The biological mechanisms regulating sperm selection by the ovine cervix

Cysteine supplementation pre-freeze and post-thaw improves integrity and reduces oxidative stress in cryopreserved ram spermatozoa

Cryopreserved ram sperm is highly sensitive to oxidative stress by reactive oxygen species (ROS) which impair sperm function and integrity. Antioxidants such as cysteine can mitigate the effect of ROS, although the optimal concentration or timing of supplementation is unknown. This study aimed to determine the effect of concentration and timing of cysteine supplementation on the integrity and function of cryopreserved ram spermatozoa. Nine ejaculates were collected from three Texel rams then cryopreserved and supplemented with cysteine (0, 0.5, or 1.0 mg/mL) added pre-freeze (PF), post-thaw (PT) or pre-freeze and post-thaw (PF + PT) generating seven treatments: 1) control 0 mg/mL, 2) PF 0.5 mg/mL, 3) PF 1 mg/mL, 4) PT 0.5 mg/mL, 5), PT 1.0 mg/mL, 6) PF + PT 0.5 mg/mL and 7) PF + PT 1.0 mg/mL. Sperm motility, viability, acrosome integrity, ROS production and penetrability through artificial cervical mucus were assessed post-thaw. Cysteine supplementation reduced ROS production which thereby improved spermatozoa motility, viability, acrosome integrity and penetrability (p < 0.001) Sperm integrity for all parameters was greatest in spermatozoa treated PF + PT with 1.0 mg/mL cysteine, although treatment pre-freeze or post-thaw also improved integrity beyond the control. This study has identified that 1.0 mg/mL cysteine is most beneficial and has highlighted the importance of preventing oxidative stress in spermatozoa post-thaw. These finding can help to mitigate the detrimental effect of cryopreservation on spermatozoa and aid the development of cryopreservation protocols in sheep.

Sperm in the Mammalian Female Reproductive Tract: Surfing Through the Tract to Try to Beat the Odds

Mammalian sperm are deposited in the vagina or the cervix/uterus at coitus or at artificial insemination, and the fertilizing sperm move through the female reproductive tract to the ampulla of the oviduct, the site of fertilization. But the destination of most sperm is not the oviduct. Most sperm are carried by retrograde fluid flow to the vagina, are phagocytosed, and/or do not pass barriers on the pathway to the oviduct. The sperm that reach the site of fertilization are the exceptions and winners of one of the most stringent selection processes in nature. This review discusses the challenges sperm encounter and how the few sperm that reach the site of fertilization overcome them. The sperm that reach the goal must navigate viscoelastic fluid, swim vigorously and cooperatively along the walls of the female tract, avoid the innate immune system, and respond to potential cues to direct their movement.

Metabolic signature of cervical mucus in ewe breeds with divergent cervical sperm transport: a focus on metabolites involved in amino acid metabolism

INTRODUCTION

Cervical artificial insemination (AI) with frozen-thawed semen in sheep has yielded unacceptably low pregnancy rates. The exception is in Norway where vaginal AI yields non-return rates in excess of 60%, which has been attributed to the ewe breed used.

OBJECTIVES AND METHODS

This study aimed to characterise, for the first time, the ovine follicular phase cervical mucus metabolome, with a focus on the amino acid profile. Cervical mucus was collected from four European ewe breeds with known differences in pregnancy rates following cervical AI with frozen-thawed semen. These were Suffolk (low fertility), Belclare (medium fertility), Norwegian White Sheep (NWS) and Fur (both high fertility).

RESULTS

A total of 689 metabolites were identified in the cervical mucus of all the four ewe breeds. Of these, 458 metabolites were altered by ewe breed, which had the greatest effect in the dataset (P < 0.05). We detected 194 metabolites involved in the amino acid pathway, of which 133, 56 and 63 were affected by ewe breed, type of cycle and their interaction, respectively (P < 0.05). N-methylhydantoin and N-carbamoylsarcosine (degradation products of creatinine pathway) exhibited the greatest fold change decrease in the Suffolk breed compared to Fur and NWS (P < 0.001). Oxidized metabolites were also decreased in Suffolk compared to high fertility breeds (P < 0.05). In contrast, other metabolites such as 3-indoxyl-sulfate, putrescine, cadaverine were significantly increased in Suffolk at the synchronised cycle.

CONCLUSION

The suboptimal amino acid profile in the cervical mucus of the low fertility Suffolk breed may have negative consequences for sperm transport.

Review: Non-surgical artificial insemination and embryo recovery as safe tools for genetic preservation in small ruminants

Artificial insemination (AI) and in vivo embryo production (or multiple ovulation and embryo transfer, MOET) programs are both instrumental in accelerating the propagation of genetically and economically superior goats and sheep. The aim of this review was to present the current gestalt of non-surgical AI and embryo recovery (NSER) procedures in small ruminants. Small body size, precluding rectal palpation, and highly limited penetrability of the uterine cervix in ewes are the major reasons for the scarce use of non-surgical assisted reproduction techniques in this species. As a result, AI and embryo recovery techniques in sheep mainly involve laparoscopy or laparotomy (LAP). In does, however, the Embrapa method of AI allows for successful intrauterine deposition of semen, resulting in pregnancy rates from 50 to 80% under field conditions (>3 000 goats inseminated) when frozen-thawed semen is used. After the administration of prostaglandin F2α (PGF2α), non-surgical (transcervical) embryo recovery is also feasible in goats, with the cervical penetration rate approaching 100%. There is a paucity of information on the efficacy of non-surgical AI using frozen semen in sheep, but the results are satisfactory with fresh, cooled, or chilled ram semen. An application of the NSER technique in ewes has greatly improved over the last decade, and cervical penetration rates of ∼90% can be achieved when a hormonal cervical dilation protocol using PGF2α, oxytocin, and/or estradiol ester (e.g., estradiol benzoate) is applied. In some genotypes of sheep, sufficient cervical dilation can be induced without estradiol ester included in the protocol. Several studies indicated that recovery of transferable quality ovine embryos using NSER is comparable to that employing a ventral midline laparotomy, and NSER is evidently a method of choice when animal welfare is concerned. Considering both the number of retrievable embryos and animal well-being, the NSER is a viable alternative for surgical procedures. With further developments, it has the makings of a primary, if not exclusive, embryo recovery technique in small ruminants worldwide.

Molecular insights to the sperm-cervix interaction and the consequences for cryopreserved sperm

Cryopreserved ram spermatozoa are limited in their capacity to traverse the ovine cervix and achieve fertilization. This altered interaction may be related to modified molecular communication between frozen-thawed ram spermatozoa, seminal plasma, and the female tract. As such, this review aims to identify the biological processes which underpin sperm maturation and transport throughout the female reproductive tract to elucidate factors which may alter this natural process in cryopreserved ram spermatozoa. We also assess critical barriers to ram spermatozoa specific to the ovine cervix and the role of seminal plasma in mitigating these barriers. Transcriptomics is explored as a new approach to understand the sperm-cervix interaction. Recent studies have demonstrated that both spermatozoa and seminal plasma contain a complex profile of coding and non-coding RNAs. These molecular species have clear links with functional fertility, and mounting evidence suggests they may be altered by cryopreservation. Emerging in vitro cell culture models are also investigated as a "next step" in studying this interaction, utilizing transcriptomics to identify subtle changes in female tract gene expression in response to spermatozoa. The application of such models is proposed as an exciting opportunity to investigate the unique challenges faced by cryopreserved spermatozoa traversing the ovine cervix prior to fertilization.

Ewe breed differences in the cervical transcriptome at the follicular phase of a synchronised oestrous cycle

Background

Cervical artificial insemination (AI) with frozen-thawed semen results in unacceptably low pregnancy rates internationally. The exception is in Norway, where vaginal deposition of frozen-thawed semen to a natural oestrous routinely yields pregnancy rates in excess of 70%. Previous studies by our group has demonstrated that this is due to differences in cervical sperm transport. However, a potentially important contributory factor is that ewes are inseminated to a natural oestrous in Norway but to a synchronised oestrous across most of the rest of the world. In this study, we interrogated the gene expression of the sheep cervix of four ewe breeds with known differences in pregnancy rates following cervical AI using frozen-thawed semen under the effect of exogenous hormones to synchronise the oestrous cycle. These four ewe breeds (n = 8 to 11 ewes per breed) are from two countries: Ireland (Belclare and Suffolk; medium and low fertility, respectively) and Norway (Norwegian White Sheep (NWS) and Fur; both with high fertility compared to the Irish ewe breeds).

Results

RNA extracted from cervical biopsies collected from these breeds was analysed by RNA-sequencing and differential gene expression analysis. Using the low-fertility Suffolk breed as a reference level; 27, 1827 and 2641 genes were differentially expressed in Belclare, Fur and NWS ewes, respectively (P <  0.05 and FC > 1.5). Gene ontology (GO) analysis revealed that Fur and NWS had an up-regulation of enriched pathways involved in muscle contraction and development compared to Suffolk. However, there was a down-regulation of the immune response pathway in NWS compared to Suffolk. In addition, GO analysis showed similar expression patterns involved in muscle contraction, extracellular matrix (ECM) development and cell-cell junction in both Norwegian ewe breeds, which differed to the Irish ewe breeds.

Conclusions

This novel study has identified a number of conserved and breed-specific biological processes under the effect of oestrous synchronisation that may impact cervical sperm transport during the follicular phase of the reproductive cycle.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08603-8.

Biochemical and molecular characterisation of sialylated cervical mucins in sheep

Sialic acid occupies terminal positions on O-glycans of cervical mucins, where they contribute to the increased viscosity of mucin thereby regulating sperm transport. This study characterised the sialylated cervical mucins from follicular phase mucus of six European ewe breeds with known differences in pregnancy rates following cervical artificial insemination using frozen-thawed semen at both synchronised and natural oestrus cycles. These were Suffolk (low fertility) and Belclare (medium fertility) in Ireland, Ile de France and Romanov (both with medium fertility) in France and Norwegian White Sheep (NWS) and Fur (both with high fertility) in Norway. Expression of mucin and sialic acid related genes was quantified using RNA-sequencing in cervical tissue from Suffolk, Belclare, Fur and NWS only. Cervical tissue was also assessed for the percentage of cervical epithelial populated by mucin secreting goblet cells in the same four ewe breeds. Biochemical analysis showed that there was an effect of ewe breed on sialic acid species, which was represented by Suffolk having higher levels of Neu5,9Ac2 compared to NWS (P < 0.05). Suffolk ewes had a lower percentage of goblet cells than Fur and NWS (P < 0.05). Gene expression analysis identified higher expression of MUC5AC, MUC5B, ST6GAL1, ST6GAL2 and lower expression of ST3GAL3, ST3GAL4 and SIGLEC10 in Suffolk compared to high fertility ewe breeds (P < 0.05). Our results indicate that specific alterations in sialylated mucin composition may be related to impaired cervical sperm transport.

Estrogenic Pastures: A Source of Endocrine Disruption in Sheep Reproduction

Phytoestrogens can impact on reproductive health due to their structural similarity to estradiol. Initially identified in sheep consuming estrogenic pasture, phytoestrogens are known to influence reproductive capacity in numerous species. Estrogenic pastures continue to persist in sheep production systems, yet there has been little headway in our understanding of the underlying mechanisms that link phytoestrogens with compromised reproduction in sheep. Here we review the known and postulated actions of phytoestrogens on reproduction, with particular focus on competitive binding with nuclear and non-nuclear estrogen receptors, modifications to the epigenome, and the downstream impacts on normal physiological function. The review examines the evidence that phytoestrogens cause reproductive dysfunction in both the sexes, and that outcomes depend on the developmental period when an individual is exposed to phytoestrogen.

Identification and characterisation of O-linked glycans in cervical mucus as biomarkers of sperm transport: A novel sheep model

Cervical mucus plays an important role in female fertility, since it allows the entry of motile and morphological normal sperm while preventing the ascent of pathogens from the vagina. The function of cervical mucus is critically linked to its rheological properties that are in turn dictated by O-glycosylated proteins, called mucins. We aimed to characterize the O-glycan composition in the cervical mucus of six European ewe breeds with known differences in pregnancy rates following cervical/vaginal artificial insemination with frozen–thawed semen, which are due to reported differences in cervical sperm transport. These were Suffolk (low fertility) and Belclare (medium fertility) in Ireland, Ile de France and Romanov (both with medium fertility) in France, and Norwegian White Sheep (NWS) and Fur (both with high fertility) in Norway (n = 28–30 ewes/breed). We identified 124 O-glycans, from which 51 were the major glycans with core 2 and fucosylated glycans as the most common structures. The use of exogenous hormones for synchronization did not affect the O-glycan composition in both high-fertility ewe breeds, but it did in the other four ewe breeds. There was a higher abundance of the sulfated glycan (Galβ1–3[SO3-GlcNAcβ1–6]GalNAc), fucosylated glycan (GlcNAcβ1–3(Fucα1–2Galβ1–3)GalNAc) and core 4 glycan (GlcNAcβ1–3[GlcNAcβ1–6]GalNAc) in the low-fertility Suffolk breed compared with NWS (high fertility). In addition, core 4 glycans were negatively correlated with mucus viscosity. This novel study has identified O-glycans that are important for cervical sperm transport and could have applications across a range of species including human.

Sperm migration in the genital tract-In silico experiments identify key factors for reproductive success

Sperm migration in the female genital tract controls sperm selection and, therefore, reproductive success as male gametes are conditioned for fertilization while their number is dramatically reduced. Mechanisms underlying sperm migration are mostly unknown, since in vivo investigations are mostly unfeasible for ethical or practical reasons. By presenting a spatio-temporal model of the mammalian female genital tract combined with agent-based description of sperm motion and interaction as well as parameterizing it with bovine data, we offer an alternative possibility for studying sperm migration in silico. The model incorporates genital tract geometry as well as biophysical principles of sperm motion observed in vitro such as positive rheotaxis and thigmotaxis. This model for sperm migration from vagina to oviducts was successfully tested against in vivo data from literature. We found that physical sperm characteristics such as velocity and directional stability as well as sperm-fluid interactions and wall alignment are critical for success, i.e. sperms reaching the oviducts. Therefore, we propose that these identified sperm parameters should be considered in detail for conditioning sperm in artificial selection procedures since the natural processes are normally bypassed in reproductive in vitro technologies. The tremendous impact of mucus flow to support sperm accumulation in the oviduct highlights the importance of a species-specific optimum time window for artificial insemination regarding ovulation. Predictions from our extendable in silico experimental system will improve assisted reproduction in humans, endangered species, and livestock.

Ewe breed differences in cervical anatomy and cervicovaginal mucus properties: An international study

In sheep, cervical artificial insemination (AI) involves depositing semen at the cervical opening, as it is not possible to traverse the cervix due to its complex anatomy. However, internationally this method yields low pregnancy rates when frozen-thawed semen is used. An exception to this is in Norway, in which vaginal deposition of frozen-thawed semen to a natural estrus yields pregnancy rates around 70%. As the cervix and its secretions are the principal factors influencing sperm transport to the site of fertilization the aim of this study was to characterise the differences in the cervical anatomy as well as the cervicovaginal mucus properties of six European ewe breeds across three countries known to have differences in pregnancy rates following cervical AI with frozen-thawed semen. These were Suffolk and Belclare in Ireland, Fur and Norwegian White Sheep (NWS) in Norway and Ile de France and Romanov in France (n = 28-30 ewes/breed). Cervicovaginal mucus was collected at the follicular and luteal phases of both a synchronized and natural cycle and assessed for mucus weight, viscosity and colour. The anatomical characteristics of the cervix (length of the cervix, number of cervical rings and the appearance of the external os) were assessed post-mortem. There was a type of the cycle by ewe breed interaction represented by no differences in mucus production between ewe breeds at the natural cycle for both the follicular and luteal phases of the cycle. However, there were differences between ewe breeds at the synchronized cycle (P < 0.05). Belclare had the lowest mucus production at the follicular phase while NWS had the lowest amount of mucus at the luteal phase of the synchronized cycle. Overall, across all ewe breeds, mucus production was higher at the follicular than at the luteal phase (P < 0.05). Despite reports of Suffolk and NWS having the most divergent pregnancy rates following cervical AI with frozen-thawed semen, both breeds had the lowest overall mucus viscosity at the follicular phase of both types of cycle with no differences between both ewe breeds (P > 0.05). The length of the cervix, number of cervical rings and the external os type were affected by ewe breed (P < 0.05). Suffolk ewes had longer cervices but lower number of cervical rings than NWS and Fur ewes (both with higher pregnancy rates). In conclusion, while mucus production and mucus viscosity was affected by breed, these changes are not consistent with the known differences between ewe breeds in their pregnancy rates following cervical AI with frozen-thawed semen.

Equilibration in freezing extender alters in vitro sperm-oviduct binding in the domestic cat (Felis catus)

For the preservation of endangered felid species, epididymal sperm may be received from valuable individuals after castration or death and they need to be cryopreserved for storage. However, pregnancy rates with epididymal or cryopreserved sperm are lower than with ejaculated and non-frozen semen even if insemination is surgically performed into the oviduct. To investigate whether equilibration, the first step of the cryopreservation procedure, has an impact on sperm-oviduct binding, we generated oviduct epithelial cell vesicles from isthmus segments of preovulatory domestic cats. Binding assays were performed with epididymal sperm in a cell culture medium (M199) without supplements, or after cooling to 15 °C in a freezing extender (TestG), supplemented with glycerol and the water-soluble fraction of hen's egg yolk mainly comprising LDL. The sperm-oviduct binding was assessed both quantitatively and qualitatively (head or tail binding of sperm with active or inactive mitochondria). Most of the bound sperm prepared in M199 had active mitochondria and were attached to the vesicles by their heads. In equilibrated samples, the proportion of bound sperm with active mitochondria and the proportion of head-bound spermatozoa were reduced. The total motility of the sperm after 1 h of incubation in the absence or presence of vesicles were also affected by the preparation (higher in equilibrated) and the incubation (lower in co-incubated), while mitochondrial activity was influenced just by the preparation. Obviously, LDL has a beneficial effect on sperm motility, but we suggest that it interferes with the molecular sperm-oviduct crosstalk and causes a reduced binding of "good" sperm.

Current challenges in sheep artificial insemination: A particular insight

Ovine artificial insemination (OAI) is not commonly performed because of specific problems related to semen application techniques, leading to highly variable results. The ideal methodology (frozen-thawed semen/vaginal route) is unfeasible under field conditions due to the cervix morphology of the ewe, which prevents the process of intrauterine insemination necessary to obtain acceptable results. Currently, OAI commercial programmes use superficial cervical insemination, CAI (vaginal), with chilled semen (15°C) and intrauterine insemination, LAI (laparoscopic), with frozen-thawed semen. The ability to improve upon these contrasting techniques may be derived from examining certain poorly studied factors such as insemination time, productive state of females and alternatives of seminal preservation, some of which we reviewed in this work. This interim solution will remain in use until AI by the vaginal route with frozen-thawed semen is developed, but it poses new challenges in optimizing the freezing of the sperm and adapting the cervical (CAI) and/or transcervical intrauterine AI (TCAI). In this review, we address the current problems and evaluate their methodological (mechanical) and chemical (dilation) alternatives. Currently, TCAI is a methodologically complex technique with poor fertility results, so further studies are needed to improve the logistics of this procedure and the results of its application.