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.

The oocyte cumulus complex regulates mouse sperm migration in the oviduct.

As the time of ovulation draws near, mouse spermatozoa move out of the isthmic reservoir, which is a prerequisite for fertilization. However, the molecular mechanism remains unclear. The present study revealed that mouse cumulus cells of oocytes-cumulus complexes (OCCs) expressed transforming growth factor-ß ligand 1 (TGFB1), whereas ampullary epithelial cells expressed the TGF-ß receptors, TGFBR1 and TGFBR2, and all were upregulated by luteinizing hormone (LH)/human chorionic gonadotropin (hCG). OCCs and TGFB1 increased natriuretic peptide type C (NPPC) expression in cultured ampullae via TGF-ß signaling, and NPPC treatment promoted spermatozoa moving out of the isthmic reservoir of the preovulatory oviducts. Deletion of Tgfb1 in cumulus cells and Tgfbr2 in ampullary epithelial cells blocked OCC-induced NPPC expression and spermatozoa moving out of the isthmic reservoir, resulting in compromised fertilization and fertility. Oocyte-derived paracrine factors were required for promoting cumulus cell expression of TGFB1. Therefore, oocyte-dependent and cumulus cell-derived TGFB1 promotes the expression of NPPC in oviductal ampulla, which is critical for sperm migration in the oviduct and subsequent fertilization.

Cervical immune activation during the luteal phase may compromise subsequent trans-cervical ram sperm transport†.

Worldwide, cervical artificial insemination using frozen-thawed semen yields low pregnancy rates. The only exception to this is in Norway, where vaginal insemination with frozen-thawed semen yields pregnancy rates in excess of 60% and which has been attributed to the specific ewe breed used. Our previous work demonstrated differences in cervical gene expression at the follicular phase of the estrous cycle in ewe breeds with known differences in pregnancy rates. In this study, we characterized the cervical transcriptome of the same ewe breeds [Suffolk, Belclare, Fur, and Norwegian White Sheep (NWS)] during the luteal phase, as an optimal environment at the luteal phase could better prepare the cervix for sperm migration through the cervix at the subsequent follicular phase. High-quality RNA extracted from postmortem cervical tissue was analyzed by RNA sequencing. After stringent filtering, 1051, 1924, and 611 differentially expressed genes (DEGs) were detected in the low-fertility Suffolk breed compared with Belclare, Fur, and NWS, respectively. Gene ontology analysis identified increased humoral adaptive immune response pathways in Suffolk. Increased expression of multiple immune genes supports the presence of an active immune response in the cervix of Suffolk ewes, which differentiates them significantly from the other three ewe breeds. Inflammatory pathways were upregulated in the Suffolk, resulting in higher expression of the potent pro-inflammatory cytokines. Therefore, higher levels of pro-inflammatory cytokines indicate unresolved inflammation in the cervix of the low-fertility Suffolk breed that could contribute to reduced cervical sperm transport in the next follicular phase.

An ex-vivo assessment of differential sperm transport in the female reproductive tract between high and low fertility bulls.

Despite passing all quality control checks at animal breeding centres, bulls with apparently normal semen quality can yield unacceptably low field fertility rates. This study took an ex-vivo approach to assess if bulls of divergent field fertility differ in the ability of their spermatozoa to interact with the female reproductive tract and its secretions. Six high and six low fertility Holstein Friesian bulls (+4.0 ± 0.2 and -15.7 ± 3.13, respectively; adjusted mean fertility ± s.e.m. mean of the bull population was 0) were selected from a population of 840 bulls with >500 field inseminations per bull. Thawed spermatozoa from each bull were analysed across a range of in vitro assays to assess their ability to transverse the female reproductive tract including; motility and kinematic parameters using computer-assisted sperm analysis, viability, membrane fluidity and acrosomal integrity using flow cytometry as well as mucus penetration tests, rheotactic behaviour and sperm binding ability to the oviductal epithelium. While there was no significant difference between high and low fertility bulls in most of the sperm motility, kinematic and sperm functional parameters (namely, motility, average path velocity, linearity, straightness, amplitude of lateral head movement), viability, membrane fluidity or acrosome intactness, high fertility bulls had higher curvilinear velocity compared to the low fertility group (P < 0.05) and a higher straight-line velocity was observed although it did not reach statistical significance (P = 0.08). There was no difference between treatment groups in the ability of spermatozoa to penetrate periovulatory cervical mucus or in their rheotactic response (P > 0.05). Interestingly, there was a positive correlation between the straight-line velocity of spermatozoa and their rheotactic response (r = 0.45, P < 0.001) and further linear regression analysis indicated 18.9% of the variance in sperm rheotaxis was accounted for by straight line velocity. A higher number of spermatozoa from the high fertility group compared to the low fertility group bound to oviductal explants (15.1 ±â€¯0.98 and 12.5 ±â€¯0.76, respectively; mean ±â€¯s.e.m; P < 0.05). In conclusion, the differences in the kinematics of sperm motility and ability to bind to oviductal explants between high and low fertility bulls were modest and are unlikely to explain the inherent differences in fertility between these cohorts of bulls.

Sperm transport and male pregnancy in seahorses: An unusual model for reproductive science.

The Syngnathidae (seahorses and pipefishes) are a group of teleost fishes in which, uniquely, developing embryos are hosted throughout pregnancy by males, using a specialized brood pouch situated on the abdomen or tail. Seahorses have evolved the most advanced form of brood pouch, whereby zygotes and embryos are intimately connected to the host's circulatory system and also bathed in pouch fluid. The pouch is closed to the external environment and has to perform functions such as gaseous exchange, removal of waste and maintenance of appropriate osmotic conditions, much like the mammalian placenta. Fertilization of the oocytes occurs within the brood pouch, but unlike the mammalian situation the sperm transport mechanism from the ejaculatory duct towards the pouch is unclear, and the sperm: egg ratio (about 5:1) is possibly the least of any vertebrate. In this review, there is highlighting of the difficulty of elucidating the sperm transport mechanism, based on studies of Hippocampus kuda. The similarities between seahorse pouch function and the mammalian placenta have led to suggestions that the pouch provides important nutritional support for the developing embryos, supplementing the nutritional functions of the yolk sac provided by the oocytes. In this review, there is a description of the recent evidence in support of this hypothesis, and also emphasis, as in mammals, that embryonic development depends on nutritional support from the placenta-like pouch at important stages of the gestational period ("critical windows").

Identification and characterization 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 (Galß1-3[SO3-GlcNAcß1-6]GalNAc), fucosylated glycan (GlcNAcß1-3(Fucα1-2Galß1-3)GalNAc) and core 4 glycan (GlcNAcß1-3[GlcNAcß1-6]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.

ATP activation of peritubular cells drives testicular sperm transport.

Spermatogenesis, the complex process of male germ cell proliferation, differentiation, and maturation, is the basis of male fertility. In the seminiferous tubules of the testes, spermatozoa are constantly generated from spermatogonial stem cells through a stereotyped sequence of mitotic and meiotic divisions. The basic physiological principles, however, that control both maturation and luminal transport of the still immotile spermatozoa within the seminiferous tubules remain poorly, if at all, defined. Here, we show that coordinated contractions of smooth muscle-like testicular peritubular cells provide the propulsive force for luminal sperm transport toward the rete testis. Using a mouse model for in vivo imaging, we describe and quantify spontaneous tubular contractions and show a causal relationship between peritubular Ca2+ waves and peristaltic transport. Moreover, we identify P2 receptor-dependent purinergic signaling pathways as physiological triggers of tubular contractions both in vitro and in vivo. When challenged with extracellular ATP, transport of luminal content inside the seminiferous tubules displays stage-dependent directionality. We thus suggest that paracrine purinergic signaling coordinates peristaltic recurrent contractions of the mouse seminiferous tubules to propel immotile spermatozoa to the rete testis.

As sperm develop in the testis, the immature cells must make their way through a maze of small tubes known as seminiferous tubules. However, at this stage, the cells do not yet move the long tails that normally allow them to 'swim'; it is therefore unclear how they are able to move through the tubules. Now, Fleck, Kenzler et al. have showed that, in mice, muscle-like cells within the walls of seminiferous tubules can create waves of contractions that push sperm along. Further experiments were then conducted on cells grown in the laboratory. This revealed that a signaling molecule called ATP orchestrates the moving process by activating a cascade of molecular events that result in contractions. Fleck, Kenzler et al. then harnessed an advanced microscopy technique to demonstrate that this mechanism occurs in living mice. Together, these results provide a better understanding of how sperm mature, which could potentially be relevant for both male infertility and birth control.

Revealing mechanisms of mating plug function under sexual selection.

Mating plugs are produced by many sexually reproducing animals and are hypothesized to promote male fertilization success under promiscuous mating. However, tests of this hypothesis have been constrained by an inability to discriminate ejaculates of different males in direct competition. Here, we use stable isotope labeling in vivo and proteomics to achieve this in a promiscuous rodent, Myodes glareolus We show that, although the first male's plug is usually dislodged, it can be retained throughout the second male's copulation. Retained plugs did not completely block rival sperm but did significantly limit their numbers. Differences in the number of each male's sperm progressing through the female reproductive tract were also explained by natural variation in the size of mating plugs and reproductive accessory glands from which major plug proteins originate. Relative sperm numbers in turn predicted the relative fertilization success of rival males. Our application of stable isotopes to label ejaculates resolves a longstanding debate by revealing how rodent mating plugs promote fertilization success under competitive conditions. This approach opens new opportunities to reveal cryptic mechanisms of postcopulatory sexual selection among diverse animal taxa.

Sperm transport and endometrial inflammatory response in mares after artificial insemination with cryopreserved spermatozoa.

This study aimed to determine whether the insemination site and dose with cryopreserved sperm of reproductively normal mares affect the sperm population in uterine tubes and the intensity of endometrial inflammatory response. Experimental subjects were estrous mares inseminated, in the mid-uterine body (Body) or the tip of the uterine horn (Tip), ipsilateral to the dominant follicle, with one 0.5 mL straw with 50 × 106 sperm (50) or with eight straws with 50 × 106 sperm/straw (400). Mares were slaughtered 2 h, 4 h and 12 h after artificial insemination (AI) and randomly assigned to following groups: Body 50 (n = 19) (2 h, 4 h or 12 h); Tip 50 (n = 29) (2 h, 4 h, or 12 h); Body 400 (n = 24) (2 h, 4 h, or 12 h); Tip 400 (n = 21) (2 h, 4 h, or 12 h). A Control group (n = 16) was not inseminated. After slaughter, uterine tubes were separated from uterus, and uteri and tubes flushed with phosphate-buffered saline (PBS). After flushing, an endometrial sample was collected from ipsilateral and contralateral horns and mid-uterus body for further histopathological examination. A sample of each uterine tube flushing was examined for sperm count, and a sample of each uterine flushing was used for polymorphonuclear neutrophils (PMNs) count. Data were analyzed using PROC GLM from SASv9.4. Insemination time, site, sperm dose, and their interactions were considered independent variables and sperm and PMNs numbers dependent variables. Deep horn insemination increased ipsilateral uterine tube sperm number without an increase in the inflammatory reaction compared with the uterine body insemination. The higher the insemination dose, the higher the uterine tubes' sperm number and inflammatory reaction, with a quicker resolution. In conclusion, the insemination site and dose affected sperm in the uterine tubes, while post-insemination time and dose influenced the inflammatory reaction.

Getting to and away from the egg, an interplay between several sperm transport mechanisms and a complex oviduct physiology.

In mammals, the architecture and physiology of the oviduct are very complex, and one long-lasting intriguing question is how spermatozoa are transported from the sperm reservoir in the isthmus to the oocyte surface. In recent decades, several studies have improved knowledge of the factors affecting oviduct fluid movement and sperm transport. They report sperm-guiding mechanisms that move the spermatozoa towards (rheotaxis, thermotaxis, and chemotaxis) or away from the egg surface (chemorepulsion), but only a few provide evidence of their occurrence in vivo. This gives rise to several questions: how and when do the sperm transport mechanisms operate inside such an active oviduct? why are there so many sperm guidance processes? is one dominant over the others, or do they cooperate to optimise the success of fertilisation? Assuming that sperm guidance evolved alongside oviduct physiology, in this review we propose a theoretical model that integrates oviduct complexity in space and time with the sperm-orienting mechanisms. In addition, since all of the sperm-guidance processes recruit spermatozoa in a better physiological condition than those not selected, they could potentially be incorporated into assisted reproductive technology (ART) to improve fertility treatment and/or to develop innovative contraceptive methods. All these issues are discussed in this review.

The role of semen and seminal plasma in inducing large-scale genomic changes in the female porcine peri-ovulatory tract.

Semen modifies the expression of genes related to immune function along the porcine female internal genital tract. Whether other pathways are induced by the deposition of spermatozoa and/or seminal plasma (SP), is yet undocumented. Here, to determine their relative impact on the uterine and tubal transcriptomes, microarray analyses were performed on the endocervix, endometrium and endosalpinx collected from pre-ovulatory sows 24 h after either mating or artificial insemination (AI) with specific ejaculate fractions containing spermatozoa or sperm-free SP. After enrichment analysis, we found an overrepresentation of genes and pathways associated with sperm transport and binding, oxidative stress and cell-to-cell recognition, such as PI3K-Akt, FoxO signaling, glycosaminoglycan biosynthesis and cAMP-related transcripts, among others. Although semen (either after mating or AI) seemed to have the highest impact along the entire genital tract, our results demonstrate that the SP itself also modifies the transcriptome. The detected modifications of the molecular profiles of the pre/peri-ovulatory endometrium and endosalpinx suggest an interplay for the survival, transport and binding of spermatozoa through, for instance the up-regulation of the Estrogen signaling pathway associated with attachment and release from the oviductal reservoir.

Modification of genital kidney nephrons for sperm transport in a plethodontid salamander, Eurycea longicauda.

Salamanders possess kidneys with two distinct regions: a caudal pelvic portion and cranial genital portion. Nephrons of the pelvic region are responsible for urine formation and transport. Nephrons of the genital region transport sperm from testes to Wolffian ducts; however, nephrons of the genital region possess all the same functional regions found in pelvic kidney nephrons that are involved with urine formation and transport (renal corpuscles, proximal tubules, distal tubules, and collecting ducts). Morphological similarities between pelvic and genital regions stimulated past researchers to hypothesize that nephrons of genital kidneys possess dual function; that is, sperm transport and urine formation/transport. Considering size of glomeruli is directly related to the total amount of blood plasma filtered into the Bowman's space, we tested the hypothesis that nephrons of genital kidneys have reduced urine formation function by comparing glomerular size between nephrons of pelvic and genital kidney regions in Eurycea longicauda with general histological techniques. Light microscopy analysis revealed that glomeruli of pelvic kidneys were significantly larger than those measured from genital kidneys. Transmission electron microscopy analysis also revealed modifications in genital kidney nephrons when compared to pelvic kidney nephrons that suggested a decrease in urine formation function in genital kidneys. Such modifications included a decrease in basal and lateral plasma membrane folding in genital kidney proximal and distal tubules compared to that of pelvic kidney proximal and distal tubules. Genital kidney proximal tubules were also ciliated, which was not observed in pelvic kidney proximal tubules. In conclusion, although structurally similar at the histological level, it appears that nephrons of genital kidneys have decreased urine formation function based on glomerular size comparison and nephron ultrastructure.

Active peristaltic movements and fluid production of the mouse oviduct: their roles in fluid and sperm transport and fertilization†.

To study how the oviduct behaves in relation to fluid secretion and sperm transport, ovary-oviduct-uterus complexes of the mouse were installed in a fluid-circulating chamber without disturbing the blood circulation or parasympathetic innervation. Injection of a bolus of Indian ink into the lower isthmus revealed very active adovarian peristalsis of the isthmus, which was most prominent during the periovulatory period. Oviduct fluid, secreted by the entire length of the isthmus, was rapidly transported to the ampulla and ovarian bursa before draining into the peritoneal cavity. The upper isthmus, in particular the isthmic-ampullary junction, was responsible for this adovarian fluid flow. Peristalsis of the oviduct, undisturbed flow of oviduct fluid from the isthmus to the peritoneal cavity, and the spermatozoon's own motility all contribute to efficient sperm ascent and to fertilization within the oviduct. Therefore, chemotaxis, rheotaxis, and thermotaxis of spermatozoa toward oocyte-cumulus complexes in the ampulla are all unlikely mechanisms for explaining sperm-oocyte contact and successful fertilization, given the rapid adovarian flow of oviduct fluid in this species.

Deciphering the oviductal extracellular vesicles content across the estrous cycle: implications for the gametes-oviduct interactions and the environment of the potential embryo.

BACKGROUND: The success of early reproductive events depends on an appropriate communication between gametes/embryos and the oviduct. Extracellular vesicles (EVs) contained in oviductal secretions have been suggested as new players in mediating this crucial cross-talk by transferring their cargo (proteins, mRNA and small ncRNA) from cell to cell. However, little is known about the oviductal EVs (oEVS) composition and their implications in the reproductive success. The aim of the study was to determine the oEVs content at protein, mRNA and small RNA level and to examine whether the oEVs content is under the hormonal influence of the estrous cycle. RESULTS: We identified the presence of oEVs, exosomes and microvesicles, in the bovine oviductal fluid at different stages of the estrous cycle (postovulatory-stage, early luteal phase, late luteal phase and pre-ovulatory stage) and demonstrated that their composition is under hormonal regulation. RNA-sequencing identified 903 differentially expressed transcripts (FDR < 0.001) in oEVs across the estrous cycle. Moreover, small RNA-Seq identified the presence of different types of ncRNAs (miRNAs, rRNA fragments, tRNA fragments, snRNA, snoRNA, and other ncRNAs), which were partially also under hormonal influence. Major differences were found between post-ovulatory and the rest of the stages analyzed for mRNAs. Interesting miRNAs identified in oEVs and showing differential abundance among stages, miR-34c and miR-449a, have been associated with defective cilia in the oviduct and infertility. Furthermore, functional annotation of the differentially abundant mRNAs identified functions related to exosome/vesicles, cilia expression, embryo development and many transcripts encoding ribosomal proteins. Moreover, the analysis of oEVs protein content also revealed changes across the estrous cycle. Mass spectrometry identified 336 clusters of proteins in oEVs, of which 170 were differentially abundant across the estrous cycle (p-value< 0.05, ratio < 0.5 or ratio > 2). Our data revealed proteins related to early embryo development and gamete-oviduct interactions as well as numerous ribosomal proteins. CONCLUSIONS: Our study provides with the first molecular signature of oEVs across the bovine estrous cycle, revealing marked differences between post- and pre-ovulatory stages. Our findings contribute to a better understanding of the potential role of oEVs as modulators of gamete/embryo-maternal interactions and their implications for the reproductive success.

The flagellar protein Enkurin is required for mouse sperm motility and for transport through the female reproductive tract.

Enkurin was identified initially in mouse sperm where it was suggested to act as an intracellular adaptor protein linking membrane calcium influx to intracellular signaling pathways. In order to examine the function of this protein, a targeted mutation was introduced into the mouse Enkurin gene. Males that were homozygous for this mutated allele were subfertile. This was associated with lower rates of sperm transport in the female reproductive tract, including reduced entry into the oviduct and slower migration to the site of fertilization in the distal oviduct, and with poor progressive motility in vitro. Flagella from wild-type animals exhibited symmetrical bending and progressive motility in culture medium, and demembranated flagella exhibited the "curlicue" response to Ca2+ in vitro. In contrast, flagella of mice homozygous for the mutated allele displayed only asymmetric bending, nonprogressive motility, and a loss of Ca2+-responsiveness following demembrantion. We propose that Enkurin is part of a flagellar Ca2+-sensor that regulates bending and that the motility defects following mutation of the locus are the proximate cause of subfertility.

In vivo three-dimensional tracking of sperm behaviors in the mouse oviduct.

Mammalian sperm evolutionarily acquired complex mechanisms to regulate their behaviors, which are thought to be crucial in navigating through the female reproductive tract toward fertilization. However, all current knowledge of this process is largely extrapolated from in vitro and ex vivo studies, because in vivo analysis of sperm in their native fertilization environment has not been possible. Here, we report a functional optical coherence tomography approach that allows, for the first time, in vivo three-dimensional tracking of sperm behaviors in the mouse oviduct. Motile sperm are identified with their intrinsic dynamic characteristics. Sperm trajectories are reconstructed in three dimensions with a ∼5 µm spatial resolution, allowing for quantitative analysis of the sperm velocity and location relative to the oviduct. Using this method, we found different behavior patterns, including sperm collection by the oviduct epithelium, spatial dependence of sperm velocity, and sperm grouping and separation as the first in vivo evidence of sperm cooperation in the ampulla, the site of fertilization. This approach opens new avenues to study sperm-oviduct interactions in vivo toward a more complete understanding of fertility and reproductive disorders.

Exceptionalism is not exceptional in relation to sexual and reproduction mechanisms: Contrasts of human and animal sexuality.

Speculation that the release of oxytocin by orgasm in the human female during coitus facilitates fertility by enhancing uterine sperm transport has been criticized as having no unequivocal empirical human evidence. However, a counter claim that this supports human "exceptionalism" as some form of uterine sperm transport occurs in other species. This is a misconception as it ignores that human uterine peristalsis, powered by contractions of the smooth muscle of the archimyometrium, facilitates sperm transport even without any systemic oxytocin involvement. Moreover, examination of various unique reproductive mechanisms in numerous animals also indicates that the claim is misjudged and rests on a biased interpretation of what "exceptionalism" means in this biological context. Ten chosen aspects of our sexuality are presented as being exceptional to humans. Clin. Anat. 30:940-945, 2017. © 2017 Wiley Periodicals, Inc.

Biological basis for human capacitation-revisited.

BACKGROUND: A little more than a decade ago a review entitled 'Biological basis for human capacitation' was published. A primary conclusion of the review was that with all the technological advances that have been made since the first experiments demonstrated the in vivo requirement of capacitation for fertilization, very little progress had since been made, most significantly for human. OBJECTIVE AND RATIONALE: The present review was carried out to provide an update on the biological basis for human capacitation. It briefly revisits the original schema, presents a review of the literature that urged research interest in human sperm capacitation and puts under the spotlight the original definition of capacitation balanced against the limitations of experiments in vitro to characterize a complex process that necessarily mandates a female component, and very recent findings in the mouse. It also includes proposed considerations for new thinking regarding capacitation, and progress toward understanding the biology of human capacitation. SEARCH METHODS: The PubMed, Google Scholar and Scopus literature databases were reviewed extensively using inclusive, broad and multispecies search terms without publication date limitation. OUTCOMES: Comprehensive screening of the literature database showed that no papers regarding human sperm capacitation in vivo have been published in the past 20 years. Recent experiments in the mouse have provided compelling and unanticipated data regarding capacitation and in vivo fertilization. Questions were posed and addressed regarding: stimuli for initiation of capacitation, capacitation relative to the cumulus-oocyte complex, comparison between in vivo and in vitro capacitation, and potential species-specific differences in location and timing of capacitation. WIDER IMPLICATIONS: There has been no progress on the in vivo biology of human sperm capacitation since before the turn of the century. Human IVF and its technologies may likely have inhibited, and continue to hold back, any future in vivo experiments that would address one or more questions regarding acquisition of fertilizing capacity in human. The limiting factor for progress in the area is access to funding and human subjects.

Deciphering the mechanisms involving cenexin, ninein and centriolin in sperm maturation during epididymal transit in the domestic cat.

The sperm centrosome is an essential organelle with a key role in organizing the sperm aster for proper syngamy and formation of the first mitotic spindle. The sperm cell acquires the functional capability during epididymal transit by incorporation of key factors. The objective of the study was to identify these key maturation proteins, such as ninein and centriolin as well as cenexin-a scaffold protein that serves to bind ninein and centriolin. Epididymal samples were dissected from 17 adult cat testes (>1 year old) and spermatozoa were extracted from the different regions, including rete testis, caput, corpus, cauda and vas deferens. Tissue samples and sperm cells were fixed separately in 4% paraformaldehyde before immunostaining with anticenexin, ninein or centriolin antibodies. Results showed that the proportion of sperm cells with cenexin localized at the centrosome progressively increased along the tract with the lowest percentage of stained cells in the testis (mean = 45%) and highest in the cauda (mean = 81%). Although not significant, the intensity of cenexin immunofluorescence in positive cells increased twofold from the testis to vas deferens. There was no significant difference in the proportion of sperm labelled with centriolin or ninein (ranges of 21%-26% and 33%-48% between segments, respectively) or the intensity (±58% and ±63% change as compared to testis, respectively). Cenexin may serve as a scaffold protein for centriolin and ninein, as the vast majority of spermatozoa only displayed colocalization of these proteins when cenexin was also present (mean = 85% and 91% colocalization, respectively). In summary, these results could be applied to future efforts to create an in vitro culture system capable of rescuing the impaired centrosome of an infertile male, with particular potential for wild felid conservation.

Importance of sperm morphology during sperm transport and fertilization in mammals.

After natural or artificial insemination, the spermatozoon starts a journey from the site of deposition to the place of fertilization. However, only a small subset of the spermatozoa deposited achieves their goal: to reach and fertilize the egg. Factors involved in controlling sperm transport and fertilization include the female reproductive tract environment, cell-cell interactions, gene expression, and phenotypic sperm traits. Some of the significant determinants of fertilization are known (i.e., motility or DNA status), but many sperm traits are still indecipherable. One example is the influence of sperm dimensions and shape upon transport within the female genital tract towards the oocyte. Biophysical associations between sperm size and motility may influence the progression of spermatozoa through the female reproductive tract, but uncertainties remain concerning how sperm morphology influences the fertilization process, and whether only the sperm dimensions per se are involved. Moreover, such explanations do not allow the possibility that the female tract is capable of distinguishing fertile spermatozoa on the basis of their morphology, as seems to be the case with biochemical, molecular, and genetic properties. This review focuses on the influence of sperm size and shape in evolution and their putative role in sperm transport and selection within the uterus and the ability to fertilize the oocyte.