Effect of initial cumulus morphology on meiotic dynamic and status of mitochondria in horse oocytes during IVM

PKD regulates mitophagy to prevent oxidative stress and mitochondrial dysfunction during mouse oocyte maturation

Mitochondria play dominant roles in various cellular processes such as energy production, apoptosis, calcium homeostasis, and oxidation-reduction balance. Maintaining mitochondrial quality through mitophagy is essential, especially as its impairment leads to the accumulation of dysfunctional mitochondria in aging oocytes. Our previous research revealed that PKD expression decreases in aging oocytes, and its inhibition negatively impacts oocyte quality. Given PKD's role in autophagy mechanisms, this study investigates whether PKD regulates mitophagy to maintain mitochondrial function and support oocyte maturation. When fully grown oocytes were treated with CID755673, a potent PKD inhibitor, we observed meiosis arrest at the metaphase I stage, along with decreased spindle stability. Our results demonstrate an association with mitochondrial dysfunction, including reduced ATP production and fluctuations in Ca2+ homeostasis, which ultimately lead to increased ROS accumulation, stimulating oxidative stress-induced apoptosis and DNA damage. Further research has revealed that these phenomena result from PKD inhibition, which affects the phosphorylation of ULK, thereby reducing autophagy levels. Additionally, PKD inhibition leads to decreased Parkin expression, which directly and negatively affects mitophagy. These defects result in the accumulation of damaged mitochondria in oocytes, which is the primary cause of mitochondrial dysfunction. Taken together, these findings suggest that PKD regulates mitophagy to support mitochondrial function and mouse oocyte maturation, offering insights into potential targets for improving oocyte quality and addressing mitochondrial-related diseases in aging females.

Description of a new quantitative method to assess mitochondrial distribution pattern in mature equine oocytes

The Mitochondrial distribution pattern or MDP in mammalian oocytes serves as an indicator of their cytoplasmic maturity, with a heterogeneous pattern associated with mature cytoplasm. Currently, MDP assessment involves fluorescent labelling of mitochondria followed by visual evaluation, as no quantitative method exists. Our objective was to develop a quantitative approach to assess MDP in mature equine oocytes. Equine oocytes, obtained by ovum pick up (OPU) were matured in vitro, and only metaphase II oocytes were used in the study (n = 56). Following denudation, oocytes were fixed, stained with MitoTracker™ Red CMXRos (50 nM in TCM-199 with Hank´s salts and 10% FBS) for 15 min at 38 °C, and then incubated with 2.5 µg/ml Hoechst 33342 for 10 min at 38 °C. Confocal microscope images were acquired, and the oocyte's MDP was visually classified as either homogeneous (HoD; n = 17) or heterogeneous (HeD; n = 39). For quantitative analysis, Fiji-ImageJ software was employed. Background subtraction was performed, and a 1-pixel line along the diameter was drawn to calculate the intensity profile. Fluorescence intensities were normalized, and ratios of peripheral to central fluorescence intensity were determined. Student´s t-test was used for comparations; MDP ratio was (mean ± standard error of the mean): 0.8 ± 0.02 for HoD and 0.3 ± 0.02 for HeD (p < 0.001). These results demonstrate concordance between quantitative and qualitative MDP assessment in mature equine oocytes. Our study describes a new approach to quantify mitochondrial distribution pattern and cytoplasmic maturation in mature equine oocytes.

Establishing a 3D-cultured system based on alginate-hydrogel embedding benefits the in vitro maturation of porcine Oocytes

The in vitro maturation (IVM) quality of oocytes is directly related to the subsequent developmental potential of embryos and a fundamental of in vitro embryo production. However, conventional IVM methods fail to maintain the gap-junction intercellular communication (GJIC) between cumulus-oocyte complexes (COCs), which leads to insufficient oocyte maturation. Herein, we investigated the effects of three different three-dimensional (3D) culture methods on oocyte development in vitro, optimized of the alginate-hydrogel embedding method, and assessed the effects of the alginate-hydrogel embedding method on subsequent embryonic developmental potential of oocytes after IVM and parthenogenetic activation (PA). The results showed that Matrigel embedding and alginate-hydrogel embedding benefited the embryonic developmental potential of oocytes after IVM and PA. With the further optimization of alginate-hydrogel embedding, including crosslinking and decrosslinking of parameters, we established a 3D culture system that can significantly increase oocyte maturation and the blastocyst rate of embryos after PA (27.2 ± 1.5 vs 36.7 ± 2.8, P < 0.05). This 3D culture system produced oocytes with markedly increased mitochondrial intensity and membrane potential, which reduced the abnormalities of spindle formation and cortical granule distribution. The alginate-hydrogel embedding system can also remarkably enhance the GJIC between COCs. In summary, based on alginate-hydrogel embedding, we established a 3D culture system that can improve the IVM quality of porcine oocytes, possibly by enhancing GJIC.

Insights into the NAD+ biosynthesis pathways involved during meiotic maturation and spindle formation in porcine oocytes

Treatments that elevate NAD+ levels have been found to improve oocyte quality in mice, cattle, and pigs, suggesting that NAD+ is vital during oocyte maturation. This study aimed to examine the influence of different NAD+ biosynthetic pathways on oocyte quality by inhibiting key enzymes. Porcine oocytes from small antral follicles were matured for 44 h in a defined maturation system supplemented with 2-hydroxynicotinic acid [2-HNA, nicotinic acid phosphoribosyltransferase (NAPRT) inhibitor], FK866 [nicotinamide phosphoribosyltransferase (NAMPT) inhibitor], or gallotannin [nicotinamide mononucleotide adenylyltransferase (NMNAT) inhibitor] and their respective NAD+ pathway modulators (nicotinic acid, nicotinamide, and nicotinamide mononucleotide, respectively). Cumulus expansion was assessed after 22 h of maturation. At 44 h, maturation rates were determined and mature oocytes were fixed and stained to assess spindle formation. Each enzyme inhibitor reduced oocyte maturation rate and adversely affected spindle formation, indicating that NAD+ is required for meiotic spindle assembly. Furthermore, NAMPT and NMNAT inhibition reduced cumulus expansion, whereas NAPRT inhibition affected chromosomal segregation. Treating oocytes with gallotannin and nicotinamide mononucleotide together showed improvements in spindle width, while treating oocytes with 2-HNA and nicotinic acid combined showed an improvement in both spindle length and width. These results indicate that the salvage pathway plays a vital role in promoting oocyte meiotic progression, while the Preiss-Handler pathway is essential for spindle assembly.

The Role of Mitochondria in Oocyte Maturation

With the nucleus as an exception, mitochondria are the only animal cell organelles containing their own genetic information, called mitochondrial DNA (mtDNA). During oocyte maturation, the mtDNA copy number dramatically increases and the distribution of mitochondria changes significantly. As oocyte maturation requires a large amount of ATP for continuous transcription and translation, the availability of the right number of functional mitochondria is crucial. There is a correlation between the quality of oocytes and both the amount of mtDNA and the amount of ATP. Suboptimal conditions of in vitro maturation (IVM) might lead to changes in the mitochondrial morphology as well as alternations in the expression of genes encoding proteins associated with mitochondrial function. Dysfunctional mitochondria have a lower ability to counteract reactive oxygen species (ROS) production which leads to oxidative stress. The mitochondrial function might be improved with the application of antioxidants and significant expectations are laid on the development of new IVM systems supplemented with mitochondria-targeted reagents. Different types of antioxidants have been tested already on animal models and human rescue IVM oocytes, showing promising results. This review focuses on the recent observations on oocytes’ intracellular mitochondrial distribution and on mitochondrial genomes during their maturation, both in vivo and in vitro. Recent mitochondrial supplementation studies, aiming to improve oocyte developmental potential, are summarized.

A Nuclear and Cytoplasmic Characterization of Bovine Oocytes Reveals That Cysteamine Partially Rescues the Embryo Development in a Model of Low Ovarian Reserve

Simple Summary

Women’s reproductive performance starts declining in the mid-30s, and by age 40–45, the possibility of becoming pregnant becomes very small. Reproductive aging is a physiological process of fertility decline characterized by a decrease in quality and stockpile of eggs (also called ovarian reserve) in most mammals. However, young individuals too can show an accelerated reproductive aging that similarly results in a low ovarian reserve and hypofertility. This syndrome, called premature ovarian failure (POF), is becoming a relevant problem due to the general tendency to postpone the first pregnancy. In this study, we used bovine ovaries that were classified in two categories, according to the number of follicles visible on the ovarian surface, and analyzed some parameters of egg maturation. We observed that eggs from the ‘aging-like’ ovaries carry several defects that impair maturation. However, one of the parameters was improved upon supplementation with a scavenger of free radicals, providing a proof of concept that in-depth knowledge of the cellular mechanisms is essential to find solutions to everyday-life problems.

Abstract

Decreased oocyte quality is a major determinant of age-associated fertility decline. Similarly, individuals affected by early ovarian aging carry low-quality oocytes. Using an established bovine model of early ovarian aging, we investigated key features of ‘quality’ oocyte maturation, associated with the onset of egg aneuploidy and reproductive aging, such as histone modifications, mitochondria distribution and activity, reduced glutathione (GSH) content, and gap junction functionality. Bovine ovaries were classified according to the antral follicle count (AFC), and the retrieved oocytes were processed immediately or matured in vitro. We observed alterations in several cellular processes, suggesting a multifactorial etiology of the reduced oocyte quality. Furthermore, we performed a rescue experiment for one of the parameters considered. By adding cysteamine to the maturation medium, we experimentally increased the free radical scavenger ability of the ‘low competence’ oocytes and obtained a higher embryo development. Our findings show that adopting culture conditions that counteract the free radicals has a positive impact on the quality of ‘compromised’ oocytes. Specifically, cysteamine treatment seems to be a promising option for treating aging-related deficiencies in embryo development.

Lipid Metabolism in Bovine Oocytes and Early Embryos under In Vivo, In Vitro, and Stress Conditions

Lipids are a potential reservoir of energy for initial embryonic development before activation of the embryonic genome and are involved in plasma membrane biosynthesis. Excessive lipid droplet formation is detrimental to cryotolerance and is related to alterations in mitochondrial function, which likely affects lipid metabolism. Increased lipid accumulation in in vitro produced embryos is a consequence of the stress during in vitro embryonic development process. There are several open questions concerning embryo lipid metabolism and developmental potential. Oocyte maturation and embryo development in vivo and in vitro may vary if the donors are subjected to any type of stress before follicle puncture because crucial changes in oocyte/embryonic metabolism occur in response to stress. However, little is known about lipid metabolism under additional stress (such as heat stress). Therefore, in this review, we aimed to update the information regarding the energy metabolism of oocytes and early bovine embryos exhibiting developmental competence, focusing on lipid metabolic pathways observed under in vivo, in vitro, and stress conditions.

Altered morphokinetics in equine embryos from oocytes exposed to DEHP during IVM

Di-(2-ethylhexyl) phthalate (DEHP) is a commonly used plasticizer with endocrine-disrupting properties. In this study, we used an equine model to investigate DEHP concentrations in ovarian follicular fluid (FF), and to determine the effects of exposure of oocytes to potentially toxic concentrations of DEHP during in vitro maturation (IVM) on embryo development and quality. Embryo development was evaluated using time-lapse monitoring (TLM), a photomicroscopic tool that reveals abnormalities in cleavage kinetics unobservable by conventional morphology assessment. Blastocyst bioenergetic/oxidative status was assessed by confocal analysis. The possibility that verbascoside (VB), a bioactive polyphenol with antioxidant activity, could counteract DEHP-induced oocyte oxidative damage, was investigated. DEHP was detected in FF and in IVM media at concentrations up to 60 nM. Culture of oocytes in the presence of 500 nM DEHP delayed second polar body extrusion, reduced duration of the second cell cycle, and increased the percentage of embryos showing abrupt multiple cleavage, compared with controls. Mitochondrial activity and intracellular levels of reactive oxygen species were reduced in blastocysts from DEHP-exposed oocytes. VB addition during IVM limited DEHP-induced blastocyst damage. In conclusion, DEHP is detectable in equine FF and culture medium, and oocyte exposure to increased concentrations of DEHP during IVM affects preimplantation embryo development. Moreover, TLM, reported for the first time in the horse in this study, is an efficient tool for identifying altered morphokinetic parameters and cleavage abnormalities associated with exposure to toxic compounds.

Resveratrol supplementation during in vitro maturation improves embryo development of prepubertal goat oocytes selected by brilliant cresyl blue staining

This study aimed to investigate the effect of resveratrol supplementation in maturation medium on the developmental ability and bioenergetic\oxidative status of prepubertal goat oocytes selected by brilliant cresyl blue (BCB). Oocytes collected from slaughterhouse-derived ovaries were selected by 13 µM BCB staining and classified as grown BCB+ and growing BCB- oocytes. All oocytes were matured in vitro in our conventional maturation medium and supplemented with 1 µM (BCB+R and BCB-R) and without (Control groups: BCB+C and BCB-C) resveratrol. After 24 h, IVM-oocytes were fertilized with fresh semen and presumptive zygotes were in vitro cultured for 8 days. Oocytes were assessed for blastocyst development and quality, mitochondrial activity and distribution, and levels of GSH, ROS, and ATP. BCB+R (28.3%) oocytes matured with resveratrol presented significantly higher blastocyst development than BCB+C (13.0%) and BCB- groups (BCB-R: 8.3% and BCB-C: 4.7%). Resveratrol improved blastocyst development of BCB-R oocytes at the same rate as BCB+C oocytes. No differences were observed in blastocyst quality among groups. GSH levels were significantly higher in resveratrol groups (BCB+R: 36554.6; BCB-R: 34946.7 pixels/oocyte) than in control groups (BCB+C: 27624.0; BCB-C: 27655.4 pixels/oocyte). No differences were found in mitochondrial activity, ROS level, and ATP content among the groups. Resveratrol-treated oocytes had a higher proportion of clustered active mitochondria in both BCB groups (BCB+R: 73.07%; BCB-R: 79.16%) than control groups (BCB+C: 19.35%; BCB-C: 40%). In conclusion, resveratrol increased blastocyst production from oocytes of prepubertal goats, particularly in better quality oocytes (BCB+).

Exposure to cadmium during in vitro maturation at environmental nanomolar levels impairs oocyte fertilization through oxidative damage: A large animal model study

Cadmium is a highly toxic heavy metal with negative effects on oocyte fertilization. The aim of this study was to analyse whether cadmium-induced impairment of fertilization is caused by mitochondria dysfunction and oxidative stress in the cumulus-oocyte complex (COC). Preliminarily, 19 trace element levels were measured in ovaries from juvenile and adult ewes and age-related cadmium ovarian bioaccumulation at nanomolar concentrations was found. COCs from juvenile and adult ewes, exposed during in vitro maturation to 1nM or 100nM CdCl₂, and subjected to in vitro fertilization showed significantly lower fertilization rates in exposed COCs compared with controls. In vitro matured exposed and control COCs underwent confocal microscopy analysis of mitochondria activity and reactive oxygen species (ROS) levels and lipid peroxidation (LPO) assay at cumulus cell and oocyte level. In both age groups, cadmium at nanomolar concentrations induced cumulus-oocyte mitochondria over-activity and oxidative damage which were related to impaired oocyte fertilization.

In vivo and in vitro maturation of rabbit oocytes differently affects the gene expression profile, mitochondrial distribution, apoptosis and early embryo development

In vivo-matured cumulus–oocyte complexes are valuable models in which to assess potential biomarkers of rabbit oocyte quality that contribute to enhanced IVM systems. In the present study we compared some gene markers of oocytes and cumulus cells (CCs) from immature, in vivo-matured and IVM oocytes. Moreover, apoptosis in CCs, nuclear maturation, mitochondrial reallocation and the developmental potential of oocytes after IVF were assessed. In relation to cumulus expansion, gene expression of gap junction protein, alpha 1, 43 kDa (Gja1) and prostaglandin-endoperoxide synthase 2 (Ptgs2) was significantly lower in CCs after in vivo maturation than IVM. In addition, there were differences in gene expression after in vivo maturation versus IVM in both oocytes and CCs for genes related to cell cycle regulation and apoptosis (V-Akt murine thymoma viral oncogene homologue 1 (Akt1), tumour protein 53 (Tp53), caspase 3, apoptosis-related cysteine protease (Casp3)), oxidative response (superoxide dismutase 2, mitochondrial (Sod2)) and metabolism (glucose-6-phosphate dehydrogenase (G6pd), glyceraldehyde-3-phosphate dehydrogenase (Gapdh)). In vivo-matured CCs had a lower apoptosis rate than IVM and immature CCs. Meiotic progression, mitochondrial migration to the periphery and developmental competence were higher for in vivo-matured than IVM oocytes. In conclusion, differences in oocyte developmental capacity after IVM or in vivo maturation are accompanied by significant changes in transcript abundance in oocytes and their surrounding CCs, meiotic rate, mitochondrial distribution and apoptotic index. Some of the genes investigated, such as Gja1, could be potential biomarkers for oocyte developmental competence in the rabbit model, helping improve in vitro culture systems in these species.

Cloned foal derived from in vivo matured horse oocytes aspirated by the short disposable needle system

Transvaginal ultrasound-guided follicle aspiration is one method of obtaining recipient oocytes for equine somatic cell nuclear transfer (SCNT). This study was conducted: (1) to evaluate the possibility of oocyte aspiration from pre-ovulatory follicles using a short disposable needle system (14-G) by comparing the oocyte recovery rate with that of a long double lumen needle (12-G); (2) to investigate the developmental competence of recovered oocytes after SCNT and embryo transfer. The recovery rates with the short disposable needle vs. the long needle were not significantly different (47.5% and 35.0%, respectively). Twenty-six SCNT embryos were transferred to 13 mares, and one mare delivered a live offspring at Day 342. There was a perfect identity match between the cloned foal and the cell donor after analysis of microsatellite DNA, and the mitochondrial DNA of the cloned foal was identical with that of the oocyte donor. These results demonstrated that the short disposable needle system can be used to recover oocytes to use as cytoplasts for SCNT, in the production of cloned foals and for other applications in equine embryology.

Differences in the Kinetic of the First Meiotic Division and in Active Mitochondrial Distribution between Prepubertal and Adult Oocytes Mirror Differences in their Developmental Competence in a Sheep Model

Our aim is to verify if oocyte developmental potential is related to the timing of meiotic progression and to mitochondrial distribution and activity using prepubertal and adult oocytes as models of low and high developmental capacity respectively. Prepubertal and adult oocytes were incorporated in an in vitro maturation system to determine meiotic and developmental competence and to assess at different time points kinetic of meiotic maturation, 2D protein electrophoresis patterns, ATP content and mitochondria distribution. Maturation and fertilization rates did not differ between prepubertal and adult oocytes (95.1% vs 96.7% and 66.73% vs 70.62% respectively for prepubertal and adult oocytes). Compared to adults, prepubertal oocytes showed higher parthenogenesis (17.38% vs 2.08% respectively in prepubertals and adults; P<0.01) and polispermy (14.30% vs 2.21% respectively in prepubertals and adults; P<0.01), lower cleavage rates (60.00% vs 67.08% respectively in prepubertals and adults; P<0.05) and blastocyst output (11.94% vs 34.% respectively in prepubertals and adults; P<0.01). Prepubertal oocytes reached MI stage 1 hr later than adults and this delay grows as the first meiotic division proceeds. Simultaneously, the protein pattern was altered since in prepubertal oocytes it fluctuates, dropping and rising to levels similar to adults only at 24 hrs. In prepubertal oocytes ATP rise is delayed and did not reach levels comparable to adult ones. CLSM observations revealed that at MII, in the majority of prepubertal oocytes, the active mitochondria are homogenously distributed, while in adults they are aggregated in big clusters. Our work demonstrates that mitochondria and their functional aggregation during maturation play an active role to provide energy in terms of ATP. The oocyte ATP content determines the timing of the meiotic cycle and the acquisition of developmental competence. Taken together our data suggest that oocytes with low developmental competence have a slowed down energetic metabolism which delays later development.

Developmental competence of equine oocytes: impacts of zona pellucida birefringence and maternally derived transcript expression

In the present study, equine oocytes were classified into groups of presumably high and low developmental competence according to cumulus morphology, as well as oocyte ability to metabolise brilliant cresyl blue (BCB) stain. All oocytes were evaluated individually in terms of morphometry, zona pellucida birefringence (ZPB) and relative abundance of selected candidate genes. Oocytes with an expanded cumulus (Ex), representing those with presumably high developmental competence, had a significantly thicker zona (18.2 vs 17.3µm) and a significantly higher ZPB (64.6 vs 62.1) than oocytes with a compacted cumulus (Cp). Concurrently, oocytes classified as highly developmentally competent (BCB+) had a significantly thicker zona (18.8 vs 16.1µm) and significantly higher ZPB (63.1 vs 61.3) compared with oocytes classified as having low developmental competence. Expression of TFAM, STAT3 and CKS2 was significantly higher in Ex compared with Cp oocytes, whereas expression of COX1, ATPV6E and DNMT1 was lower. Together, the data reveal that developmentally competent equine oocytes are larger in size, have higher ZPB values and exhibit a typical genetic signature of maternally derived transcripts compared with oocytes with lower in vitro developmental competence.

Effect of holding equine oocytes in meiosis inhibitor-free medium before in vitro maturation and of holding temperature on meiotic suppression and mitochondrial energy/redox potential

BACKGROUND

Evaluation of mitochondrial function offers an alternative to evaluate embryo development for assessment of oocyte viability, but little information is available on the relationship between mitochondrial and chromatin status in equine oocytes. We evaluated these parameters in immature equine oocytes either fixed immediately (IMM) or held overnight in an Earle's/Hank's' M199-based medium in the absence of meiotic inhibitors (EH treatment), and in mature oocytes. We hypothesized that EH holding may affect mitochondrial function and that holding temperature may affect the efficiency of meiotic suppression.

METHODS

Experiment 1 - Equine oocytes processed immediately or held in EH at uncontrolled temperature (22 to 27°C) were evaluated for initial chromatin configuration, in vitro maturation (IVM) rates and mitochondrial energy/redox potential. Experiment 2 - We then investigated the effect of holding temperature (25°C, 30°C, 38°C) on initial chromatin status of held oocytes, and subsequently repeated mitochondrial energy/redox assessment of oocytes held at 25°C vs. immediately-evaluated controls.

RESULTS

EH holding at uncontrolled temperature was associated with advancement of germinal vesicle (GV) chromatin condensation and with meiotic resumption, as well as a lower maturation rate after IVM. Holding did not have a significant effect on mitochondrial distribution within chromatin configurations. Independent of treatment, oocytes having condensed chromatin had a significantly higher proportion of perinuclear/pericortical mitochondrial distribution than did other GV configurations. Holding did not detrimentally affect oocyte energy/redox parameters in viable GV-stage oocytes. There were no significant differences in chromatin configuration between oocytes held at 25°C and controls, whereas holding at higher temperature was associated with meiosis resumption and loss of oocytes having the condensed chromatin GV configuration. Holding at 25°C was not associated with progression of mitochondrial distribution pattern and there were no significant differences in oocyte energy/redox parameters between these oocytes and controls.

CONCLUSIONS

Mitochondrial distribution in equine GV-stage oocytes is correlated with chromatin configuration within the GV. Progression of chromatin configuration and mitochondrial status during holding are dependent on temperature. EH holding at 25°C maintains meiotic arrest, viability and mitochondrial potential of equine oocytes. This is the first report on the effects of EH treatment on oocyte mitochondrial energy/redox potential.

A specific adenylyl cyclase inhibitor (DDA) and a cyclic AMP-dependent protein kinase inhibitor (H-89) block the action of equine growth hormone on in vitro maturation of equine oocytes

The objectives of this study were firstly to determine whether the stimulatory function of equine growth hormone (eGH) on equine oocyte maturation in vitro is mediated via cyclic adenosine monophosphate (cAMP); and secondly if the addition of eGH in vitro influences oocyte nuclear maturation and if this effect is removed when GH inhibitors are added to the culture. Cumulus–oocyte complexes (COCs) were recovered from follicles <25 mm in diameter and randomly allocated as follows: (i) control (no additives); and (ii) 400 ng/ml of eGH. A specific inhibitor against cyclic AMP-dependent protein kinase (H-89; 10−9, 10−11 or 10−15 M concentration) and a specific adenylate cyclase inhibitor, 2′,3′-dideoxyadenosine (DDA; 10−8, 10−10 or 10−14 M concentration) were used to observe whether they could block the eGH effect. After 30 h of in vitro maturation at 38.5°C with 5% CO2 in air, oocytes were stained with 10 μg/ml of Hoechst to evaluate nuclear status. More mature oocytes (P < 0.05) were detected when COCs were incubated with eGH (29 of 84; 34.5%) than in the control group (18 of 82; 21.9%). The H-89 inhibitor used at a concentration of 10−9 M (4 of 29; 13.8%) decreased (P < 0.05) the number of oocytes reaching nuclear maturation when compared with eGH (11 of 29; 38%). The DDA inhibitor at a concentration of 10−8 M (2 of 27; 7.4%) also reduced (P < 0.05) the number of oocytes reaching maturity when compared with the eGH group (9 of 30; 30%). Results from the present study show that H-89 and DDA can be used in vitro to block the eGH effect on equine oocyte maturation.

Prooxidant effects of verbascoside, a bioactive compound from olive oil mill wastewater, on in vitro developmental potential of ovine prepubertal oocytes and bioenergetic/oxidative stress parameters of fresh and vitrified oocytes

Verbascoside (VB) is a bioactive polyphenol from olive oil mill wastewater with known antioxidant activity. Oxidative stress is an emerging problem in assisted reproductive technology (ART). Juvenile ART is a promising topic because, in farm animals, it reduces the generation gap and, in human reproductive medicine, it helps to overcome premature ovarian failure. The aim of this study was to test the effects of VB on the developmental competence of ovine prepubertal oocytes and the bioenergetic/oxidative stress status of fresh and vitrified oocytes. In fresh oocytes, VB exerted prooxidant short-term effects, that is, catalase activity increase and uncoupled increases of mitochondria and reactive oxygen species (ROS) fluorescence signals, and long-term effects, that is, reduced blastocyst formation rate. In vitrified oocytes, VB increased ROS levels. Prooxidant VB effects in ovine prepubertal oocytes could be related to higher VB accumulation, which was found as almost one thousand times higher than that reported in other cell systems in previous studies. Also, long exposure times of oocytes to VB, throughout the duration of in vitro maturation culture, may have contributed to significant increase of oocyte oxidation. Further studies are needed to identify lower concentrations and/or shorter exposure times to figure out VB antioxidant effects in juvenile ARTs.

Confocal fluorescence assessment of bioenergy/redox status of dromedary camel (Camelus dromedarius) oocytes before and after in vitro maturation

BACKGROUND

Reproductive biotechnologies in dromedary camel (Camelus dromedarius) are less developed than in other livestock species. The in vitro maturation (IVM) technology is a fundamental step for in vitro embryo production (IVP), and its optimization could represent a way to increase the success rate of IVP. The aim of the present study was to investigate the bioenergy/oxidative status of dromedary camel oocytes before and after IVM by confocal microscopy 3D imaging.

METHODS

Oocytes were retrieved by slicing ovaries collected at local slaughterhouses. Recovered oocytes were examined before and after IVM culture for nuclear chromatin configuration and bioenergy/oxidative status, expressed as mitochondria (mt) distribution and activity, intracellular Reactive Oxygen Species (ROS) levels and distribution and mt/ROS colocalization.

RESULTS

The mean recovery rate was 6 oocytes/ovary. After IVM, 61% of oocytes resumed meiosis and 36% reached the Metaphase II stage (MII). Oocyte bioenergy/redox confocal characterization revealed changes upon meiosis progression. Immature oocytes at the germinal vesicle (GV) stage were characterised by prevailing homogeneous mt distribution in small aggregates while MI and MII oocytes showed significantly higher rates of pericortical mt distribution organized in tubular networks (P<0.05). Increased mt activity in MI (P<0.001) and MII (P<0.01) oocytes compared to GV stage oocytes was also observed. At any meiotic stage, homogeneous distribution of intracellular ROS was observed. Intracellular ROS levels also increased in MI (P<0.01) and MII (P<0.05) oocytes compared to GV stage oocytes. The mt/ROS colocalization signal increased in MI oocytes (P<0.05).

CONCLUSIONS

This study provides indications that qualitative and quantitative indicators of bioenergy and oxidative status in dromedary camel oocytes are modified in relation with oocyte meiotic stage. These data may increase the knowledge of camel oocyte physiology, in order to enhance the efficiency of IVP procedures.

Influence of equine growth hormone, insulin-like growth factor-I and its interaction with gonadotropins on in vitro maturation and cytoskeleton morphology in equine oocytes

In horses, successful in vitro fertilization procedures are limited by our inability to consistently mature equine oocytes by in vitro methods. Growth hormone (GH) is an important regulator of female reproduction in mammals, playing an important role in ovarian function, follicular growth and steroidogenesis. The objectives of this research were to investigate: the effects of equine growth hormone (eGH) and insulin-like growth factor-I (IGF-I) on the in vitro maturation (IVM) of equine oocytes, and the effects of eGH in addition to estradiol (E2), gonadotropins (FSH and LH) and fetal calf serum (FCS) on IVM. We also evaluated the cytoskeleton organization of equine oocytes after IVM with eGH. Equine oocytes were aspirated from follicles <30 mm in diameter and matured for 30 h at 38.5°C in air with 5% CO2. In experiment 1, selected cumulus-oocyte complexes (COCs) were randomly allocated as follows: (a) control (no additives); (b) 400 ng/ml eGH; (c) 200 ng/ml IGF-I; (d) eGH + IGF-I; and (e) eGH + IGF-I + 200 ng/ml anti-IGF-I. In addition to these treatment groups, we also added 1 μg/ml E2, 5 IU/ml FSH, 10 IU/ml LH and 10% FCS in vitro (experiment 2). Oocytes were stained with markers for microtubules (anti-α-tubulin antibody), microfilaments (AlexaFluor 488 Phalloidin) and chromatin (TO-PRO3-iodide) and assessed via confocal microscopy. No difference was observed when eGH and IGF-I was added into our IVM system. However, following incubation with eGH alone (40%) and eGH, E2, gonadotropins and FCS (36.6%) oocytes were classified as mature v. 17.6% of oocytes in the control group (P < 0.05). Matured equine oocytes showed that a thin network of filaments concentrated within the oocyte cortex and microtubules at the metaphase spindle showed a symmetrical barrel-shaped structure, with chromosomes aligned along its midline. We conclude that the use of E2, gonadotropins and FCS in the presence of eGH increases the number of oocytes reaching oocyte competence.

G6PDH-activity in equine oocytes correlates with morphology, expression of candidate genes for viability, and preimplantative in vitro development

Efficiencies for in vitro production of equine embryos are still low due to highly variable developmental competences of equine immature oocytes. In contrast to the equine, in vitro developmental competence of immature oocytes has been predicted successfully by the activity of glucose-6-phosphate dehydrogenase (G6PDH) indicated by brilliant cresyl blue (BCB) dye in a range of different species. Therefore, the aim of the present study was to test the association between G6PDH activity in equine oocytes with: (1) cumulus morphology and oocyte properties in terms of diameter and volume; (2) maturational competence; (3) gene expression of certain molecular markers; and (4) in vitro embryo development after intracytoplasmic sperm injection. Equine oocytes were exposed to BCB stain and were classified as BCB+ or BCB- according to their ability to convert the dye from blue to colorless. Additionally, BCB+ and BCB- oocytes were subclassified as having a compact (Cp) or expanded (Ex) cumulus complex. As a result, BCB+ oocytes had a greater proportion of expanded cumulus oocyte complexes compared with BCB- oocytes (71.2% vs. 49.5%). Moreover, we observed a significant difference in oocyte diameter and volume between BCB+ and BCB- oocytes irrespective of cumulus morphology. BCB+ oocytes reached a higher maturation rate compared with BCB- oocytes (59.0% vs. 28.7%). Regarding the analyzed candidate genes, relative transcript abundance was significantly different for nine genes. The expression of eight genes was significantly higher (P < 0.05) for BCB+ oocytes, including ATPV6E, IF-3, TFAM, DNMT1, STAT3, Aurora-A, ODC1, and CKS2 whereas BCB- oocytes showed higher in expression of COX1. These results are in line with the observed developmental competence. Cleavage rate (45.9% vs. 29.0%) and percentage of embryos that reached the blastocyst stage (9.2% vs. 1.4%) were significantly higher for embryos derived from BCB+ oocytes compared with BCB- oocytes. In conclusion, the present study provides evidence that G6PDH-activity in immature equine oocytes is a useful predictor for subsequent in vitro developmental competence.

Brilliant Cresyl Blue stain selects largest oocytes with highest mitochondrial activity, maturation-promoting factor activity and embryo developmental competence in prepubertal sheep

The aim of this study was to test the Brilliant Cresyl Blue (BCB) stain to select prepubertal sheep oocytes for in vitro blastocyst production. Oocyte diameter, mitochondrial activity, maturation-promoting factor (MPF) activity and mRNA relative expression (RE) of genes related to metabolism (ATPase Na(+)/K(+) transporting α 1 (ATP1A1) and cytochrome c oxidase subunit 1 (COX1)) and constitutive function of the cell (cytoplasmic polyadenylation-element-binding protein (CPEB) and S100A10) were assessed. Immature oocytes were exposed to different BCB concentrations (13, 26, 39 and 52  μM) and classified according to their cytoplasm colouration as grown BCB+ (blue cytoplasm) and growing BCB- (colourless cytoplasm). Staining oocytes with 13  μM BCB during 60  min allows selection of (BCB+) the largest (123.66  μm) and most competent oocytes to develop to the blastocyst stage (21%) with a higher number of cells (69.71 ± 6.19 s.e.m.) compared with non-stained BCB- oocytes (106.82  μm, 9% and 45.91 ± 3.35 s.e.m. respectively). Mitochondrial activity, assessed by MitoTracker Orange CMTMRos probe, was significantly higher in BCB+ than in BCB- oocytes after in vitro maturation (3369 and 1565  AU respectively). MPF activity was assessed by CDC2 kinase activity assay showing significantly higher activity at metaphase II stage in BCB+ than in BCB- oocytes (1.479 ± 0.09 and 1.184 ± 0.05 optical density respectively). The genes analysed in this work, ATP1A1, COX1, CPEB and S100A 10, did not show significant effect in mRNA RE between BCB selected oocytes. In conclusion, BCB stains larger and more competent oocytes to develop to the blastocyst stage with more active mitochondria and MPF activity and higher blastocyst cell number.

Mitochondrial aggregation patterns and activity in in vitro cultured bovine oocytes recovered from early antral ovarian follicles

The low developmental competence seen in in vitro cultured oocytes collected from early antral follicles may be related to their mitochondrial status. The aim of this study was to examine the chromatin configuration, pattern of mitochondrial aggregation and mitochondrial activity of non-cultured and in vitro-cultured bovine oocytes originating from early antral ovarian follicles. Cumulus-oocyte complexes with adjacent granulosa cells (COCGs) were recovered from early antral follicles of 0.4 to 0.8 mm diameter. Control (Day 0) oocytes were recovered from freshly collected COCGs and fixed and stained. Selected COCGs were placed in growth culture for 7 days (Day 7) or 14 days (Day 14). Following growth culture, COCs with normal appearance were placed in maturation medium (IVM) for 24 h and then fixed and stained with MitoTracker CMTM Ros Orange and Hoechst 33258. The percentage of oocytes with an immature meiotic configuration after growth culture decreased with the time of growth culture, being 96.7; 72.5 and 35.4% respectively for Day 0, Day 7 and Day 14 of culture; the remaining oocytes were degenerating or resuming meiosis. After subsequent IVM the highest proportion of oocytes in diakinesis or metaphase I was found in the D7+IVM group (59.4%). When growth culture was prolonged to day 14 and IVM, the number of degenerated oocytes increased dramatically after IVM. The mitochondrial distribution in the oocytes changed from homogeneous to heterogeneous as growth culture time increased. The respiratory activity as measured by fluorescence intensity increased over the time of growth culture, and was highest in oocytes that had resumed GVBD. In conclusion, for oocytes in isolated COCGs from early antral follicles, culture conditions longer than 7 days should be more adapted for a slow nuclear maturation accompanied by a decreased energy metabolism to prevent chromatin pycnosis.

Mitochondrial distribution and meiotic progression in canine oocytes during in vivo and in vitro maturation

The objective was to evaluate mitochondrial distribution, and its relationship to meiotic development, in canine oocytes during in vitro maturation (IVM) at 48, 72, and 96 h, compared to those that were non-matured or in vivo matured (ovulated). The distribution of active mitochondria during canine oocyte maturation (both in vitro and in vivo) was assessed with fluorescence and confocal microscopy using MitoTracker Red (MT-Red), whereas chromatin configuration was concurrently evaluated with fluorescence microscopy and DAPI staining. During IVM, oocytes exhibited changes in mitochondrial organization, ranging from a fine uniform distribution (pattern A), to increasing clustering spread throughout the cytoplasm (pattern B), and to a more perinuclear and cortical distribution (pattern C). Pattern A was mainly observed in germinal vesicle (GV) oocytes (96.4%), primarily in the non-matured group (P < 0.05). Pattern B was seen in all ovulated oocytes which were fully in second metaphase (MII), whereas in IVM oocytes, ∼64% were pattern B, irrespective of duration of culture or stage of nuclear development (P > 0.05). Pattern C was detected in a minor percentage (P < 0.05) of oocytes (mainly those in first metaphase, MI) cultured for 72 or 96 h. In vitro matured oocytes had a minor percentage of pattern B (P < 0.05) and smaller mitochondrial clusters in IVM oocytes than ovulated oocytes, reaching only 4, 11, and 17% of MII at 48, 72, and 96 h, respectively. Thus, although IVM canine oocytes rearranged mitochondria, which could be related to nuclear maturation, they did not consistently proceed to MII, perhaps due to incomplete IVM, confirming that oocytes matured in vitro were less likely to be competent than those matured in vivo.

Meiotic progression, mitochondrial features and fertilisation characteristics of porcine oocytes with different G6PDH activities

The aim of the present study was to investigate the developmental competence, mitochondrial characteristics and chromatin status of immature follicular porcine oocytes selected for their glucose-6-phosphate dehydrogenase (G6PDH) activity by brilliant cresyl blue (BCB) staining. In Experiment 1, the oocyte parameters were determined in parallel right after BCB staining (T(0)), after 22 h of in vitro maturation (IVM) (T(22)) and after 44 h of IVM (T(44)) (n = 496). BCB-stained oocytes (BCB+) at T(0) were characterised by fibrillated chromatin filaments in their germinal vesicles (GV) and diakinesis stages whereas unstained (BCB-) oocytes at T(0) contained in their GV mainly condensed stages of chromatin (P < 0.05). After 22 h of IVM BCB+ oocytes showed a prominent chromatin configuration of metaphase I and after 44 h the majority developed a M II nuclear configuration in contrast to the BCB- group (P < 0.0001). Differences were also observed between the two oocyte populations in their mitochondrial activity (P < 0.05). At the beginning of IVM BCB+ oocytes were characterised by high mitochondrial activity in their cytoplasm. The BCB+ oocytes showed clear visible homogenous distributions of mitochondria (P < 0.005) and contained more aggregated clusters of mitochondria in contrast to BCB- oocytes (P < 0.005). In Experiment 2, 318 oocytes were tested for their G6PDH activity and introduced to IVM and IVF. Only oocytes from the BCB+ group, which were matured after 44 h up to the stage of M II (81.6%) were fertilised (17.4%), penetrated (46%) or activated (15.6%) after IVF. These results indicate a relationship between the G6PDH activity of porcine oocytes before IVM and their subsequent nuclear development, mitochondrial activity and aggregation.

Expression of the mu opioid receptor and effects of the opioid antagonist naloxone on in vitro maturation of oocytes recovered from anoestrous bitches

The mu-opioid receptor (MOR) is expressed in bovine, human, equine and canine oocytes, and in seasonal breeders, it is expressed with higher intensity during the anoestrous phase. Supplementation of in vitro maturation (IVM) medium with opioid agents, agonists or antagonists, was shown to affect oocyte maturation in several species such as rat, bovine and equine. This study reports the effects of supplementing IVM medium with naloxone (Nx), an opioid antagonist, on nuclear and cytoplasmic maturation rate of oocytes recovered from anoestrous bitches. Cytoplasmic maturation was examined in terms of mitochondrial (mt) distribution. In order to confirm the receptor-mediated action of Nx, in oocytes of anoestrous bitches, MOR expression was analyzed by Western blot. Cumulus-oocyte complexes, recovered from the ovaries of bitches in anoestrous, were cultured in vitro and Nx was added at the concentrations of 1 x 10(-6), 1 x 10(-8) and 1 x 10(-10) M. The rate of oocytes resuming meiosis after culture in presence of 1 x 10(-6) M Nx (29%) was significantly higher than that of oocytes of control group (12%; p < 0.05). However, treatment with Nx did not affect mt distribution pattern. In denuded oocytes and in corresponding cumulus cells, a doublet of 65 and 50 kDa was observed. We conclude that, in oocytes of anoestrous bitches, MOR is expressed and Nx significantly improves nuclear maturation rate. Further studies should be performed to elucidate the expression of other opioid receptors, such as delta and kappa, and possible interactive effects of their antagonists on canine oocyte maturation.

Changes in the distribution of mitochondria before and after in vitro maturation of human oocytes and the effect of in vitro maturation on mitochondria distribution

OBJECTIVE

To clarify the relationship between oocyte maturation and mitochondria distribution and assess the effects of in vitro maturation (IVM) on the distribution of mitochondria in human oocytes.

DESIGN

Prospective randomized trial.

SETTING

Hospital-based IVF center.

PATIENT(S)

One hundred fifty-eight patients undergoing intracytoplasmic sperm injection (ICSI) treatment for male factors or combined with oviduct infertility and fifteen patients undergoing controlled ovarian hyperstimulation followed by coitus or IUI.

INTERVENTION(S)

Of all the 284 immature oocytes, 140 were fixed directly. The others were prepared for IVM before they were fixed. All the 21 oocytes matured in vivo were fixed directly and stained for mitochondria. Both immature and mature oocytes were stained by Mito Tracker Green FM. The distribution of mitochondria was observed using a confocal laser scanning microscope.

MAIN OUTCOME MEASURE(S)

Mitochondrial distribution.

RESULT(S)

Three mitochondria distribution patterns were identified: peripheral, semiperipheral, and evenly diffused. A peripheral distribution of mitochondria was presented by 64.1% (50/78) of the germinal vesicle (GV) oocytes; 45.2% (28/62) of the meiosis I oocytes maintained the peripheral distribution; and 38.7% (24/62) presented a diffused status. After IVM, 75.5% (80/106) of the oocytes displayed an evenly diffused type of distribution. The mitochondria were more abundant in the inner cytoplasm than in the peripheral region in most of the oocytes matured in vivo.

CONCLUSION(S)

There are obvious changes in the distribution of mitochondria in human oocytes before and after maturation. Distribution of mitochondria in oocytes matured in vitro is slightly different from that of oocytes matured in vivo. The results may partially explain the reduced developmental potential of oocytes matured in vitro compared with those matured in vivo.

Mitochondrial distribution patterns in canine oocytes as related to the reproductive cycle stage

This study investigates the mitochondrial (mt) distribution in canine ovarian oocytes examined at recovery time, as related to the reproductive cycle stage, and in oviductal oocytes. Ovarian Germinal Vesicle (GV) stage oocytes were recovered from bitches in anestrous (A, n=2), follicular phase (F, n=4), ovulation (O, n=2), early luteal (EL, n=7) and mid/late luteal phase (MLL, n=2). Oviductal GV, metaphase I (MI) or MII stage oocytes were recovered from six bitches between 56 and 110 h after ovulation. Mitochondria were revealed by using MitoTracker Orange CMTM Ros and confocal microscopy. In ovarian oocytes, three mt distribution patterns were found: (I) small aggregates diffused throughout the cytoplasm; (II) diffused tubular networks; (III) pericortical tubular networks. Significantly higher rates of oocytes showing heterogeneous mt patterns (II+III) were obtained from bitches in F (75%) and in O (96%) compared with bitches in A (31%; F vs. A: P<0.05; O vs. A: P<0.001), in EL (61%; O vs. EL: P<0.01), or in MLL (0%; F vs. MLL: P<0.05; O vs. MLL: P<0.001). Fluorescence intensity did not vary according to mt distribution pattern except that it was lower in oocytes recovered in EL phase and showing small mt aggregations (P<0.001). The majority of ovulated MII stage oocytes (79%) showed diffused tubular mt network. We conclude that mt distribution pattern of canine ovarian immature oocytes changes in relation to reproductive cycle stage and that patterns observed in oocytes recovered from bitches in periovulatory phases are heterogeneous and similar to those of in vivo matured oocytes.

Cytoplasmic lipid droplets and mitochondrial distribution in equine oocytes: Implications on oocyte maturation, fertilization and developmental competence after ICSI

Lipid droplets (LDs) and mitochondria in the ooplasm are essential for energy production required for maturation, fertilization and embryo development. This study investigates the correlations between cytoplasmic LDs polar aggregation and: (1) nuclear maturation (Experiment 1); (2) mitochondrial (mt) distribution pattern and localization (Experiment 2); (3) fertilization and embryonic development after intracytoplasmic sperm injection (ICSI; Experiment 3) in equine oocytes recovered from slaughtered mares and matured in vitro. Morphologically normal oocytes were selected after culture and categorized as having polar (P) aggregation or uniform (U) distribution of LDs. In Experiment 1, the maturation rate was significantly higher in P compared with U oocytes (69%, 40/58 vs. 32%, 13/41; P<0.001). In Experiment 2, it was observed that P and U oocytes showed heterogeneous mt distribution at comparable rates (68%, 25/37 vs. 50%, 2/4 for P and U respectively; NS). Moreover, only in 8/25 (32%) of P oocytes, LDs overlapped with mt aggregates in the area containing meiotic spindle. In Experiment 3, normal fertilization (51%, 19/37 vs. 60%, 6/10, for P and U) and cleavage rates (83%, 20/24 vs. 67%, 4/6, for P and U) did not differ between groups, also in oocytes with LDs located nearby the polar body. Overall, P aggregation of LDs was related to cumulus expansion at collection. In conclusion, in equine matured oocytes, P aggregation of LDs is related with cumulus expansion and nuclear maturation. However, it is not related with heterogeneous mt distribution and cannot be considered a predictive indicator for normal fertilization and embryo development.

Molecular and subcellular characterisation of oocytes screened for their developmental competence based on glucose-6-phosphate dehydrogenase activity

Oocyte selection based on glucose-6-phosphate dehydrogenase (G6PDH) activity has been successfully used to differentiate between competent and incompetent bovine oocytes. However, the intrinsic molecular and subcellular characteristics of these oocytes have not yet been investigated. Here, we aim to identify molecular and functional markers associated with oocyte developmental potential when selected based on G6PDH activity. Immature compact cumulus-oocyte complexes were stained with brilliant cresyl blue (BCB) for 90 min. Based on their colouration, oocytes were divided into BCB(-) (colourless cytoplasm, high G6PDH activity) and BCB(+) (coloured cytoplasm, low G6PDH activity). The chromatin configuration of the nucleus and the mitochondrial activity of oocytes were determined by fluorescence labelling and photometric measurement. The abundance and phosphorylation pattern of protein kinases Akt and MAP were estimated by Western blot analysis. A bovine cDNA microarray was used to analyse the gene expression profiles of BCB(+) and BCB(-) oocytes. Consequently, marked differences were found in blastocyst rate at day 8 between BCB(+) (33.1+/-3.1%) and BCB(-) (12.1+/-1.5%) oocytes. Moreover, BCB(+) oocytes were found to show higher phosphorylation levels of Akt and MAP kinases and are enriched with genes regulating transcription (SMARCA5), cell cycle (nuclear autoantigenic sperm protein, NASP) and protein biosynthesis (RPS274A and mRNA for elongation factor 1alpha, EF1A). BCB(-) oocytes, which revealed higher mitochondrial activity and still nucleoli in their germinal vesicles, were enriched with genes involved in ATP synthesis (ATP5A1), mitochondrial electron transport (FL405), calcium ion binding (S100A10) and growth factor activity (bone morphogenetic protein 15, BMP15). This study has evidenced molecular and subcellular organisational differences of oocytes with different G6PDH activity.

Effect of recombinant bovine somatotropin (rbST) on cytoplasmic maturation of bovine oocytes and their developmental competence in vitro

The objectives of this study were to evaluate the effects of recombinant bovine somatotropin (rbST) on the nuclear and cytoplasmic maturation of bovine oocytes and their further developmental competence to blastocysts in vitro. We analyzed the mitochondrial activity and concentration of intracellular stored calcium ([Ca(2+)](is)) in matured oocytes and the morphology and chromatin status of produced embryos after in vitro fertilization. Cumulus-oocyte complexes were incubated in TCM 199 containing 10% fetal calf serum (control medium 1: CM 1) or 10% estrus cow serum (control medium 2: CM 2). The culture medium of the treatment groups was modified by supplementation of the control medium with 10 ng/ml rbST (CM 1A and CM 2A), 10(6)/ml granulosa cells (CM 1B and CM 2B), or 10 ng/ml rbST plus 10(6)/ml granulosa cells (CM 1C and CM 2C). No differences were observed in the percentages of oocytes reaching metaphase II between the groups. However, the proportion of blastocysts was highest in treatment groups CM 1C and CM 2C (P<0.05). The type of serum did not alter the positive effect of rbST on the developmental competence of embryos. The fluorescence intensity of metabolically active mitochondria measured by intensity per oocyte (Em 570) after MitoTracker CMTM Ros Orange labeling was significantly increased in oocytes matured in the presence of 10 ng/ml rbST and granulosa cells (309.21 vs. 119.97 microA; P<0.01). In parallel, the concentration of [Ca(2+)](is) in oocytes, determined using fluorophore chlortetracycline, was significantly decreased (0.85 +/- 0.02 vs. 0.97 +/- 0.03 AU; P<0.05). Based on these results, we concluded that rbST, in interaction with granulosa cells stimulates the oxidative activity of ooplasmic mitochondria and decreases the content of [Ca(2+)](is) in oocytes. These facts support the hypothesis that somatotropin influences the developmental competence of bovine oocytes during maturation in vitro, and this effect can be modulated by granulosa cells.