Embryo quality is most frequently evaluated at the blastocyst stage, although quality parameters further back along the developmental axis, such as early developmental kinetics or oocyte quality, can be equally valuable. Despite the fact that previous studies in bovine have linked oocyte diameter and early developmental kinetics with blastocyst formation and viability, their relation with the incidence of apoptosis during embryo development remains relatively unexplored. Therefore, we related non-invasive parameters of oocyte and embryo quality, such as embryo kinetics, embryo morphology, and oocyte diameter, to the incidence of apoptosis throughout embryo development using fluorescent detection of active caspase-3 and -7. First, bovine in vitro embryos were selected according to developmental kinetics and morphology at four set times during culture and subjected to fluorescent detection of active caspase-3 and -7. Caspase activity was significantly higher in slow developing embryos in comparison with fast cleavers (P < 0.05), but was not related to embryo morphology. Second, bovine oocytes were divided into three groups on the basis of oocyte diameter and the resulting embryos were used for staining at the same four set times. Caspase activity was significantly higher in embryos derived from growing oocytes compared with those of fully grown oocytes at 45, 80, and 117 hours post-insemination (hpi; P < 0.05), but not at 168 hpi.

Recently, new culture devices such as Corral and Primo Vision dishes have been designed for the culture of human embryos to allow the combination of group culture plus follow-up of individual embryos. Bovine inseminated oocytes were allocated to Primo Vision dishes, Corral dishes, individual culture or classical group culture. Blastocyst development in Primo Vision dishes was similar to classical group culture (34.3 and 39.0% respectively), and better than Corral dishes or individual culture (28.9 and 28.5% respectively). In Primo Vision dishes, a higher number of ‘slow’ embryos developed to the blastocyst stage compared with their individually cultured counterparts, while no differences were observed for ‘fast’ embryos. ‘Slow’ embryos in a ‘standard drop’ had a higher chance of becoming a blastocyst compared with individual culture (OR: 2.3), whereas blastulation of ‘fast’ embryos was less efficient in a ‘delayed drop’ than in individual culture (OR: 0.3). The number of non-cleaved embryos in Primo Vision dishes did not negatively influence blastocyst development. Likewise, removing non-cleaved embryos (NC removed) and regrouping the cleaved embryos afterwards (ReGR) did not affect blastocyst development and quality compared with group culture in Primo Vision dishes (CTRL, 31.6%, NC removed, 29.3% and ReGR, 29.6%). The experiments revealed that group culture of bovine embryos in Primo Vision dishes is superior to individual culture, primarily because of the higher blastocyst rate achieved by slow embryos. Non-cleaved or arrested embryos do not hamper the ability of co-cultured bovine embryos to reach the blastocyst stage in group culture.

Vitrification of immature germinal vesicle-stage oocytes is a promising method in assisted reproduction but is associated with reduced developmental potential and low birth rates. Cumulus-oocyte complexes (COCs) express several connexins that form hexameric hemichannels, which interact head to head to create a gap junction or exist as unopposed free hemichannels. The latter are normally closed but open under stress conditions and may exert detrimental effects. We determined whether minimizing hemichannel opening and cell death during vitrification could improve COC quality. Bovine immature COCs underwent vitrification, storage and warming, followed by dye uptake to assess hemichannel opening and TUNEL staining to detect cell death. Based on these scores, we optimized the procedure by tuning the equilibration time, temperature, cryoprotectant concentration and extracellular Ca²⁺ concentration and assessed its impact on maturation, cleavage and blastocyst formation after parthenogenetic activation. We found that the major stressor resides in the cooling/warming phase of the vitrification procedure and observed that hemichannel opening and cell death in cumulus cells measure different aspects of cell stress. Optimization of the hemichannel and cell death readouts demonstrated that combined minimal hemichannel opening/cell death gave the highest cleavage rates but had no effect on maturation and blastocyst formation. Neither hemichannel nor cell death optimization performed better than the non-optimized protocol, leading to the conclusion that cell stress factors other than those detected by hemichannel dye uptake or TUNEL positivity are involved.

Induction of hyperactivated motility is considered essential for triggering the release of oviduct-bound mammalian spermatozoa in preparation for fertilization. In this study, oviduct-bound stallion spermatozoa were exposed for 2 h to: i) pre-ovulatory and ii) post-ovulatory oviductal fluid; iii) 100% and iv) 10% follicular fluid (FF); v) cumulus cells, vi) mature equine oocytes, vii) capacitating and viii) non-capacitating medium. None of these triggered sperm release or hyperactivated motility. Interestingly, native FF was detrimental to sperm viability, an effect that was negated by heat inactivation, charcoal treatment and 30 kDa filtration alone or in combination. Moreover, sperm suspensions exposed to treated FF at pH 7.9 but not pH 7.4 showed Ca²⁺-dependent hypermotility. Fluo-4 AM staining of sperm showed elevated cytoplasmic Ca²⁺ in hyperactivated stallion spermatozoa exposed to treated FF at pH 7.9 compared to a modest response in defined capacitating conditions at pH 7.9 and no response in treated FF at pH 7.4. Moreover, 1 h incubation in alkaline, treated FF induced protein tyrosine phosphorylation in 20% of spermatozoa. None of the conditions tested induced widespread release of sperm pre-bound to oviduct epithelium. However, the hyperactivating conditions did induce release of 70–120 spermatozoa per oviduct explant, of which 48% showed protein tyrosine phosphorylation and all were acrosome-intact, but capable of acrosomal exocytosis in response to calcium ionophore. We conclude that, in the presence of elevated pH and extracellular Ca²⁺, a heat-resistant, hydrophilic, <30 kDa component of FF can trigger protein tyrosine phosphorylation, elevated cytoplasmic Ca²⁺ and hyperactivated motility in stallion sperm, but infrequent release of sperm pre-bound to oviduct epithelium.

Fibronectin (Fn) is a 440 kDa glycoprotein assumed to participate in sperm–egg interaction in human. Recently, it has been demonstrated that Fn – when present during bovine IVF – strongly inhibits sperm penetration. The present study was conducted firstly to evaluate the expression of Fn and its integrin receptor (α₅β₁) on male and female bovine gametes using indirect immunofluorescence and secondly, to determine the function of Fn during bovine IVF. Endogenous Fn was detected underneath the zona pellucida (ZP) and integrin α₅ on the oolemma of cumulus-denuded oocytes. Bovine spermatozoa displayed integrin α₅ at their equatorial segment after acrosome reaction. We established that the main inhibitory effect of exogenously supplemented Fn was located at the sperm–oolemma binding, with a (concurrent) effect on fusion, and this can probably be attributed to the binding of Fn to spermatozoa at the equatorial segment, as shown by means of Alexa Fluor 488-conjugated Fn. Combining these results, the inhibitory effect of exogenously supplemented Fn seemed to be exerted on the male gamete by binding to the exposed integrin α₅β₁ receptor after acrosome reaction. The presence of endogenous Fn underneath the ZP together with integrin α₅ expression on oolemma and acrosome-reacted (AR) sperm cell surface suggests a ‘velcro’ interaction between the endogenous Fn ligand and corresponding receptors on both (AR) sperm cell and oolemma, initiating sperm–egg binding.

In contrast to man and many other mammalian species, conventional in vitro fertilization (IVF) with horse gametes is not reliably successful. The apparent inability of stallion spermatozoa to penetrate the zona pellucida in vitro is most likely due to incomplete activation of spermatozoa (capacitation) because of inadequate capacitating or fertilizing media. In vivo, the oviduct and its secretions provide a microenvironment that does reliably support and regulate interaction between the gametes. This review focuses on equine sperm–oviduct interaction. Equine sperm–oviduct binding appears to be more complex than the presumed species-specific calcium-dependent lectin binding phenomenon; unfortunately, the nature of the interaction is not understood. Various capacitation-related events are induced to regulate sperm release from the oviduct epithelium and most data suggest that exposure to oviduct secretions triggers sperm capacitation in vivo. However, only limited information is available about equine oviduct secreted factors, and few have been identified. Another aspect of equine oviduct physiology relevant to capacitation is acid–base balance. In vitro, it has been demonstrated that stallion spermatozoa show tail-associated protein tyrosine phosphorylation after binding to oviduct epithelial cells containing alkaline secretory granules. In response to alkaline follicular fluid preparations (pH 7.9), stallion spermatozoa also show tail-associated protein tyrosine phosphorylation, hyperactivated motility and (limited) release from oviduct epithelial binding. However, these ‘capacitating conditions’ are not able to induce the acrosome reaction and fertilization. In conclusion, developing a defined capacitating medium to support successful equine IVF will depend on identifying as yet uncharacterized capacitation triggers present in the oviduct.

In contrast to various other mammalian species, conventional in vitro fertilization (IVF) with horse gametes is not reliably successful. In particular, stallion spermatozoa fails to penetrate the zona pellucida, most likely due to incomplete activation of stallion spermatozoa (capacitation) under in vitro conditions. In other mammalian species, specific capacitation triggers have been described; unfortunately, none of these is able to induce full capacitation in stallion spermatozoa. Nevertheless, knowledge of capacitation pathways and their molecular triggers might improve our understanding of capacitation-related events observed in stallion sperm. When sperm cells are exposed to appropriate capacitation triggers, several molecular and biochemical changes should be induced in the sperm plasma membrane and cytoplasm. At the level of the sperm plasma membrane, (1) an increase in membrane fluidity, (2) cholesterol depletion and (3) lipid raft aggregation should occur consecutively; the cytoplasmic changes consist of protein tyrosine phosphorylation and elevated pH, cAMP and Ca²⁺ concentrations. These capacitation-related events enable the switch from progressive to hyperactivated motility of the sperm cells, and the induction of the acrosome reaction. These final capacitation triggers are indispensable for sperm cells to migrate through the viscous oviductal environment, penetrate the cumulus cells and zona pellucida and, finally, fuse with the oolemma. This review will focus on molecular aspects of sperm capacitation and known triggers in various mammalian species. Similarities and differences with the horse will be highlighted to improve our understanding of equine sperm capacitation/fertilizing events.

In many species, sperm binding to oviduct epithelium is believed to be an essential step in generating a highly fertile capacitated sperm population primed for fertilization. In several mammalian species, this interaction is based on carbohydrate-lectin recognition. d-galactose has previously been characterized as a key molecule that facilitates sperm–oviduct binding in the horse. We used oviduct explant and oviduct apical plasma membrane (APM) assays to investigate the effects of various carbohydrates; glycosaminoglycans; lectins; S-S reductants; and the capacitating factors albumin, Ca²⁺ and HCO₃ ⁻ on sperm–oviduct binding in the horse. Carbohydrate-specific lectin staining indicated that N-acetylgalactosamine, N-acetylneuraminic acid (sialic acid) and d-mannose or d-glucose were the most abundant carbohydrates on equine oviduct epithelia, whereas d-galactose moieties were not detected. However, in a competitive binding assay, sperm–oviduct binding density was not influenced by any tested carbohydrates, glycosaminoglycans, lectins or d-penicillamine, nor did the glycosaminoglycans induce sperm tail-associated protein tyrosine phosphorylation. Furthermore, N-glycosidase F (PNGase) pretreatment of oviduct explants and APM did not alter sperm–oviduct binding density. By contrast, a combination of the sperm-capacitating factors albumin and HCO₃ ⁻ severely reduced (>10-fold) equine sperm–oviduct binding density by inducing rapid head-to-head agglutination, both of which events were independent of Ca²⁺ and an elevated pH (7.9). Conversely, neither albumin and HCO₃ ⁻ nor any other capacitating factor could induce release of oviduct-bound sperm. In conclusion, a combination of albumin and HCO₃ ⁻ markedly induced sperm head-to-head agglutination which physically prevented stallion sperm to bind to oviduct epithelium.

Porcine embryo selection prior to transfer is mainly influenced by morphological criteria. However, the relationship between embryonic morphology, developmental potential and cell death by apoptosis in porcine embryos is still unclear. The aim of this study was to establish embryo quality parameters for in vivo fertilised porcine embryos based on timing of development in vitro, embryo morphology and the presence of apoptosis. The kinetics of development and morphological parameters were investigated in a time-lapse cinematographic experiment. Possible links between embryo morphology and apoptosis were examined via a confocal laser scanning experiment, analysing nuclear changes, annexin V and terminal dUTP nick-end labelling. The timing of early cleavages was firmly linked to embryo developmental competence in vitro. Attainment of at least the 5-cell stage before 77 h post insemination and attainment of the morula stage before 102 h post insemination significantly increased the odds for reaching the early blastocyst stage. Overall, a negative effect of fragmentation percentage and fragmentation pattern on subsequent embryonic development was observed, but the developmental potential of embryos experiencing slight fragmentation (0–5%) was not different from embryos without fragmentation. Correlations detected between developmental arrest and fragmentation, and fragmentation and apoptosis were 0.60 and 0.87 (P < 0.05) respectively. Only a minority of the embryos arrested between the 1- and 4-cell stage displayed biochemical characteristics of apoptosis. Consequently, a significant correlation (0.57) between developmental arrest and apoptosis could only be established for embryos arrested after embryonic genome activation.