Ovarian control of very low sperm/egg ratios at the commencement of mammalian fertilisation to avoid polyspermy

Evolutionary, proteomic, and experimental investigations suggest the extracellular matrix of cumulus cells mediates fertilization outcomes

Studies of fertilization biology often focus on sperm and egg interactions. However, before gametes interact, mammalian sperm must pass through the cumulus layer; in mice, this consists of several thousand cells tightly glued together with hyaluronic acid and other proteins. To better understand the role of cumulus cells and their surrounding matrix, we perform proteomic experiments on cumulus oophorus complexes (COCs) in house mice (Mus musculus), producing over 24,000 mass spectra to identify 711 proteins. Seven proteins known to stabilize hyaluronic acid and the extracellular matrix were especially abundant (using spectral counts as an indirect proxy for abundance). Through comparative evolutionary analyses, we show that three of these evolve rapidly, a classic signature of genes that influence fertilization rate. Some of the selected sites overlap regions of the protein known to impact function. In a follow-up experiment, we compared COCs from females raised in two different social environments. Female mice raised in the presence of multiple males produced COCs that were smaller and more resistant to sperm-derived hyaluronidase compared to females raised in the presence of a single male, consistent with a previous study that demonstrated such females produced COCs that were more resistant to fertilization. Although cumulus cells are often thought of as enhancers of fertilization, our evolutionary, proteomic, and experimental investigations implicate their extracellular matrix as a potential mediator of fertilization outcomes.

Selection of Boar Sperm by Reproductive Biofluids as Chemoattractants

Simple Summary

Both in natural breeding and some assisted reproduction technologies, spermatozoa are deposited into the uterus. The journey the spermatozoa must take from the place of semen deposition to the fertilization site is long, hostile, and selective of the best spermatozoa. For the fertilization to succeed, spermatozoa are guided by chemical stimuli (chemoattractants) to the fertilization site, mainly secreted by the oocyte, cumulus cells, and other substances poured into the oviduct in the periovulatory period. This work studied some sources of chemotactic factors and their action on spermatozoa functionality in vitro, including the fertility. A special chemotactic chamber for spermatozoa selection was designed which consists of two wells communicated by a tube. The spermatozoa are deposited in well A, and the chemoattractants in well B. This study focuses on the use of follicular fluid (FF), periovulatory oviductal fluid (pOF), conditioned medium from the in vitro maturation of oocytes (CM), and progesterone (P4) as chemoattractants to sperm. The chemotactic potential of these substances is also investigated as related to their action on CatSper which is a calcium channel in the spermatozoa known to be sensitive to chemoattractants and essential for motility.

Abstract

Chemotaxis is a spermatozoa guidance mechanism demonstrated in vitro in several mammalian species including porcine. This work focused on follicular fluid (FF), periovulatory oviductal fluid (pOF), the medium surrounding oocytes during in vitro maturation (conditioned medium; CM), progesterone (P4), and the combination of those biofluids (Σ) as chemotactic agents and modulators of spermatozoa fertility in vitro. A chemotaxis chamber was designed consisting of two independent wells, A and B, connected by a tube. The spermatozoa are deposited in well A, and the chemoattractants in well B. The concentrations of biofluids that attracted a higher proportion of spermatozoa to well B were 0.25% FF, 0.25% OF, 0.06% CM, 10 pM P4 and 0.25% of a combination of biofluids (Σ2), which attracted between 3.3 and 12.3% of spermatozoa (p < 0.05). The motility of spermatozoa recovered in well B was determined and the chemotactic potential when the sperm calcium channel CatSper was inhibited, which significantly reduced the % of spermatozoa attracted (p < 0.05). Regarding the in vitro fertility, the spermatozoa attracted by FF produced higher rates of penetration of oocytes and development of expanded blastocysts. In conclusion, porcine reproductive biofluids show an in vitro chemotactic effect on spermatozoa and modulate their fertilizing potential.

Porcine model for the study of sperm capacitation, fertilization and male fertility

Mammalian fertilization remains a poorly understood event with the vast majority of studies done in the mouse model. The purpose of this review is to revise the current knowledge about semen deposition, sperm transport, sperm capacitation, gamete interactions and early embryonic development with a focus on the porcine model as a relevant, alternative model organism to humans. The review provides a thorough overview of post-ejaculation events inside the sow's reproductive tract including comparisons with humans and implications for human fertilization and assisted reproductive therapy (ART). Porcine methodology for sperm handling, preservation, in vitro capacitation, oocyte in vitro maturation, in vitro fertilization and intra-cytoplasmic sperm injection that are routinely used in pig research laboratories can be successfully translated into ART to treat human infertility. Last, but not least, new knowledge about mitochondrial inheritance in the pig can provide an insight into human mitochondrial diseases and new knowledge on polyspermy defense mechanisms could contribute to the development of new male contraceptives.

Why doesn't conventional IVF work in the horse? The equine oviduct as a microenvironment for capacitation/fertilization

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.

An alkaline follicular fluid fraction induces capacitation and limited release of oviduct epithelium-bound stallion sperm

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 Ca(2+)-dependent hypermotility. Fluo-4 AM staining of sperm showed elevated cytoplasmic Ca(2+) 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 Ca(2+), a heat-resistant, hydrophilic, <30 kDa component of FF can trigger protein tyrosine phosphorylation, elevated cytoplasmic Ca(2+) and hyperactivated motility in stallion sperm, but infrequent release of sperm pre-bound to oviduct epithelium.

Outstanding questions concerning sperm-epithelial binding in the mammalian oviduct

Preovulatory binding of viable spermatozoa in the oviduct isthmus is widely accepted as a preliminary to fertilization, but details of physiological events associated with epithelial binding and release from binding are themselves little understood. Important questions include the potential number, distribution and stability of such sites in the caudal isthmus, whether multiple molecular forms of binding exist within a single-mated individual, and whether some sites are more favourable than others for the maintenance of preovulatory sperm viability. Also to be resolved is whether spermatozoa interact with the first available binding sites in the isthmus, whether spermatozoa from second or subsequent matings bind closer to the site of fertilization, and whether the first spermatozoa entering the oviduct are those that will be released first with impending ovulation. Ideally, future research needs to focus on a fertilizing spermatozoon monitored in vivo and not on spermatozoa destined to remain on or in the zona pellucida or in the lower reaches of the oviduct.

A high incidence of chromosome abnormalities in two-cell stage porcine IVP embryos

In pigs, in vitro production is difficult with a high occurrence of polyspermy and low blastocyst formation rates. To test the hypothesis that this may, at least in part, be due to chromosomal errors, we employed whole genome amplification and comparative genomic hybridization, performing comprehensive chromosome analysis to assess both cells of the two-cell stage in vitro porcine embryos. We thus described the incidence, nature and origin of chromosome abnormalities, i.e. whether they derived from incorrect meiotic division during gametogenesis or aberrant mitotic division in the zygote. We observed that 19 out of 51 (37%) of two-cell stage early pig IVP embryos had a chromosome abnormality, mostly originating from an abnormal division in the zygote. Moreover, we frequently encountered multiple aneuploidies and segmental chromosome aberrations. These results indicate that the pig may be particularly sensitive to in vitro production, which may, in turn, be due to incorrect chromosome segregations during meiosis and early cleavage divisions. We thus accept our hypothesis that chromosome abnormality could explain poor IVP outcomes in pigs.

Cross-talk between free and bound spermatozoa to modulate initial sperm:egg ratios at the site of fertilization in the mammalian oviduct

This essay proposes that highly localized communication between free and bound spermatozoa in the caudal portion of the oviduct acts to regulate the numbers detaching from the epithelium and progressing to the site of fertilization close to the time of ovulation. Low initial sperm:egg ratios are essential for monospermic fertilization. Liberation of surface macromolecules and metabolic prompting from activated spermatozoa, together with altered patterns of sperm movement and dynamic differences in intracellular Ca(2+) ion status between neighboring sperm cells, would influence the progressive release of spermatozoa from the reservoir in the oviduct isthmus. Different intensities of preovulatory epithelial binding, reflecting a range of states in the sperm surface membranes and associated proteins, would provide a further explanation for a chronologically staggered periovulatory detachment of spermatozoa. Intimate sperm-sperm interactions within the confines of the oviduct isthmus offer a sensitive means of fine-tuning the vanguard of competent male gametes reaching the isthmo-ampullary junction.

Sperm preparation: state-of-the-art--physiological aspects and application of advanced sperm preparation methods

For assisted reproduction technologies (ART), numerous techniques were developed to isolate spermatozoa capable of fertilizing oocytes. While early methodologies only focused on isolating viable, motile spermatozoa, with progress of ART, particularly intracytoplasmic sperm injection (ICSI), it became clear that these parameters are insufficient for the identification of the most suitable spermatozoon for fertilization. Conventional sperm preparation techniques, namely, swim-up, density gradient centrifugation and glass wool filtration, are not efficient enough to produce sperm populations free of DNA damage, because these techniques are not physiological and not modeled on the stringent sperm selection processes taking place in the female genital tract. These processes only allow one male germ cell out of tens of millions to fuse with the oocyte. Sites of sperm selection in the female genital tract are the cervix, uterus, uterotubal junction, oviduct, cumulus oophorus and the zona pellucida. Newer strategies of sperm preparation are founded on: (i) morphological assessment by means of 'motile sperm organelle morphological examination (MSOME)'; (ii) electrical charge; and (iii) molecular binding characteristics of the sperm cell. Whereas separation methods based on electrical charge take advantage of the sperm's adherence to a test tube surface or separate in an electrophoresis, molecular binding techniques use Annexin V or hyaluronic acid (HA) as substrates. Techniques in this category are magnet-activated cell sorting, Annexin V-activated glass wool filtration, flow cytometry and picked spermatozoa for ICSI (PICSI) from HA-coated dishes and HA-containing media. Future developments may include Raman microspectrometry, confocal light absorption and scattering spectroscopic microscopy and polarization microscopy.

Most fertilizing mouse spermatozoa begin their acrosome reaction before contact with the zona pellucida during in vitro fertilization

To fuse with oocytes, spermatozoa of eutherian mammals must pass through extracellular coats, the cumulus cell layer, and the zona pellucida (ZP). It is generally believed that the acrosome reaction (AR) of spermatozoa, essential for zona penetration and fusion with oocytes, is triggered by sperm contact with the zona pellucida. Therefore, in most previous studies of sperm-oocyte interactions in the mouse, the cumulus has been removed before insemination to facilitate the examination of sperm-zona interactions. We used transgenic mouse spermatozoa, which enabled us to detect the onset of the acrosome reaction using fluorescence microscopy. We found that the spermatozoa that began the acrosome reaction before reaching the zona were able to penetrate the zona and fused with the oocyte's plasma membrane. In fact, most fertilizing spermatozoa underwent the acrosome reaction before reaching the zona pellucida of cumulus-enclosed oocytes, at least under the experimental conditions we used. The incidence of in vitro fertilization of cumulus-free oocytes was increased by coincubating oocytes with cumulus cells, suggesting an important role for cumulus cells and their matrix in natural fertilization.

Polyspermy prevention: facts and artifacts?

The purpose of this review is to open a debate as to whether or not oocytes actively repel supernumerary sperm or in nature final sperm: oocyte ratios are so low that polyspermy preventing mechanisms are not necessary. Before encountering the oocyte, spermatozoa need to be primed, either by environmental factors as in animals exhibiting external fertilization, or by factors from the female reproductive tract, as in mammals. The spermatozoon must then recognize and interact with the outer layers of the oocyte and progression of the fertilizing spermatozoon through these layers is further controlled and modulated by a precise sequence of signals in situ. Removal of these outer coats may not inhibit fertilization, however does interfere with the dynamics of sperm-oocyte interaction. We propose that monospermy in mammals and sea urchins, under natural conditions, is ensured by the controlled and gradual encounter of a minimum number of spermatozoa with the oocyte and that fine tuning is ensured by the structural and molecular organization of the oocyte and its surrounding coats. We suggest that laboratory experiments using oocytes deprived of their investments and exposed to unnaturally high concentrations of spermatozoa are artifactual and argue that the conclusions leading to the hypothesis of a fast electrical block to polyspermy are unfounded. Under laboratory conditions the majority of spermatozoa, although motile and capable of attaching to the oocyte surface, are either physiologically incompetent or attach to areas of the oocyte surface that do not support entry.

Sperm head binding to epithelium of the oviduct isthmus is not an essential preliminary to mammalian fertilization - review

In endeavouring to understand the nature of sperm-oviduct interactions in mammals, attention was focused on experimental models in which fertilization can occur without a preliminary phase of sperm head binding to the isthmus epithelium. The ovarian endocrine milieu imposed on the oviduct tissues plays an important role in the binding phenomenon, although less so after the time of ovulation. Nonetheless, a sperm suspension introduced into the peritoneal cavity or surgical insemination directly into the oviduct ampulla before ovulation can result in fertilization, as can a surgical model in which the isthmus has been resected and the remaining portions of the duct reanastomosed. Mating or artificial insemination after ovulation in pigs permits rapid sperm transport to the site of fertilization, and the frequency of polyspermic penetration increases with the post-ovulatory age of eggs.Strategies underlying sperm binding were considered, especially in terms of preovulatory sperm storage and suppression of full membranous maturation. These, in turn, raised the problem of how sperm binding in vitro to oviduct cells from prepuberal animals or to cells harvested during the luteal phase of the estrous cycle, or to cells from the ampulla or even the tracheal epithelium, can act to regulate sperm storage and maturation with precision. In an evolutionary perspective, preovulatory binding of diverse populations of cells to the endosalpinx may have developed as a form of fine tuning to assist in sperm selection, to synchronize completion of capacitation with the events of ovulation, and to promote monospermic fertilization by a controlled release of competent gametes.

Fertilization: a sperm's journey to and interaction with the oocyte

Mammalian fertilization comprises sperm migration through the female reproductive tract, biochemical and morphological changes to sperm, and sperm-egg interaction in the oviduct. Recent gene knockout approaches in mice have revealed that many factors previously considered important for fertilization are largely dispensable, or if they are essential, they have an unexpected function. These results indicate that what has been observed in in vitro fertilization (IVF) differs significantly from what occurs during "physiological" fertilization. This Review focuses on the advantages of studying fertilization using gene-manipulated animals and highlights an emerging molecular mechanism of mammalian fertilization.

Zona-free embryo culture: is it a viable option to improve pregnancy rates?

Sporadic reports published during the previous decade have documented pregnancies achieved with transfer of zona-free human embryos. Although the overall efficiency seems to be good and some authors have suggested systematic application for special infertility problems, there have been only a few attempts to compare the benefits of zona-free embryo culture and transfer with the traditional approach using zona-intact embryos. So far, the majority of instances in which zona-free culture has been applied have occurred accidentally. This review summarizes the known functions of the zona pellucida, analyses natural and artificial situations where its function is compromised, including zona hardening and difficult hatching that seem to be related to in-vitro embryo culture, and discusses possible methods and timing for artificial zona removal. With the availability of in-vitro systems capable of replacing important functions of the zona pellucida, routine use of zona-free culture for the whole in-vitro period, after or even before fertilization, is a realistic possibility with potential additional benefits. Based on the increasing amount of animal studies, a systematic comparison is suggested that may eventually diminish the handicaps of the in-vitro situation and lead to simplification of manipulations as well as higher success rates after embryo transfer.

What controls polyspermy in mammals, the oviduct or the oocyte?

A block to polyspermy is required for successful fertilisation and embryo survival in mammals. A higher incidence of polyspermy is observed during in vitro fertilisation (IVF) compared with the in vivo situation in several species. Two groups of mechanisms have traditionally been proposed as contributing to the block to polyspermy in mammals: oviduct-based mechanisms, avoiding a massive arrival of spermatozoa in the proximity of the oocyte, and egg-based mechanisms, including changes in the membrane and zona pellucida (ZP) in reaction to the fertilising sperm. Additionally, a mechanism has been described recently which involves modifications of the ZP in the oviduct before the oocyte interacts with spermatozoa, termed "pre-fertilisation zona pellucida hardening". This mechanism is mediated by the oviductal-specific glycoprotein (OVGP1) secreted by the oviductal epithelial cells around the time of ovulation, and is reinforced by heparin-like glycosaminoglycans (S-GAGs) present in oviductal fluid. Identification of the molecules contributing to the ZP modifications in the oviduct will improve our knowledge of the mechanisms of sperm-egg interaction and could help to increase the success of IVF systems in domestic animals and humans.

Male infertility, female fertility and extrapair copulations

Females that are socially bonded to a single male, either in a social monogamy or in a social polygyny, are often sexually polyandrous. Extrapair copulations (EPC) have often been suggested or rejected, on both empirical and theoretical grounds, as an important mechanism that enables females to avoid fertility risks in case their socially bonded male is infertile. Here, we explore this possibility in two steps. First, we present a mathematical model that assumes that females have no precopulatory information about male fertility, and shows that a female EPC strategy increases female reproductive success only if certain specific conditions are upheld in the nature of male infertility. In particular, these conditions require both (i) that fertile sperm precedence (FSP) is absent or incomplete within ejaculates of the same male (i.e. that an infertile male is, at least partly, truly infertile), and (ii) the existence of FSP among ejaculates of different males (such that infertile spermatozoa of the infertile male are at a disadvantage when competing against spermatozoa of a fertile male). Second, to evaluate their potential role in the evolution of female EPC, we review the abundance and FSP patterns of the different male infertility types. The conclusion is drawn that some common infertility types, such as poor sperm count or motility, contribute to the evolution of female EPC, whereas other common infertility types, such as sperm depletion or allocation in a social monogamy (but not in a social polygyny), and in particular male driven polyspermy, do not. Also, a deeper look at the arms race between sperm fertilization efficiency and female barriers to sperm may answer the non-trivial question: "why are some types of infertility so common?"

Fertilization and early embryonic development in the porcine fallopian tube

Fertilization and early embryo development relies on a complex interplay between the Fallopian tube and the gametes before and after fertilization. Thereby the oviduct, as a dynamic reproductive organ, enables reception, transport and maturation of male and female gametes, their fusion, and supports early embryo development. This paper reviews current knowledge regarding physiological processes behind the transport of boar spermatozoa, their storage in and release from the functional sperm reservoir (SR), and of the interactions that newly ovulated oocytes play within the tube during their transport to the site of fertilization. Experimental evidence of an ovarian control on sperm release from the SR is highlighted. Furthermore, the impact of oviductal secretion on sperm capacitation, oocyte maturation, fertilization and early embryo development is stressed.

The human progesterone receptor shows evidence of adaptive evolution associated with its ability to act as a transcription factor

The gene encoding the progesterone receptor (PGR) acts as a transcription factor, and participates in the regulation of reproductive processes including menstruation, implantation, pregnancy maintenance, parturition, mammary development, and lactation. Unlike other mammals, primates do not exhibit progesterone withdrawal at the time of parturition. Because progesterone-mediated reproductive features vary among mammals, PGR is an attractive candidate gene for studies of adaptive evolution. Thus, we sequenced the progesterone receptor coding regions in a diverse range of species including apes, Old World monkeys, New World monkeys, prosimian primates, and other mammals. Adaptive evolution occurred on the human and chimpanzee lineages as evidenced by statistically significant increases in nonsynonymous substitution rates compared to synonymous substitution rates. Positive selection was rarely observed in other lineages. In humans, amino acid replacements occurred mostly in a region of the gene that has been shown to have an inhibitory function (IF) on the ability of the progesterone receptor to act as a transcription factor. Moreover, many of the nonsynonymous substitutions in primates occurred in the N-terminus. This suggests that cofactor interaction surfaces might have been altered, resulting in altered progesterone-regulated gene transcriptional effects. Further evidence that the changes conferred an adaptive advantage comes from SNP analysis indicating only one of the IF changes is polymorphic in humans. In chimpanzees, amino acid changes occurred in both the inhibitory and transactivation domains. Positive selection provides the basis for the hypothesis that changes in structure and function of the progesterone receptor during evolution contribute to the diversity of primate reproductive biology, especially in parturition.

Enigmas of mammalian gamete form and function

The gametes of man and some other Eutheria have been manipulated successfully for practical reasons, but many gaps remain in our basic understanding of the way that they function. This situation stems not least from a failure to recognize the extent to which eutherian spermatozoa and eggs, and elements related to their operation, have come to differ from those of other groups. Novel features in the male that reflect this include a radical design of the sperm head with the acrosome seeming to function primarily in egg-coat binding rather than its lysis, a multifaceted post-testicular sperm maturation and an androgen/low-temperature-regulated system of sperm storage--both tied to the epididymis, a variable male accessory sex gland complex, and descent of the testis and epididymis to a scrotum. In the female, such novelties are represented in a need for sperm capacitation, in an unusual regulation of sperm transport within the oviduct, in the cumulus oophorus and character of the zona pellucida around the small egg, and in a unique configuration of gamete fusion. The collective evidence now suggests that many of these features reflect a new fertilisation strategy or its consequences, with most being causally linked. One initial 'domino' in this regard appears to be the small yolkless state of the egg and its intolerance for polyspermy, as determinants of the unusual mode of oviductal sperm transport and possibly the existence and form of the cumulus oophorus. However, a particularly influential first 'domino' appears to be the physical character of the eutherian zona pellucida. This differs from the egg coats of other animal groups by virtue of a resilient elasticity and thickness. These qualities allow this primary and often only coat to stretch and so persist during later expansion of the blastocyst, usually until close to implantation. At the same time, the dimensions, physical character, and particularly the relative protease-insensitivity of the zona appear to have had profound effects on sperm form and function and, more indirectly, on sperm-related events in the male and the female tract. Marsupials display some similarities and also some strikingly different features, against which the enigmas of the eutherian situation can be evaluated.

Experimental Evidence for the Influence of Cumulus-Oocyte-Complexes on Sperm Release from the Porcine Oviductal Sperm Reservoir

In the pig, a temporal relationship is suggested between sperm release from the sperm reservoir (SR) and ovulation, but the mechanism(s) is still under discussion. In two experiments, the influence of transferred ova on the release of SR-spermatozoa at ovulation and the effect of supplementation with non-sulfated glycosaminoglycan hyaluronan (HA) on embryo development and the number of accessory spermatozoa, respectively, were examined. PMSG/hCG primed ovectomized gilts that had previously received endoscopic low-dose insemination into the cranial uterine horn were used as an experimental model. After salpingectomy, tubal segments (ampulla, cranial, and caudal isthmus) were flushed and sperm numbers or respective accessory spermatozoa were counted. In Experiment 1, the distribution of the sperm population was altered in the presence of cumulus-oocyte-complexes (COCs). A higher proportion of spermatozoa was found after transfer of COCs into one oviduct in the ampulla and cranial isthmus segments compared with the controls (17.5 vs. 4.9%, p<0.05). In Experiment 2, the quality of the transferred ova and treatment influenced the presence of accessory spermatozoa. Transfer of COCs together with HA increased (p<0.05) the number of accessory spermatozoa compared with the other treatment groups and was similar to those in the "undisturbed" controls. No modifications were obtained regarding mean blastomere numbers (2.6 +/- 0.2 to 3.1 +/- 0.2). In summary, this study was demonstrated that cumulus-oocyte-complexes may be involved in triggering sperm release from the pig oviductal SR and that HA might be related to sperm release.

Intrafallopian transfer of gametes and early stage embryos for in vivo culture in cattle

It may be possible to avoid inadequate in vitro culture conditions by incubating gametes or embryos in the oviducts for a short time. Ideally, an optimized procedure should be devised, combining in vitro and in vivo systems, in order to achieve synchronization in cattle. We transferred gametes as well as embryos in various stages of development and placed them into the oviducts. Embryos were recovered on Day 7 by flushing of oviducts and uterine horns. Blastocyst rates were determined on Day 7 and on Day 8. Experimental designs included transfer of in vitro matured cumulus oocyte complexes into previously inseminated heifers (COCs group), transfer of in vitro matured COCs simultaneously with capacitated spermatozoa (GIFTs group), transfer of four to eight cell stage embryos developed in vitro after IVM/IVF (Cleaved Stages group) and a group of solely in vitro produced embryos (IVP control group). Our results indicate that in vivo culture of IVM/IVF embryos in the homologous bovine oviduct has a positive influence on subsequent pre-implantation development. In addition, we have evidence that in vitro maturation and in vivo fertilization cannot be synchronized.

Spermadhesin AQN1 is a candidate receptor molecule involved in the formation of the oviductal sperm reservoir in the pig

Sperm are stored in the isthmic region of the oviduct under conditions that maintain viability and suppress early capacitation steps until ovulation occurs. The initial contact between sperm and oviductal epithelium is mediated by carbohydrate-protein interactions. In the pig, the carbohydrate recognition system has been shown to involve oligomannosyl structures. The spermadhesins AWN and AQN1 are the dominant porcine carbohydrate-binding sperm proteins. The objective of this study was to demonstrate that AQN1 contributes to sperm binding to the oviductal epithelium. AQN1 showed a broad carbohydrate-binding pattern as it recognizes both alpha- and beta-linked galactose as well as Manalpha1-3(Manalpha1-6)Man structures, whereas AWN bound only the galactose species. Binding of ejaculated sperm to oviductal epithelium was inhibited by addition of AQN1 but not by AWN. Mannose-binding sites were localized over the rostral region of the sperm head. Flow cytometry showed that, under capacitating conditions, the population of live sperm was shifted within 30 min toward an increase in the proportion of cells with low mannose- and high galactose-binding. The loss of mannose-binding sites was accompanied by the loss of AQN1 in sperm extracts and the significant reduction in the sperm-oviduct binding. The oviductal epithelium was shown by GNA-lectin histochemistry and by SDS-PAGE and lectin blotting of the apical membrane fraction to express mannose components that could be recognized by AQN1. These results demonstrate that the sperm lectin AQN1 fulfils the criteria for an oviduct receptor in the pig and may play a role in the formation of the oviductal sperm reservoir.

Boar spermatozoa in the oviduct

In the pig, a functional tubal sperm reservoir (SR) is established before ovulation to ensure availability of suitable numbers of viable spermatozoa for fertilization. The boar's large ejaculate is split: most spermatozoa are delivered in a sperm-rich fraction (SRF) followed by a post-SRF fraction containing increasing amounts of the spermadhesin PSP-I/PSP-II-rich seminal vesicle secretion. This heterodimer acts as leukocyte chemoattractant both in vitro and in vivo, contributing to the phagocytosis of those spermatozoa not reaching the SR. Sequential ejaculate deposition of marked spermatozoa and SR screening showed that most spermatozoa in the SR arose from the fortuitous PSP-poor, first portion of the SRF fraction, escaping phagocytosis and replenishing the SR within 2-3 h. The SR-sperm numbers diminish gradually in relation to ovulation, spermatozoa being continuously redistributed toward the upper isthmus. In vitro, only uncapacitated spermatozoa bind to epithelial explants, suggesting that the SR influences sperm capacitation. In vivo, most viable spermatozoa--usually harbored in the deep furrows in the pre- or peri-ovulatory SR during spontaneous standing estrus--are uncapacitated, but capacitation significantly increases after ovulation. Pre-/peri-ovulatory SR spermatozoa promptly capacitate in vitro when exposed to the effector bicarbonate, an influence that can be reversed by co-incubation with SR fluid or its component hyaluronan. Fluid collected from the ampullar segment (rich in bicarbonate) induces capacitation in vitro. In conclusion, the lack of massive sperm capacitation in the SR and the diverse individual response to capacitation shown by tubal spermatozoa would relate both to the insurance of full sperm viability before ovulation and the presence of spermatozoa at different stages of capacitation in the upper oviduct, thus maximizing the chances of normal fertilization.

How does polyspermy happen in mammalian oocytes?

Polyspermy is one of the most commonly observed abnormal types of fertilization in mammalian oocytes. In vitro fertilization (IVF) provides approaches to study the mechanisms by which oocytes block polyspermic fertilization. Accumulated data indicate that oocyte, sperm and insemination conditions are all related to the occurrence of polyspermic fertilization. A high proportion of immature and aged oocytes showed polyspermy as compared with mature oocytes. Preincubation of oocytes and/or sperm with oviductal epithelial cells or collected oviductal fluid before IVF reduces polyspermic penetration. Recently, it was found that an abnormal zona pellucida is one of main causes of polyspermy in human eggs. A high proportion of polyspermy has resulted from the use of a high concentration of capacitated spermatozoa at the site of fertilization, irrespective of in the in vivo or in vitro environment. Oviductal secretions or oviductal epithelial cells themselves can regulate the number of spermatozoa reaching or binding to the zona pellucida thus reducing multiple sperm penetration. Suboptimal in vitro conditions, such as supplementations in IVF media, pH, and temperature during IVF, also induce polyspermic fertilization in some mammals. Species-specific differences are present regarding the relationship between insemination conditions and polyspermy.

The effect of oviductal epithelial cell co-culture during in vitro maturation on sow oocyte morphology, fertilization and embryo development

In vitro embryo production in the sow is challenged by poor cytoplasmic maturation, low sperm penetration and low normal fertilization, leading to the development of poor quality blastocysts containing a small number of nuclei. In prepubertal gilt oocytes, the presence of porcine oviductal epithelial cells (pOECs) during maturation increases cytoplasmic maturation and blastocyst development. These aspects, as well as blastocyst quality, may be improved when adult sow oocytes are matured with pOEC. Therefore, the effect of the presence of pOEC on sow oocyte morphology, fertilization and the progression of embryo development was evaluated. The pOEC were cultured in M199 for 18 h, then cultured in NCSU23 for 4 h before the oocytes were added. Oocytes from 2 to 6 mm follicles were matured in 500 microl NCSU23, with eCG and hCG, for 24 h, and then cultured with or without pOEC, in NCSU23 without hormones, for 18 h. In vitro fertilization took place in modified Tris-buffered medium, for 6 h, and the presumptive zygotes were then cultured for 162 h in NCSU23. Morphology of the IVM oocytes was compared to that of immature oocytes and in vivo matured MII oocytes from slaughtered sows in estrus. The in vitro matured oocytes had a greater diameter and a wider perivitelline space than the immature and in vivo matured MII oocytes (P < 0.01). Penetration, polyspermy and pronucleus formation did not differ between the pOEC and Control groups, although the total penetration rate was higher for the Control oocytes (26% versus 39%; P < 0.01). Fewer blastocysts developed in the pOEC group than in the Control group (19% versus 27%; P < 0.01), but blastocyst growth was accelerated, leading to a higher percentage of hatched blastocysts (3% versus 10%; P < 0.01). Finally, the average blastocyst cell number was higher in the pOEC group (47 versus 40; P < 0.05) and a greater percentage of blastocysts contained a superior number of nuclei. In conclusion, the addition of pOEC during the second half of in vitro maturation resulted in fewer blastocysts formed, but of those blastocysts that did form the quality was improved.

Reflections upon sperm-endosalpingeal and sperm-zona pellucida interactions in vivo and in vitro

In terms of experimental studies of the process of fertilization in mammals, this essay attempts to give a balanced assessment of current interpretations and to raise questions of direct relevance to two fields of research. First, the nature of pre-ovulatory sperm-endosalpingeal binding is set in a physiological perspective, and examination of this dynamic process in vitro is shown to suffer from serious shortcomings. In particular, sperm-epithelial binding reactions demonstrated in cultures of endosalpingeal cells may represent only a portion of the binding reaction in vivo or, as revealed by sperm binding to the tracheal epithelium, could be largely non-specific. Second, concerning the subsequent phase of binding of a fertilizing spermatozoon to the zona pellucida, the nature of the initial contact arrest of the sperm head is evaluated, especially in the light of the highly uneven or 'pitted' surface of this outer coating of the oocyte. Bearing in mind the active phase of oviduct macromolecular secretion that continues after ovulation, the question is posed as to how functional sperm binding sites are preserved on the surface of the zona pellucida rather than masked by epithelial and cumulus cell secretions. Finally, it is recalled that initial sperm : egg ratios at the site of fertilization in the ampullary-isthmic region of the oviduct are close to unity in diverse species of mammal, such fertilizing spermatozoa being released from prominent reserves in the caudal isthmus shortly before the time of ovulation. On strictly quantitative grounds, therefore, the extent to which biochemical or molecular studies portray changes in the vanguard of fertilizing spermatozoa needs to be questioned. Such studies are more likely to be descriptive of overall changes in very large numbers of sperm cells at diverse stages of maturation.

Capacitation of mammalian spermatozoa in vivo, with a specific focus on events in the fallopian tubes

This essay argues strongly that for those sperm cells involved in fertilisation, the process of capacitation represents an active and specific coordination within succeeding regions of the female tract and one whose completion is synchronised with the events of ovulation. Observations on the time-course of capacitation when spermatozoa are first exposed to the uterus and then progress to the Fallopian tubes indicate a synergistic influence of these adjoining portions of the female tract on the rate of capacitation. Three concepts on the control of capacitation are introduced to emphasise the importance of integration in vivo, namely that (1) completion of capacitation is a peri-ovulatory event, (2) suppression of completion of capacitation is an essential storage strategy during a long pre-ovulatory interval, and (3) the process of capacitation comes under the influence of local and systemic ovarian control mechanisms, especially the secretion of progesterone from Graafian follicles soon to ovulate. The last would act to coordinate the final maturation and meeting of male and female gametes. Despite the preceding points, the requirement for such integrated in vivo programming of sperm cell maturation can clearly be overridden in systems of culture. The most reasonable interpretation here would be that a microdrop of culture medium containing eggs, follicular cells and components of follicular fluid would to a considerable extent represent a post-ovulatory environment. Within such a preparation, there would be leaching of the sperm surface among the relatively vast and heterogeneous population of cells, and a proportion of spermatozoa could then respond to 'post-ovulatory signals', not least to molecular influences of the zona pellucida and vitelline products for completion of capacitation. Nonetheless, a physiologically meaningful interpretation of capacitation calls for a stepwise analysis of the dynamic interactions between sperm cell and female tract at successive stages between the uterus and ampullary-isthmic junction.

Vital aspects of Fallopian tube physiology in pigs

This essay reviews four topical aspects of Fallopian tube physiology that bear on either successful fertilization or early development of the zygote. An initial focus is on glycoprotein secretions of the duct that accumulate as a viscous mucus in the caudal isthmus. Because this is the site of the pre-ovulatory sperm reservoir, an involvement of the secretions is considered in: preventing uterine and ampullary tubal fluids from entering the functional sperm reservoir; removing residual male secretions from the sperm surface; deflecting spermatozoa towards endosalpingeal organelles and reducing flagellar beat before ovulation. The subtle prompting of flagellar movement with impending ovulation is examined in terms of potential reactivation mechanisms, with overall control attributed to increasing secretion of progesterone. The site of full capacitation and the acrosome reaction in a fertilizing spermatozoon is then debated, with strong arguments pointing to completion of these processes in the specific fluids at the ampullary-isthmic junction. Finally, the synthetic activity of cumulus cells released at ovulation as a paracrine tissue in the Fallopian tube is highlighted with reference to steroid hormones, peptides and cytokines. Not only does the suspension of granulosa-derived cells influence the process of fertilization, but also it may amplify oocyte or embryonic signals to the endosalpinx and ipsilateral ovary.

Efficiency of gamete usage in nature: sperm storage, fertilization and polyspermy

Gamete production for both males and females can be energetically expensive such that selection should maximize fertilization opportunities while minimizing fertilization costs. In laboratory studies of Drosophila reproduction, however, the failure of eggs to yield adult progeny can be quite high, suggesting that female control over gamete utilization is surprisingly inefficient. We examined gamete utilization in D. pseudoobscura from nature and compared our observations to those for laboratory populations. In natural populations 100% of oviposited eggs effectively produce adult progeny, and fertilization is exclusively monospermic, indicating that in nature, D. pseudoobscura females maintain a very strict control over their reproduction such that gamete usage is extremely efficient. The potential reasons for the inefficient gamete utilization in the laboratory, as well as the potential impact on laboratory studies of sperm competition, sexual conflict, and the evolution of reproductive barriers are discussed. Furthermore, in this sperm-heteromorphic species, our observations show definitively that in nature, as well as in the laboratory, only the long sperm morph participates in fertilization.

Heparin and Ca2+-free medium can enhance release of bull sperm attached to oviductal epithelial cell monolayers

The success of assisted reproductive techniques, such as IVF, could be enhanced by being able to select the most competent spermatozoa in a sample. Attachment and subsequent release of spermatozoa from oviductal epithelial cells (OEC) could provide populations of functionally superior spermatozoa for use in these protocols. The objective of the present study was to investigate the ability of heparin and Ca2+-free medium to induce spermatozoa release from bovine OEC. Epithelial cells were grown to confluence in 24-well plates and pooled frozen bull semen was added to a final concentration of 1 x 10(6) spermatozoa/well. Spermatozoa were allowed to bind to OEC for 2 h. Medium with unbound spermatozoa was removed and replaced by Sperm-TALP, only (control), with heparin (5, 10, or 15 IU/mL), or Ca2+-free with 2 mM EGTA. Treatments were left on sperm-OEC co-cultures for 0.5, 1, 2, 3, or 5 h. At each time, the media were recovered and spermatozoa from each treatment were counted and evaluated for acrosome integrity and motility. The total number of spermatozoa attached to OEC after 2 h of co-culture was considered 100%. Spermatozoa release is expressed as percentage of the total number of sperm cells bound to OEC after 2 h of co-culture. Data were analyzed by ANOVA and results are expressed as mean +/- SEM from three independent replicates. Beginning at 0.5 h, more sperm cells (P < 0.05) were released from OEC in the heparin groups (10 and 15 IU/mL, 77.3 +/- 6.2% and 84.0 +/- 6.2%, respectively) as compared to the control (46.4 +/- 6.2%). The Ca2+-free medium also induced spermatozoa release when compared with the control, but the effect was not significant until 3 h (38.2 +/- 1.9% vs 59.5 +/- 6.9%; P < 0.05). The percentage of acrosome reacted spermatozoa was not affected by heparin treatment. Heparin at 10 IU/mL increased (P < 0.05) the percentage of motile spermatozoa, whereas Ca2+-free medium caused the opposite effect at 0.5 h after addition of treatments. We conclude that both heparin and Ca2+-free medium are able to promote spermatozoa displacement from OEC attachment. Based on motility and acrosome status data, we predict that released sperm cells may be used for IVF and other assisted reproductive techniques.

Study of the functional anatomy of bovine oviductal mucosa

The oviducts of 31 cyclic cows were examined to study the structure and nature of the oviductal mucosa. The general distribution of spermatozoa within the oviductal mucosa was studied in five additional cows. The oviductal infundibulum is an asymmetric funnel-shaped structure surrounding the ostium. It is divided along the free boarder of the mesosalpinx and presents one wide and one narrow side. The mucosa of the wide side possesses a system of low interconnected cords that converge distally forming primary folds. The folds on the narrow side start sharply from the free margin and fuse toward the ostium abdominale. Areas between folds throughout the lumen of the oviduct show a high degree of complex organization. Interfold spaces are occupied by secondary and small interconnected folds which join to form a system of cul-de-sacs. In the infundibulum, these cul-de-sacs open toward the ovary, while cul-de-sacs present in the caudal isthmus and in the UTJ open toward the uterus. Marked variations were observed in the oviductal epithelium depending on the oviductal segment, basal or apical areas of the folds, and phase of the oestrous cycle. Near to the time of ovulation, numerous spermatozoa were found in the periphery of the caudal isthmus within pockets of basal interfold areas, as well as within pockets and cul-de-sacs of the tubo-uterine junction. Individual spermatozoa were also observed in peripheral areas of the ampullary-isthmic junction and ampulla. The topography of the oviduct provides a complex system of regulation which may influence not only the passage of gametes and/or embryos, but also movement of fluid within the oviductal canal.

The causes of reduced fertility with cryopreserved semen

Cryopreserved mammalian semen is generally acknowledged to have an impaired fertility by comparison with fresh semen. The reduction arises from both a lower viability post-thaw and sublethal dysfunction in a proportion of the surviving subpopulation. The reasons for the loss of fertility are various. In this paper, factors affecting the proportion of survivors (e.g., cold shock susceptibility, cooling rate, diluent composition and osmotic stress) and factors influencing functional status of survivors (e.g., membrane stability, oxidative damage, membrane receptor integrity, nuclear structure) are briefly reviewed. The possible effects of cryopreservation on the role of spermatozoa in the early stages of embryogenesis are considered. In the light of this review, indications for new approaches for improving the performance of cryopreserved semen are offered.

Effect of hyaluronan on monospermic penetration of porcine oocytes fertilized in vitro

An experimental series was carried out to examine the ability of exogenous hyaluronan (HA) to facilitate monospermic penetration of in vitro-matured (IVM) oocytes during conventional porcine in vitro fertilization (IVF). IVM oocytes were co-cultured in fertilization medium with frozen-thawed, ejaculated boar spermatozoa. Addition of 0.5 mg/mL HA to the fertilization medium increased the rates of monospermic penetration (p < 0.05) on cumulus-denuded oocytes, without detrimentally affecting penetration rates (rates of monospermy were 57% and 78% in 2 mM caffeine-containing fertilization medium without and with HA, respectively). Caffeine and cumulus cells affected the rate of monospermic fertilization (p < 0.02-0.03), and caffeine affected the rate at which oocytes were penetrated (p < 0.02). Moreover, exogenous HA (0.5 mg/mL) in the sperm pre-incubation medium promoted monospermic penetration rates without affecting overall penetration rates. The effects of exogenous HA on sperm penetration and monospermy were also compared among semen batches. After semen was pooled from three fertile boars and held in homologous seminal plasma (SP) for 3 (batch A), 10 (batch B) or 20 (batch C) h prior to being cooled, spermatozoa from each semen batch were co-cultured with IVM oocytes. Monospermy of cumulus-denuded oocytes was increased (p < 0.05) by the addition of exogenous HA within the same semen batch (batches B and C). Furthermore, co-culture with cumulus cells did not mask the effects of HA on batch-B semen. The concentration of exogenous HA (0, 0.05 and 0.5 mg/mL) during sperm-oocyte co-culture differentially affected the rates of penetration and monospermy of cumulus-intact oocytes, depending on the time spermatozoa were held in SP before being frozen. The highest penetration rate was obtained by exposing semen to SP for 20 h prior to being cooled (0.05 mg/mL HA). The results suggest that exogenous HA decreases polyspermy during conventional porcine IVF. Furthermore, it appears that the action of HA is dependent upon the priming action of seminal plasma.

Ovarian follicular fluid, progesterone and Ca2+ ion influences on sperm release from the fallopian tube reservoir

As a means of determining whether ovarian follicular fluid reaches the functional sperm reservoir in the caudal isthmus of the Fallopian tube shortly after ovulation, 0.01-0.02 ml aliquots of whole or steroid-free follicular fluid were introduced into the distal extremity of the isthmus within 1 hr before ovulation. Eggs were recovered during a second intervention 4 hr 45 min-6 hr 10 min after treatment and examined by phase-contrast microscopy for the normality of fertilisation. In a separate experiment, 0.01-0.02 ml aliquots of 10 microM calcium ionophore solution were introduced into the same site in comparable animals. Sixty-nine fertilised eggs were recovered from 12 fallopian tubes treated with whole follicular fluid, of which 24 (34.8%) were polyspermic. The 12 contralateral control tubes (PBS-treated) yielded 47 fertilised eggs, of which only one (2.1%) was polyspermic (P < 0.001). Steroid-free aliquots of the same follicular fluid introduced bilaterally into eight fallopian tubes (4 animals) resulted in recovery of 59 fertilised eggs, of which only one (1.7%) was polyspermic. Treatment with ionophore solution yielded a 41.6% incidence of polyspermy (10 of 24 eggs from four tubes) compared with 3.8% polyspermy (1 egg) from the control tubes (P < 0.01). Dispermy was the principal form of polyspermy. The numbers of accessory spermatozoa on/in the zona pellucida were increased by the experimental treatment. Follicular fluid passing down the fallopian tube ampulla at ovulation was therefore considered not to be the physiological stimulus for an initial, tightly-controlled release of spermatozoa from epithelial binding in the caudal isthmus. Indeed, because such sperm activation commences shortly before ovulation, a locally transmitted ovarian programming with relatively high concentrations of follicular hormones remains the favoured model. Although pre-ovulatory progesterone is considered to be the coordinating steroid of increasing influence in these pre-fertilisation events, its effects are proposed to be modulated in the endosalpinx by mobilisation of Ca2+ ions into a discrete population of bound spermatozoa. Results of the steroid-free follicular fluid and calcium ionophore treatments stand in support.

Pronuclear location before the first cell division determines ploidy of polyspermic pig embryos

Polyspermy occurs frequently in the fertilization of mammalian eggs, but little is known about whether polyspermic eggs have developmental ability in vitro or in vivo. We previously reported that poly-pronuclear (PPN; 3 or more pronuclei) pig eggs developed normally to the blastocyst stage despite having fewer inner cell mass cell numbers as compared to blastocysts derived from two-pronuclear (2PN) eggs. Here it is shown that most PPN pig eggs have abnormal cleavage patterns (having 3 or more cells) in the first cell division and retarded development of pronuclei prior to syngamy as compared to 2PN eggs. Most blastocysts (14 of 18) that developed from PPN eggs showed abnormal ploidy (were haploid, triploid, and tetraploid) whereas 20 of 22 blastocysts derived from 2PN embryos were diploid. The size and morphology of most Day 40 fetuses that developed from PPN eggs appeared to be normal. Of 8 Day 40 fetuses analyzed, 1 was triploid (XXY) and another was a mosaic with both diploid (XX) and tetraploid cells (frequency of less than 10%, XXXX), and the others were diploid. Anomalies of chromosomal composition were not detected in these fetuses. Five live piglets and one dead piglet were born from two recipients of PPN eggs. It is proposed that not all pronuclei of PPN pig eggs participate in syngamy, resulting in diploid cells in the conceptus. Our data suggest that there are two types of pronuclei location in polyspermic pig eggs and that the resulting ploidy is determined at the zygote stage before the first cell division according to pronuclear location.

In vitro production of pig embryos: comparisons of culture media and boars

The utilization of in vitro produced pig embryos for commercial production or research is dependent upon the development of improved methodology. Our objective was to establish a consistent in vitro embryo production (IVP) system and subsequently utilize the procedures to evaluate culture system components and boar effects. To summarize the IVP system, 403 inseminated oocytes from a total of 2243 were analyzed across 17 replicates for maturation and fertilization efficiency, while 1838 zygotes were cultured in 26 replicates for developmental data. Penetration, cleavage and blastocyst development rates were determined at 18, 44 and either 144 or 168 h post insemination, respectively. Monospermic penetration averaged 31.8+/-7.3% while polyspermy was 30.8+/-17.2%. Cleavage rate was 44.9+/-16.1%, with 21.8+/-7.5% of fertilized oocytes and 51.9+/-15.9% of cleaved embryos developing to blastocysts. For culture medium comparison, fertilized oocytes were cultured in either BECM-6, BECM-7, NCSU-23 or NCSU-23aa and supplemented on Day 5 post insemination (pi) with 10% FCS. These treatments resulted in 4.0, 4.9, 19.8 and 13.6% (+/-3.2%) blastocysts by Day 7 pi, with an average cell number of 44.4+/-9.0, 65.1+/-8.2, 61.3+/-4.5 and 64.4+/-4.8, respectively. These IVP procedures consistently produced zygotes from semen of several different boars, capable of forming blastocysts in vitro. Comparison of developmental rates among the boars indicated that this system is variable among boars but not strictly boar-dependent. Culture media comparisons suggest that NCSU-23 yielded a higher percentage of blastocysts than the other media in this IVP system.

Long-term stability of the bovine block to polyspermy

Under most circumstances in eutherian mammals, mating occurs before ovulation and a defense against multiple sperm penetration of the zygote is established rapidly after fertilisation. It is generally assumed that this block to polyspermy is both stable and long-lasting, but these points have not been specifically tested. They assume particular significance in domestic farm animals in which procedures of artificial insemination could mistakenly introduce sperm suspensions into a genital tract already containing a fertilised egg. Bovine blastocysts generated in vitro were further exposed in vitro to new suspensions of capacitated bull spermatozoa containing 1.5 x 10(6) cells per ml on days 7 or 8 of development, that is, shortly before or after hatching from the zona pellucida. Inseminated and control blastocysts were examined by light or electron microscopy. Whereas accessory spermatozoa were frequently bound to the surface of the zona pellucida, non had penetrated into the substance of the zona. Similarly, whilst a few spermatozoa had attached to the trophoblast of hatched blastocysts on day 8, none had penetrated through to the blastocoele. No difference was found in the incidence of hatching or rate of expansion between experimental (re-inseminated) and control groups. Accordingly, it is concluded that the bovine block to polyspermy is both stable and long-lasting when tested in vitro in the presence of freshly capacitated bull spermatozoa. Early embryonic loss, which may reach a level of 30% or more in this species, is thus unlikely to be a consequence of instability in the block to polyspermy and accessory sperm penetration of the zygote.