Germinal vesicle configurations and patterns of polypeptide synthesis of procine oocytes from antral follicles of different size, as related to their competency for spontaneous maturation

Lab partners: oocytes, embryos and company. A personal view on aspects of oocyte maturation and the development of monozygotic twins

The present review addresses the oocyte and the preimplantation embryo, and is intended to highlight the underlying principle of the "nature versus/and nurture" question. Given the diversity in mammalian oocyte maturation, this review will not be comprehensive but instead will focus on the porcine oocyte. Historically, oogenesis was seen as the development of a passive cell nursed and determined by its somatic compartment. Currently, the advanced analysis of the cross-talk between the maternal environment and the oocyte shows a more balanced relationship: Granulosa cells nurse the oocyte, whereas the latter secretes diffusible factors that regulate proliferation and differentiation of the granulosa cells. Signal molecules of the granulosa cells either prevent the precocious initiation of meiotic maturation or enable oocyte maturation following hormonal stimulation. A similar question emerges in research on monozygotic twins or multiples: In Greek and medieval times, twins were not seen as the result of the common course of nature but were classified as faults. This seems still valid today for the rare and until now mainly unknown genesis of facultative monozygotic twins in mammals. Monozygotic twins are unique subjects for studies of the conceptus-maternal dialogue, the intra-pair similarity and dissimilarity, and the elucidation of the interplay between nature and nurture. In the course of in vivo collections of preimplantation sheep embryos and experiments on embryo splitting and other microsurgical interventions we recorded observations on double blastocysts within a single zona pellucida, double inner cell masses in zona-enclosed blastocysts and double germinal discs in elongating embryos. On the basis of these observations we add some pieces to the puzzle of the post-zygotic genesis of monozygotic twins and on maternal influences on the developing conceptus.

Dibutyryl-cAMP and roscovitine differently affect premature meiotic resumption and embryo development of vitrified immature porcine oocytes

Vitrification and warming can trigger premature meiosis in immature porcine oocytes. Our aim was to compare the efficacies of two meiotic inhibitors, dibutyryl-cAMP and roscovitine for the meiosis synchronization during in vitro maturation (IVM) of porcine oocytes vitrified at the germinal vesicle (GV) stage. We first compared the efficacy of 1 mM dibutyryl-cAMP and 25 μM roscovitine on meiotic arrest during the first 22 h of IVM. Dibutyryl-cAMP could maintain the GV stage in 83.5% of oocytes; however, roscovitine was even more effective (96.6%), whereas only 17.4% of the oocytes remained at the GV stage without these additives. Temporal meiotic arrest for 22 h by roscovitine did not reduce the percentage of oocytes reaching the Metaphase II stage during subsequent IVM. However, after parthenogenetic stimulation or in vitro fertilization, subsequent embryo development to the blastocyst stage was compromised after roscovitine treatment, whereas dibutyryl-cAMP improved the percentage of blastocyst development. In conclusion, dibutyryl-cAMP could derogate but not completely prevent premature meiosis in vitrified oocytes, whereas roscovitine could more efficiently prevent it. However, for embryo production, the use of roscovitine was disadvantageous, whereas the use of dibutyryl-cAMP was beneficial.

Expression and Impact of Vaspin on In Vitro Oocyte Maturation through MAP3/1 and PRKAA1 Signalling Pathways

Oocyte maturation is a critical stage in embryo production and female reproduction. The aims of this study were to determine: (i) the mRNA and protein expression of vaspin and its receptor 78-kDa glucose-regulated (GRP78) in porcine cumulus-oocyte complexes (COCs) by real-time PCR and Western blot analysis, respectively, and their localisation by immunofluorescence; and (ii) the effects of vaspin on in vitro oocyte maturation (IVM) and the involvement of mitogen ERK1/2 (MAP3/1)- and AMPKα (PRKAA1)-activated kinases in the studied processes. Porcine COCs were matured in vitro for 22 h or 44 h with vaspin at a dose of 1 ng/mL and nuclear maturation assessed by Hoechst 33342 or DAPI staining and the measurement of progesterone (P4) level in the maturation medium. We showed that vaspin and GRP78 protein expression increased in oocytes and cumulus cells after IVM. Moreover, vaspin enhanced significantly porcine oocyte IVM and P4 concentration, as well as MAP3/1 phosphorylation, while decreasing PRKAA1. Using pharmacological inhibitors of MAP3/1 (PD98059) and PRKAA1 (Compound C), we observed that the effect of vaspin was reversed to the control level by all studied parameters. In conclusion, vaspin, by improving in vitro oocyte maturation via MAP3/1 and PRKAA1 kinase pathways, can be a new factor to improve in vitro fertilisation protocols.

Synchronization of In Vitro Maturation in Porcine Oocytes

When removed from the follicles, during the 44 h process of in vitro maturation (IVM) fully grown porcine oocytes resume meiosis spontaneously from the late diplotene stage of the first meiotic prophase and proceed to the metaphase-II (MII) stage at which they remain arrested until fertilization. However, the spontaneous resumption may start at various times causing heterogeneity in the nuclear stage and also in cytoplasmic characteristics within a population. Those oocytes that reach the MII stage earlier than others undergo an aging process which is detrimental for further embryo development. The synchronization of nuclear progression of porcine oocytes can be achieved by a transient inhibition of meiotic resumption during the first 20-22 h of IVM by the elevation of intracellular levels of cyclic adenosine monophosphate (cAMP) using the cellular membrane-permeable analog of cAMP, dibutyryl cyclic AMP. A simple and efficient protocol for such treatment is described below.

Synergistic effect of porcine follicular fluid and dibutyryl cyclic adenosine monophosphate on development of parthenogenetically activated oocytes from pre-pubertal gilts

This study investigated the effect of porcine follicular fluid (PFF) and dibutyryl cyclic adenosine monophosphate (dbcAMP) during in vitro maturation (IVM) of porcine oocytes on meiotic maturation, fertilization and embryo development, and compared the effect of supplementing the embryo culture media with PFF or foetal bovine serum (FBS) on embryo development. Oocytes from pre-pubertal gilts were IVM for 44 h, and parthenogenetically activated or in vitro-fertilized. Embryos were cultured in porcine zygote medium (PZM3) for 7 days. Cleavage and blastocyst rates were evaluated at 48 h and 7 days of culture. The supplementation of the IVM medium with 25% PFF and 1 mm dbcAMP for the first 22 h resulted in more (p < 0.05) embryos developing to the blastocyst stage as compared with the inclusion of dbcAMP alone. The dbcAMP + PFF combination increased (p < 0.05) the average number of nuclei per blastocyst as compared with either of these components alone or in its absence. A synergistic effect of dbcAMP + PFF during IVM was also reflected in the capacity of oocytes to regulate sperm penetration and prevent polyspermy, as twice as many oocytes from the control group were penetrated by more than one sperm as compared with those matured in the presence of both dbcAMP and PFF. The supplementation of PZM3 with 10% FBS from days 5 to 7 of culture significantly improved the total cell quantity in embryos derived either from control or dbcAMP + PFF matured oocytes. There was no effect on the total cell quantity when FBS was replaced by the same concentration of PFF. These studies showed that dbcAMP, PFF and FBS can improve both the quantity (57.3% vs 41.5%) and quality (74.8 vs 33.3 nuclei) of porcine blastocysts derived from oocytes recovered of pre-pubertal gilts.

Synchronization of in vitro maturation in porcine oocytes

When removed from the follicles, during the 44 h process of in vitro maturation (IVM) fully grown porcine oocytes resume meiosis spontaneously from the late diplotene stage of the first meiotic prophase and proceed to the metaphase-II (MII) stage at which they remain arrested until fertilization. However, the resumption may start at various times causing heterogeneity in the nuclear stage and also in cytoplasmic characteristics (i.e., the activity of certain protein kinases) within a population. Those oocytes that reach the MII stage earlier than others undergo an ageing process which is detrimental for further embryo development. The synchronization of nuclear progression is possible by a transient inhibition of meiotic resumption during the first 20-22 h of IVM either by (1) the elevation of intracellular levels of cyclic adenosine monophosphate (cAMP) or (2) suppressing the activity of the metaphase promoting factor (MPF). A protocol for each approach is described.

Developmental regulation of histone H3 methylation at lysine 4 in the porcine ovary

Follicular growth and oogenesis involve highly dynamic changes in morphogenesis, chromatin structure, and gene transcription. The tight coordination of these events leads to ovulation of a mature oocyte and formation of the luteal tissue necessary to regulate embryo implantation and development. This entire process is regulated by numerous endocrine and in situ mechanisms. The role of epigenetic mechanisms in folliculogenesis, such as the biochemical modification of the DNA packaging proteins, the histones, is not well understood. Our objective was to determine the cellular and follicular stage-specific patterns of histone H3 methylation at lysine 4 (K4) in porcine preovulatory follicles and during luteinization in pig ovaries. Ovary tissues were collected from slaughtered prepubertal and cyclic gilts at various stages of the estrous cycle, pregnancy, and from ovaries recovered from gonatropin-treated gilts at 0, 24, and 38 h post human chorionic gonadotropin (hCG) injection. Samples were fixed in 4% paraformaldehyde and processed for embedding in paraffin and sectioned using standard histological protocols. Immunofluorescent staining was performed on 3 microm thick sections. The immunostaining pattern of mono-, di-, and tri-methylated histone H3-K4 and lysine-specific demethylase 1 (LSD1, also known as KDM1 or AOF1) was assessed. Interestingly, H3-K4 mono-, di-, and tri-methylation in follicles of prepubertal gilts was specifically distributed and developmentally regulated. While granulosa cells of primary, secondary, and early antral follicles were negative for H3-K4 methylation those from large antral follicles showed a striking upregulation in the cells located in the proximity to the oocyte. Specifically, the cumulus oophorus displayed intense staining for H3-K4 methylation and signals were strongest in the granulosa cells in the inner two cell layers of the follicular wall. Although all oocytes from primary to large antral stage follicles were positive for H3-K4 mono-, di-, and tri-methylation, the patterns of distribution were altered through oocyte follicle development. H3-K4 methylation in granulosa cells was dramatically reduced as time to ovulation approached and was low to undetected at 38 h post hCG treatment. H3-K4 mono-, di-, and tri-methylation in large luteal cells increased as differentiation evolved but remained low in small luteal cells. Strikingly, LSD1 (KDM1) expression was found to be restricted to the corpus luteum. In summary, this study provides new information on histone H3-K4 methylation patterns in the oocyte and follicle during folliculogenesis, which suggests that these epigenetic markers serve an essential regulatory role during folliculogenesis.

Germinal vesicle chromatin configuration and meiotic competence is related to the oocyte source in canine

This study investigated the effect of deriving oocytes from different stages of the estrous cycle on oocyte diameter, germinal vesicle (GV) chromatin configuration, and in vitro meiotic competence in canine oocytes. Cumulus oocyte complexes (COCs) were recovered from both ovaries during anestrous, follicular, and luteal phases and in vivo ovulated oocytes. The diameter of canine oocyte was compared with or without the zona pellucida (ZP) before in vitro maturation (IVM). Also, GV chromatin configuration was evaluated before (0 h) or 72 h after IVM by fixation with 3.7% formaldehyde supplemented with 10 microg/ml Hoechst 33342 for 30 min. COCs were matured in TCM199 supplemented with 10% fetal bovine serum (FBS), 0.6 mM cysteine, 0.2 mM pyruvic acid, 50 microg/ml gentamycin sulfate, and 20 microg/ml 17beta-estradiol (E(2)) at 39 degrees C and 5% CO(2) in air for 72 h. The diameter of in vivo ovulated oocytes with the ZP (167.5+/-12.7 microm) or without ZP (133.9+/-5.3 microm) was significantly greater (p<0.05) than those of anestrous, follicular, and luteal oocytes (with ZP, 151.2+/-7.4, 153.1+/-8.8 and 152.8+/-5.4 microm, respectively; without ZP, 115.3+/-7.6, 122.1+/-4.9 and 114.3+/-6.6 microm, respectively). At 0 h, the GV-II configuration was more prevalent in oocytes from anestrual ovaries than from follicular or luteal ovaries or in vivo ovulated oocytes (63.6% versus 14.8%, 33.0%, and 0.0%; p<0.05), whereas the proportion of oocytes with the GV-V configuration was higher in follicular phase and ovulated oocytes than in oocytes from anestrus and luteal phase (57.4% and 100% versus 2.0% and 22.7%; p<0.05). However, oocytes in luteal phase exhibited diverse GV configurations (10.3%, 33.0%, 16.5%, 13.4%, and 22.7% in GV-I, GV-II, GV-III, GV-IV, and GV-V, respectively). After 72 h post-IVM, a greater percentage of in vivo ovulated oocytes progressed to MII than those oocytes collected during anestrous, follicular, and luteal phases (50.0% versus 5.5%, 11.5%, and 9.1%; p<0.05). In conclusion, the oocyte diameter, GV chromatin configuration, and meiotic maturation of canine COCs are related to the oocyte source. These results indicated that the oocyte source could be critical to nuclear progression to MII stage in canines.

Nuclear morphology, diameter and meiotic competence of buffalo oocytes relative to follicle size

The nuclear morphology, diameter and in vitro meiotic competence of buffalo oocytes was compared relative to follicle size. Cumulus-oocyte complexes (COCs) were collected from 1-<2, 2-<3, 3-<4, 4-<6 and 6-<8 mm follicles from abattoir ovaries. Cumulus cells were removed using 3 mg mL(-1) hyaluronidase in saline and repeated pipetting. Denuded oocytes were measured, fixed in 3% glutaraldehyde, stained with 4,6-diamidoino-2-phenylindole and evaluated for nuclear morphology, namely the stage of germinal vesicle (GV) development before in vitro maturation (IVM). The COCs from >2-mm follicles were matured in vitro in their respective size groups for 24 h in Medium 199 supplemented with 10 microg mL(-1) follicle-stimulating hormone, 10 microg mL(-1) luteinizing hormone, 1.5 microg mL(-1) oestradiol, 75 microg mL(-1) streptomycin, 100 IU mL(-1) penicillin, 10 mM HEPES and 10% fetal bovine serum. Matured oocytes were fixed, stained and evaluated for GV status and meiotic development. The number of oocytes collected from follicles 1-<8 mm in diameter averaged 1.82 per ovary. Oocytes from follicles 1-<2 mm (107.7 +/- 1.6 microm), 2-<3 mm (108 +/- 1.1 microm) and 3-<4 mm (114.6 +/- 1.3 microm) in diameter were smaller in diameter (P < 0.05) than oocytes from follicles 4-<6 mm (124.4 +/- 1.3 microm) and 6-<8 mm (131.9 +/- 1.4 microm) in diameter. A majority of oocytes (P< 0.05) from <4-mm follicles was at the initial stages of GV development (GV-I, II and III), whereas oocytes from 4-<6- and 6-<8-mm follicles were at the final stages of GV-IV (35.0 and 21.6% respectively) and GV-V (49.1 and 67.5% respectively). Poor IVM rates of 32.0% and 32.7% to metaphase (M)-II were observed for oocytes isolated from 2-<3- and 3-<4-mm follicles, respectively, whereas significantly (P< 0.05) more oocytes from 4-<6- and 6-<8-mm follicles reached M-II (67.1% and 79.1% respectively). In conclusion, buffalo oocytes displayed a size-dependent ability to undergo meiotic maturation and we suggest that oocytes from >4-mm follicles should be considered in buffalo in vitro fertilization systems for better results.

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.

Chromatin modifications in the germinal vesicle (GV) of mammalian oocytes

The nucleus of eukaryotic cells is organized into functionally specialized compartments that are essential for the control of gene expression, chromosome architecture and cellular differentiation. The mouse oocyte nucleus or germinal vesicle (GV) exhibits a unique chromatin configuration that is subject to dynamic modifications during oogenesis. This process of 'epigenetic maturation' is critical to confer the female gamete with meiotic as well as developmental competence. In spite of its biological significance, little is known concerning the cellular and molecular mechanisms regulating large-scale chromatin structure in mammalian oocytes. Here, recent findings that provide mechanistic insight into the complex relationship between large-scale chromatin structure and global transcriptional repression in pre-ovulatory oocytes will be discussed. Post-translational modifications of histone proteins such as acetylation and methylation are crucial for heterochromatin formation and thus play a key role in remodeling the oocyte genome. This strategy involves multiple and hierarchical chromatin modifications that regulate nuclear dynamics in response to a developmentally programmed signal(s), presumably of paracrine origin, before the resumption of meiosis. Models for the experimental manipulation of large-scale chromatin structure in vivo and in vitro will be instrumental to determine the key cellular pathways and oocyte-derived factors involved in genome-wide chromatin modifications. Importantly, analysis of the functional differentiation of chromatin structure in the oocyte genome with high resolution and in real time will have wide-ranging implications to understand the role of nuclear organization in meiosis, the events of nuclear reprogramming and the spatio-temporal regulation of gene expression during development and differentiation.

Chromatin, microtubule and microfilament configurations in the canine oocyte

In the present study, we observed chromatin, microtubule and microfilament distribution in canine oocytes. The germinal vesicle (GV) chromatin of canine oocytes was classified into four configurations (GV-I, -II, -III and -IV) based on the degree of chromatin separation and condensation. Oocytes recovered from follicular phase ovaries had a greater amount (68%, P < 0.05) of GV-III or GV-IV chromatin than did those from non-follicular phase ovaries (35%). The majority (86.7%) of in vivo ovulated oocytes were at GV-IV. The rates of development to GV breakdown/metaphase I/metaphase II were higher in oocytes recovered from follicular ovaries than from non-follicular ovaries. Immunostaining results revealed cytoplasmic microtubules present in all GV-stage oocytes. Following GV breakdown, microtubular asters were produced from condensed chromatin. The asters appeared to be elongated, and encompassed condensed chromatin particles to form meiotic metaphase chromatin. Microfilaments were located in the cortex and around the GV. During meiotic maturation, a microfilament-rich area, in which the chromatin is allocated, was observed in the oocyte. Our results indicate that oocytes recovered from follicular ovaries were in an advanced stage of GV, and were more competent to complete maturation compared to those from non-follicular phase ovaries. Both microtubules and microfilaments are closely associated with reconstruction of chromatin during meiotic maturation in canine oocytes.

Meiotic competence of bovine fetal oocytes following in vitro maturation

The in vitro ability between fetal and cow oocytes to resume meiosis and progression to metaphase-II (M-II) was compared. Cumulus oocyte complexes (COCs) were harvested from 2 to 6 mm follicles from ovaries of 7.5 month to term fetuses and adult cows. Cumulus cells were removed using 3 mg/ml hyaluronidase and repeated pipetting. Denuded oocytes were fixed in 3% glutaraldehyde, stained with DAPI and evaluated under fluorescent microscopy for nuclear status before in vitro maturation (IVM). COCs from fetal and adult ovaries were also matured in 200 microl droplets of medium 199 supplemented with 10 microg/ml FSH, 10/ml LH, 1.5 microg/ml estradiol, 75 microg/ml streptomycin, 100 IU/ml penicillin, 10 mM hepes and 10% FBS for 24 h at 39 degrees C and 5% CO(2). Matured oocytes were fixed, stained and evaluated as explained above for nuclear status namely stage of germinal vesicle (GV) development and subsequent meiotic competence. Data were analyzed using chi-square analysis. The majority of fetal oocytes (P<0.05) before IVM were at GV stages GV-I (27.7%), GV-II (37.6%) and GV-V (22.8%) compared to cow oocytes, which were at GV stages IV (28.3%) and V (46.7%). After IVM, fewer fetal oocytes were at earlier stages of GV development and majority (P<0.05) were at GV-V (24.0%), premetaphase (17.4%) and metaphase-I (M-I: 7.2%) stages. However, after IVM, more cow oocytes matured to M-II than did fetal oocytes (93.7% versus 26.9%; P<0.05). In conclusion, fetal oocytes do not mature in vitro as well as cow oocytes. Our findings suggest that the low meiotic competence of fetal oocytes can be attributed to their being at earlier stages of GV development before IVM.

Effects of Culture Medium, Serum Type, and Various Concentrations of Follicle-Stimulating Hormone on Porcine Preantral Follicular Development and Antrum Formation In Vitro1

Developing a culture system for preantral follicles has important biotechnological implications due to the potential to produce large number of oocytes for embryo production and transfer. As an initial step toward accomplishing this long-term goal, a study was conducted to determine the effects of culture medium, serum type, and different concentrations of FSH on preantral follicular development in vitro. Specific endpoints included follicular growth rate, antrum formation, recovery rate of cumulus cell-oocyte complexes (COCs) from follicles, and oocyte meiotic competence. Compared with the North Carolina State University medium 23 (NCSU23), preantral follicles cultured in TCM199 medium for 4 days grew faster (P < 0.02). However, more follicles cultured in NCSU23 differentiated to form an antrum than in TCM199 (P < 0.01). For this reason, NCSU23 was chosen to investigate the role of FSH and serum type in regulating preantral follicular growth. Compared with the 0 mIU/ml FSH control, addition of 2 mIU/ml FSH to the medium stimulated follicular growth and antrum formation and suppressed apoptosis of granulosa cells (P < 0.05), supporting the essential role of FSH in preantral follicular growth and development. Another experiment compared fetal calf serum (FCS) with prepubertal gilt serum (PGS) and studied different concentrations of FSH in the culture medium (0.5, 1, and 2 mIU/ml). The best follicular growth rate was obtained with 2 mIU/ml compared with 0.5 or 1 mIU/ml FSH. Compared with PGS, FCS supplementation increased the cumulative percentage of antral follicles and COC recovery rate (P < 0.04). None of the oocytes recovered from any of these experiments reached metaphase II stage after maturation in vitro. In summary, culture medium, serum type, and FSH concentration in the medium interacted to affect follicular growth and antrum formation in vitro. These results suggest that a longer term culture of preantral follicles (>4 days) may be needed to produce oocytes capable of undergoing meiosis in vitro.

Follicular factors regulating oocyte maturation and quality

Maturation of the oocyte can be divided into two different aspects: nuclear maturation and cytoplasmic maturation. The spontaneous nature of nuclear maturation in oocytes removed from the follicle and cultured in vitro was observed in mammals as early as 1935. However, oocytes cultured in basic conditions are deficient in some cytoplasmic factors and are, therefore, developmentally incompetent. Data from large domestic species indicate that although oocytes matured in vitro in supplemented media can develop after fertilization, they require the presence of follicular factors during culture to ensure their developmental competence. The importance of follicular maturation on the capacity of oocytes to achieve fertilization and early embryonic development can be studied by reproducing some important events in vitro. Follicular supplementation may be in the form of follicular fluid, granulosa cells or follicle-conditioned media, and there is evidence that the maturational status of the follicle used for co-culture influences subsequent male pronuclear formation. Studies in this laboratory on the prolific Chinese Meishan pig, which has significantly higher early embryonic survival than conventional European breeds, indicate crucial differences in the pattern of follicle development and, therefore, the intrafollicular environment in which the oocytes are nurtured. It is suggested that this produces oocytes of improved 'quality' and this hypothesis is supported by experiments both in vivo and in vitro. Ultimately, it is hoped that these studies on large domestic animals will lead to identification of the follicular factors that influence oocyte quality.

Transcriptional activity of the mouse oocyte genome: companion granulosa cells modulate transcription and chromatin remodeling

Chromatin configuration in the germinal vesicle (GV) undergoes dynamic changes during oocyte growth, yet little is known about the mechanisms regulating chromatin remodeling in mouse oocytes. The hypothesis that companion granulosa cells play a role in modulating chromatin configuration and subsequent transcriptional activity in the oocyte genome was tested. Analysis of transcriptional activity, as determined by Br-UTP incorporation, revealed a similar percentage of transcriptionally active and inactive oocytes present in the large antral follicles of mature females. However, gonadotropin stimulation of follicular development induced an increase in the proportion of transcriptionally inactive oocytes. Interestingly, a similar proportion of stage-matched, oocyte-granulosa cell complexes grown in vitro without gonadotropin stimulation displayed chromatin redistribution around the nucleolus and no transcriptional activity. In contrast, when cultured in the absence of companion granulosa cells, transcriptional activity remained unabated in the majority of denuded GV stage oocytes. Extended prophase arrest in fully grown transcriptionally inactive oocyte-granulosa cell complexes had no effect on the progression of meiosis after in vitro maturation. However, it reduced the competence to complete preimplantation embryo development. These results indicate that chromatin redistribution around the nucleolus is associated with transcriptional repression in the GV of both fully grown in vivo-derived oocytes and cultured oocyte-granulosa cell complexes. Moreover, the results presented here suggest that some aspects of intraovarian control mechanisms were abrogated during culture of oocyte-granulosa cell complexes, resulting in a higher proportion of oocytes with "mature" chromatin. Most importantly, companion granulosa cells played an active role in modulating the transcriptional activity of the oocyte genome.

Nuclear configuration of bovine oocytes derived from fresh and in vitro-cultured preantral and early antral ovarian follicles

The aim of this experiment was to characterize the growth and nuclear configuration of oocytes isolated from late preantral and early antral bovine ovarian follicles immediately after recovery and after the in vitro culture. Individual follicles were isolated by microdissection from slices of the ovarian cortex. Follicles were sorted by diameter into 175 to 224, 225 to 274 and 275 to 325 microm-size classes. The follicles selected for in vitro culture were placed singly into 40 microL droplets of medium (TCM 199 enriched with FCS, insulin, transferrin, sodium selenite, sodium pyruvate, 1-glutamine, hypoxanthine, FSH and estradiol-17beta) and cultured for 6, 8, 11, 14 or 17 d. The sizes of follicles and oocytes were related to the duration of culture and gradually increased as culture duration was prolonged. The analysis of the relationship between mean diameters of oocytes at the time of recovery and after the in vitro culture, has shown significant differences after culture lasting 8 d (76.9+/-9.9 vs. 86.1+/-11.1 microm; P < 0.05), 11 d (77.0+/-9.9 vs. 91.9+/-17.5 microm; P < 0.01), 14 d (80.0+/-9.5 vs. 97.9+/-16.5 microm; P < 0.01) and 17 d (82.6+/-6.6 vs. 97.2+/-11.5 microm; P< 0.01). No statistical differences were shown among oocytes in the 5 pre-culture groups (79.5+/-8.8; 76.9+/-9.9; 77.1+/-9.9; 80.1+/-9.5 and 82.6+/-6.6 microm). Meiotic arrest was preserved in 71.9% of oocytes in our culture system up to 14 d. Frequency of the germinal vesicle (GV) stage did not significantly differ among oocytes evaluated "fresh" or cultured for 6, 8, 11 or 14 d. No relationship was observed between the size class of follicles and the frequency of the GV-stage. Prolonging the culture period to 17 d drastically decreased the percentage of oocytes in the GV-stage (18.7%) and increased the percentage of oocytes having premature initiation of meiosis (GVBD; 46.3%) and degeneration (25.0%). These results suggest that out of all culture periods used in our experiment, Day 14 was found to be the longest culture time allowing for both oocyte growth and maintenance of nuclear configuration at the GV-stage.

In vitro maturation of domestic dog oocytes cultured in advanced preantral and early antral follicles

Initial studies in our laboratory demonstrated that a large proportion of domestic dog advanced preantral (APAN) and early antral (EAN) follicles contained grown oocytes that had acquired the dense cytoplasmic lipid characteristic of preovulatory oocytes. The objective of this study was to assess nuclear maturation of those oocytes after in vitro culture. Both APAN and EAN follicles (152 to 886 microns in diameter) were isolated from ovaries by treatment with collagenase and DNase. The follicles were cultured in Dulbecco's Modified Eagle's medium/nutrient mixture F-12 Ham culture medium supplemented with 20% (v/v) fetal bovine serum (FBS), 2 mM L-glutamine, 1% (v/v) antibiotic-antimycotic, 1 microgram FSH/ml, 10 IU hCG/ml and 1 microgram estradiol/ml. Within each group (APAN or EAN), control follicles were not cultured (0 h), and 2 to 12 follicles per well were incubated under a humidified atmosphere of 5% CO2 in air at 37 degrees C for 24, 48 or 72 h. After 24 h of culture, significantly more (5.3%, 20/374; P < 0.05) oocytes from APAN follicles reached the metaphase I to metaphase II stages (MI to MII) than the percentage of control follicles observed at 0 h (0.9%, 3/318). Continued culture resulted in a further increase (P < 0.05) in the percentage of oocytes reaching MI to MII by 48 h (11.5%, 47/407), which remained unchanged at 72 h (9.9%, 40/404). The percentage of oocytes from EAN follicles reaching MI to MII did not significantly increase after 24 h of culture. However, there was an increase (P < 0.05) by 48 h of culture (8.7%, 11/126), which remained unchanged at 72 h (7.5%, 8/106). These results show that dog oocytes cultured within advanced preantral and early antral follicles in vitro are competent to resume meiosis to the metaphase stage.

In vitro penetration assay of boar sperm fertility: Effect of various factors on the penetrability of immature pig oocytes

The present study was designed 1) to examine the influence of cumulus cells, ovary storage time and oocyte size on the penetrability of immature pig oocytes, and 2) to investigate the effect of 2 methods of treating the semen from different boars on the inter-assay variability of homologous in vitro penetration tests of boar sperm fertility. In Experiment 1, cumulus oocyte complexes, oocytes with spontaneous loss of the cumulus cells during collection, and oocytes mechanically stripped of cumulus cells were used. No differences were observed in oocyte penetrability among the 3 types of oocyte, although mechanical removal of the cumulus caused an increase (P < 0.005) in the degeneration rate compared with the other oocyte types. In Experiment 2, the oocytes were recovered from ovaries kept in PBS (30 degrees C) for 2, 4 or 6 h after slaughter of prepuberal gilts. Ovary storage did not modify the penetrability of oocytes but increased (P < 0.02) their degeneration rates. In Experiment 3, the diameters of fresh oocytes were determined after co-incubation with spermatozoa. They were classified into 4 groups according to diameter: A) < 105 microm, B) 105-115 microm, C) 116-120 microm and D) > 120 microm. Oocytes from Groups C and D exhibited higher (P < 0.05) penetrability than oocytes from the other groups. In Experiment 4, stored, diluted spermatozoa from 4 boars were pretreated by centrifugation at 50 x g for 3 min and subsequent concentration of the supernatants at 1,200 x g for 3 min. The pellets were treated (washed twice and preincubated for 40 minutes) before co-incubation with immature oocytes or used directly as untreated samples (unwashed and non-preincubated). A boar effect (P < 0.001) was evident for the parameters of in vitro penetration, independently of sperm treatment. When the oocytes were inseminated with untreated spermatozoa, the effects of the replicate and the boar-by-replicate interaction on the variability in oocyte penetrability were not significant. The results of this study indicate that the use of standardized immature pig oocytes and stored untreated, diluted spermatozoa can provide a useful method for optimizing the homologous in vitro penetration (hIVP) assay of boar fertility.

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.

Oocyte recovery and maturation in the American black bear (Ursus americanus): a model for endangered ursids

A study was conducted to determine if meiotic maturation could be induced in ovarian oocytes of the American black bear (Ursus americanus), a model for gamete "rescue" techniques for endangered ursids. Ovaries obtained from 48 black bears yielded 2,403 oocytes (51.1 +/- 4.9/female), of which 777 (32.3%) were morphologically classified as excellent quality. More total oocytes were recovered from donors that were anestrous compared to luteal/pregnant (P < 0.05) at the time of ovarian excision. Delaying the recovery of oocytes from antral follicles within excised ovaries from 12-24 hr to 25-36 hr had no effect (P > 0.05) on the overall number of high quality oocytes recovered or subsequent maturational ability. The highest incidence of metaphase II was reached between 48 and 60 hr of in vitro incubation. Donor status (anestrous vs. luteal/pregnant) had no influence on the oocyte maturation rate by 24 or 48 hr, but by 60 hr, more (P < 0.05) oocytes recovered from anestrous females (43.9%) had achieved metaphase II compared to luteal/pregnant counterparts (23.1%). In preliminary trials involving endangered ursids, 54 ovarian oocytes were recovered from three aged sun bears (Helarctos malayanus), of which 72.2% were excellent quality and 15.4% matured in vitro to metaphase II. Similarly, 119 antral oocytes were recovered from two aged sloth bears (Melursus ursinus), of which 41.2% were excellent and 17.5% matured in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Patterns of intercellular connectivity in the mammalian cumulus-oocyte complex

Electron and fluorescence microscopic techniques have been used in a complementary fashion to study the patterns of follicle cell-oocyte interactions within cumulus-oocyte-complexes of various mammals. The principal findings are: (1) two distinct types of transzonal processes exist that are distinguishable on the basis of cytoskeletal composition; (2) in some of the species examined (pig, goat, primate), corkscrew-shaped processes rich in tubulin, traverse the zona pellucida and are invaginated into the oocyte cortex; (3) actin-rich processes either ramify as a network at the outer surface of the zona pellucida or penetrate the zona and make contact with the oolemma in a species specific manner. These results are discussed with respect both to the need to employ complementary optical methods in assessing connectivity patterns within COC and to the possible role that extracellular matrix-cell interactions play in the homeostatic control of oocyte growth and maturation.

Defining a role for calcium in the resumption and progression of meiosis in the pig oocyte

The role of calcium (Ca++) during spontaneous meiotic maturation of pig oocytes was examined. The hypothesis that elevations of endogenously derived intracellular Ca++ are prerequisite for germinal vesicle breakdown (GVBD) and progression through meiosis was tested. In addition, investigations were carried out to determine whether GVBD and meiotic progression were dependent upon extracellular Ca++ influx. Elevation of endogenously derived Ca++ was inhibited directly by loading cells with BAPTA, a specific Ca++-chelator, or indirectly with neomycin. Extracellular Ca++ influx was prevented by culturing oocytes in Ca(++)-deficient medium, with EGTA, or in the presence of the Ca++ channel blocker verapamil. Pretreatment with BAPTA/AM and subsequent culture in the absence of added exogenous Ca++ resulted in a similar inhibition of GVBD (1 microM BAPTA/AM vs. untreated, P < 0.01). After 4 h following follicular release, oocytes were no longer sensitive to BAPTA/AM treatment. Neomycin also significantly inhibited GVBD (0.5 mM neomycin vs. untreated, P < 0.05) as well as meiotic progression past metaphase I (0.25 mM neomycin vs. untreated, P < 0.05). The incidence of GVBD was not significantly affected when oocytes were simply cultured in Ca++ deficient medium or when cultured in the presence of EGTA or verapamil. However, progression of meiosis past GVBD to metaphase II was suppressed by reducing levels of Ca++ in the culture medium (0.68 mM Ca++ vs. 1.7 mM Ca++, P < 0.05) and by treatment with verapamil (0.25 mM verapamil vs. untreated, P < 0.05). Meiotic progression was completely blocked at metaphase I in the presence of 0.85 mM EGTA.(ABSTRACT TRUNCATED AT 250 WORDS)

Phospholipid turnover and ultrastructural correlates during spontaneous germinal vesicle breakdown of the bovine oocyte: effects of a cyclic AMP phosphodiesterase inhibitor

The turnover of [32P]orthophosphate in bovine oocyte phospholipids was studied during the early stages of spontaneous meiotic maturation, and during inhibition of this process by the cAMP phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX). Radioactive lipids were separated by TLC and the meiotic stage was determined cytogenetically. Ultrastructure of the nuclear membrane was examined using transmission EM. During the commitment period to meiotic resumption, which precedes germinal vesicle breakdown (GVBD), small localized convolutions appeared in the intact nuclear membrane. This was accompanied by a decrease in [32P]phosphatidic acid (PA) and an increase in [32P]-phosphatidylcholine (PC). This was followed by extensive convolutions, and subsequent dissociation, of the nuclear membrane, concomitant with a tremendous surge in [32P]PC and [32P]phosphatidylethanolamine (PE). The cAMP-mediated maintenance of meiotic arrest involved retention of entire nuclear envelope integrity and total inhibition of the surge in [32P]PC and [32P]PE which accompanied GVBD. The increase in [32P]phosphatidylinositol (PI) associated with all stages of early meiotic resumption was unaffected by IBMX. Microinjection of heparin inhibited GVBD, and injection of inositol 1,4,5-trisphosphate (IP3) overrode IBMX-maintained meiotic arrest in almost 40% of the oocytes. The results suggest that there may be several functions for phospholipid turnover in the regulation of spontaneous meiotic resumption in the bovine oocyte. The first precedes the commitment period, and involves IP3 generation to serve as the primary signal for meiotic resumption. The second occurs concomitant with the commitment period, is unaffected by the level of intracellular cAMP, and is associated with the general turnover of phospholipid. The third is associated with GVBD, and is cAMP-sensitive, and may represent stimulation of de novo synthesis of phospholipid, thereby permitting disruption of the nuclear membrane.

Neomycin, an inhibitor of phosphoinositide hydrolysis, inhibits the resumption of bovine oocyte spontaneous meiotic maturation

The possibility that the intracellular signals generated upon phosphoinositide hydrolysis are involved in regulating bovine oocyte spontaneous meiotic resumption was investigated. Oocytes were mass-harvested and cultured in 2A-BMOC medium supplemented with 0.5% bovine serum albumin in the presence or absence of neomycin (an inhibitor of phosphoinositide hydrolysis) or phorbol myristate acetate (an activator of protein kinase C). The role of intracellular calcium was examined by preloading with BAPTA/AM (a calcium chelator) prior to culture. Meiotic maturation was scored cytogenetically. 1) Neomycin induces an irreversible inhibition of germinal vesicle breakdown which does not exceed 60% and is apparent at concentrations of 5 mM or above. Progression of meiosis past metaphase I is inhibited at concentrations of 2.5 mM or above. The full effect of neomycin is only apparent if it is presented to the oocytes within 3 h of follicular release, although germinal vesicle breakdown is not observed until 9 h culture under control conditions. 2) PMA alone has negligible effect on germinal vesicle breakdown, but it acts synergistically with 2 mM IBMX to inhibit this process. PMA has a dual effect on the progression of meiosis past metaphase I: 1 nM PMA has a stimulatory effect while 1 microM PMA blocks the ability of oocytes to reach anaphase I or beyond. These observations are not found with a non-tumor-promoting phorbol ester. 3) Spontaneous meiotic resumption is not significantly affected in the absence of added exogenous calcium. However, oocytes preloaded with BAPTA/AM exhibit a dose-dependent inhibition of germinal vesicle breakdown, even in the presence of extracellular calcium.(ABSTRACT TRUNCATED AT 250 WORDS)

Meiotic competence acquisition is associated with the appearance of M-phase characteristics in growing mouse oocytes

To determine whether the acquisition of meiotic competence during the growth phase of oogenesis is associated with the appearance of M-phase characteristics, oocytes obtained from 13- to 30-day-old mice were evaluated by fluorescence microscopy with respect to chromatin and microtubule organization , in vitro maturation ability, and the distribution of M-phase phosphoproteins. Meiotically incompetent oocytes were distinguished from their competent counterparts in displaying elaborate interphase-like arrays of cytoplasmic microtubules and dispersed germinal vesicle chromatin. Meiotically competent oocytes were larger in size, exhibited condensation of chromatin around the nucleolus, and displayed a progressive diminution of cytoplasmic microtubules in conjunction with the appearance of multiple microtubule organizing centers. After 24 hr in culture, medium- to large-sized oocytes exhibiting perinucleolar chromatin condensation resume meiosis whereas smaller meiotically incompetent oocytes retain GVs with diffuse chromatin. Moreover, indirect immunofluorescence studies using the M-phase phosphoprotein specific monoclonal antibody MPM-2 indicate that the appearance of reactive cytoplasmic foci is directly correlated with nuclear changes characteristic of meiotically competent oocytes. Thus, the earliest transition to a meiotically competent state during oocyte growth in the immature mouse ovary is characterized by stage-specific and coordinated modifications of nuclear and cytoplasmic components.

Ultrastructural correlates of meiotic maturation in mammalian oocytes

Immature mammalian oocytes reside in ovarian follicles with junctionally coupled granulosa cells. When released from a currently undefined meiotic arresting influence, these oocytes resume meiosis to progress from late diplotene (germinal vesicle stage) through the first meiotic division to metaphase II. Oocytes remain at metaphase II until fertilization activates them to complete meiosis. This review summarizes ultrastructural events that occur during meiotic maturation in mammals. Developmental correlates that promise a clearer understanding of regulatory mechanisms operating to control maturation are emphasized. By use of TEM of thin sections, freeze-fracture analysis, and replicated oocyte cortical patches, we demonstrate stage-specific changes in the oocyte nucleus, reorganization of cytoplasmic organelles, correlations between oocyte maturational commitment and the junctional integrity of associated granulosa cells, and definition of the components comprising the oocyte cortical cytoplasm.

Effects of cyclic AMP on the spontaneous meiotic maturation of cumulus-free bovine oocytes cultured in chemically defined medium

The rate of spontaneous meiotic maturation and the period of commitment to this process were determined in bovine oocytes devoid of surrounding cumulus cells, cultured in chemically defined medium with bovine serum albumin in the absence of serum. The effects of compounds that are known to elevate levels of intracellular cyclic adenosine monophosphate (cAMP) on the resumption and progression of meiosis were investigated. Bovine oocytes were mass-harvested, denuded of cumulus cells, and cultured in 2A-BMOC medium supplemented with 0.5% bovine serum albumin. Intracellular cAMP levels were indirectly modified using 8-bromo-cAMP, dibutyryl cAMP (dbcAMP), forskolin, or 3-isobutyl-1-methyl xanthine (IBMX). Meiotic maturation was scored cytogenetically. Ninety percent of denuded bovine oocytes mature after 24 h, with 65% progressing beyond anaphase I. These oocytes remain at the germinal vesicle (GV) stage for up to 8 h in culture. GV breakdown (GVBD) occurs in 40.5% of oocytes at 9 h. The peak times for the different meiotic stages were 12 h for diakinesis, 15 h for late diakinesis to metaphase I, 20 h for metaphase I, and 24 h for telophase I. By 48 h, most had reached metaphase II. There is a 2-h lag period between the time at which they become irreversibly committed to mature (at 7 h) and when they demonstrate GVBD (at 9 h). Incubation for 12 h with high concentrations of 8-bromo-cAMP and forskolin significantly inhibited GVBD, while the effect of dbcAMP was similar but less pronounced.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino acids can induce nondisjunction during maturation of mammalian oocytes in vitro

Certain chromosomal disorders in mammalian embryos are traceable to meiotic errors during oocyte maturation. This report evaluates the influence of amino acids on meiotic maturation in vitro of oocytes from pigs, hamsters, and rats. The results indicate that maturing porcine oocytes respond not only to 1-glutamine (1-gln) but also to 1-isoleucine (1-ileu) in complex or chemically defined media by exhibiting significantly (P less than .05) increased incidences of nondisjunction when compared with oocytes in control medium. Nondisjunction was highly correlated (r = 0.981) with dose of 1-gln in porcine oocytes (incidences of maturing oocytes exhibiting nondisjunction were 19.3%, 39.7%, 41.5%, 66.2%, and 88.5% at 1-gln concentrations of 0 mM, 0.5 mM, 1.0 mM, 3.0 mM and 10.0 mM, respectively). Hamster oocytes also exhibited significantly (P less than .05) increased nondisjunction when cultured in medium containing 1-gln (52% of maturing cumulus-enclosed oocytes exhibited nondisjunction in medium with 3.0 mM 1-gln vs. 6.3% in control medium). Denuded hamster oocytes also responded to 1-gln (35% exhibited nondisjunction in 4.0 mM 1-gln, 8% in control medium). In contrast the incidence of nondisjunction in rat oocytes was not increased significantly by 1-gln over a concentration range of 0-12.0 mM. This study demonstrates that maturing oocytes respond to certain environmental conditions by undergoing chromosomally abnormal maturation. Specifically, amino acids can induce nondisjunction when present in elevated concentrations during oocyte maturation. The amino acid influence not only was dose dependent for porcine oocytes but also was species specific.(ABSTRACT TRUNCATED AT 250 WORDS)

Gene expression during oogenesis and oocyte development in mammals

Mouse oocytes progress through early meiotic prophase during fetal life and reach the diplotene stage by birth. During prepubertal and reproductive life, oocytes are continuously selected to grow from the pool of small primordial oocytes. Growing oocytes reach full size in 2 weeks, and full-grown oocytes are present in rapidly enlarging follicles for about 5 days before meiotic maturation and ovulation. RNA synthesis during early meiotic prophase, as estimated from [3H]uridine incorporation followed by autoradiography and from electron microscopic analysis of nuclear components, proceeds at a moderate rate throughout except for a brief period in early pachytene when synthesis is low or absent. RNA synthesis continues in primordial oocytes at a moderate rate. Incorporation studies, electron microscopic analyses, and particularly measurements of ongoing RNA polymerase activity (completion of initiated chains as analysed in tissue sections) indicate a distinctly increased rate of synthesis during oocyte growth over that of primordial oocytes, followed by a decline in full-grown oocytes. During growth, this rate increases severalfold. The absolute rate of synthesis of heterogeneous nuclear RNA (using rRNA as a standard) during mid-growth is very rapid, but nevertheless still much lower than that in typical lampbrush chromosomes. Most of the hnRNA turns over with a half-life of about 20 min, as is typical in somatic cells. Newly synthesized mRNA-like RNA enters the cytoplasm at about one-half the rate of rRNA, and about one-third of the ribosomes and one-fourth of the mRNA appear in polysomes. In full-grown oocytes, the rate of synthesis falls distinctly, but a significant level of synthesis continues until it essentially ceases at breakdown of the germinal vesicle. During meiotic prophase, chromosomes are most compact at pachytene and unfold lateral projections as RNA synthesis increases in late pachytene-early diplotene. In primordial oocytes, the diplotene state of chromosomes is obvious in most mammals, but in rodents the chromosomes are more evenly dispersed and are said to be in a dictyate state, although they are still presumably in a diplotene configuration. The chromosome core, which is present in leptotene through early diplotene stages, apparently disappears in the dictyate stage.(ABSTRACT TRUNCATED AT 400 WORDS)

In the absence of protein, estradiol suppresses meiosis of porcine oocytes in vitro

The effect of estradiol on the spontaneous maturation of porcine oocytes was investigated. Cumulus-enclosed (intact) and cumulus-free (denuded) oocytes were cultured in the presence of estradiol-17 beta (0 to 10 microgram/ml) in a chemically defined bicarbonate-buffered medium that contained either dextran or BSA, or in a complex Hepes-buffered medium that was supplemented with serum. After 24 hr, chromatin spreads were prepared and meiotic maturation was scored. The biochemical integrities of the cumulus cells were assessed by determination of the estradiol and progesterone content of spent media after culture of intact oocytes in the presence of 0.5 X 10(-6) M testosterone and 10 microgram/ml follicle-stimulating hormone. Estradiol did not significantly affect the onset of maturation of either intact or denuded oocytes that were cultured in medium containing either BSA or serum. In serum-supplemented medium, however, the progression of maturation beyond metaphase I was significantly affected by the steroid in a dose-dependent manner. The steroid significantly inhibited the release from meiotic arrest of both types of oocyte cultured in medium supplemented with dextran. Supplementation of all media with testosterone and FSH significantly stimulated the synthesis of estradiol by the cumulus cells, compared with that of control groups. The synthesis of progesterone, however, was significantly stimulated by testosterone and FSH only in the BSA and serum-supplemented media. It is concluded that exogenous estradiol has the capacity to arrest meiosis in vitro but that this capacity can only be expressed if no exogenous protein(s) is present. In the absence of exogenous protein, progesterone synthesis by the adherent cumulus cells is minimal.