Association of MPF, MAPK, and nuclear progression dynamics during activation of young and aged bovine oocytes

Effects of MEK1/2 inhibitor U0126 on FGF10-enhanced buffalo oocyte maturation in vitro

Fibroblast growth factor 10 (FGF10) plays critical roles in oocyte maturation and embryonic development; however, the specific pathway by which FGF10 promotes in vitro maturation of buffalo oocytes remains elusive. The present study was aimed at investigating the mechanism underlying effects of the FGF10-mediated extracellular regulated protein kinases (ERK) pathway on oocyte maturation and embryonic development in vitro. MEK1/2 (mitogen-activated protein kinase kinase) inhibitor U0126, alone or in combination with FGF10, was added to the maturation culture medium during maturation of the cumulus oocyte complex. Morphological observations, orcein staining, apoptosis detection, and quantitative real-time PCR were performed to evaluate oocyte maturation, embryonic development, and gene expression. U0126 affected oocyte maturation and embryonic development in vitro by substantially reducing the nuclear maturation of oocytes and expansion of the cumulus while increasing the apoptosis of cumulus cells. However, it did not have a considerable effect on glucose metabolism. These findings suggest that blocking the MEK/ERK pathway is detrimental to the maturation and embryonic development potential of buffalo oocytes. Overall, FGF10 may regulate the nuclear maturation of oocytes and cumulus cell expansion and apoptosis but not glucose metabolism through the MEK/ERK pathway. Our findings indicate that FGF10 regulates resumption of meiosis and expansion and survival of cumulus cells via MEK/ERK signaling during in vitro maturation of buffalo cumulus oocyte complexes. Elucidation of the mechanism of action of FGF10 and insights into oocyte maturation should advance buffalo breeding. Further studies should examine whether enhancement of MEK/ERK signaling improves embryonic development in buffalo.

Chronological age enhances aging phenomena and protein nitration in oocyte

Background

The average age of childbearing has increased over the years contributing to infertility, miscarriages, and chromosomal abnormalities largely invoked by an age-related decline in oocyte quality. In this study, we investigate the role of nitric oxide (NO) insufficiency and protein nitration in oocyte chronological aging.

Methods

Mouse oocytes were retrieved from young breeders (YB, 8-14 weeks [w]), retired breeders (RB, 48-52w) and old animals (OA, 80-84w) at 13.5 and 17 hours after ovulation trigger. They were assessed for zona pellucida dissolution time (ZPDT); ooplasmic microtubule dynamics (OMD); cortical granule (CG) status and spindle morphology (SM), as markers of oocyte quality. Sibling oocytes from RB were exposed to NO supplementation and assessed for aging phenomena (AP). All oocyte cumulus complexes were subjected to fluorescence nitrotyrosine (NT) immunocytochemistry and confocal microscopy to assess morphology and protein nitration.

Results

At 13.5 h from hCG trigger, oocytes from RB compared to YB had significantly increased ZPDT (37.8 ± 11.9 vs 22.1 ± 4.1 seconds [s]), OMD (46.9 vs 0%), CG loss (39.4 vs 0%), and decreased normal SM (30.3 vs 81.3%), indicating premature AP that worsened among oocytes from RB at 17 hours post-hCG trigger. When exposed to SNAP, RB AP significantly decreased (ZPDT: 35.1 ± 5.5 vs 46.3 ± 8.9s, OMD: 13.3 vs 75.0% and CG loss: 50.0 vs 93.3%) and SM improved (80.0 vs 14.3%). The incidence of NT positivity was significantly higher in cumulus cells (13.5 h, 46.7 ± 4.5 vs 3.4 ± 0.7%; 17 h, 82.2 ± 2.9 vs 23.3 ± 3.6%) and oocytes (13.5 h, 57.1 vs 0%; 17 h, 100.0 vs 55.5%) from RB compared to YB. Oocytes retrieved decreased with advancing age (29.8 ± 4.1 per animal in the YB group compared to 10.2 ± 2.1 in RB and 4.0 ± 1.6 in OA). Oocytes from OA displayed increased ZPDT, major CG loss, increased OMD and spindle abnormalities, as well as pronuclear formation, confirming spontaneous meiosis to interphase transition.

Conclusions

Oocytes undergo zona pellucida hardening, altered spindle and ooplasmic microtubules, and premature cortical granule release, indicative of spontaneous meiosis-interphase transition, as a function of chronological aging. These changes are also associated with NO insufficiency and protein nitration and may be alleviated through supplementation with an NO-donor.

DNA damage repair is suppressed in porcine aged oocytes

This study sought to evaluate DNA damage and repair in porcine postovulatory aged oocytes. The DNA damage response, which was assessed by H2A.X expression, increased in porcine aged oocytes over time. However, the aged oocytes exhibited a significant decrease in the expression of RAD51, which reflects the DNA damage repair capacity. Further experiments suggested that the DNA repair ability was suppressed by the downregulation of genes involved in the homologous recombination (HR) and nonhomologous end-joining (NHEJ) pathways. The expression levels of the cell cycle checkpoint genes, CHEK1 and CHEK2, were upregulated in porcine aged oocytes in response to induced DNA damage. Immunofluorescence results revealed that the expression level of H3K79me2 was significantly lower in porcine aged oocytes than in control oocytes. In addition, embryo quality was significantly reduced in aged oocytes, as assessed by measuring the cell proliferation capacity. Our results provide evidence that DNA damage is increased and the DNA repair ability is suppressed in porcine aged oocytes. These findings increase our understanding of the events that occur during postovulatory oocyte aging.

FGF18 modulates CTGF mRNA expression in cumulus-oocyte complexes and early bovine embryos: preliminary data

The Hippo pathway is involved in the proliferation of intrafollicular cells and in early embryonic development, mainly because effectors of this pathway are key transcription regulators of genes such as CTGF and CYR61, which are involved in cell proliferation. Recent studies by our group found that fibroblast growth factor 18 (FGF18) is present in the fallopian tube during early embryonic development, leading to the hypothesis that FGF18 may have a role during embryonic development. Therefore, the aim of the following study was to determine whether FGF18 modulates the expression of Hippo pathway target genes, CTGF and CYR61, during oocyte maturation and early embryonic development. Three experiments were carried out, with in vitro maturation (IVM) of cumulus-oocyte complexes (COCs) and embryo culture. In experiment one, FGF18 (100 ng/ml) induced an increase (P < 0.05) in CTGF gene expression at 12 h post-exposure. In experiment two, FGF18 (100 ng/ml) induced a reduction (P < 0.05) in CTGF expression at 3 h post-exposure. In the third experiment, day 7 embryos exposed to FGF18 during oocyte IVM expressed greater CTGF mRNA abundance, whereas FGF18 exposure during embryo in vitro embryo culture did not alter CTGF expression in comparison with untreated controls. The preliminary data presented here show that FGF18 modulates CTGF expression in critical periods of oocyte nuclear maturation, cumulus expansion and early embryonic development in cattle.

Functions and dysfunctions of the mammalian centrosome in health, disorders, disease, and aging

Since its discovery well over 100 years ago (Flemming, in Sitzungsber Akad Wissensch Wien 71:81-147, 1875; Van Beneden, in Bull Acad R Belg 42:35-97, 1876) the centrosome is increasingly being recognized as a most impactful organelle for its role not only as primary microtubule organizing center (MTOC) but also as a major communication center for signal transduction pathways and as a center for proteolytic activities. Its significance for cell cycle regulation has been well studied and we now also know that centrosome dysfunctions are implicated in numerous diseases and disorders including cancer, Alstrom syndrome, Bardet-Biedl syndrome, Huntington's disease, reproductive disorders, and several other diseases and disorders. The present review is meant to build on information presented in the previous review (Schatten, in Histochem Cell Biol 129:667-686, 2008) and to highlight functions of the mammalian centrosome in health, and dysfunctions in disorders, disease, and aging with six sections focused on (1) centrosome structure and functions, and new insights into the role of centrosomes in cell cycle progression; (2) the role of centrosomes in tumor initiation and progression; (3) primary cilia, centrosome-primary cilia interactions, and consequences for cell cycle functions in health and disease; (4) transitions from centrosome to non-centrosome functions during cellular polarization; (5) other centrosome dysfunctions associated with the pathogenesis of human disease; and (6) centrosome functions in oocyte germ cells and dysfunctions in reproductive disorders and reproductive aging.

Centrosome and microtubule functions and dysfunctions in meiosis: implications for age-related infertility and developmental disorders

The effects of oocyte aging on meiotic spindle dynamics have been well recognised, but the mechanisms underlying the effects are not well understood. In this paper we review the role of centrosomes and the microtubule cytoskeleton in meiotic spindle formation and maintenance, and the impact of oocyte aging on spindle integrity resulting in centrosome and microtubule dysfunctions that are associated with aneuploidy. Loss of spindle integrity includes dispersion of proteins from the centrosome core structure and loss of attachment of microtubules to centrosomes and kinetochores, which will result in abnormal chromosome separation. The inability of centrosomal proteins to accurately associate with the centrosome structure may be the result of destabilisation of the core structure itself or of microtubule destabilisation at the centrosome-facing microtubule areas that are acetylated in fresh oocytes but may not be acetylated in aging oocytes. Microtubule destabilisation prevents accurate motor-driven transport of centrosomal proteins along microtubules to form and maintain a functional centrosome. Other factors to form and maintain the MII spindle include signal transductions that affect microtubule dynamics and stability. Understanding the mechanisms underlying centrosome and microtubule dysfunctions during oocyte aging will allow diagnosis and analysis of oocyte quality and abnormalities as important aspects for targeted treatment of aging oocytes to extend or restore viability and developmental capacity. New therapeutic approaches will allow improvements in reproductive success rates in IVF clinics, as well as improvements in reproductive success rates in farm animals. This review is focused on: (1) centrosome and microtubule dynamics in fresh and aging oocytes; (2) regulation of centrosome and/or microtubule dynamics and function; and (3) possible treatments to extend the oocyte's reproductive capacity and viability span.

Mitogen-Activated Protein Kinase Activity Is Not Essential for the First Step of Nuclear Reprogramming in Bovine Somatic Cell Nuclear Transfer

For reprogramming a somatic nucleus during mammalian cloning, metaphase of the second meiotic division (MII) oocytes has been widely used as recipient cytoplasm. High activity of maturation-promoting factor (MPF) and mitogen-activated protein kinase (MAPK) is believed to accelerate the remodeling and/or reprogramming of a somatic nucleus introduced into the ooplasm by somatic cell nuclear transfer. We demonstrated previously that the first step in nuclear reprogramming is not directly regulated by MPF and MAPK because activated oocytes in which MPF activity is diminished and MAPK activity is maintained can develop to the blastocyst stage after receiving an M phase somatic nucleus in bovine cloning. In this study, our aim was to test whether MAPK activity is necessary for the first step in nuclear reprogramming and/or chromatin remodeling (phosphorylation of histone H3 at Ser3, trimethylation of histone H3 at Lys 9, and acetylation of histone H3 at Lys14) in bovine somatic cloning. We found that it was not necessary, and neither was MPF activity.

Oocyte aging in a marine protostome worm: The roles of maturation-promoting factor and extracellular signal regulated kinase form of mitogen-activated protein kinase

The roles of maturation-promoting factor (MPF) and an extracellular signal regulated kinase form of mitogen-activated protein kinase (ERK MAPK) are analyzed during oocyte aging in the marine protostome worm Cerebratulus. About a day after removal from the ovary, unfertilized metaphase-I-arrested oocytes of Cerebratulus begin to flatten and swell before eventually lysing, thereby exhibiting characteristics of a necroptotic mode of regulated cell death. Based on immunoblots probed with phospho-specific antibodies, MPF and ERK are initially active in freshly mature specimens. However, as oocytes age, both kinase activities decline, with ERK deactivation occurring well before MPF downregulation. Experiments using pharmacological modulators indicate that oocyte degradation is promoted by the maturation-initiated activation of ERK as well as by the deactivation of MPF that occurs in extensively aged specimens. The potential significance of these findings is discussed relative to previously published results for apoptotic eggs and oocytes of echinoderm and vertebrate deuterostomes.

Changes of spontaneous parthenogenetic activation and development potential of golden hamster oocytes during the aging process

The golden hamster is an excellent animal experimental model for oocyte research. The hamster oocytes are very useful in clinical examination of human spermatozoan activity. Non-fertile oocytes can lead to time-dependent processes of aging, which will affect the results of human spermatozoa examination. As a consequence there is a need to investigate the aging and anti-aging processes of golden hamster oocytes. In order to study the aging processes and parthenogenetic activation of golden hamster oocytes, in vivo oocytes, oocytes cultured with or without cumulus cells, and oocytes treated with Trichostatin A (TSA) or caffeine were collected and investigated. We found that: (1) spontaneous parthenogenetic activation, developmental potential (cleavage rate), and zona pellucida (ZP) hardening undergo age-dependent changes in in vivo, in vitro, and after TSA or caffeine treatment; (2) in vivo, oocytes became spontaneously parthenogenetic 25 h post-hCG treatment; (3) in vitro, cumulus cells did not significantly increase the parthenogenetic activation rate of cultured hamster oocytes; and (4) TSA or caffeine could delay spontaneous oocyte parthenogenetic activation and the aging processes by at least 5h, but also accelerated the hardening of the ZP. These results define the conditions for the aging and anti-aging processes in golden hamster oocytes. TSA and caffeine play roles in controlling spontaneous activation, which could facilitate the storage and use of golden hamster oocytes for studying processes relevant to human reproduction.

Leukemia inhibitory factor enhances bovine oocyte maturation and early embryo development

The present study was conducted to examine the effects of leukemia inhibitory factor (LIF) on bovine oocyte maturation and early embryo development in vitro. Results showed that LIF supplementation (25 ng/ml) enhanced nuclear maturation of intact cumulus-oocyte complexes (COCs) compared to the vehicle control. Similar results were observed in denuded oocytes, indicating that LIF directly influences oocyte development. LIF-treated oocytes showed a higher cortical-granule-migration rate and increased expression of CD9, a tetraspanin transmembrane protein essential for fertilization. After in vitro fertilization, oocytes receiving LIF supplementation exhibited a higher cleavage rate and yielded a significantly higher number of blastocysts. To further dissect the molecular mechanism underlying this LIF-induced bovine oocyte maturation phenotype, we examined the involvement of two signaling cascades, mitogen-activated protein kinases (MAPK3/1)- and the signal transducer and activator of transcription 3 (STAT3)-dependent pathways. Western blot results revealed that LIF phosphorylated MAPK3/1 and STAT3. Inhibition of MAPK3/1 activation with MEK inhibitor U0126 only partially blocked LIF-induced nuclear maturation, although it attenuated oocyte cytoplasmic maturation. Inhibition of JAK/STAT3 activation with a specific pharmacological inhibitor completely abolished the LIF-response in bovine oocyte. In summary, these data revealed a novel role for LIF in bovine oocyte maturation subsequent embryonic development.

Functional role of the bovine oocyte-specific protein JY-1 in meiotic maturation, cumulus expansion, and subsequent embryonic development

Oocyte-expressed genes regulate key aspects of ovarian follicular development and early embryogenesis. We previously demonstrated a requirement of the oocyte-specific protein JY-1 for bovine early embryogenesis. Given that JY-1 is present in oocytes throughout folliculogenesis, and oocyte-derived JY-1 mRNA is temporally regulated postfertilization, we hypothesized that JY-1 levels in oocytes impact nuclear maturation and subsequent early embryogenesis. A novel model system, whereby JY-1 small interfering RNA was microinjected into cumulus-enclosed germinal vesicle-stage oocytes and meiotic arrest maintained for 48 h prior to in vitro maturation (IVM), was validated and used to determine the effect of reduced oocyte JY-1 expression on nuclear maturation, cumulus expansion, and embryonic development after in vitro fertilization. Depletion of JY-1 protein during IVM effectively reduced cumulus expansion, percentage of oocytes progressing to metaphase II, proportion of embryos that cleaved early, total cleavage rates and development to 8- to 16-cell stage, and totally blocked development to the blastocyst stage relative to controls. Supplementation with JY-1 protein during oocyte culture rescued effects of JY-1 depletion on meiotic maturation, cumulus expansion, and early cleavage, but did not rescue development to 8- to 16-cell and blastocyst stages. However, effects of JY-1 depletion postfertilization on development to 8- to 16-cell and blastocyst stages were rescued by JY-1 supplementation during embryo culture. In conclusion, these results support an important functional role for oocyte-derived JY-1 protein during meiotic maturation in promoting progression to metaphase II, cumulus expansion, and subsequent embryonic development.

Aging-related changes in in vitro-matured bovine oocytes: oxidative stress, mitochondrial activity and ATP content after nuclear maturation

The objective of this research was to clarify the aging-related changes in in vitro-matured bovine oocytes. Firstly, we examined the fertilization and embryonic development of bovine oocytes after 22 and 30–34 h of in vitro maturation (IVM). The oocytes after 30–34 h of IVM (penetrated by sperm at around 40 h after starting IVM) showed a lower developmental rate to blastocysts (P<0.01), although normal fertilization rates were similar regardless of IVM duration. In the next experiment, reactive oxygen species (ROS), mitochondrial activity and ATP content in oocytes after 20, 30 and 40 h of IVM were examined. The lowest level of ROS was found in the group subjected to 30 h of IVM. The mitochondrial activity and ATP content in the group subjected to 40 h of IVM were higher than in the group subjected to 20 h of IVM (P<0.01), and those in the group subjected to 30 h of IVM showed intermediate values. Thereafter, the mitochondrial activities at 3 days after in vitro fertilization in embryos derived from the oocytes subjected to 22 and 34 h of IVM were evaluated. In the group subjected to 34 h of IVM, high-polarized mitochondria were frequently observed at the periphery of blastomeres. The present results suggest that high mitochondrial activity observed in oocytes after prolonged IVM culture and localization of high-polarized mitochondria at the periphery of blastomeres during early embryonic development may be associated with the low developmental competence in aged bovine oocytes.

Estimation of the optimal timing of fertilization for embryo development of in vitro-matured bovine oocytes based on the times of nuclear maturation and sperm penetration

The objective of this research was to estimate the optimal timing for fertilization to achieve proper embryonic development of in vitro-matured bovine oocytes. First, cumulus-oocyte complexes were subjected to in vitro maturation (IVM) for 14–22 hr. The timing when 50% of oocytes reached metaphase II stage was estimated to be 17.5 hr after IVM start. Next, using oocytes subjected to IVM for 12–30 hr, sperm penetration was examined after 4–18 hr of in vitro fertilization (IVF). A significant negative correlation between IVM duration and the timing when 50% of oocytes were penetrated by sperm after IVF start was observed (P<0.01). Finally, oocytes subjected to 12–30 hr of IVM were inseminated and cultured for 6 days to examine embryonic development. In the group with 22 hr of IVM, the percentages of cleaved embryos and blastocysts were the highest values in all groups. According to the regression equation describing the time from nuclear maturation to sperm penetration (x) and the percentage of blastocysts (y) (y=7.23x − 0.297x², P<0.01), the blastocyst rate peaked when sperm penetration occurred at 12.2 hr after achieving nuclear maturation. In conclusion, under the present IVM/IVF conditions, it was estimated that oocytes acquired their highest developmental competence at about 30 hr after IVM start, and thus, the optimal IVM duration was calculated to be about 21 hr.

Degradation of actin nucleators affects cortical polarity of aged mouse oocytes

OBJECTIVE

To investigate the molecular mechanism of mouse oocyte polarity loss during aging.

DESIGN

Experimental study.

SETTING

Academic basic research laboratory.

ANIMAL(S)

Mice.

INTERVENTION(S)

Oocytes were collected 16 hours after injection of hCG and cultured in M16 medium for an additional 14 hours with or without caffeine.

MAIN OUTCOME MEASURE(S)

Expression and localizations of actin nucleators actin-related protein 2/3 complex, JMY, and WAVE2 were examined by immunofluorescence staining, and their messenger RNA levels were examined by real-time reverse transcription-polymerase chain reaction.

RESULT(S)

The protein and messenger RNA levels of actin-related protein 2/3 complex, JMY, and WAVE2 were decreased in aged oocytes, but the levels were normal in caffeine-treated aged oocytes.

CONCLUSION(S)

Our data indicated that the loss of oocyte polarity may be due to the degradation of actin nucleators in aged oocytes.

Follicular oocytes better support development in rabbit cloning than oviductal oocytes

This study was conducted to determine the effect of rabbit oocytes collected from ovaries or oviducts on the developmental potential of nuclear transplant embryos. Donor nuclei were obtained from adult skin fibroblasts, cumulus cells, and embryonic blastomeres. Rabbit oocytes were flushed from the oviducts (oviductal oocytes) or aspirated from the ovaries (follicular oocytes) of superovulated does at 10, 11, or 12 h post-hCG injection. The majority of collected oocytes were still attached to the sites of ovulation on the ovaries. We found that follicular oocytes had a significantly higher rate of fusion with nuclear donor cells than oviductal oocytes. There was no difference in the cleavage rate between follicular and oviductal groups, but morula and blastocyst development was significantly higher in the follicular group than in the oviductal group. Two live clones were produced in follicular group using blastomere and cumulus nuclear donors, whereas one live clone was produced in the oviductal group using a cumulus nuclear donor. These results demonstrate that cloned rabbit embryos derived from follicular oocytes have better developmental competence than those derived from oviductal oocytes.

Disparate consequences of heat stress exposure during meiotic maturation: embryo development after chemical activation vs fertilization of bovine oocytes

Consequences of heat stress exposure during the first 12 h of meiotic maturation differed depending on how and when bovine oocytes were activated. If heat-stressed oocytes underwent IVF at ~24 h, blastocyst development was less than for respective controls and similar to that obtained for nonheat-stressed oocytes undergoing IVF at 30 h (i.e. slightly aged). In contrast, if heat-stressed oocytes underwent chemical activation with ionomycin/6-dimethylaminopurine at 24 h, blastocyst development was not only higher than respective controls, but also equivalent to development obtained after activation of nonheat-stressed oocytes at 30 h. Developmental differences in chemically activated vs IVF-derived embryos were not related to fertilization failure or gross alterations in cytoskeletal components. Rather, ionomycin-induced calcium release and MAP kinase activity were less in heat-stressed oocytes. While underlying mechanisms are multifactorial, ability to obtain equivalent or higher development after parthenogenetic activation demonstrates that oocytes experiencing heat stress during the first 12 h of meiotic maturation have the necessary components to develop to the blastocyst stage, but fail to do so after fertilization.

Cellular and molecular mechanisms of various types of oocyte aging

It is well established that age-related decline of a woman's fertility is related to the poor developmental potential of her gametes. The age-associated decline in female fertility is largely attributable to the oocyte aging caused by ovarian aging. Age-associated oocyte aging results in a decrease in oocyte quality. In contrast to ovarian aging, there is a concept of postovulatory oocyte aging. Postovulatory aging of oocytes, not being fertilized for a prolonged time after ovulation, is known to significantly affect the development of oocytes. Both categories of oocyte aging have similar phenotypes of reproductive failure. However, the mechanisms of the decline in oocyte quality are not necessarily equivalent. An age-dependent increase in aneuploidy is a key determinant of oocyte quality. The reduced expression of molecules regulating cell cycle control during meiosis might be involved in the age-dependent increase in aneuploidy. The mechanism of age-associated oocyte aging might be involved in mitochondrial dysfunction, whose etiologies are still unknown. Alternatively, the mechanism of postovulatory oocyte aging might be involved in reactive oxygen species-induced mitochondrial injury pathways followed by abnormal intracellular Ca²⁺ regulation of the endoplasmic reticulum. We suggest that future research into the mechanism of oocyte aging will be necessary to develop a method to rescue the poor developmental potential of aged oocytes.

Effect of maternal age on the developmental competence and progression of nuclear maturation in bovine oocytes

Progression of meiotic division in oocytes and early embryonic development are affected by oocytes quality. In most mammals, oocyte quality declines with increase in maternal age. The main aim of the present study is to investigate the effect of maternal age on developmental competence, progression of meiotic division, and associated kinetics of maturation promoting factor (MPF) activity in bovine oocytes. Oocytes were collected from the ovaries of young and old cows (here after referred to as young cow oocytes and old cow oocytes, respectively). When old cow oocytes were matured and fertilized in vitro, the rate of abnormal fertilization was greater than that in young cow oocytes. Moreover, progression of nuclear maturation and activation of MPF during oocyte maturation (or inactivation of MPF and formation of pronucleus after insemination) were faster in old cow oocytes than in young cow oocytes. Relative expression of cyclin B, cyclin-dependent kinase 1 and MAD2 transcripts in either immature or mature oocytes did not differ between the two groups. When cumulus cells (CC) were removed and denuded oocytes were cultured, there was no difference in the progression of nuclear maturation between the two age groups. Moreover gap junctions between oocytes and CC disappeared more rapidly during maturation of old cow oocytes than of young cow oocytes. These results suggest that the fertilization ability of old cow oocytes is low and that premature progression of meiotic division in these oocytes is partly due to impaired oocyte-CC gap junctions communication.

Impact of linoleic acid on bovine oocyte maturation and embryo development

Linoleic acid (LA; 18:2 n-6) is the most abundant fatty acid in bovine follicular fluid, and it was previously reported that LA concentration significantly decreases when follicle size increases. This suggests that LA may have a role in the regulation of oocyte maturation. The present study investigated the effect of LA supplementation on bovine oocyte maturation and early embryo development in vitro. Treatment of cumulus-oocyte complexes (COCs) with LA significantly inhibited cumulus cell expansion and retarded development of the oocytes to the metaphase II (MII) stage in a dose-dependent manner. This effect was reversible, and the oocytes developed to the MII stage after extended culture in the absence of LA. Treatment of COCs with LA also resulted in a significantly lower percentage of cleaved embryos and blastocyst yield. Furthermore, COCs treated with LA had significant effects compared with controls in i) increasing prostaglandin E(2) concentration in the medium, ii) decreasing intracellular cAMP at 6 and 24 h of maturation and iii) decreasing phosphorylation of the MAPK1 and 3 at 24 h, and AKT at 6 h of maturation. In conclusion, LA supplementation to bovine oocytes during maturation altered the molecular mechanisms regulating oocyte maturation and resulted in decreased percentage of oocytes at MII stage and inhibition of the subsequent early embryo development. These data provide evidence for adverse effects of LA on oocyte development, which can be associated with dietary increased level of LA in the follicular fluid and the decline in fertility in farm animals and human.

Colcemid-treatment of heifer oocytes enhances nuclear transfer embryonic development, establishment of pregnancy and development to term

Four experiments were designed to examine the effects of colcemid, a microtubule assembly inhibitor, on the development of bovine nuclear transfer (NT) embryos in vitro and in vivo. Recipient oocytes matured at different times were exposed to colcemid. Approximately 80-93% of the exposed oocytes, with or without the first polar body (PB1), developed obvious membrane projections. In Experiment 1, oocytes matured for either 14-15 or 16-17 hr, treated with colcemid and used as recipient cytoplasm for NT resulted in over 40% blastocyst development. In Experiment 2, oocytes matured for 16-17 hr were treated with either 0.2 or 0.4 microg/ml colcemid for 2-3 or 5-6 hr, respectively. The percentages of blastocyst development (39-42%) were not statistically different among the different colcemid treatment groups, but were both higher (P < 0.05) than the control group (30%). Colcemid concentrations and length of colcemid treatment of oocytes did not affect their ability to support NT embryo development to the blastocyst and hatched blastocyst stages. Results from Experiment 3 indicate that semi-defined medium increases morula and blastocyst development of NT embryos derived from colcemid-treated oocytes under 5% CO2 in air atmosphere. In addition, cell numbers of blastocysts in colcemid-treated groups were numerically higher than the control groups. After embryo transfer, higher (P < 0.05) pregnant rates were obtained from the colcemid-treated group than the nontreated group. Five of 40 recipients (12.5%) which received embryos from colcemid-treated oocytes delivered healthy calves, significantly higher than those recipients (3.3%) that received embryos derived from nontreated oocytes.

The effect of linolenic Acid on bovine oocyte maturation and development

Dietary polyunsaturated fatty acids can influence reproductive performance. In dairy cattle, some high-fat diets resulted in higher blastocyst rates and improved embryo quality. These effects may partly be mediated by a direct action of fatty acids on oocyte development. The present study investigated the effect of linolenic acid (ALA; 18:3 n-3) supplementation on bovine oocyte maturation and early embryo development in vitro. Treatment of cumulus-oocyte complexes (COCs) with 50 muM ALA significantly increased the percentage of oocytes at the metaphase II (MII) stage compared with untreated controls (95% +/- 2% vs. 84% +/- 2%, respectively). Higher doses of ALA were detrimental. Treatment of COCs with 50 muM ALA compared with controls also resulted in a significantly higher percentage of cleaved embryos (77% +/- 9% vs. 69% +/- 9%, respectively) and blastocyst rate (36% +/- 4% vs. 23% +/- 5%, respectively) and better-quality embryos. Furthermore, COCs treated with ALA had significant increases compared with controls in: 1) prostaglandin E(2) (PGE(2)) concentration (233% +/- 41%) in the medium, 2) intracellular cAMP at 3 h of maturation, and 3) phosphorylation of the mitogen-activated protein kinases (MAPKs) during the first 6 h of maturation. Moreover, ALA overcame the suppressive effects of the prostaglandin-endoperoxide synthase 2 inhibitor (NS-398) on oocyte maturation and partially improved the maturation rate in the presence of the MAPK kinase inhibitor (U-0126). Linolenic acid could not, however, recover maturation in the presence of both inhibitors. In conclusion, treatment of bovine COCs with ALA during oocyte maturation affects the molecular mechanisms controlling oocyte nuclear maturation, leading to an increased number of MII-stage oocytes and improved subsequent early embryo development. This effect is mediated both directly through MAPK pathway and indirectly through PGE(2) synthesis.

Beneficial effect of young oocytes for rabbit somatic cell nuclear transfer

This study was designed to examine the effect of the age of rabbit oocytes on the developmental potential of cloned embryos. The metaphase II oocytes used for nuclear transfer (NT) were collected at 10, 12, 14, and 16 h post-hCG injection (hpi). The total number of oocytes collected per donor (21.4-23.7) at 12 to 16 hpi was similar, but significantly higher than that collected at 10 hpi (16.2). Additionally, a significant improvement in blastocyst development was achieved with embryos generated by electrically mediated cell fusion (56.0%), compared to those from nuclear injection (13.1 %) (Experiment 1). Markedly higher blastocyst development (45.8-54.5%) was also achieved with oocytes collected at 10-12 hpi than from those collected 14-16 hpi (8.3-14.3%) (Experiment 2). In Experiment 3, the blastocyst rates of NT embryos derived from oocytes harvested 12 hpi (39.2-42.8 %) were significantly higher than from those collected at 16 hpi (6.8-8.4 %) (p < 0.05), regardless of the donor cell age. Kinase activity assays showed variable changes of activity in rabbit oocytes over the period of 10-16 hpi; however, there was no correlation with preimplantational development (blastocyst rate vs. MPF, R = 0.326; blastocyst rate vs. MAPK, R = -0.131). Embryo transfer of NT embryos utilizing 12 hpi oocytes resulted in one full-term but stillborn, and one live cloned rabbit; thus, an efficiency of 1.7 % (n = 117) (Experiment 4). These results demonstrated that NT utilizing relatively young rabbit oocytes, harvested at 10-12 h after hCG injection, was beneficial for the development of NT embryos.

Kinetics of nuclear status and kinase activities during in vitro maturation of canine oocytes

In contrast to those of other mammals, canine oocytes are ovulated at the germinal vesicle (GV) stage and then progress to the metaphase II (MII) stage in the oviduct. In other species, oocytes at the MII are widely used for in vitro fertilization or as recipients in somatic cell nuclear transfer. Many researchers have tried to improve the in vitro maturation (IVM) of canine oocytes. However, the proportion of MII oocytes remains low, resulting in poor efficiency of embryogenesis in vitro. This leads us to the possibility that the in vitro cytoplasmic maturation of canine oocytes is insufficient. Furthermore, the optimal culture period for IVM of canine oocytes is controversial, and physiological evaluation is required to improve canine IVM. We show here the time-dependent changes in mitogen-activated protein kinase (MAPK) and p34(cdc2) kinase activities in canine oocytes during IVM, since it is well known that both MAPK and p34(cdc2) kinase are activated following meiotic progression and show high activities in the MII stage in other species. Immediately after collection from ovaries, most oocytes were arrested at the GV stage, which was maintained until 24 h of culture. At 48 h of culture, more than half of the oocytes had progressed beyond the MI stage. A higher proportion of MII oocytes were observed with 72 h of culture compared with other culture periods. MAPK activity was found to increase in a time-dependent manner and reached a plateau at 72 h of culture. The level of p34(cdc2) kinase activity also increased in a time-dependent manner, with its maximal level observed after 72 h of culture. Activity was decreased with 96 h of culture, although there was no significant difference in the proportion of MII oocytes between 72 and 96 h. Our data thus show that the optimal culture period for IVM of canine oocytes is 72 h because both MAPK and p34(cdc2) kinase showed high activities at that time.

Demecolcine-assisted enucleation of goat oocytes: protocol optimization, mechanism investigation, and application to improve the developmental potential of cloned embryos

Although demecolcine-assisted enucleation has been performed successfully in porcine and cattle, the mechanism and protocol optimization of chemically assisted enucleation need further investigation. The present study optimized the protocol for goat oocyte enucleation and demonstrated that a 30-min treatment with 0.8 ng/mL demecolcine-induced cytoplasmic protrusions in over 90% of the oocytes. Rates of enucleation, cell fusion, and blastocyst formation were significantly higher after demecolcine-assisted than after blind aspiration enucleation, although differences in rates of live births remain to be unequivocally determined between the two treatments. The ability to form protrusions decreased significantly as spindles became less organized in aged oocytes and the oocytes with a poor cumulus expansion. More than 93% of the demecolcine-induced protrusions persisted for 2 h in the absence of cytochalasin B (CB) but most disappeared within 30 min of CB treatment. The spindle disintegrated, an actin-rich ring formed around the chromosome mass and the MAP kinase activity increased significantly after demecolcine treatment. When oocytes with induced protrusions were treated with CB, however, the contractile ring disappeared, the spindle reintegrated, and both MPF and MAP kinase activities decreased significantly. It is concluded that (1) cytoplasmic protrusions can be induced in goat oocytes with a very low concentration of demecolcine; (2) oocyte selection and enucleation can be achieved simultaneously with demecolcine treatment; and (3) an interactive effect between the MAP kinase, MPF, microfilaments and microtubules might be implicated in the control of cytoplasmic protrusion formation after demecolcine treatment.

Oocyte spontaneous activation in different rat strains

Oocyte spontaneous activation (OSA) has been reported to occur during in vitro culture of ovulated rat oocytes. The objective of this study was to compare the rate of oocyte spontaneous activation and the level of maturation promoting factor (MPF) activity in oocytes from different strains. Twelve strains were selected from two commercial sources. Females were superovulated and oocytes collected 17 h after hCG injection. Denuded oocytes were cultured in M16 medium under oil at 37 degrees C and 5% CO(2) in air. The proportion of activated oocytes was determined after 6 h of in vitro culture. Data were compared by analysis of variance (ANOVA), considering each animal as an experimental unit. MPF activity was determined in oocytes from the different strains at 0, 1.5, and 3 h after oocyte collection. The log ratio of the MPF activity at 1.5 and 3 h relative to 0 hours for each animal was analyzed by ANOVA. While significant (p < 0.01) differences were observed between strains in the rate of OSA, there were no differences between strains in the level of MPF during the time points measured (p > 0.3).

The effect of activation treatments on the development of reconstructed bovine oocytes

The purpose of this study was to investigate the effect of different activation treatments on the development of IVM-derived and cloned bovine embryos. The effect of oocyte age (20h versus 24h after IVM) on the blastocyst rate was also investigated. No differences in the percentage of blastocyst development were observed between the oocytes matured for 20 and 24h (15% versus 27%, p>0.05). Reconstructed oocytes activated 4h after fusion (fusion before activation, FBA) had a higher blastocyst rate than those oocytes activated immediately after electrofusion (fusion and activation simultaneously, FAS) (26% versus 5%, p<0.01). Blastocyst rates were significantly greater (p<0.01) for the reconstructed oocytes activated by calcium ionophore (A23187) combined with 6-dimethylaminopurine (6-DMAP) (51.6%) than for those activated with cycloheximide (CHX) plus cytochalasin B (CB) treatment (1h, 8.2%; 5h, 14.3%). However, the blastocyst rates were similar among reconstructed oocytes activated by electric pulses and A23187 (30.5% versus 42.2%) or by A23187 and ionomycin (36.7% versus 33.3%) combined with 6-DMAP, respectively. Blastocysts that developed from reconstructed oocytes activated by A23187 and 6-DMAP resulted in three pregnancies (3/9) and one live birth from 18 embryos transferred to recipient cows. Genotypic analysis of six bovine microsatellite markers by polymerase chain reaction confirmed that the cloned calf was genetically identical to the nuclear donor. In conclusion, reconstructed oocytes that derived from oocytes exposed to activation treatment 4h after electrofusion are more likely to develop to the blastocyst stage. The best treatment to activate reconstructed bovine oocytes in this study was A23187 combined with 6-DMAP.

Activation of Akt (protein kinase B) stimulates metaphase I to metaphase II transition in bovine oocytes

In somatic cells, the serine/threonine kinase Akt (or protein kinase B) was shown to contribute to processes linked to cellular growth, cell survival and cell cycle regulation. In contrast to these findings, the function of Akt during the meiosis of mammalian oocytes remains to be investigated. We analysed the phosphorylation pattern and the activity of Akt during meiotic maturation (transition from prophase I to metaphase II) of bovine oocytes. The oocytes were maturedin vitro(IVM) for 0, 10 and 24 h to reach the germinal vesicle (GV), metaphase I (M I) and metaphase II (M II) stages respectively. The abundance and phosphorylation pattern of Akt was revealed by Western blotting using total Akt or phosphoso-Akt-specific antibodies. The activity of this particular kinase was determined by anin vitrokinase assay. Furthermore, functional properties were analysed by cultivating oocytes in the presence of the Akt inhibitor SH6. The results showed that the overall abundance of Akt did not change significantly during IVM. On the other hand, Akt became phosphorylated at Thr 308 and Ser 473, reaching its maximum at the M I phase. In the GV and M II stages, only low basal phosphorylation levels were observed on both sides. This phosphorylation profile corresponded strictly to the activity of the kinase. The cultivation of oocytes in the presence of the phosphatidylinositol analogue SH6 for 24 h showed that, with higher concentrations, up to 65% of the oocytes were arrested in the M I stage. This result indicated that Akt is involved in the M I/M II transition during the meiotic maturation of bovine oocytes. The physiological aspects of the Akt function will be discussed.

Effects of duration, concentration, and timing of ionomycin and 6-dimethylaminopurine (6-DMAP) treatment on activation of goat oocytes

The protocol of ionomycin followed by 6-dimethylaminopurine (6-DMAP) is commonly used for activation of oocytes and reconstituted embryos. Since numerous abnormalities and impaired development were observed when oocytes were activated with 6-DMAP, this protocol needs optimization. Effects of concentration and treatment duration of both drugs on activation and development of goat oocytes were examined in this study. The best oocyte activation (87-95%), assessed by pronuclear formation, was obtained when oocytes matured in vitro for 27 hr were treated with 0.625-20 microM ionomycin for 1 min before 6-hr incubation in 2 mM 6-DMAP. Progressional reduction of time for 6-DMAP-exposure showed that the duration of 6-DMAP treatment can be reduced to 1 hr from the second up to the fourth hour after ionomycin, to produce activation rates greater than 85%. Activation rates of oocytes in vitro matured for 27, 30, and 33 hr were higher (P < 0.05) than that of oocytes matured for 24 hr when treated with ionomycin plus 1-hr (the third hour) 6-DMAP, but a 4-hr incubation in 6-DMAP enhanced activation of the 24-hr oocytes. Goat activated oocytes began pronuclear formation at 3 hr and completed it by 5-hr post ionomycin. An extended incubation in 6-DMAP (a) impaired the development of goat parthenotes, (b) quickened both the release from metaphase arrest and the pronuclear formation, and (c) inhibited the chromosome movement at anaphase II (A-II) and telophase II (T-II), leading to the formation of one pronucleus without extrusion of PB2. In conclusion, duration, concentration, and timing of ionomycin and 6-DMAP treatment had marked effects on goat oocyte activation, and to obtain better activation and development, goat oocytes matured in vitro for 27 hr should be activated by 1 min exposure to 2.5 microM ionomycin followed by 2 mM 6-DMAP treatment for the third hour.

Parthenogenetic activation of mouse oocytes by strontium chloride: a search for the best conditions

Strontium has been successfully used to induce activation of mouse oocytes in nuclear transfer and other experiments, but the optimum treatment conditions have not been studied systematically. When cumulus-free oocytes were treated with 10mM SrCl(2) for 0.5-5h, activation rates (88.4+/-4.1 to 91.2+/-2.7%) did not differ (mean+/-S.E.; P>0.2), but rate of blastulation (57.3+/-3.5%) and cell number per blastocyst (45.0+/-2.4) were the highest after treatment for 2.5h. When treated with 1-20mM SrCl(2) for 2.5h, the activation rate and cell number per blastocyst were higher (P<0.02) after 10mM SrCl(2) treatment than other treatments. The best activation and development were obtained with Ca(2+)-free Sr(2+) medium, but the activation rate was low (37.7+/-1.6%) in Ca(2+)-containing medium. Activation rates were the same, regardless of the presence or absence of cytochalasin B (CB) in the activating medium, but the blastulation rate was higher (P<0.001) in the presence of CB. Only 70% of the cumulus-enclosed oocytes were activated and 10% blastulated after a 10 min exposure to 1.6mM SrCl(2), and many lysed, with increased intensity of Sr(2+) treatment. The presence of CB in SrCl(2) medium markedly reduced lysis of cumulus-enclosed oocytes. Media M16 and CZB did not differ when used as activating media. Only 10.5% of the oocytes collected 13 h post hCG were activated by Sr(2+) treatment alone, with 34% blastulating, but rates of activation and blastulation increased (P<0.001) to 94 and 60%, respectively, when they were further treated with 6-dimethylaminopurine (6-DMAP). The total and ICM cell numbers were less (P<0.001) in parthenotes than in the in vivo fertilized embryos. In conclusion, the concentration and duration of SrCl(2) treatment and the presence or absence of CB in activating medium and cumulus cells had marked effects on mouse oocyte activation and development. To obtain the best activation and development, cumulus-free oocytes collected 18 h post hCG should be treated for 2.5h with 10mM SrCl(2) in Ca(2+)-free medium supplemented with 5 microg/mL of CB.

Activated bovine cytoplasts prepared by demecolcine-induced enucleation support development of nuclear transfer embryos in vitro

Demecolcine-induced enucleation (IE) of mouse oocytes has been shown to improve development to term of cloned mice. In this study, we characterized the kinetics and morphological progression of bovine oocytes subjected to IE, and evaluated their ability to support embryo development to the blastocyst stage after nuclear transfer (NT). In vitro matured bovine oocytes were parthenogenetically activated and subsequently exposed to demecolcine at various times post-activation. Onset and duration of demecolcine treatment significantly altered activation and IE frequencies, which varied from 7.1% to 100% and 33.3% to 91.7%, respectively, at 5 hr post-activation. A significant decrease in IE frequencies was observed at 17 hr post-activation (3.4%-46.1%), possibly due to reincorporation of chromosomes into the oocyte after incomplete second polar body (PB) extrusion. Oocytes were reconstructed by NT before (treatment 1) or after (treatment 2) activation and demecolcine treatment, and cultured in vitro. Cleavage (48.1%-54.2%) and blastocyst rates (15.7%-19%) were equivalent for the two treatments, as well as the total cell number in NT blastocysts. Furthermore, most of the blastocysts were completely diploid (treatment 2) or heteroploid but with a majority of diploid nuclei (treatment 1). Our results demonstrate that the IE method can be successfully used to produce enucleated bovine cytoplasts that are competent to support development to the blastocyst stage after NT. This technically simple approach may provide a more efficient method to enhance the success rate of NT procedures. Further studies are needed to improve the in vitro development efficiency and to expand our understanding of the mechanism(s) involved in demecolcine-induced enucleation.

Nuclear Dynamics of Parthenogenesis of Human Oocytes: Effect of Oocyte Aging in vitro

We investigated in detail the nuclear kinetics of oocyte activation of aged human oocytes following combined activation treatment with calcium ionophore and puromycin. Two types of oocytes were used: (a) 1-day-old oocytes after 20-24 h retrieval, and (b) 2-day-old oocytes after 44-50 h retrieval. A total of 185 unfertilized aged oocytes, 91 1-day-old and 94 2-day-old oocytes, were fixed at 1, 2, 4, 6 and 8 h after activation treatment and then metaphase II (MII), anaphase or telophase II (A/T II) or pronuclear stage were recorded. We demonstrated that a combined calcium ionophore and puromycin treatment induced a high activation rate in both 1-day-old (95.6%) and 2-day-old oocytes (95.2%). Our results also demonstrated that the nuclear progression was faster in 2-day-old oocytes than in 1-day-old oocytes, although nuclear progression in parthenogenetically activated human oocytes requires the longer time periods compared with ICSI fertilization. It is concluded that combined treatment of the calcium ionophore and puromycin allows a high rate of parthenogenetic activation and the nuclear kinetics of parthenogenetically activated human oocytes appears to be more rapid in in vitro aging oocytes.

Effects of post-treatment with 6-Dimethylaminopurine (6-DMAP) on ethanol activation of mouse oocytes at different ages

To study the effect of post-treatment with 6-Dimethylaminopurine (6-DMAP) on oocyte activation and development, mouse oocytes collected at different times post human chorion gonadotropin (hCG) injection were incubated in 6-DMAP-containing Chatot-Ziomek-Bavister (CZB) medium for different periods after ethanol exposure, and activation and development were observed. When oocytes were cultured in 6-DMAP without prior ethanol exposure, the highest activation rate was only 40%. Incubation in 6-DMAP for 6 h following ethanol exposure significantly (P < 0.05) increased the activation rate in oocytes recovered 15 and 18 h post hCG, but this effect was not significant in the 21 h oocytes. When oocytes were incubated in 6-DMAP for 1 h at different time points after ethanol, a 6-DMAP susceptible temporal window was found to be located from the second to the fifth h in the 18 h oocytes and from the fourth to the fifth h in the 15 h oocytes, and within the window, the duration of 6-DMAP incubation can be reduced to 0.5 h with more than 80% activation. With the 13 h oocytes, however, 6-DMAP-incubation can only be shortened to 3 h and no specific temporal window was identified. Oocytes that were incubated in 6-DMAP for 1 or 2 h after ethanol exposure developed to morula/blastocyst stages at significantly (P < 0.05) higher rates than those incubated in 6-DMAP for 6 h. Our results suggested that (i) long duration of 6-DMAP incubation impaired the development of mouse parthenogenotes; (ii) the effect of 6-DMAP alone was limited without prior ethanol exposure; (iii) the egg age affected both the timing of 6-DMAP susceptibility and the duration of exposure required to obtain a maximal activating effect; (iv) the most effective activating protocols varied for oocytes of different ages.

Activation of equine nuclear transfer oocytes: methods and timing of treatment in relation to nuclear remodeling

Early development of embryos produced by transfer of equine nuclei to bovine cytoplasts is superior to that of intraspecies equine nuclear transfer embryos. This may be related to differences in chromatin remodeling or efficiency of activation between the two oocyte types. The pattern of donor nucleus remodeling was examined in equine-equine and equine-bovine reconstructed oocytes. Chromosome condensation occurred in equine cytoplasts by 2 h but was not seen in bovine cytoplasts until 4 h. We investigated the effect of activation of equine-equine reconstructed oocytes at <30 min or at 2 h after reconstruction. Four activation treatments were evaluated at each time point: injection of sperm extract alone, or in combination with 6-dimethylaminopurine (6-DMAP), cytochalasin B, or 1% dimethylsulphoxide. There was no significant difference in normal cleavage rate or average nucleus number of embryos between equine oocytes activated <30 min or at 2 h after reconstruction. The combination of 6-DMAP with sperm extract significantly (P < 0.01) improved cleavage rate compared with the other three treatments. Activation with sperm extract and 6-DMAP 2 h after donor nucleus injection gave the highest cleavage (79%) and the highest cleavage with normal nuclei (40%). Sperm extract and 6-DMAP also effectively activated oocytes parthenogenetically, yielding 83% cleavage and 73% cleavage with normal nuclei. These results indicate that although nuclear remodeling occurs rapidly in equine cytoplasts, early activation does not improve embryonic development after reconstruction.

Development of parthenogenetic rat embryos

In an effort to establish cloning technology for the rat, we tested several methods (electric stimulation, treatment with ethanol or strontium) for the parthenogenetic activation of rat oocytes. We observed marked individual differences among rats of the outbred Wistar strain in their ability to yield activatable oocytes. These differences were independent of the activation protocol and may be due to a genetic predisposition that is crucial for the parthenogenetic activation of oocytes. The activation of oocytes was dependent upon the time between superovulation of the donor animal and the collection of the embryos. Aged oocytes (derived about 24 h after superovulation) were more prone to activation by each method than were younger oocytes, and some even underwent spontaneous activation without treatment and exhibited pronuclear formation and blastocyst development. All activation methods were effective in generating parthenogenetic rat embryos, and rat parthenotes developed until implantation. However, in general, short-term (15 min) and long-term (2 h) strontium treatment was superior to stimulation by ethanol or electric pulse for parthenogenetic activation. These results will be helpful in achieving successful cloning in the rat.