Rhesus monkey oocyte maturation and fertilization in vitro: roles of the menstrual cycle phase and of exogenous gonadotropins

Amphiregulin promotes the maturation of oocytes isolated from the small antral follicles of the rhesus macaque

BACKGROUND

In non-primates, the epidermal growth factor (EGF) and EGF-related ligands such as amphiregulin (AREG) serve as critical intermediates between the theca/mural cells and the cumulus-oocyte-complex (COC) following the mid-cycle LH surge. Studies were designed in primates (1) to analyze AREG levels in follicular fluid (follicular fluid) obtained from pre-ovulatory follicles, as well as (2) to assess dose-dependent effects of AREG on oocytes from small antral follicles (SAFs) during culture, including meiotic and cytoplasmic maturation.

METHODS

Controlled ovulation protocols were performed on rhesus monkeys (n=12) to determine AREG content within the single, naturally selected dominant follicle after an ovulatory stimulus. Using healthy COCs (n=271) obtained from SAFs during spontaneous cycles (n=27), in vitro maturation (IVM) was performed in the absence or presence of physiological concentrations of AREG (10 or 100 ng/ml) with or without gonadotrophins (FSH, 75 mIU/ml; LH, 75 mIU/ml). At the end of the culture period, oocyte meiotic maturation was evaluated and ICSI was performed (n=111), from which fertilization and early embryo development was followed in vitro.

RESULTS

AREG levels in follicular fluid from pre-ovulatory follicles increased (P<0.05) following an ovulatory bolus of hCG at 12, 24 and 36 h post-treatment. At 12 h post-hCG, AREG levels in follicular fluid ranged from 4.8 to 121.4 ng/ml. Rhesus macaque COCs incubated with 10 ng/ml AREG in the presence of gonadotrophins displayed an increased percentage of oocytes that progressed to the metaphase II (MII) stage of meiosis (82 versus 56%, P<0.05) and a decreased percentage of metaphase I (MI) oocytes (2 versus 23%, P<0.05) relative to controls, respectively. The percentage of either MI or MII oocytes at the end of the culture period was not different between oocytes cultured with 100 ng/ml AREG or in media alone. Fertilization and first cleavage rates obtained by ICSI of all IVM MII oocytes were 93 and 98%, respectively, and did not vary among treatment groups. Of the MII oocytes that fertilized (n=103), 37 were randomly selected and maintained in culture to assess developmental potential. A total of 13 early blastocysts were obtained, with four embryos developing to expanded blastocysts.

CONCLUSIONS

These data indicate that AREG levels increase in rhesus macaque pre-ovulatory follicles after an ovulatory stimulus, and a specific concentration of AREG (10 ng/ml) enhances rhesus macaque oocyte nuclear maturation but not cytoplasmic maturation from SAFs obtained during the natural menstrual cycle. However, owing to the small number of samples in some treatment groups, further studies are now required.

Cumulus-oocyte complexes from small antral follicles during the early follicular phase of menstrual cycles in rhesus monkeys yield oocytes that reinitiate meiosis and fertilize in vitro

The stage at which follicle-enclosed cumulus-oocyte complexes achieve developmental competence in primates is unknown. Therefore, studies were designed to characterize the ability of oocytes in small antral follicles present during the menstrual cycle to spontaneously resume meiosis, fertilize, and support early embryo development. Ovaries were removed from adult rhesus monkeys (n = 12) during the early follicular phase (Days 3-4) of spontaneous cycles. Small antral follicles were divided into five groups according to their diameter; group I: <0.5 mm; group II: 0.5-0.99 mm; group III: 1.0-1.49 mm; group IV: 1.5-1.99 mm; and group V: 2.0-2.5 mm. The cumulus-oocyte complex from healthy small antral follicles (devoid of dark oocytes or granulosa cells) were extracted (n = 199) and cultured for 48 h under different conditions: in TALP (tyrode, albumin, lactate, pyruvate) medium alone, SAGE medium alone, or plus gonadotropins. At 48 h, oocyte meiotic status and diameter were measured after treatment of cumulus-oocyte complexes with hyaluronidase. Cumulus-oocyte complexes derived from follicles of 0.5- to 2-mm diameter contain oocytes that typically reinitiate meiosis in the absence or presence of gonadotropins and fertilize via in vitro fertilization or intracytoplasmic sperm injection. Moreover, the inseminated oocytes can reach the morula stage but arrest. Thus, the ability of these oocytes to complete maturation, as monitored from subsequent embryonic development after fertilization, is suboptimal. Further studies on primate IVM of oocytes from SAFs are warranted in order for them to be considered as an additional, novel source of gametes for fertility preservation in cancer patients.

WEE2 is an oocyte-specific meiosis inhibitor in rhesus macaque monkeys

WEE1 homolog 2 (WEE2, also known as WEE1B) is a newly identified member of the WEE kinase family that is conserved from yeast to humans. The aim of the present study was to determine the spatiotemporal expression pattern and the function of WEE2 during oocyte maturation in a nonhuman primate species, the rhesus macaque. Among 11 macaque tissues examined, WEE2 transcript is predominantly expressed in the ovary and only weakly detectable in the testis. Within the ovary, WEE2 mRNA is exclusively localized in the oocyte and appears to accumulate during folliculogenesis, reaching the highest level in preovulatory follicles. Microinjection of a full-length WEE2-GFP (green fluorescent protein) fusion mRNA indicates a specific nuclear localization of WEE2 protein in both growing and fully grown germinal vesicle (GV)-intact oocytes. Taking the long double-stranded RNA-mediated RNA interference approach, we found that down-regulation of WEE2 led to meiotic resumption in a subset of GV oocytes even in the presence of a phosphodiesterase 3 inhibitor. On the other hand, overexpression of WEE2 delays the reentry of oocytes into meiosis in both mice and monkeys. These findings suggest that WEE2 is a conserved oocyte-specific meiosis inhibitor that functions downstream of cAMP in nonhuman primates.

The menstrual cycle of the spider monkey (Ateles geoffroyi)

The ovarian cycles of four adult female spider monkeys (Ateles geoffroyi) were followed daily throughout 30 days by means of vaginal swabs and blood samplings. Cytological analyses of the vaginal swabs and radioimmunoassay determination of the daily levels of estradiol-17 beta (E2) and progesterone (P4) wer done in order to classify the kind of ovarian cycle of this species. Our results show that Ateles geoffroyi females display menstrual cycles of about 24 days on average. By comparison with the well-known menstrual cycles of women, apes, and Old World monkeys, the four distinctive cytological phases (bleeding, follicular, periovulatory, and luteal) could be recognized; mid-cycle E2 peaks followed by mid-luteal increases of the same hormone were present in all four females. P4 levels were higher after the E2 peak, although both hormones were present throughout the cycles. Also, age-dependent features, hormone profiles, and changes in menstrual phases lengths were detected.

In vitro manipulation of nonhuman primate gametes for embryo production and embryo transfer

Since nonhuman primates are closely related to humans and share many physical similarities, they are important for use in research areas such as human infectious diseases, reproduction, physiology, endocrinology, metabolism, neurology and longevity. To develop and maintain these animals, we must establish techniques for in vitro manipulation of spermatozoa and eggs. For a decade my research group has been conducting basic research to establish embryo manipulation techniques and to clarify the reproductive phenomena in nonhuman primates. This article summarizes the past research on in vitro manipulation of nonhuman primate gametes, from collection of reproductive cells and in vitro fertilization to the birth of offspring after embryo transfer, as well as the current status of these research areas. The studies summarized here will directly lead to the development of standard techniques for practical and comprehensive use in nonhuman primates.

A macaque model for studying mechanisms controlling oocyte development and maturation in human and non-human primates

A model to study mechanisms controlling nuclear and cytoplasmic maturation of primate oocytes is being developed in our laboratory. The high incidence of pregnancy failure in women following in-vitro fertilization (IVF) may be partly attributed to inadequate cytoplasmic maturation of oocytes. Advancement of knowledge of mechanisms controlling primate oocyte maturation would have important implications for treatment of human infertility, and would potentially increase numbers of viable non-human primate embryos for biomedical research. Use of a non-human primate model to study oocyte and embryo biology avoids legal, ethical and experimental limitations encountered in a clinical situation. Using this model, the meiotic and developmental capacity of oocytes from three sources have been compared: (i) in-vivo matured oocytes from monkeys stimulated with follicle-stimulating hormone (FSH) and human chorionic gonadotrophin, (ii) in-vitro matured oocytes from monkeys primed with FSH, and (iii) in-vitro matured oocytes from non-stimulated monkeys. This work demonstrates that oocyte developmental competence is likely acquired both during follicle development, before meiotic resumption, and during meiotic progression, concurrent with nuclear maturation. Potential causes of developmental failure of in-vitro matured oocytes, implications for human infertility, and future strategies to study the regulation of primate oocyte maturation are discussed.

Low oxygen inhibits but complex high-glucose medium facilitates in vitro maturation of squirrel monkey oocyte-granulosa cell complexes

PURPOSE

The objectives of these in vitro maturation studies in primate cumulus-oocyte complexes (COCs) were to evaluate the effect of a reduced-oxygen environment and to compare medium with a high-glucose concentration to medium with pyruvate but no glucose.

METHODS

COCs were retrieved from squirrel monkeys stimulated with 1 mg of follicle-stimulating hormone (FSH) for 4-6 days. Experiment 1 examined maturation after 48 hr in 5% O2/5% CO2/90% N2 compared with 5% CO2/air. The medium was CMRL-1066 containing moderate glucose (5.5 mM) supplemented with 1 mM glutamine, 0.33 mM pyruvate, 0.075 IU/ml human FSH, 5 IU/ml human chorionic gonadotropin, 75 U penicillin G/ml, and 20% fetal bovine serum. Experiment 2 in 5% CO2/air, compared P-1 medium (pyruvate and lactate but no glucose) to Waymouth's medium (27.5 mM glucose), both with identical supplements.

RESULTS

Only 3 (8%) of 37 COCs matured in 5% O2, while 39 (49%) of 80 matured in ambient O2. Fourteen (22%) of 64 complexes matured in P-1 medium, compared to 47 (49%) of 96 meiosis II oocytes in Waymouth's medium (P < 0.05).

CONCLUSIONS

These are the first primate studies showing detrimental effects of reduced-oxygen culture on in vitro maturation. Additionally, maturation was enhanced with complex high-glucose medium suggesting that the predominant metabolism is aerobic glycolysis.

Effects of ovarian source, patient age, and menstrual cycle phase on in vitro maturation of immature human oocytes

OBJECTIVE

To determine the efficiency of in vitro maturation, expressed by nuclear maturation, of oocytes aspirated during gynecologic surgeries or collected from excised ovaries. To assess the effect of patient age and cycle phase at collection on the oocyte's ability to mature in vitro. To examine the time course of oocyte maturation in vitro.

DESIGN

Nuclear maturation based on patient criteria compared.

SETTING

University-based IVF program and research center.

PATIENT(S)

Consented patients undergoing gynecologic surgeries or patients undergoing oophorectomy.

INTERVENTION(S)

Oocytes were maintained in culture for 48 hours and evaluated for maturation.

MAIN OUTCOME MEASURE(S)

Nuclear maturation evaluated as germinal vesicle breakdown (GVBD) or progression to the metaphase II (MII) stage.

RESULT(S)

A significantly higher percentage of oocytes collected during the follicular phase of the menstrual cycle underwent GVBD than did oocytes collected during the luteal phase (60% versus 48%, respectively). The percentage of oocytes reaching the MII stage, from these two groups, was not different. No statistically significant differences in maturation were observed in oocytes from different ovarian sources or from patients >40 or <40 years of age.

CONCLUSION(S)

These data suggest that oocytes collected during the follicular phase are more likely to undergo GVBD than oocytes collected during the luteal phase. In this study, ovarian source, age, or cycle phase did not influence the final meiotic maturation of oocytes to metaphase II.

Activin A stimulates meiotic maturation of human oocytes and modulates granulosa cell steroidogenesis in vitro

OBJECTIVE

To test the hypothesis that activin A promotes in vitro human oocyte meiotic maturation while inhibiting steroid secretion by nonluteinized antral granulosa cells.

DESIGN

Prospective randomized controlled study.

SETTING

A university medical center.

PATIENT(S)

Nine women ranging in age from 31-44 years who were undergoing oophorectomy for nonovarian pathology.

INTERVENTION(S)

Analysis of meiotic maturation of oocytes and steroid secretion by granulosa cells cultured in the presence or absence of activin A.

MAIN OUTCOME MEASURE(S)

Germinal vesicle breakdown (GVBD) and attainment of metaphase II (MII) in oocytes, and progesterone and E2 secretion by granulosa cells.

RESULT(S)

Activin A significantly enhanced GVBD (91% vs. 65%) for control and maturation to MII (56% vs. 35% for control) of immature oocytes. Activin A significantly suppressed basal, and inhibin A-and FSH-stimulated progesterone and E2 secretion by nonluteinized granulosa cells.

CONCLUSION(S)

Activin A is a promoter of oocyte maturation in vitro and a modulator of granulosa cell steroidogenesis in culture.

Primate embryonic stem cells

Primate embryonic stem (ES) cells are derived from preimplantation embryos, have a normal karyotype, and are capable of indefinite, undifferentiated proliferation. Even after culture for more than a year, primate ES cells maintain the potential to differentiate to trophoblast and derivatives of embryonic endoderm, mesoderm, and ectoderm. In this review, we compare the characteristics of ES cell lines from two primate species, the rhesus monkey (Macaca mulatta) and the common marmoset (Callithrix jacchus), with the characteristics of mouse ES cells and human embryonal carcinoma cells. We also discuss the implications of using primate ES cells to understand early human development and discuss the practical and ethical implications for the understanding and treatment of human disease.

Use of purified porcine follicle-stimulating hormone for ovarian stimulation of macaque monkeys

At present, in nonhuman primates, ovarian stimulation with heterologous gonadotropin preparations is the only reliable way to produce substantial numbers of competent ova for in vitro fertilization and embryo development studies. Preparations such as equine chorionic gonadotropin (eCG) and human menopausal gonadotropins (hMG or hFSH) have been used successfully, but eCG is crude and contains variable amounts of LH activity, while hMG/hFSH is very expensive and the supply is not stable. This study examined the use of a purified porcine FSH preparation (Folltropin V) for ovarian stimulation in rhesus monkeys. Twice-daily intramuscular injections of this preparation resulted in good follicular development, and was followed by a single intramuscular injection of hCG. Ova were collected laparoscopically 30 h post hCG, fertilized in vitro and then cultured until development ceased. Stimulation of 9 monkeys with Folltropin V yielded a mean of 20 ova per animal, of which 71% reached metaphase II and were inseminated; of these, 92% were fertilized in vitro and 48% developed into blastocysts in vitro. These results are similar to those reported by us and by others using eCG, hMG or an hFSH/hMG combination for ovarian stimulation of macaque monkeys. We conclude that Folltropin V is a suitable alternative preparation for ovarian stimulation in nonhuman primates and one that also has the advantages of being readily available and much less expensive than human gonadotropin preparations.

Maturity and fertility of rhesus monkey oocytes collected at different intervals after an ovulatory stimulus (human chorionic gonadotropin) in in vitro fertilization cycles

In rhesus monkeys undergoing ovarian stimulation for in vitro fertilization (IVF), a midcycle injection of human chorionic gonadotropin (hCG) substitutes for the LH surge and induces preovulatory oocyte maturation. The time interval between injection and oocyte collection, ideally, allows for the completion of oocyte maturation without ovulation, which would reduce the number of oocytes available for harvest. To evaluate the influence of this time interval on oocyte parameters following hCG administration, we conducted a series of gonadotropin treatment protocols in 51 animals in which the interval from hCG administration to follicular aspiration was systematically varied from 27 to 36 hr. Follicle number and size, evaluated prior to hCG administration by sonography, did not vary significantly or consistently with preovulatory maturation time. Oocytes were harvested by laparotomy or laparoscopy, and scored for maturity before insemination. The percentage of mature, metaphase II (MII) oocytes at recovery increased significantly with increasing preovulatory time and was inversely proportional to that of metaphase I (MI) oocytes. However, oocyte yield tended toward a progressive decrease with increasing preovulatory maturation times from a high of 27 oocytes at 27 hr to a low of 17 oocytes/animal at the 36 hr time interval. Fertilization levels declined significantly from a high of 50% at 27 hr to a low of 30% at 36 hr. Thus, although higher percentages of mature oocytes were recovered at the longer time intervals, optimal oocytes/embryo harvests were realized after the shorter time intervals (27 and 32 hr) and are most compatible with the goal of achieving high yields of fertile oocytes and embryos following gonadotropin stimulation in rhesus monkeys.