Impact of litter size on the hematological and iron status of gilts, sows and newborn piglets: a comparative study of domestic pigs and wild boars.

BACKGROUND: The critically low hepatic iron stores of newborn piglets are considered to be a major cause of neonatal iron deficiency in modern breeds of domestic pig (Sus domestica). The main factor believed to contribute to this phenomenon is large litter size, which has been an objective of selective breeding of pigs for decades. As consequence, iron transferred from the pregnant sow has to be distributed among a greater number of fetuses. RESULTS: Here, we investigated whether litter size influences red blood cell (RBC) indices and iron parameters in Polish Large White (PLW) piglets and gilts. Small and large litters were produced by the transfer of different numbers of embryos, derived from the same superovulated donor females, to recipient gilts. Piglets from large litters obtained following routine artificial insemination were also examined. Our results clearly demonstrated that varying the number of piglets in a litter did not affect the RBC and iron status of 1-day-old piglets, with all showing iron deficiency anemia. In contrast, gilts with small litters displayed higher RBC and iron parameters compared to mothers with large litters. A comparative analysis of the RBC status of wild boars (having less than half as many piglets per litter as domestic pigs) and PLW pigs, demonstrated higher RBC count, hemoglobin level and hematocrit value of both wild boar sows and piglets, even compared to small-litter PLW animals. CONCLUSIONS: These findings provide evidence that RBC and iron status in newborn PLW piglets are not primarily determined by litter size, and indicate the need to study the efficiency of iron transport across the placenta in domestic pig and wild boar females.

Effect of high hydrostatic pressure on mitochondrial activity, reactive oxygen species level and developmental competence of cultured pig embryos.

High hydrostatic pressure (HHP) has been previously used to increase mammalian oocyte and embryo tolerance on subsequent stresses related with different assisted reproductive technologies. Nevertheless, the mechanisms for HHP-induced stress responses in early embryos have not been yet well understood. Previous studies focused mainly on HHP-modified gene expression while possible changes in cellular functions, including modification of energy metabolism and oxidative stress were neglected. Therefore, we aimed to analyze the effect of HHP treatment on the efficiency of subsequent in vitro pig embryos culture in NCSU-23 medium, on mitochondrial membrane potential (ΔΨm) and reactive oxygen species (ROS) level during their pre-implantation development. Porcine embryos were exposed to the hydrostatic pressure of 20 MPa and their quick response to such stress was analyzed 1 h later. In comparison with control embryos, we detected lower ΔΨm by ∼13% only in expanded blastocysts as well as decreased ROS level by ∼30% and ∼42% at the morula and expanded blastocyst stages, respectively. After HHP-treatment at transcriptionally inactive zygote stage and subsequent embryo culture, long-time responses were found: (1) at expanded blastocyst stage manifesting by ΔΨm decrease by ∼16%, (2) at the morula and expanded blastocyst stages in the form of ROS level reduction by ∼38% and ∼33% respectively. Following HHP stress applied at the transcriptionally active morula stage the long-time response in the expanded blastocysts as a decrease of ΔΨm by ∼19% and ROS level by ∼37% was observed. The percentage of obtained expanded blastocysts was higher after culture of HHP-treated zygotes in comparison to the control. Moreover, expanded blastocysts developed in vitro from both HHP-treated zygotes or morulae, exhibited higher total number of cells per blastocyst, higher number of cells in the inner cell mass as well as lower number of TUNEL-positive nuclei per blastocyst and lower TUNEL index, when compared to untreated embryos. Therefore, the HHP stress applied at the zygote stage, enhances developmental potential and quality of in vitro obtained porcine blastocysts due to the both decreased ΔΨm and ROS level. Our findings may contribute to better understanding of the mechanism of HHP-mediated modifications of energy metabolism and oxidative stress during in vitro development of pig embryos.

Correction to: The production of UL16-binding protein 1 targeted pigs using CRISPR technology.

[This corrects the article DOI: 10.1007/s13205-018-1107-4.].

Improved quality of porcine embryos cultured with hyaluronan due to the modification of the mitochondrial membrane potential and reactive oxygen species level.

Although considerable progress has been made in pig embryo culture systems, the developmental competence and quality of the produced embryos are still lower than their in vivo-derived counterparts. Because hyaluronan (HA) regulates various cellular processes and possesses antioxidant properties, this glycosaminoglycan seems to be a promising supplement in culture media. However, until now, its beneficial influence on in vitro pig embryo development has been debatable. Hence, we aimed to investigate the effect of 0.25 mg/mL, 0.5 mg/mL and 1 mg/mL concentrations of HA on the developmental potential and quality of cultured porcine embryos. We found that 1 mg/mL HA supplementation significantly increased the obtained percentages of cleaved embryos to ∼95%, morulae to ∼87% and blastocysts to ∼77%. At 0.5 mg/mL and 1 mg/mL HA concentrations, we observed a significantly improved blastocyst quality, expressed as the total number of cells per blastocyst, number of cells in the inner cell mass, number of TUNEL-positive nuclei per blastocyst, the TUNEL index and the blastocyst diameter. Because the inner mitochondrial membrane potential (ΔΨm) and reactive oxygen species (ROS) level are important for proper embryo development, for the first time, we measured these two parameters in cultured embryos at various HA concentrations and during their development up to the expanded blastocyst stage. For blastocysts cultured with 1 mg/mL HA, the ΔΨm and ROS level were ∼1.6 and 2.7 times lower, respectively, than those of the control blastocysts. Both ΔΨm and the ROS level were increased in parallel during in vitro embryo development with and without HA, but this increase was less pronounced in the presence of HA. Hence, our quantitative data unequivocally show that supplementation of NCSU-23 culture medium with 1 mg/mL HA improves the developmental potential and quality of pig embryos. This effect results from a significant decrease in the ROS level induced by the HA-dependent ΔΨm reduction.

Successful production of piglets derived from mature oocytes vitrified using OPS method.

OBJECTIVE: Examination of effect of vitrification solution with or without foetal calf serum (FCS) on the in vitro and in vivo survival of matured pig oocytes. MATERIALS AND METHODS: Exp. 1: oocytes were exposed to vitrification solutions: VSa (15% DMSO, 15% EG, 0.5 M sucrose dissolved in TCM-199 with FCS) or VSb (VSa without FCS). Exp. 2: oocytes were vitrified in VSa or VSb using OPS. A fraction of vitrified oocytes were transferred to 6 synchronised and inseminated recipients.. RESULTS: Survival rate after exposure and vitrification was the same for VSa and VSb. Transfer of 48 oocytes vitrified in VSb resulted with two pregnancies and 12 live piglets. Molecular analysis results: eight piglets originated from the surrogate mother's oocytes, four piglets from vitrified oocytes.. CONCLUSION: The use of DMSO and EG as cryoprotectants without serum supplementation was advantageous for the in vivo development of vitrified mature porcine oocytes.

Fertility of boar semen cryopreserved in extender supplemented with butylated hydroxytoluene.

The present study was to determine the effect of butylated hydroxytoluene (BHT) on quality and fertilizing ability of frozen-thawed boar semen. In the first experiment, five crossbreds of Polish Landrace and Large White boars (five ejaculates per boar) were frozen in 0.5 mL straws after dilution with lactose-egg yolk-glycerol extender supplemented with 0 (control), 0.5, 1.0, and 2.0 mM BHT. The sperm quality was verified based on the motility (computer-assisted sperm analysis; total motility, %; progressive motility, %), membrane integrity (YO-PRO-1/propidium iodide [PI] assay), acrosome integrity (fluorescein isothiocyanate-conjugated with peanut agglutinin/PI), and lipid peroxidation (chemiluminescence method) at 15 minutes postthaw. In the second experiment, the semen cryopreserved in extender supplemented with 1.0 and 2.0 mM BHT were selected for intrauterine artificial insemination of synchronized gilts. An intrauterine artificial insemination with low numbers of spermatozoa (500 × 10(6)) was surgically infused into each uterine horn. The highest (P < 0.001) progressive motility (%), membrane integrity, and acrosomal integrity were noted by the addition of 1.0 and 2.0 mM BHT to the freezing extender. Moreover, the various concentrations (0.5-2.0 mM) of BHT caused a considerable decrease in lipid peroxidation in relation to the control extender (P < 0.001). The highest reproductive performance of inseminated gilts (farrowing rate, 86.7%; litter size, 10.8 ± 1.6) was observed when semen was cryopreserved in extender supplemented with 1.0 mM BHT. These findings demonstrate that the addition of 1.0 mM BHT to the freezing extender efficiently improves the fertilizing ability of postthaw boar spermatozoa.

Influence of embryo transfer on embryo preimplantation development.

OBJECTIVE: To investigate the impact of injection speeds of the transferred load on embryo development. DESIGN: A laboratory model for in vitro simulation of ET was developed to investigate the impact of varying injection speeds of the transferred load on embryo development. SETTING: Academic research institutes of reproduction biotechnology and private centers of reproductive medicine. ANIMAL(S): Mouse hybrid F(1) females (C57bl/10 J × CBA-H; N = 15) aged 2-3 months. INTERVENTION(S): In vitro exposure of mouse embryos with either the fast ET (ejection speed, >1 m/s) or slow ET (ejection speed, <0.1 m/s) and consecutive culture for 36 hours. MAIN OUTCOME MEASURE(S): Development rate, morphology and apoptotic index of embryos. RESULT(S): The development rate was the slowest in embryos exposed to the fast ET. Morphological changes in response to ET were observed only among embryos exposed to the fast ET. The mean apoptotic index was 17.6% in the group exposed to the fast ET, 5.6% in the group exposed to the slow ET, and 2.58% in the control group. CONCLUSION(S): A reduction of the ejection speed of the transferred load allows avoidance of a developmental delay and diminishes injury of the embryos. Therefore, it is reasonable to suggest transferring the embryos at the lowest possible ejection speed.

Fluid dynamics during embryo transfer.

OBJECTIVE: To study fluid dynamics during ET. DESIGN: Computational fluid dynamics were applied to calculate fluid velocity changes, dynamic pressure differences, and shear stress in the transferred load for the following injection speeds: 0.1, 1, 6, 12, and 20 m/sec. SETTING: Academic research institute of mechanical engineering and reproduction biotechnology and private centers of reproductive medicine. PATIENT(S): None. INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): Fluid velocity, dynamic pressure, and shear stress during injection of the transferred load. RESULT(S): An increase of injection speed for the transferred load increased the shear stress, dynamic pressure, and velocity differences acting on the embryo. The narrowing of the catheter lumen diameter by 20% amplified the transferred fluid velocity by 78%. An embryo positioned in proximity to the catheter's wall was exposed to considerably higher shear stress, dynamic pressure, and velocity difference than an embryo in the center of the catheter's lumen. CONCLUSION(S): The transfer of an embryo should be conducted gently and with minimal injection speed. Any narrowing of the catheter lumen should be eliminated. Preferably the embryo should be kept far from the catheter's wall during injection of the transferred load.

Lipid content in pig blastocysts cultured in the presence or absence of protein and vitamin E or phenazine ethosulfate.

In the present study, total lipid content and content of triglycerides, phospholipids and cholesterol were determined in pig blastocysts cultured in medium without protein, supplemented with bovine serum albumin (BSA), with fetal calf serum (FCS), vitamin E or phenazine ethosulfate (PES). In comparison to blastocysts cultured in NCSU-23 with BSA, we observed a decrease of the total lipid content in PES-treated embryos. Triglyceride content in FCS-, vitamin E- and PES-treated embryos as well as in blastocysts cultured without protein was 81.9%, 70.2%, 57.2% and 74.8% of that found in the blastocysts cultured in NCSU-23 with BSA, respectively. Nevertheless the content of phospholipids remained unchanged. This decrease of triglyceride content in the porcine blastocyst after in vitro culture may be explained by altered lipid metabolism in embryos.

Pressure induced nucleus DNA fragmentation.

PURPOSE: The present study was designed to investigate the impact of pressure on nuclear DNA integrity in viable cells of mouse blastocysts. METHODS: The blastocysts of hybrid F1 females [(C57Bl/10 J × CBA-H);N = 15] aged 2-3 months were exposed into the pressure impulse lasting ~0.021 s and characterized by a positive pressure peak of ~76 mmHg. The nuclear DNA fragmentation index of mouse blastocysts was assessed by TUNEL assay within 60 s after exposure to pressure impulse. RESULTS: The mean nuclear DNA fragmentation index was significantly higher in the experimental group (83%) than in the control group (19.7%); p < 0.001. CONCLUSION(S): A low magnitude pressure impulse can induce nuclear DNA fragmentation in mouse blastocysts. The compression and decompression forces appearing during pressure fluctuations are responsible for the observed DNA shearing.

New technique to quantify the lipid composition of lipid droplets in porcine oocytes and pre-implantation embryos using Nile Red fluorescent probe.

The principal objective of this study was to develop a novel method based on confocal microscopy and a solvatochromic fluorescent dye, Nile red (NR) to quantify the main types of lipids in a single mammalian oocyte and embryo. We hypothesize that NR staining followed by the decomposition of NR-spectra identifies and quantifies the triglycerides, phospholipids, and cholesterol in a single oocyte and embryo. We analyzed the lipid droplets in porcine oocytes and pre-implantation embryos up to the hatched blastocyst stage developed in vivo and in cultured blastocysts. The emission spectrum of NR-stained mixture of different lipid types is a convolution of several component spectra. The principal component analysis (PCA) and a multivariate curve resolution-alternating least squares method (MCR-ALS) allowed to decompose the emission spectrum and quantify the relative amount of each lipid type present in mixture. We reported here that the level of the triglycerides, phospholipids and cholesterol in lipid droplets significantly decreases by 17.7%, 26.4% and 23.9%, respectively, from immature to mature porcine oocytes. The content of triglycerides and phospholipids remains unchanged in droplets of embryos from the zygote up to the morula stage. Then the triglyceride level decreases in the blastocyst by 15.1% and in the hatched blastocyst by 37.3%, whereas the amount of phospholipids decreases by 10.5% and 12.5% at the blastocyst and hatched blastocyst stages, respectively. In contrast, the content of cholesterol in droplets does not change during embryo cleavage. The lipid droplets in the blastocyst produced in vivo contain lower amounts of triglycerides (by 26.1%), phospholipids (by 14.2%) and cholesterol (by 34.8%) than those in the blastocyst cultured in NCSU-23 medium. In conclusion, our new technique is suitable to quantify the content of triglycerides, phospholipids and cholesterol in individual mammalian oocytes and embryos. Our findings indicate an important role for lipids during porcine oocyte maturation and early embryonic development, and suggest an altered lipid metabolism in cultured embryos.

Influence of embryo transfer on blastocyst viability.

OBJECTIVE: To investigate the impact of injection speeds of the transferred load on embryo viability. DESIGN: Laboratory model for in vitro simulation of embryo transfer (ET). SETTING: Academic research institutes of reproduction biotechnology and private centers of reproductive medicine. ANIMAL(S): Mouse hybrid F1 females, C57bl/10J × CBA-H (N = 15), aged 2 to 3 months. INTERVENTION(S): In vitro exposure of mouse blastocysts to either fast ET with an ejection speed of the transferred load of >1 m/s or slow ET with an ejection speed of <0.1 m/s. MAIN OUTCOME MEASURE(S): Morphologic changes and apoptotic index of blastocysts. RESULT(S): Morphologic changes in response to ET were most prevalent in blastocysts exposed to fast ET. The mean apoptotic index was 52% in the group exposed to fast ET, 25% in the group exposed to slow ET, and 12.8% in control group. CONCLUSION(S): Fast ejection of the transferred load can trigger both morphologic changes and apoptosis in mouse blastocysts. A reduction of the ejection speed of the transferred load minimizes injury to the embryos. Therefore, embryos should be transferred at the lowest possible speed.

Factors and methods of pig oocyte and embryo quality improvement and their application in reproductive biotechnology.

Compared to other mammalian species, pig oocytes and embryos are characterized by high lipid contents stored mainly as lipid droplets in the cytoplasm. This fact has a negative influence on manipulations on oocytes and embryos and, in general, biotechnological procedures are much less advanced in pigs than in cows. This paper discusses current methods for modifying porcine oocytes and embryos using in vitro culture or microsurgical manipulation, chemical agents such as cytochalasin B or D, physical means such as centrifugation or increased pressure and the biotechnological implications of these procedures. The presented methods make it possible to modify the characteristics of oocytes and embryos and thus increase their susceptibility to cryopreservation and cloning.

Effects of protein source, vitamin E and phenazine ethosulfate on developmental competence and quality of porcine embryos cultured in vitro.

Effects of fetal calf serum (FCS) or bovine serum albumin (BSA), with or without vitamin E (vit. E) or phenazine ethosulfate (PES) supplementation on developmental competence and quality of cultured porcine embryos were examined. The experiment was done on zygotes and 2-cell embryos obtained from superovulated gilts. Morphologically normal zygotes were cultured in vitro in NCSU-23 medium supplemented with: experiment 1-0.004 g/ml BSA, 10% FCS, protein-free (control); experiment 2-0 (control), 25, 50 or 100 microM vit. E; experiment 3-0 (control), 0.025, 0.05 or 0.075 microM PES. Embryo quality criteria were developmental competence (cleavage, morula and blastocyst rates), total cell number per blastocyst and degree of apoptosis as assessed by TUNEL staining. Presence of BSA in culture medium increased significantly morula and blastocysts production as compared to FCS (P < 0.001) and protein-free group (P < 0.05 and P < 0.001, respectively). The blastocysts cultured in protein-free medium had higher average number of apoptotic nuclei and DNA fragmented nucleus index as compared to the BSA (P < 0.05 and P < 0.01, respectively) and FCS (P < 0.5) group. Supplementation in culture medium of 100 microM vit. E increased blastocyst production as compared to control and 50 microM vit-E (P < 0.05). Both the number of cells per and percentage of TUNEL positive nuclei per blastocyst were slightly lower in PES treated than control groups.

Animal reproduction biotechnology in Poland.

The key research areas of the Department are: in vitro production of embryos, embryo cryopreservation, animal transgenesis, cloning, cytometric semen sexing and evaluation. Research has been focused on the in vitro production of animal embryos, including the development of complex methods for oocyte maturation, fertilization and embryo culture. Moreover, experiments on long-term culturing of late preantral and early antral bovine ovarian follicles have been developed. Studies on the cloning of genetically modified pigs with "humanized" immunological systems have been undertaken. A cloned goat was produced from oocytes reconstructed with adult dermal fibroblast cells. The novel technique of rabbit chimeric cloning for the production of transgenic animals was applied; additionally, the recipient-donor-cell relationship in the preimplantation developmental competences of feline nuclear transfer embryos has been studied. Regarding transgenic animal projects, gene constructs containing growth hormone genes connected to the mMt promoter were used. Modifications of milk composition gene constructs with tissue-specific promoters were performed. Moreover, pigs for xenotransplantation and animal models of human vascular diseases have been produced. Over the last 15 years, our flow cytometry research group has focused its work on new methods for sperm quality assessment and sex regulation. In the 1970s, our team initiated studies on embryo cryopreservation. As a result of vitrification experiments, the world's first rabbits and sheep produced via the transfer of vitrified embryos were born.

Survival of bovine fibroblasts and cumulus cells after vitrification.

The aim of the experiment was to investigate the effect of vitrification on viability and the cell cycle of bovine cumulus cells and fibroblasts after culture with or without serum starvation. In all vitrified-thawed bovine somatic cells, the number of samples that reached the confluence stage was high (50 to 100%). The viability of vitrified somatic cells depended on the concentration of the cells. The viability was higher for cells vitrified at the concentration of 10 x 10(6) per ml than for cells vitrified at a concentration of 1 x 10(6) per ml (p < 0.05; for cumulus cell, and fibroblast). Time of cell starving has had no impact on their susceptibility to vitrification in case of vitrified cumulus cells. Starving time caused shifts in proportions of subsequent cell cycle phases of vitrified fibroblasts and cumulus cells. In conclusion, the bovine cumulus and fibroblast cells can be cryopreserved successfully by vitrification procedure.

Effect of atosiban on rabbit embryo development and human sperm motility.

OBJECTIVE: To investigate embryotoxic potential and effects on human sperm motility of the mixed vasopressin V(1a)/oxytocin receptor antagonist atosiban considered for novel indication of improvement of uterine receptivity in embryo-transfer recipients. DESIGN: One-cell rabbit embryo bioassay and human sperm motility bioassay were performed in control media or in media containing atosiban. SETTING: Private center of reproductive medicine and academic research institute of reproduction biotechnology. ANIMAL(S): Rabbit females (New Zealand and California, N = 15) aged 4.5-6.5 months. INTERVENTION(S): In vitro exposure of one-cell rabbit embryos and human sperm to atosiban in the range of therapeutic concentrations clinically occurring in human beings. MAIN OUTCOME MEASURE(S): Embryo development and sperm motility. RESULT(S): Preimplantation development of one-cell rabbit embryos was not affected by atosiban in the concentrations < or =15,000 nM, which was 50-fold higher than the mean plasma concentration reached during regular therapy (300 nM). Atosiban did not affect human sperm motility in concentrations of < or =3,000 nM, in other words, 10 times the human mean plasma concentration. CONCLUSION(S): Clinical application of atosiban in the proposed indication may be safe for embryos because it is compatible with preimplantation rabbit embryo development and human sperm motility.

Distribution of porcine endogenous retroviruses (PERVs) DNA in organs of a domestic pig.

OBJECTIVES: Domestic pig may serve as the most appropriate organ source for human xenotransplantation in the future. However, there is a serious threat of xenogeneic pathogens transmission, especially porcine endogenous retroviruses (PERVs) which are present in genomes of all pigs. The aim of this study was to monitor the prevalence and distribution of PERV DNA in organs of a domestic pig. METHODS: We used a primer set for a highly conserved fragment of PERV gag sequence to monitor a total PERV DNA copy number and genotype-specific primer sets to study PERV subtypes distribution using Real-Time QPCR (SYBR Green I). RESULTS: Our results showed that PERV DNA was present in all studied pigs, however, most PERV DNA molecules carried numerous mutations thus indicating inability to express functional retroviral particles. The level of PERV DNA in kidney was much higher than in heart (p = 0.007) and in the liver (p = 0.009). CONCLUSIONS: It indicates that kidney is potentially the biggest PERV reservoir which makes it the organ of particular concern in xenotransplantation. We also conclude it is possible to monitor pig herds for individuals with the lowest PERV DNA prevalence, especially lacking PERV-C, and perhaps with only defective PERV proviruses that are unable to express functional RNA.

Effect of donor stimulation, frozen semen and heparin treatment on the efficiency of in vitro embryo production in goats.

Investigations on in vitro embryo production in goats in comparison with other domestic species, especially cattle, have been the subject of few reports despite their usefulness for both basic research and commercial application. The objectives of this study were to compare the efficiency of IVP in goats using immature follicular oocytes recovered from FSH-primed and control goats. After IVM, oocytes were fertilized with fresh or frozen-thawed semen capacitated with or without heparin. Mature oocytes were fertilized in vitro with fresh and frozen-thawed sperm of a single buck. Sperm preparation included swim-up separation and heparin treatment (50 micrograms/ml of sperm suspension for 45 min) before spermatozoa were added to oocytes in TALP-IVF. After IVF, the zygotes were cultured for 24h and cleaved embryos were further cultured with goat oviduct epithelial cells or transferred to synchronized recipients. Mean number of oocytes recovered from FSH-primed goats (24.5 +/- 8.6) was significantly higher (P < 0.01; t test) in comparison to control does (14.7 +/- 4.7). Irrespective of fresh or frozen semen, no differences were observed in blastocyst yield when sperm was treated with heparin. However, the highest cleavage rate (99/126; 79.4%) as well as blastocyst yield (47/126; 37.3%) was obtained after IVF with fresh sperm capacitated without heparin. Contrary to fresh sperm, heparin treatment of frozen-thawed sperm significantly improved (P < 0.01) embryo cleavage. No differences between in vivo developmental competence of embryos related to sperm origin were found after transferring into recipients. Overall, more than 60% of the recipients became pregnant and 20% of all transferred embryos survived delivering 13 healthy kids. Our caprine IVP system allows obtaining embryos with developmental competence comparable to bovine IVP.

Timing of nuclear maturation of nonstored and stored domestic cat oocytes.

In this study we compared the effects of preculture storage of ovaries, IVM medium, a reduced O(2) atmosphere and duration of culture on in vitro maturation (IVM) of domestic cat oocytes. One randomly selected ovary of each pair (69 pairs) was stored in PBS at 10 degrees C for 16-24h before oocyte recovery. The second ovary from each pair was used as a nonstored control. In Experiment I, we investigated the effect of culture media (TCM 199 versus SOF) and a reduced O(2) atmosphere (a humidified gas atmosphere of either 5% CO(2) in air or 5% CO(2):5% O(2):90% N(2)) on IVM of both stored and nonstored oocytes. In the second experiment, we compared timing of nuclear maturation of both stored and nonstored oocytes cultured for 17-18, 20-21, 24-26, 28-30, 33-34 or 42-45 h before being evaluated for meiotic status. In both, Experiments I and II, the recovery rate, quality and competence for maturation of oocytes originating from stored ovaries did not differ (P>0.05) compared with nonstored. In Experiment I, neither culture medium (37.5 versus 43.2% of Metaphase II, respectively in TCM 199 versus SOF) or gas atmosphere (40.0 versus 32.5% of Metaphase II, respectively in 5% CO(2) in air versus 5% CO(2):5% O(2):90% N(2)) affected oocyte maturation. In Experiment II, the mean proportion of oocytes achieving Metaphase II within 17-18 h of culture was 36.1% and did not significantly increase (P>0.05) over time up to 28 h. The highest proportion of oocytes (67.3%) reached Metaphase II stage after 42-45 h of culture. Therefore, we conclude that two "waves" of nuclear maturation of cat oocytes can be distinguished. The first wave takes place within 26 h and it is likely that most oocytes of this wave mature by 17-18 h; the second wave occurs after 28-30 h of IVM. It can be assumed that this double wave may reflect the presence of two oocyte populations with two different degrees of "prematuration" which require different lengths of IVM.