD competence in the absence of hypoxanthine underwent spontaneous GVBD. In contrast, 95% of the GVBD-competent oocytes were maintained in meiotic arrest by hypoxanthine. Following withdrawal of the hypoxanthine after the 12-day culture, 75% of the GVBD-competent oocytes underwent GVBD. These results show that hypoxanthine, and/or its metabolites, maintains meiotic arrest in oocytes that grow and acquire GVBD competence in vitro. Follicle-stimulating hormone (FSH), but not luteinizing hormone or human chorionic gonadotropin, induced oocyte GVBD in the continued presence of hypoxanthine. FSH stimulated oocyte maturation at a significantly (P less than 0.01) higher frequency than coculture of the granulosa cell-denuded oocytes with granulosa cells in the continued presence of hypoxanthine. FSH did not induce the maturation of denuded oocytes cocultured with granulosa cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Meiotic competence in horse oocytes: interactions among chromatin configuration, follicle size, cumulus morphology, and season

Horse oocytes were collected from an abattoir over a 15-mo period. After classification of follicle size and cumulus morphology, oocytes were either fixed immediately (0 h) or matured in vitro (24 h). There was no effect of season on the number of antral follicles present on the ovaries, or on oocyte maturation rate for any class of oocyte. The proportion of oocytes having condensed chromatin at 0 h increased with increasing follicle size. The oocyte maturation rate also increased with follicle size, and for follicles 相似文献   

Meiotic competence of in vitro grown goat oocytes

The objective of the present study was to grow meiotically incompetent goat oocytes from early antral follicles in vitro and to render them competent to undergo germinal vesicle breakdown. Cumulus-oocyte complexes with pieces of parietal granulosa cells were isolated from follicles 0.35-0.45 mm in diameter using both mechanical and enzymatic methods. The cumulus-oocyte complexes were divided into two groups according to oocyte diameter (group A: < 95 microm; group B: > 95 microm) and cultured for 8 or 9 days on granulosa cell monolayers. Within 8 days of culture, the mean oocyte diameter increased from 86 +/- 0.4 microm to 95 +/- 0.7 microm in group Aand from 106 +/- 0.2 microm to 109 +/- 0.5 microm in group B. After 9 days of culture, the mean diameter of oocytes from groups A and B were 99 +/- 0.5 microm and 112 +/- 0.4 microm, respectively. The meiotic competence of oocytes grown in vitro was evaluated by in vitro maturation. Within 8 days of culture, only 3% of oocytes from group A and 6% of oocytes from group B acquired the ability to undergo germinal vesicle breakdown. After 9 days of culture, 7% of group A oocytes and 42% of group B oocytes were competent to resume meiosis. The expression of p34(cdc2) in oocytes grown in vitro was analysed by the western blot technique. During 9 days of culture, p34(cdc2) accumulated in both groups of growing oocytes, but its concentration was lower than in fully grown oocytes used as controls. The results showed for the first time that goat oocytes from early antral follicles can grow, accumulate p34(cdc2) and acquire the ability to resume meiosis, when cultured for 9 days on granulosa cell monolayers.     

绵羊卵泡液和次黄嘌呤对卵母细胞体外减数分裂的影响

目的　利用在培养液中添加绵羊卵泡液和次黄嘌呤 ,抑制卵母细胞GVBD发生 ,延长转录活性 ,从而使卵母细胞真正成熟 ,提高胚胎质量及生产效率。方法　利用体外成熟技术对有屠宰采集的绵羊卵母细胞进行培养 ,培养液中添加卵泡液及次黄嘌呤 ,检查成熟效果。结果　将卵母细胞培养在 5 0 %和 10 0 %的卵泡液中 ,2 4h后处于GV期的卵母细胞分别为 19% (8 4 2 )和 33 3% (13 39)。在含有 4mmol L次黄嘌呤的培养液中 ,2 4h后有2 1 6 % (16 74 )的卵母细胞处GV期 ,而对照组中只有 6 % (3 5 0 ) ,经过次黄嘌呤处理的卵母细胞多数都停滞于PⅠ期(44 6 % ,33 74 )。在 4mmol L次黄嘌呤培养液中添加FSH并未使受到抑制的卵母细胞诱导成熟。结论　卵泡液和次黄嘌呤只能在有限的程度上抑制减数分裂的重新启动 ,并对减数分裂的全过程都有影响 ,这种影响程度与抑制因子的浓度相关 ,存在明显的剂量效应。     

The ability to achieve meiotic maturation in the dog oocyte is linked to glycolysis and glutamine oxidation

We tested the hypothesis that meiotic competence of dog oocytes is tightly linked with donor follicle size and energy metabolism. Oocytes were recovered from small (<1 mm diameter, n = 327), medium (1–<2 mm, n = 292) or large (≥2 mm, n = 102) follicles, cultured for 0, 24, or 48 hr, and then assessed for glycolysis, glucose oxidation, pyruvate uptake, glutamine oxidation, and nuclear status. More oocytes (P < 0.05) from large follicles (37%) reached the metaphase‐II (MII) stage than from the small group (11%), with the medium‐sized class being intermediate (18%; P > 0.05). Glycolytic rate increased (P < 0.05) as oocytes progressed from the germinal vesicle (GV) to MII stage. After 48 hr of culture, oocytes completing nuclear maturation had higher (P < 0.05) glycolytic rates than those arrested at earlier stages. GV oocytes recovered from large follicle oocytes had higher (P < 0.05) metabolism than those from smaller counterparts at culture onset. MII oocytes from large follicles oxidized more (P < 0.05) glutamine than the same stage gametes recovered from smaller counterparts. In summary, larger‐sized dog follicles contain a more metabolically active oocyte with a greater chance of achieving nuclear maturation in vitro. These findings demonstrate a significant role for energy metabolism in promoting dog oocyte maturation, information that will be useful for improving culture systems for rescuing intraovarian genetic material. Mol. Reprod. Dev. 79: 186–196, 2012. Published 2011. This article is a U.S. Government work and is in the public domain in the USA.     

Trichlorfon-induced polyploidy and nondisjunction in mouse oocytes from preantral follicle culture

Trichlorfon (TCF), an organophosphate insecticide and potent inhibitor of choline esterases, was previously shown to induce first meiotic nondisjunction and spindle aberrations in isolated, follicle cell-denuded mouse oocytes maturing in vitro. To explore dose-response and direct and indirect, potentially synergistic effects of TCF on the somatic cells and the oocyte within a follicle, we presently employed preantral follicle culture. 100 microg/ml TCF added at the time of hormonally stimulated resumption of meiosis of follicle cell-enclosed mouse oocytes, 16 h before in vitro ovulation, induced significant rises in first meiotic nondisjunction in oocytes from preantral follicle culture. Lower concentrations (6 microg/ml TCF) disturbed polar body formation. Nuclear maturation to meiosis II in absence of cytokinesis resulted in significant increases in polyploidy. Oocytes maturing in follicles in the presence of TCF had aberrant second meiotic spindles. Influences of TCF on somatic cell function were evident by reduced follicular mucification in vitro and deceased progesterone production. In contrast to TCF, acetylcholine (0.1-100 microM) increased progesterone production. The observations therefore suggest that TCF influences oocyte maturation and folliculogenesis directly and indirectly. High TCF is aneugenic at first meiotic division in oocytes, irrespective of the presence or absence of follicle cells. At lower concentrations TCF interferes with spindle formation, chromosome congression at meiosis II, and coordination of nuclear and cytoplasmic maturation, posing risks for second meiotic errors. The observations suggest that chronic TCF exposure during maturation in the follicle may predispose oocytes to the formation of chromosomally unbalanced preimplantation embryos after fertilization.     

Developmental control of oocyte maturation and egg activation in metazoan models

Production of functional eggs requires meiosis to be coordinated with developmental signals. Oocytes arrest in prophase I to permit oocyte differentiation, and in most animals, a second meiotic arrest links completion of meiosis to fertilization. Comparison of oocyte maturation and egg activation between mammals, Caenorhabditis elegans, and Drosophila reveal conserved signaling pathways and regulatory mechanisms as well as unique adaptations for reproductive strategies. Recent studies in mammals and C. elegans show the role of signaling between surrounding somatic cells and the oocyte in maintaining the prophase I arrest and controlling maturation. Proteins that regulate levels of active Cdk1/cyclin B during prophase I arrest have been identified in Drosophila. Protein kinases play crucial roles in the transition from meiosis in the oocyte to mitotic embryonic divisions in C. elegans and Drosophila. Here we will contrast the regulation of key meiotic events in oocytes.