Validation of a heterologous fertilization assay and comparison of fertilization rates of equine oocytes using in vitro fertilization, perivitelline, and intracytoplasmic sperm injections

Cryopreservation of equine spermatozoa reduces plasma membrane integrity and phospholipase C zeta 1 content as associated with oocyte activation

BACKGROUND

Phospholipase C zeta (PLCZ1) is considered the major sperm-borne oocyte activation factor. Cryopreserved stallion spermatozoa are commonly used for intracytoplasmic sperm injection (ICSI). However, plasma membrane damage and protein modifications caused by cryopreservation could impair sperm structure and function, leading to a reduction of PLCZ1 and oocyte activation after ICSI.

OBJECTIVES

We compared membrane integrity and PLCZ1 abundance in populations for fresh, frozen, and refrozen stallion spermatozoa, either thawed and refrozen at room or low temperature; and examined the effect of relative PLCZ1 content on cleavage after ICSI.

MATERIALS AND METHODS

Western blotting, ELISA, and immunofluorescence were conducted in stallion spermatozoa, freezing extenders, and detergent-extracted sperm fractions to detect and quantify PLCZ1. Retrospectively, PLCZ1 content and cleavage rate were analyzed. Fresh, frozen, and refrozen at room and low temperatures spermatozoa were evaluated for acrosomal and plasma membrane integrity and PLCZ1 content using flow cytometry.

RESULTS

Western blotting, ELISA, and immunofluorescence revealed significant reduction of PLCZ1 in spermatozoa after cryopreservation and confirmed PLCZ1 detection in extenders. After detergent extraction, a PLCZ1-nonextractable fraction remained in the postacrosomal region of spermatozoa. Plasma membrane integrity was significantly reduced after freezing. Acrosomal and plasma membrane integrity were similar between frozen and refrozen samples at low temperature, but both were significantly higher than samples refrozen at room temperature. Acrosomal and plasma membrane integrity significantly correlated to PLCZ1 content. Percentages of PLCZ1-labeled spermatozoa and PLCZ1 content were reduced after freezing but not after refreezing. Relative content and localization of PLCZ1 were associated with cleavage rates after ICSI.

DISCUSSION AND CONCLUSION

Sperm PLCZ1 content associates with cleavage rates after ICSI. Cryopreservation is detrimental to sperm plasma membrane integrity and PLCZ1 retention. However, refreezing did not result in additional PLCZ1 loss. Refreezing stallion spermatozoa at a low temperature resulted in better survival but did not improve PLCZ1 retention.

Phospholipase C Zeta 1 (PLCZ1): The Function and Potential for Fertility Assessment and In Vitro Embryo Production in Cattle and Horses

Phospholipase C Zeta 1 (PLCZ1) is considered a major sperm-borne oocyte activation factor. After gamete fusion, PLCZ1 triggers calcium oscillations in the oocyte, resulting in oocyte activation. In assisted fertilization, oocyte activation failure is a major cause of low fertility. Most cases of oocyte activation failures in humans related to male infertility are associated with gene mutations and/or altered PLCZ1. Consequently, PLCZ1 evaluation could be an effective diagnostic marker and predictor of sperm fertilizing potential for in vivo and in vitro embryo production. The characterization of PLCZ1 has been principally investigated in men and mice, with less known about the PLCZ1 impact on assisted reproduction in other species, such as cattle and horses. In horses, sperm PLCZ1 varies among stallions, and sperm populations with high PLCZ1 are associated with cleavage after intracytoplasmic sperm injection (ICSI). In contrast, bull sperm is less able to initiate calcium oscillations and undergo nuclear remodeling, resulting in poor cleavage after ICSI. Advantageously, injections of PLCZ1 are able to rescue oocyte failure in mouse oocytes after ICSI, promoting full development and birth. However, further research is needed to optimize PLCZ1 diagnostic tests for consistent association with fertility and to determine whether PLCZ1 as an oocyte-activating treatment is a physiological, efficient, and safe method for improving assisted fertilization in cattle and horses.

L-carnitine enhances developmental potential of bovine oocytes matured under high lipid concentrations in vitro

Maternal obesity elevates non-esterified fatty acids (NEFA) follicular concentrations. Bovine cumulus-oocyte complexes (COCs) matured in vitro under high NEFA have altered metabolism and reduced quality. Systemically, obesity promotes altered mitochondrial metabolism linked to L-carnitine insufficiency. We hypothesized that L-carnitine supplementation during IVM of bovine COCs in the presence of high NEFA would lessen the negative effects of exposure to excessive lipids on embryonic development and oxidative stress. COCs were collected from abattoir ovaries and matured in four groups: CON (control), LC (3 mM L-carnitine), HN (high NEFA: 200uM oleic, 150uM palmitic and 75uM stearic acid), and HNLC (HN and LC). Mature oocytes were assayed for aerobic and anaerobic metabolism utilizing oxygen and pH microsensors or fertilized in vitro (D0). Cleavage (D3) and blastocyst (D7, D8) rates were assessed. D3 embryos with ≥ 4 cells were stained for cytosolic and mitochondrial ROS. D8 blastocysts were assayed for gene transcript abundance of metabolic enzymes. Oocyte metabolism was not affected by IVM treatment. D3 formation of embryos with ≥ 4 cells were lower in LC or HN than CON or HNLC; blastocyst rates were greater for CON and lower for HN than LC and HNLC. D3 embryo mitochondrial and cytosolic ROS were reduced in HNLC when compared to other groups. IVM in HN altered blastocyst gene transcript abundance when compared to CON, but not LC or HNLC. In conclusion, supplementation with L-carnitine protects oocytes exposed to high NEFA during IVM and improves their developmental competence, suggesting that high lipid exposure may lead to L-carnitine insufficiency in bovine oocytes.

Acquisition of fertilization competence in guinea pig spermatozoa under different capacitation protocols

Guinea pig in vitro fertilization (IVF) are poorly developed due to the limited accessibility to oocytes and the lack of an efficient method of sperm capacitation. Thus, we aimed to evaluate different capacitation protocols that we validated through sperm analysis and using heterologous (He) IVF with zona-intact bovine oocytes. Spermatozoa of guinea pigs were collected and processed separately by 4 different protocols: A) Spermatozoa were obtained by flushing the lumen of one cauda epididymis and incubated in a minimal culture medium (MCM); B) One epididymis was placed in a prewarmed of M2 medium and gently minced with fine scissors. Spermatozoa were incubated in a modified human tubal fluid medium (HTF). In both protocols, the spermatozoa were capacitated at 37 °C under an atmosphere of 5% CO2 for 2 h. In the protocols C and D, the spermatozoa were collected by flushing the lumen of the cauda epididymis and selected by commercial density gradient Bovipure® (Nidacon Laboratories AB, Göthenborg, Sweden), according to the manufacturer's instructions. Then for Protocol C) spermatozoa were incubated in MCM medium supplemented with 10 mg/mL heparin (MCM-Hep); while for Protocol D) spermatozoa were incubated in FERT medium supplemented 10 mg/mL heparin (FERT-Hep). Incubation of C and D protocols were performed at 38.5 °C under an atmosphere of 5% CO₂ for 2 h. Capacitation protocols C and D showed a higher percentage of viability, total and hyperactive-like motility, and acrosome reaction compared to protocols A and B. For this reason, protocols C and D were used for further He-IVF analysis. Guinea pig sperm and matured zona-intact bovine oocytes were co-incubated at 5% CO₂ and 38.5 °C. Sperm-oocyte interaction was assessed at 2.5 h post-insemination (hpi) and pronuclear formation (PrF) were evaluated at 18, 20, 22, 24 and 26 hpi, while the cleavage rate was evaluated at 48 hpi. In protocol D, PrF was significantly higher than in protocol C (P ≤ 0.05) at every time point evaluated. Also, the cleavage rate at 48 hpi was higher (P ≤ 0.05) in He-IVF protocol D (69.8 ± 1.7%) compared to He-IVF protocol C (49.1 ± 1.1%). In conclusion, we determined the most adequate sperm capacitation conditions for guinea pig that allow zona-intact bovine oocyte penetration and lead to hybrid embryo formation, suggesting that these conditions could be optimal to develop IVF in guinea pigs.

A stallion spermatozoon's journey through the mare's genital tract: In vivo and in vitro aspects of sperm capacitation

Conventional in vitro fertilization is not efficacious when working with equine gametes. Although stallion spermatozoa bind to the zona pellucida in vitro, these gametes fail to initiate the acrosome reaction in the vicinity of the oocyte and cannot, therefore, penetrate into the perivitelline space. Failure of sperm penetration most likely relates to the absence of optimized in vitro fertilization media containing molecules essential to support stallion sperm capacitation. In vivo, the female reproductive tract, especially the oviductal lumen, provides an environmental milieu that appropriately regulates interactions between the gametes and promotes fertilization. Identifying these 'fertilization supporting factors' would be a great contribution for development of equine in vitro fertilization media. In this review, a description of the current understanding of the interactions stallion spermatozoa undergo during passage through the female genital tract, and related specific molecular changes that occur at the sperm plasma membrane is provided. Understanding these molecular changes may hold essential clues to achieving successful in vitro fertilization with equine gametes.

Presence of Ginsenoside Re during in vitro maturation protects porcine oocytes from heat stress

Heat stress (HS) affects the development of porcine gametes and embryos negatively, induces the decrease of reproductive ability significantly, threatens global pig production. Ginsenoside Re (GRe), is a main bioactive component of ginseng, shows very specific anti-apoptotic, antioxidant and anti-inflammatory activities. To investigate the alleviating effect of GRe on the in vitro maturation of porcine oocyte under the HS, the polar body extrusion rate, intracellular levels of reactive oxygen species (ROS) and glutathione (GSH), ATP content, mitochondrial membrane potential (MMP) were assessed. For the current study, porcine cumulus-oocyte complexes (COCs) randomly divided into four groups: the control, GRe, HS and HS+GRe group. The results showed that HS inhibited the cumulus cell expansion and polar body extrusion rate, the levels of GSH and MMP, the ATP content, the gene expression of Nrf2 of porcine oocytes and the parthenogenetic activation (PA) embryo development competence, but GRe treatment could partly neutralize these adverse effects. Moreover, HS increased the ROS formation and percentage of apoptosis, the gene expression of HSP90, CASP3 and CytoC of porcine oocytes, but GRe could weaken the effect on Cyto C and BAX expression induced by HS. Taken together, these results showed that the presence of GRe during in vitro maturation protects porcine oocytes from HS. These findings lay a foundation for GRe may be used as a potential protective drug to protect porcine oocytes against HS damage.

Effect of cholestanol and cholesterol-loaded cyclodextrin on stallion sperm function and capacitation post-cryopreservation

Cryopreservation of stallion semen is less efficient than other species such as bovine. This is mainly because of the greater susceptibility of stallion sperm to the freezing damage that generates oxidative stress and plasma membrane injury, resulting in DNA fragmentation and cell death. These data suggest the need to develop new strategies of sperm cryopreservation that can improve the efficiency of this technique in stallions by reducing or preventing membrane damage and cell death. The present study aimed to evaluate the effect of adding membrane stabilizers to the freezing medium and assess the quality and in vitro capacitation of stallion sperm after thawing. Semen samples from three stallions frozen with membrane stabilizers (cholesterol-loaded cyclodextrin and cholestanol-loaded cyclodextrin) were evaluated in two experiments: i) sperm quality and functional analysis after thawing, and ii) sperm quality and functional analysis after 4 h of post-thaw incubation in capacitating conditions. Plasma membrane integrity, mitochondrial membrane potential, membrane lipid disorder, intracellular Ca²⁺, tyrosine phosphorylation, acrosome reaction, DNA damage, sperm motility, and binding to the zona pellucida were assessed. The results showed that cholesterol-loaded cyclodextrin was the stabilizer that most efficiently reduced the membrane disruption and post-thaw cell damage. In addition, this stabilizer made it possible to obtain in vitro capacitated sperm showing higher plasma membrane integrity, mitochondrial membrane potential, sperm motility, binding to the zona pellucida and better response to in vitro capacitating conditions.

Effect of Mare Age on Transcript Abundance of Connexins-37 and -43, Zona Pellucida Proteins, and Sperm Binding

Zona pellucida (ZP) proteins are important for fertilization and sperm binding and are closely associated with cumulus cells. Communication between cumulus and oocytes is facilitated by intracellular membrane channels composed of connexins. The extent aging impacts potential differences in fertilization and reductions in fertility is not well understood. This study characterized age-related differences in transcript abundance of ZP proteins and connexins in cells from ovarian follicles. Additionally, differences in sperm binding to oocytes from old and young mares was evaluated. For experiment 1, oocytes, corona radiata, cumulus, and granulosa cells were collected from mares classified as young (4-12 years) or old (> 20 years). Transcript abundance was evaluated for connexins -37 (GJA4) and -43 (GJA1); zona pellucida glycoproteins 1, 2, 3, and 4 (ZP1, ZP2, ZP3, ZP4); Tubulin (TUBA1A), and equine chorionic gonadotropin β. For experiment 2, oocytes that failed to cleave following intracytoplasmic sperm injection (ICSI) were stored in salt solution for up to 4 years and used for sperm binding assays. Transcript abundance for GJA1 was decreased in oocytes, corona radiata, and granulosa cells while GJA4 was decreased in cumulus cells from old compared to young mares. Additionally, ZP1 tended to be decreased in corona radiata and cumulus cells from old mares. Oocytes from old mares tended to bind less spermatozoa compared young mares. Oocytes that failed to cleave following ICSI can be used for sperm binding studies for up to 2 years without losses in sperm binding. Our findings suggest that maternal age may contribute to changes in cellular communication and the ZP that could impact sperm binding.

Intracytoplasmic Sperm Injection in Cattle

Intracytoplasmic sperm injection (ICSI) involves the microinjection of sperm into a matured oocyte. Although this reproductive technology is successfully used in humans and many animal species, the efficiency of this procedure is low in the bovine species mainly due to failed oocyte activation following sperm microinjection. This review discusses various reasons for the low efficiency of ICSI in cattle, potential solutions, and future directions for research in this area, emphasizing the contributions of testis-specific isoforms of Na/K-ATPase (ATP1A4) and phospholipase C zeta (PLC ζ). Improving the efficiency of bovine ICSI would benefit the cattle breeding industries by effectively utilizing semen from elite sires at their earliest possible age.

Fertilizing capacity of vitrified stallion sperm assessed utilizing heterologous IVF after different semen warming procedures

The aim of this study was to evaluate the fertilizing capacity of frozen or vitrified stallion sperm after assessing different warming procedures. In Experiment 1, different warming procedures were compared after sperm vitrification: immersion in extender at 43 °C (C), or in a water bath at 37 °C/30 s (W37), 43 °C/10 s (W43) or 60 °C/5 s (W60). With the W60 treatment, there were greater values (P < 0.05) for VCL (83.93 ± 3.6 μm/s) and ALH (3.00 ± 0.2 μm) than freezing and with the C group, and greater values (P < 0.001) for PM (35.33 ± 2.5 %) than with the W43 treatment. In Experiment 2, the fertilizing capacity of vitrified and frozen sperm was assessed utilizing heterologous IVF procedures, using cattle oocytes. Vitrification resulted in greater values (P < 0.05) than freezing for the number of bound sperm (1.36 ± 0.3 and 0.69 ± 0.2, respectively). There were no differences between frozen or vitrified sperm in pronuclear formation (26 hours post-insemination - hpi; 14.08 ± 4.2 % and 22.78 ± 4.8 %, respectively) or cleavage rate (32.77 ± 4.3 % and 39.66 ± 4.6 %, respectively). In conclusion, vitrified stallion sperm warmed in a water bath at 60 ºC had the capacity to penetrate cattle oocytes, leading to pronuclear formation and hybrid embryo cleavage after heterologous IVF.

Localisation of phospholipase Cζ1 (PLCZ1) and postacrosomal WW-binding protein (WBP2 N-terminal like) on equine spermatozoa and flow cytometry quantification of PLCZ1 and association with cleavage in vitro

Oocyte activation is initiated when a fertilising spermatozoon delivers sperm-borne oocyte-activating factor(s) into the oocyte cytoplasm. Candidates for oocyte activation include two proteins, phospholipase Cζ1 (PLCZ1) and postacrosomal WW-binding protein (PAWP; also known as WBP2 N-terminal like (WBP2NL)). We localised PLCZ1 and WBP2NL/PAWP in stallion spermatozoa and investigated the PLCZ1 content and sperm parameters as well as cleavage after intracytoplasmic sperm injection (ICSI). PLCZ1 was identified as 71-kDa protein in the acrosomal and postacrosomal regions, midpiece and principal piece of the tail. Anti-WBP2NL antibody identified two WBP2NL bands (~28 and ~32kDa) in the postacrosomal region, midpiece and principal piece of the tail. PLCZ1 and WBP2NL expression was positively correlated (P=0.04) in sperm heads. Flow cytometry evaluation of PLCZ1 revealed large variations in fluorescence intensity and the percentage of positively labelled spermatozoa among stallions. PLCZ1 expression was significantly higher in viable than non-viable spermatozoa, and DNA fragmentation was negatively correlated with PLCZ1 expression and the percentage of positively labelled spermatozoa (P<0.05). The use of equine sperm populations considered to have high versus low PLCZ1 content resulted in significantly higher cleavage rates after ICSI of bovine and equine oocytes, supporting the importance of PLCZ1 for oocyte activation.

Cryopreservation of stallion semen: Effect of adding antioxidants to the freezing medium on sperm physiology

Cryopreservation of stallion semen has not reached the level of efficiency and positive results described in other species. This is mainly due to the greater sensitivity of stallion sperm to the freezing process, showing higher rates of oxidative stress and plasma membrane damage, which trigger the activation of several cell damage pathways that ultimately culminate in DNA fragmentation and cell death. Therefore, finding molecules that improve the efficiency of this technique in stallion by preventing oxidative stress and cell damage is required. Thus, the aim of the present study was to evaluate the effect of adding three antioxidants (MnTBAP, NAC and FeTPPS) to the freezing medium on the quality and functional parameters of stallion sperm. Semen samples from three stallions frozen with the antioxidants were evaluated in two conditions: (a) adding the antioxidants before freezing, and (b) before and after freezing. Plasma membrane integrity, mitochondrial membrane potential, lipid peroxidation, intracellular ROS levels, membrane lipid disorder, DNA damage, sperm motility and binding to the zona pellucida were assessed. The results showed that MnTBAP was the antioxidant treatment that best controlled the oxidative stress process and post-thaw cell damage, showing higher plasma membrane integrity, mitochondrial membrane potential, sperm motility, number of spermatozoa bound to the zona pellucida of bovine oocytes and lower lipid disorder. Additionally, it was determined that a second post-thaw application of antioxidants is detrimental since induced higher cell damage and lower sperm motility, without showing any beneficial effect on the spermatozoa.

Effects of extender, cryoprotectants and thawing protocol on motility of frozen-thawed stallion sperm that were refrozen for intracytoplasmic sperm injection doses

We examined the effects of different freezing extenders, cryoprotectant agents (CPA) and initial thawing temperatures for preparing doses of refrozen stallion sperm for intracytoplasmic sperm injection (ICSI). Single ejaculates, from twelve stallions, were frozen in lactose-EDTA-egg yolk extender (LE) with 5% glycerol. In experiment 1, sperm were initially thawed to 5 °C or 37 °C, before being diluted in LE or skim milk-egg yolk extender (SMEY) containing either 5% glycerol (GLY), 5% methylformamide (MF) or 5% of a combination of both (GMF). In experiment 2, frozen sperm were initially thawed to 5 °C, diluted and refrozen in SMEY containing 2, 4, 6 or 8% GLY or GMF. In Experiment 1, sperm motility was reduced after each cryopreservation cycle (P < 0.05). Extender type did not affect motility after refreezing (P > 0.05), but sperm initially thawed to 5 °C exhibited higher motility than sperm thawed to 37 °C (P < 0.05). In addition, sperm refrozen in SMEY containing MF or GMF exhibited higher motility than sperm refrozen in GLY alone (P < 0.05). In experiment 2, there was an interaction between CPA and CPA concentration (P < 0.05). Sperm refrozen with GMF had higher motility than refrozen sperm with GLY (P < 0.05), and while GLY concentration did not affect post-thaw motility (P > 0.05). Sperm refrozen with 6 or 8% GMF exhibited the highest motility (P < 0.05). In conclusion, sperm motility is best maintained when thawing and refreezing stallion sperm in low sperm concentration ICSI doses by initially thawing the sperm to 5 °C and diluting the sperm in a freezing extender with 8% GMF.

Impact of Equine and Bovine Oocyte Maturation in Follicular Fluid From Young and Old Mares on Embryo Production in Vitro

Equine follicular fluid (FF) provides autocrine and paracrine factors from theca, granulosa, and cumulus cells, both reflecting and impacting oocyte and follicle maturation. We hypothesized that maturation of oocytes in FF from old versus young mares has a deleterious effect on oocyte maturation and their subsequent developmental potential. Follicular fluid was collected from the large, dominant follicle from young mares (4-13 years) or old mares (21-26 years) and classified as: (1) Noninduced follicular fluid (NFF), FF from noninduced follicle 33 ± 3 mm, or (2) Induced follicular fluid (IFF), FF collected ∼24 hours after administration of ovulation-inducing drugs when a follicle 33 ± 3 mm was observed. In experiment 1, immature equine oocytes were collected, matured in vitro for 30 ± 2 hours in 100% IFF, collected from young or old mares, with the addition of follicle stimulating hormone (5 mU/mL), then fertilized by intracytoplasmic sperm injection. In experiment 2, immature bovine oocytes were collected, matured in 100% IFF or NFF, collected from young mares or old mares, then fertilized via in vitro fertilization. In experiment 1, more blastocysts tended (P = .08) to be produced from equine oocytes that were matured in old versus young mare FF. In experiment 2, when IFF and NFF groups were combined, cleavage rates were higher (P = .001) when bovine oocytes were matured in FF from young than old mares. In contrast to our hypothesis, we observed no conclusive evidence that FF from old mares has a deleterious impact on oocytes and their early developmental potential.

Cytoskeletal alterations associated with donor age and culture interval for equine oocytes and potential zygotes that failed to cleave after intracytoplasmic sperm injection

Intracytoplasmic sperm injection (ICSI) is an established method to fertilise equine oocytes, but not all oocytes cleave after ICSI. The aims of the present study were to examine cytoskeleton patterns in oocytes after aging in vitro for 0, 24 or 48h (Experiment 1) and in potential zygotes that failed to cleave after ICSI of oocytes from donors of different ages (Experiment 2). Cytoplasmic multiasters were observed after oocyte aging for 48h (P<0.01). A similar increase in multiasters was observed with an increased interval after ICSI for young mares (9-13 years) but not old (20-25 years) mares. Actin vesicles were observed more frequently in sperm-injected oocytes from old than young mares. In the present study, multiasters appeared to be associated with cell aging, whereas actin vesicles were associated with aging of the oocyte donor.

Designing 3-Dimensional In Vitro Oviduct Culture Systems to Study Mammalian Fertilization and Embryo Production

The oviduct was long considered a largely passive conduit for gametes and embryos. However, an increasing number of studies into oviduct physiology have demonstrated that it specifically and significantly influences gamete interaction, fertilization and early embryo development. While oviduct epithelial cell (OEC) function has been examined during maintenance in conventional tissue culture dishes, cells seeded into these two-dimensional (2-D) conditions suffer a rapid loss of differentiated OEC characteristics, such as ciliation and secretory activity. Recently, three-dimensional (3-D) cell culture systems have been developed that make use of cell inserts to create basolateral and apical medium compartments with a confluent epithelial cell layer at the interface. Using such 3-D culture systems, OECs can be triggered to redevelop typical differentiated cell properties and levels of tissue organization can be developed that are not possible in a 2-D culture. 3-D culture systems can be further refined using new micro-engineering techniques (including microfluidics and 3-D printing) which can be used to produce ‘organs-on-chips’, i.e. live 3-D cultures that bio-mimic the oviduct. In this review, concepts for designing bio-mimic 3-D oviduct cultures are presented. The increased possibilities and concomitant challenges when trying to more closely investigate oviduct physiology, gamete activation, fertilization and embryo production are discussed.

Effects of α-tocopherol and freezing rates on the quality and heterologous in vitro fertilization capacity of stallion sperm after cryopreservation

The effects of supplementation of α-tocopherol and different freezing rates (FRs) on the ability of stallion sperm to fertilize bovine oocytes with intact zona pellucida were investigated, in an attempt to develop a model to assess cryopreserved sperm function. Semen was obtained from four purebred Lusitano stallions (n = 4). Each ejaculate was subjected to cryopreservation with a commercial extender (Ghent, Minitub Iberia, Spain), without any supplementation (control) or supplemented with 2-mM α-tocopherol. The semen was exposed to two different FRs between 5 °C and -15 °C: slow (5 °C/min) and moderate (10 °C/min). After thawing, the viability (SYBR®-14 and propidium iodide [PI]), mitochondrial membrane potential (JC-1, 5,5',6,6'-tetrachloro-1,1',3,3'tetraethylbenzimidazolyl carbocyanine iodine) and membrane lipid peroxidation (C11-BODIPY(581/591)) of each sample were determined by flow cytometry. Moreover, the heterologous IVF rate was measured to evaluate the fertilization capacity of postthaw semen in the four different treatments. For both extenders, the viability was higher for spermatozoa cooled slowly (39.40 ± 2.17 vs. 17.59 ± 2.25-control; 31.96 ± 2.19 vs. 11.46 ± 1.34-Tocopherol; P < 0.05). The α-tocopherol extender improved (P < 0.05) postthaw lipid peroxidation (10.28 ± 0.70 vs. 15.40 ± 0.95-slow FR; 10.14 ± 0.40 vs. 13.48 ± 0.34-moderate FR); however, it did not improve viability and mitochondrial membrane potential. Regarding the IVF rate, in the moderate FR, α-tocopherol supplementation reported a higher percentage of IVF (20.50 ± 2.11; P < 0.05), comparing with the control (14.00 ± 1.84). Regarding the slow FR, no significance differences were observed for percentage of IVF between the two extenders and the FRs. However, it seems that the α-tocopherol supplementation improved the IVF rate. In conclusion, this research reported that bovine oocytes intact zona pellucida can be used to evaluate the quality of postthaw stallion semen and α-tocopherol supplementation in the stallion freezing extender might exert a protective effect against oxidative damage during heterologous IVF.

Heterologous murine and bovine IVF using bottlenose dolphin (Tursiops truncatus) spermatozoa

Assisted reproductive technologies are of great importance for increasing the genetic diversity in captive animals. The use of bovine or murine oocytes in heterologous IVF provides advantages compared to homologous IVF in nondomestic animals, such as the accessibility to oocytes and the availability of well-developed in vitro maturation systems. The aim of this study was to determine the heterologous IVF parameters using cryopreserved dolphin spermatozoa and zona-intact bovine or murine oocytes and to examine the nuclear chromatin status of the dolphin spermatozoa. All the processes involved in the fertilization including embryo cleavage were observed by confocal microscopy and hybrid embryo formation was confirmed by polymerase chain reaction. Heterologous bovine IVF showed no polyspermy, lower percentages of pronuclear formation, and a lower cleavage rate compared to homologous IVF group (34.8% vs. 89.3%). Heterologous murine IVF showed a lower cleavage rate than homologous IVF (9.6% vs. 77.1%). With respect to dolphin sperm chromatin, it was more stable, i.e. more resistant to EDTA-SDS decondensation than the bovine sperm chromatin. This study revealed the stability of the dolphin sperm chromatin and the ability of the dolphin spermatozoa to penetrate zona-intact bovine and murine oocytes, leading to hybrid embryo formation.