Factors affecting the developmental capacity of mouse oocytes undergoing maturation in vitro

Acquisition of oocyte competence to develop as an embryo: integrated nuclear and cytoplasmic events

Infertility affects ~7% of couples of reproductive age with little change in incidence in the last two decades. ART, as well as other interventions, have made major strides in correcting this condition. However, and in spite of advancements in the field, the age of the female partner remains a main factor for a successful outcome. A better understanding of the final stages of gamete maturation yielding an egg that can sustain embryo development and a pregnancy to term remains a major area for improvement in the field. This review will summarize the major cellular and molecular events unfolding at the oocyte-to-embryo transition. We will provide an update on the most important processes/pathways currently understood as the basis of developmental competence, including the molecular processes involved in mRNA storage, its recruitment to the translational machinery, and its degradation. We will discuss the hypothesis that the translational programme of maternal mRNAs plays a key role in establishing developmental competence. These regulations are essential to assemble the machinery that is used to establish a totipotent zygote. This hypothesis further supports the view that embryogenesis begins during oogenesis. A better understanding of the events required for developmental competence will guide the development of novel strategies to monitor and improve the success rate of IVF. Using this information, it will be possible to develop new biomarkers that may be used to better predict oocyte quality and in selection of the best egg for IVF.

FSH Regulates mRNA Translation in Mouse Oocytes and Promotes Developmental Competence

A major challenge in assisted reproductive technology is to develop conditions for in vitro oocyte maturation yielding high-quality eggs. Efforts are underway to assess whether known hormonal and local factors play a role in oocyte developmental competence and to identify the molecular mechanism involved. Here we have tested the hypothesis that FSH improves oocyte developmental competence by regulating the translational program in the oocyte. Accumulation of oocyte proteins (targeting protein for the Xenopus kinesin xklp2 and IL-7) associated with improved oocyte quality is increased when cumulus-oocyte complexes are incubated with FSH. This increase is due to enhanced translation of the corresponding mRNAs, as indicated by microinjection of constructs in which the 3' untranslated region of the Tpx2 or Il7 transcripts is fused to the luciferase reporter. A transient activation of the phosphatidyl-inositol 3-phosphate/AKT cascade in the oocyte preceded the increase in translation. When the epidermal growth factor (EGF) receptor is down-regulated in follicular cells, the FSH-induced rate of maternal mRNA translation and AKT activation were lost, demonstrating that the effects of FSH are indirect and require EGF receptor signaling in the somatic compartment. Using Pten(fl/fl):Zp3cre oocytes in which the AKT is constitutively activated, translation of reporters was increased and was no longer sensitive to FSH stimulation. More importantly, the oocytes lacking the phosphate and tensin homolog gene showed increased developmental competence, even when cultured in the absence of FSH or growth factors. Thus, we demonstrate that FSH intersects with the follicular EGF network to activate the phosphatidyl-inositol 3-phosphate/AKT cascade in the oocyte to control translation and developmental competence. These findings provide a molecular rationale for the use of FSH to improve egg quality.

Effects of recipient oocyte age and interval from fusion to activation on development of buffalo (Bubalus bubalis) nuclear transfer embryos derived from fetal fibroblasts

The objective was to investigate the effect of recipient oocyte age and the interval from activation to fusion on developmental competence of buffalo nuclear transfer (NT) embryos. Buffalo oocytes matured in vitro for 22 h were enucleated by micromanipulation under the spindle view system, and a fetal fibroblast (pretreated with 0.1 μg/mL aphidicolin for 24 h, followed by culture for 48 h in 0.5% fetal bovine serum) was introduced into the enucleated oocyte, followed by electrofusion. Both oocytes and NT embryos were activated by exposure to 5 μM ionomycin for 5 min, followed by culture in 2 mM 6-dimethyl-aminopurine for 3 h. When oocytes matured in vitro for 28, 29, 30, 31, or 32 h were activated, more oocytes matured in vitro for 30 h developed into blastocysts in comparison with oocytes matured in vitro for 32 h (31.3 vs 19.9%, P < 0.05). When electrofusion was induced 27 h after the onset of oocyte maturation, the cleavage rate (78.0%) was higher than that of electrofusion induced at 28 h (67.2%, P < 0.05), and the blastocyst yield (18.1%) was higher (P < 0.05) than that of electrofusion induced at 25 or 26 h (7.4 and 8.5%, respectively). A higher proportion of NT embryos activated at 3 h after electrofusion developed to the blastocyst stage (18.6%) in comparison with NT embryos activated at 1 h (6.0%), 2 h (8.3%), or 4 h (10.6%) after fusion (P < 0.05). No recipient was pregnant 60 d after transfer of blastocysts developed from NT embryos activated at 1 h (0/8), 2 h (0/10), or 4 h (0/9) after fusion. However, 3 of 16 recipients were pregnant following transfer of blastocysts developed from the NT embryos activated at 3 h after fusion, and two of these recipients maintained pregnancy to term. We concluded that the developmental potential of buffalo NT embryos was related to recipient oocyte age and the interval from fusion to activation.

Effect of incubation with different concentrations and durations of FSH for in-vitro maturation of murine oocytes

The purpose of the study was to evaluate whether a lower concentration of FSH or 2-h incubation with FSH would improve the outcome of in-vitro maturation of oocytes. The immature oocytes were obtained from FVB mice, and were allocated to four groups and incubated in the maturation media for 24 h. The maturation media were supplemented with 10 mIU/ml FSH for 24 h (group 1), 10 mIU/ml FSH for 2 h (group 2), 75 mIU/ml FSH for 24 h (group 3) or 75 mIU/ml FSH for 2 h (group 4). In each group, half of the in-vitro-matured oocytes were fertilized and cultured to blastocysts and the remaining matured oocytes were analysed for growth differentiation factor (GDF)-9 and bone morphogenetic protein (BMP)-15 mRNA to assess the oocyte quality. The maturation rates and oocyte BMP-15 mRNA concentrations were similar among the four groups. The GDF-9 mRNA concentrations were similar in group 2 and group 4. The fertilization and blastocyst rates were higher in groups 2 and 4 than in groups 1 and 3. It is concluded that 2-h incubation with FSH is better than 24-h incubation in terms of the fertilization rate and blastocyst development.

Metabolic markers of developmental competence for in vitro-matured mouse oocytes

In vitromaturation of oocytes has enormous potential in assisted reproductive technology, but its use has been limited due to insufficient knowledge of oocyte physiology during this dynamic period and lack of an adequate maturation system. The aim of this study was to characterize the metabolic profiles of three groups of oocytes throughout maturation: cumulus–oocyte complexes (COCs), denuded oocytes, and denuded oocytes co-cultured with cumulus cells. Mouse oocytes were collected from 28-day-old unstimulated females and matured in a defined medium. Oocytes were matured individually and transferred into fresh 0.5 μl drops of medium at 4 h intervals until 16 h. Ultramicrofluorimetry was used to quantitate carbohydrate consumption from and metabolite release into the medium. Glucose consumption and lactate production of COCs increased (P< 0.001) over the maturation interval (0–16 h). Glucose consumption by COCs that subsequently fertilized was higher between 8–12 h of maturation than by COCs that did not fertilize (38 versus 29 pmol/COC per h, respectively;P< 0.01). Lactate production by COCs that subsequently fertilized was higher between 8–16 h of maturation, than by oocytes that did not fertilize (8–12 h, 66 versus 46 pmol/COC per h,P< 0.01; 12–16 h, 56 versus 40 pmol/COC per h, respectively;P< 0.05). These data indicate that the final hours of maturation may hold a unique marker of oocyte competence, as during this time fertilizable COCs take up more glucose and produce more lactate than those not subsequently fertilized.

Cell polarity during folliculogenesis and oogenesis

Polarity is an important aspect of oogenesis and early development for many animal groups, but only recently it has become relevant to the study of mammals. Mammalian oocyte development occurs through tight coordination and interaction between all ovarian structures. In fact, bi-directional communication between the oocyte and its companion granulosa cells (GC) in the ovarian follicle seems essential for GC proliferation, differentiation, and production of a functional female gamete. The transzonal projections (TZP), which are specialized extensions from granulosa cells that terminate on the oolema after crossing the zona pellucida, are major structural components necessary for oocyte-GC interaction. Granulosa cell polarity seems to be a necessary requisite for appropriate function of TZP, and the role of FSH as modulator of a polarized phenotype on GC is discussed. This article also discusses oocyte polarity with special reference to the partial loss of polarity that occurs during in-vitro oocyte maturation and possible implications in the modulation of oocyte competencies. Cytoskeletal markers that may account for oocyte quality were defined and found to be distinct in in-vivo and in-vitro matured oocytes. Implications of partial loss of oocyte polarity during in-vitro maturation, reflected by distinct distribution of these markers, are further discussed. It is also proposed that expression of both somatic and germ cell polarity in the ovarian follicle will ultimately determine acquisition of meiotic, fertilization and developmental competences by the oocyte.

Distinctions in meiotic spindle structure and assembly during in vitro and in vivo maturation of mouse oocytes

To better understand the differences in cytoskeletal organization between in vivo (IVO) and in vitro (IVM) matured oocytes, we analyzed remodeling of the centrosome-microtubule complex in IVO and IVM mouse oocytes. Fluorescence imaging revealed dramatic differences in meiotic spindle assembly and organization between these two populations. Metaphase spindles at both meiosis I (M-I) and meiosis II (M-II) in IVO oocytes were compact, displayed focused spindle poles with distinct gamma-tubulin foci, and were composed of acetylated microtubules. In contrast, IVM oocytes exhibited barrel-shaped spindles with fewer acetylated microtubules and gamma-tubulin diffusely distributed throughout the spindle proper. With respect to meiotic progression, IVO oocytes were more synchronous in the rate and extent of anaphase to telophase of M-I and first polar body emission than were IVM counterparts. Furthermore, IVO oocytes showed a twofold increase in cytoplasmic microtubule organizing centers (MTOCs), and constitutive MTOC proteins (gamma-tubulin and pericentrin) were excluded from the first polar body. Inclusion of MTOC constitutive proteins in the polar body and diminished number of cytoplasmic MTOCs was observed in IVM oocytes. These findings were corroborated in IVO oocytes obtained from naturally ovulated and spontaneously cycling mice and highlight a fundamental distinction in the spatial and temporal regulation of microtubule dynamics between IVO and IVM oocytes

Maturational and developmental competence of cumulus-free immature human oocytes derived from stimulated and intracytoplasmic sperm injection cycles

The present experiments compared the maturational and developmental competence of immature oocytes derived from stimulated cycles, following culture in a newly designed in-vitro maturation medium (IVM-medium) or in standard tissue culture medium (TCM-199; control). The results indicated that maturation and fertilization rates were comparable when the cumulus-free M-I stage oocytes were matured in the IVM-medium (78.6%) or the control medium (70.8%). However, there was a significant difference in blastocyst development (P < 0.05) when M-I oocytes were matured in these two media (19.6 versus 7.7%). Both maturation and early embryonic development rates of GV-stage oocytes were significantly higher (P < 0.01) in the IVM-medium (maturation: 75.7%; blastocyst: 12.9%) compared with control (maturation 55.7%; blastocyst: 0.0%). Moreover, embryos developed to the blastocyst stage at a higher rate in both media if GV-stage oocytes had matured within 24 h compared with 48 h of culture. These results demonstrate that immature human oocytes derived from stimulated ovaries can achieve maturation and early embryonic development in vitro, especially in the new IVM-medium, which may allow additional embryos to be produced for clinical use at embryo transfer.

Effects of maturational age of porcine oocytes on the induction of activation and development in vitro following somatic cell nuclear transfer

To determine the effect of maturational age of porcine oocytes on the induction of activation and development following nuclear transfer, we investigated maturation rate and efficacy of oocyte activation treatment with an electrical stimulus (ES) and cycloheximide (CHX) at various timing of maturation culture. Most oocytes developed to the metaphase II (MII) stage after 32 hr of maturation culture. Both in newly matured (32 and 36 hr of age) and MII-arrested (42 and 48 hr of age) oocytes, ES followed by exposure to CHX for 6 hr caused higher rates of pronuclear formation than ES alone. Effect of maturational age of oocytes on the development of parthenotes activated with ES and CHX was then examined. The highest percentage of parthenotes developed to blastocysts was obtained when ES was added at 42 hr of culture. Finally, newly matured (33 hr of age) and MII-arrested oocytes (44 hr of age) were enucleated, fused with serum-starved pig fetal fibroblasts and activated with ES and CHX around suitable timing (39 to 40 hr of age) or later (50 to 51 hr of age), respectively. The frequencies of cleavage (58.8%) and blastocyst formation (13.6%) of newly matured oocytes were significantly (P<0.05) higher than those of MII-arrested oocytes (38.9 and 1.8%, respectively). These results demonstrated that the development of porcine nuclear transfer embryos can be improved by using complete matured oocytes as cytoplasts and activation treatment with ES followed by CHX treatment.

Follicle stimulating hormone effects on immature human oocytes: in vitro maturation and hormone production

PURPOSE

Our purpose was (1) to determine if in vitro maturation of unstimulated oocytes could be improved with the addition of urofollitropin; (2) to evaluate the output of estradiol, testosterone, progesterone, and androstenedione by the cultured oocyte-cumulus complex; and (3) to ascertain if steroid hormone production of the oocyte-cumulus complex correlates with final oocyte maturation stage.

METHODS

Fifty-eight immature oocytes were obtained from 11 regularly cycling women undergoing oophorectomy. The oocyte-cumulus complexes were randomly assigned to control medium (Ham's F-10 supplemented with 7.5% fetal bovine serum) or test medium (control medium supplemented with 75 mIU/ml of urofollitropin).

RESULTS

(1) The addition of urofollitropin to oocyte culture medium does not significantly increase the ability of the oocyte to achieve the metaphase II stage; (2) the addition of urofollitropin significantly increases the production of progesterone, testosterone, and androstenedione by the oocyte-cumulus complex; and (3) there is no difference in the production of estradiol, progesterone, testosterone, and androstenedione by the oocyte-cumulus complex at the germinal vesicle, metaphase I or metaphase II stage of oocyte maturation.

CONCLUSIONS

This information is of importance in the use of oophorectomy specimens for patients who must undergo an oophorectomy but desire to attempt pregnancy using their oocytes, in the use of oophorectomy specimens for donor oocytes, or for patients undergoing in vitro fertilization using immature oocyte collection.

Calcium and meiotic maturation of the mammalian oocyte

The role of calcium in the regulation of both the meiotic and mitotic cell cycles has been the subject of considerable investigation in the nonmammalian field. In contrast, the mechanisms for signalling meiotic maturation in the mammalian oocyte are not as well documented nor as clearly defined. In the mammalian oocyte, calcium is associated with both spontaneous and hormone-induced meiotic maturation. A transient release of endogenously stored calcium precedes germinal vesicle breakdown and can override cyclic AMP maintained meiotic arrest; it thus may signal the resumption of meiosis. Additionally, extracellular calcium is apparently required for meiotic progression past metaphase I. The time sequence for meiotic resumption and progression is very varied between species. The timing of cell cycle protein synthesis during meiosis suggests that cyclins may be expressed in oocytes of some species much earlier in their development than in others. A generic model is proposed for the mechanism for triggering meiotic resumption in the mammalian oocyte. In this model, the critical components of meiotic resumption involve the temporal relationship of cyclin synthesis and the subsequent activation of the MPF complex by the calcium signal generated, which accounts for differences among species.

Induction of meiotic maturation in vivo in the mouse by IMP dehydrogenase inhibitors: effects on the developmental capacity of ova

The present study was conducted to examine the effects of priming duration and inosine monophosphate (IMP) dehydrogenase inhibitors on the fertilization and pre- and postimplantation development of ova from immature mice. Mice were primed with equine chorionic gonadotropin (eCG) and 1 d later were treated with mycophenolic acid (MA) or mizoribine (Mz; also known as bredinin) or an equal volume of the vehicle. Two days after priming, mice received human chorionic gonadotropin (hCG) and were mated with fertile males. Some mice received hormone but were not mated. Ova were isolated from the oviducts 23-24 hr post-hCG and cultured in Whitten's medium. Some mice were not killed until 7 or 19 d postvaginal plug to determine the extent of implantation or development to term, respectively. In superovulated control mice, 94% of ova developed to two-cells and 82% of these progressed to the blastocyst stage. Nineteen of 23 mice (83%) had implantations (24/mouse) and 20 of 24 mice (83%) had term embryos (11/mouse). Induction of meiotic maturation with Mz or MA after 1 d of priming, followed 1 d later by hCG injection and mating, resulted in a significant loss of preimplantation developmental capacity (20-22% two-cells; 29-50% blastocysts). These numbers were similar whether or not the mice were mated, indicating that the development was parthenogenetic. In addition, Mz treatment reduced the number of mice with implantation sites (13 of 19, 68%), the number of implantations per mouse (10), the viability of the implantations, and development to term (four of 24, mice, 17%; one embryo/mouse). Thus, most of the implanted embryos were resorbed after implantation. This effect could be attributed, in part, to each of the following: (1) parthenogenetic activation; (2) aging of the metaphase II oocyte before insemination; and (3) resumption of meiotic maturation before the oocyte had reached its full developmental potential. Coincident gonadotropin injection reversed the loss of development brought about by IMP dehydrogenase inhibitors. These data show that induction of premature meiotic maturation by IMP dehydrogenase inhibitors results in a significant loss of developmental capacity and indicates that active purine metabolic pathways in situ maintain meiotic arrest and prevent premature meiotic maturation that would result in compromised development.

Slow and ultrarapid freezing of fully grown germinal vesicle-stage mouse oocytes: optimization of survival rate outweighed by defective blastocyst formation

PURPOSE

The cryopreservation of mature metaphase II-stage mouse oocytes is associated with decreased fertilizability, spindle damage, and increased polyploidy. Therefore, we investigated the outcome of cryopreservation of immature germinal vesicle-stage mouse oocytes.

METHODS

Oocytes were punctured from Graafian follicles in primed F1 hybrid mice and were then released into maturation medium containing the meiotic inhibitor dibutyryl cyclic AMP. Both slow and ultrarapid freezing protocols with dimethyl sulfoxide, 1,2-proponediol, or a mixture of both agents were tested. We recorded morphological survival rates, in vitro maturation rates, and two-cell and blastocyst formation rates. Each group of frozen oocytes was compared with both unfrozen germinal vesicle-stage oocytes and metaphase II-stage oocytes.

RESULTS

An optimal cryosurvival rate of 78% was reached after ultrarapid freezing with 3 M dimethyl sulfoxide followed by one-step dilution, but a decreased rate of two-cell formation was observed. Freezing with a combination of dimethyl sulfoxide and 1,2-propanediol did not improve this fertilization-decreasing effect. Very low cryosurvival rates after freezing with 1,2-propanediol indicated its inappropriateness for ultrarapid freezing of immature oocytes. The rates of in vitro maturation were equivalent for frozen-thawed and freshly collected germinal vesicle-stage oocytes, independent of the freezing protocol used. We report, nevertheless, as a general characteristic for both slow and ultrarapid freezing of fully grown germinal vesicle-stage oocytes, that the in vitro development up to the blastocyst stage is inhibited despite full nuclear maturation.

CONCLUSION

We report that cryopreservation of immature germinal vesicle-stage oocytes is invariably associated with a low developmental capacity after fertilization. The rate of in vitro nuclear maturation did not equate with developmental competence. This in turn suggests the importance of cytoplasmic maturation for embryonic development.

In vitro maturation of oocytes from a marsupial, the tammar wallaby (Macropus eugenii)

This study examined the competence of oocytes from the tammar wallaby, Macropus eugenii to mature in vitro. Oocytes were collected from follicles > 1 mm diameter 24 h after pregnant mare serum gonadotrophin (PMSG) treatment and incubated in Eagle's minimum essential medium supplemented with 10% fetal calf serum, at 35 degrees C in 5% CO2 in air for 24, 36, or 48 h. Oocytes were incubated either granulosa cell-intact or granulosa cell-free or in the presence of 10 IU ml-1 PMSG or 10 micrograms ml-1 porcine luteinizing hormone (LH) + 10 micrograms ml-1 porcine follicle stimulating hormone (FSH). The ability of oocytes recovered from small (< 1.5-mm-diameter) and large (> or = 1.5 mm-diameter) follicles to mature in vitro was also examined. The nuclear status of oocytes was assessed using the DNA-specific dye Hoechst 33342. Initially, all oocytes examined contained a germinal vesicle. After 24 h of culture, 60% of oocytes had progressed to metaphase I or anaphase I. After 36 h, approximately 20% of oocytes possessed metaphase II chromosomes, and 20% of oocytes were at metaphase I or anaphase I. At the completion of the 48 h culture period, 40% of oocytes had completed maturation to the metaphase II stage. In vitro oocyte maturation after 48 h was not affected by the presence of granulosa cells, PMSG, or LH and FSH. More oocytes from large follicles (55%) completed maturation by 48 h than from small follicles (20%). Approximately 50% of oocytes remained at the GV stage at all times under all conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of epidermal growth factor (EGF) and defined simple media on in vitro bovine oocyte maturation and early embryonic development

The purpose of this study is to evaluate the effect of EGF and defined simple media on in vitro bovine oocyte maturation and early embryonic development. Bovine follicular oocytes were matured in vitro and co-cultured with frozen-thawed bull sperm, which was capacitated with Hepes buffered saline (HBS) solution. After incubation of oocyte-sperm complexes for 4 days, the cleavage rate was evaluated. The results obtained were as follows: 1) When bovine oocytes were matured and embryos were developed in Park-Lin medium 1 (PL(1)) containing fetal calf serum (FCS) or EGF + bovine serum albumin (BSA), the latter treatment was more effective in inducing embryonic cleavage (18%) than FCS alone (10%). 2) When bovine oocytes were matured in Park-Lin medium 2 (PL(2)) without EGF and the subsequent embryos were developed in PL(2) medium with EGF, the cleavage rate was 22.6%. 3) When bovine oocytes were matured in PL(2) medium with EGF and then the embryos were developed in PL(2) medium with EGF, the cleavage rate was 35.8%. 4) When bovine oocytes were matured in Park-Lin medium 3 (PL(3)) without EGF and then the embryos were developed in PL(3) medium, the cleavage rate was 50%. 5) When bovine oocytes and resulting embryos were matured in PL(3) medium with EGF, the cleavage rate was 53%. 6) The parthenogenesis rate induced by PL(3) medium in our current study was comparable to the findings reported by other laboratories. These results suggest that EGF stimulates in vitro bovine oocyte maturation and subsequently affects embryonic development. It is suggested that PL(3) medium is a better defined simple medium than the other media currently used by other laboratories for in vitro bovine oocyte maturation.

Meiosis, egg activation, and nuclear envelope breakdown are differentially reliant on Ca2+, whereas germinal vesicle breakdown is Ca2+ independent in the mouse oocyte

During early development, intracellular Ca2+ mobilization is not only essential for fertilization, but has also been implicated during other meiotic and mitotic events, such as germinal vesicle breakdown (GVBD) and nuclear envelope breakdown (NEBD). In this study, the roles of intracellular and extracellular Ca2+ were examined during meiotic maturation and reinitiation at parthenogenetic activation and during first mitosis in a single species using the same methodologies. Cumulus-free metaphase II mouse oocytes immediately resumed anaphase upon the induction of a large, transient Ca2+ elevation. This resumption of meiosis and associated events, such as cortical granule discharge, were not sensitive to extracellular Ca2+ removal, but were blocked by intracellular Ca2+ chelators. In contrast, meiosis I was dependent on external Ca2+; in its absence, the formation and function of the first meiotic spindle was delayed, the first polar body did not form and an interphase-like state was induced. GVBD was not dependent on external Ca2+ and showed no associated Ca2+ changes. NEBD at first mitosis in fertilized eggs, on the other hand, was frequently, but not always associated with a brief Ca2+ transient and was dependent on Ca2+ mobilization. We conclude that GVBD is Ca2+ independent, but that the dependence of NEBD on Ca2+ suggests regulation by more than one pathway. As cells develop from Ca(2+)-independent germinal vesicle oocytes to internal Ca(2+)-dependent pronuclear eggs, internal Ca2+ pools increase by approximately fourfold.

The effect of sera, bovine serum albumin and follicular cells on in vitro maturation and fertilization of porcine oocytes

The effects of fetal calf serum (FCS), estrus gilt serum (EGS) BSA, dispersed granulosa cells, hemi-sections of follicular wall, and replacement of medium after 24 hours on in vitro maturation and fertilization of porcine oocytes were studied. The results indicate that the use of BSA for 24 or 48 hours inhibited the expansion of cumulus cells and the maturation of oocytes. An incubation of 24 hours culture in FCS followed by a second 24 hours in BSA containing medium did not decrease the rate of maturation but significantly decreased the polyspermy and mean number of spermatozoa penetrated/oocyte. Renewing the medium with or without removal of cumulus cells during the second incubation increased the maturation rate. Removal of cumulus cells decreased the penetrability, the polyspermy rates of the oocyte and the mean number of spermatozoa/oocyte penetrated. The EGS-supplemented medium, dispersed granulosa cells or hemi-sections of follicular wall did not affect the maturation or fertilization rates. In conclusion, BSA, a protein supplement in maturation medium, inhibited cumulus cell expansion and maturation of porcine oocytes. After resumption of meiosis triggered by FCS, BSA did not influence maturation. The FCS-BSA treatment reduced the incidence of polyspermy and the mean number of spermatozoa penetrated/oocyte without decreasing the rate of maturation and fertilization.

Measurements of the membrane water permeability (Lp) and its temperature dependence (activation energy) in human fresh and failed-to-fertilize oocytes and mouse oocyte

The volumetric response of oocytes during rapid alterations of the extracellular osmotic environment were recorded using video microscopy. From these observations, the kinetics of water loss for human and mouse oocytes were determined over the temperature range 37 to 10 degrees C, including 37, 30, 20, and 10 degrees C. The changes in diameter of oocytes were measured over a 5-min period and a computer model was used to derive values for membrane water permeability (Lp) and inactive volume (Vb) and to compare the experimental data to the predicted values. The results for the mouse oocyte Lp were comparable to values determined by other methods. However the human data, for both failed-to-fertilize and fresh oocytes, have a wide range of values with large standard deviations. The Lp values at the various temperatures were used to calculate the Arrhenius activation energy (Ea). An Ea value of 9.48 kcal/mol was found for the fresh mouse oocyte, whereas the activation energy for human oocytes was extremely low, 3.73 kcal/mol for fresh oocytes and 1.93 kcal/mol for failed-to-fertilize oocytes.

Hormonal actions during oocyte maturation influence fertilization and early embryonic development

On the basis of evidence just reviewed, the hormonal requirements during IVM of mammalian oocytes for fertilization and developmental competence may be summarized as follows: 1. Nuclear maturation occurs "spontaneously," without follicular factors or hormones, in all mammalian species. 2. Cumulus cells are necessary during nuclear maturation in culture for fertilization and developmental competence. Their effect is enhanced by the addition of serum or follicular fluid to the maturation medium. 3. FSH retards nuclear maturation, perhaps thereby enhancing subsequent developmental competence by allowing more time for cytoplasmic maturation to occur before fertilization. The effects of FSH are mediated, in part, by soluble factors from cumulus or granulosa cells. 4. FSH in vivo before culture or added in vitro to IVM culture media enhances fertilization and stimulates steroidogenesis (progesterone and estradiol) in cumulus cells. Blockade of steroidogenesis prevents the enhancing effect of FSH on fertilization. Progesterone can substitute for FSH in enhancing fertilization of rat oocytes. 5. Other hormones enhance beneficial effects of FSH, with some apparent species differences: a. Rat: Progesterone may mediate the FSH effect. b. Cow, rabbit: Estradiol enhances the beneficial effect of FSH. c. Rabbit: Prolactin further enhances the effects of estradiol and FSH. d. Human: No evidence is available on the possible synergism between steroids and gonadotropins in enhancing in vitro maturation. It is clear that further research is required to elucidate the mechanisms of action of all the hormones that have been observed to influence oocyte maturation, and to establish the physiologic significance of most of these, particularly in species other than laboratory rodents. Major benefits to animal production and human health are likely to accrue from such research.

Direct positive effect of epidermal growth factor on the cytoplasmic maturation of mouse and human oocytes

OBJECTIVE

Immature mammalian oocytes cultured in vitro undergo inadequate cytoplasmic maturation and hence have a limited potential for fertilization. Our primary objective was to determine if the addition of epidermal growth factor (EGF) to the in vitro culture system would have a positive effect on oocyte cytoplasmic maturation.

DESIGN

We studied the effect of different EGF concentrations on both denuded and cumulus-enclosed mouse oocytes cultured in vitro.

MAIN OUTCOME MEASURES

The percentage of oocytes undergoing germinal vesicle breakdown (GVBD) and polar body one formation over time as a function of EGF concentration was determined.

RESULTS

A dose-related positive effect of EGF on both GVBD and polar body one formation over time was observed for mouse oocytes. As well, a similar effect of EGF was seen on immature human oocytes that had not been stimulated with exogenous gonadotropins.

CONCLUSIONS

The use of EGF may allow for the performance of successful in vitro fertilization procedures using immature human oocytes retrieved during unstimulated cycles.

Culture media for mouse oocyte maturation affect subsequent embryonic development

These experiments were done to determine whether the culture medium used for the spontaneous maturation of mouse oocytes can affect the subsequent capacity of the ova to become fertilized and complete preimplantation development in vitro and development to live young. Oocytes obtained from antral follicles of gonadotropin-primed immature mice underwent spontaneous maturation in control medium, i.e. Eagle's Minimum Essential Medium (MEM) supplemented with 5% fetal bovine serum, or in one of eight different media which were also supplemented with serum. All of the ova were fertilized in Whitten's medium and were assessed for cleavage to the 2-cell stage and for further preimplantation development to blastocysts during culture in Whitten's medium. Three of the eight media used for oocyte maturation improved the capacity of the ova to develop to the blastocyst stage when compared with the control: Waymouth MB 752/1, MEM with non-essential amino acids, and MEM Alpha; Waymouth medium promoted the highest frequency of development of ova to the blastocyst stage. Moreover, the blastocysts derived from oocytes that matured in Waymouth medium contained more cells than blastocysts derived from oocytes that matured in control medium. Although BGJb medium promoted the cleavage of eggs to the 2-cell stage when present during oocyte maturation, it had a detrimental effect on their subsequent preimplantation developmental capacity. Following transfer to foster mothers, more 2-cell stage embryos developed to live young after oocyte maturation in Waymouth medium (21%) than in control medium (13%).(ABSTRACT TRUNCATED AT 250 WORDS)