Hybrid motility mechanism of sperm at viscoelastic fluid-solid interface.

To fertilize eggs, sperm must pass through narrow, complex channels filled with viscoelastic fluids in the female reproductive tract. While it is known that the topography of the surfaces plays a role in guiding sperm movement, sperm have been thought of as swimmers, i.e., their motility comes solely from sperm interaction with the surrounding fluid, and therefore, the surfaces have no direct role in the motility mechanism itself. Here, we examined the role of solid surfaces in the movement of sperm in a highly viscoelastic medium. By visualizing the flagellum interaction with surfaces in a microfluidic device, we found that the flagellum stays close to the surface while the kinetic friction between the flagellum and the surface is in the direction of sperm movement, providing thrust. Additionally, the flow field generated by sperm suggests slippage between the viscoelastic fluid and the solid surface, deviating from the no-slip boundary typically used in standard fluid dynamics models. These observations point to hybrid motility mechanisms in sperm involving direct flagellum-surface interaction in addition to flagellum pushing the fluid. This finding signifies an evolutionary strategy of mammalian sperm crucial for their efficient migration through narrow, mucus-filled passages of the female reproductive tract.

Hybrid motility mechanism of sperm at viscoelastic-solid interface.

To fertilize eggs, sperm must pass through narrow, complex channels filled with viscoelastic fluids in the female reproductive tract. While it is known that the topography of the surfaces plays a role in guiding sperm movement, sperm have been thought of as swimmers, i.e., their motility comes solely from sperm interaction with the surrounding fluid, and therefore, the surfaces have no direct role in the motility mechanism itself. Here, we examined the role of solid surfaces in the movement of sperm in a highly viscoelastic medium. By visualizing the flagellum interaction with surfaces in a microfluidic device, we found that the flagellum stays close to the surface while the kinetic friction between the flagellum and the surface is in the direction of sperm movement, providing thrust. Additionally, the flow field generated by sperm suggests slippage between the viscoelastic fluid and the solid surface, deviating from the no-slip boundary typically used in standard fluid dynamics models. These observations point to hybrid motility mechanisms in sperm involving direct flagellum-surface interaction in addition to flagellum pushing the fluid. This finding signifies an evolutionary strategy of mammalian sperm crucial for their efficient migration through narrow, mucus-filled passages of the female reproductive tract.

Suspension of hostility: Positive interactions between spermatozoa and female reproductive tracts.

Interactions between spermatozoa and the female reproductive tract (FRT) are complex, in many cases poorly understood, and likely to contribute to the mechanistic basis of idiopathic infertility. As such, it is not surprising that the FRT was often viewed historically as a "hostile" environment for spermatozoa. The FRT has also been touted as a selective environment to ensure that only the highest quality spermatozoa progress to the oocyte for the opportunity to participate in fertilization. Recent advances, however, are giving rise to a far more nuanced view in which supportive spermatozoa × FRT interactions-in both directions-contribute to beneficial, even essential, effects on fertility. In this perspective article, we discuss several examples of positive spermatozoa × FRT interactions. We believe that these examples, arising in part from studies of taxonomically diverse nonmammalian systems, are useful to efforts to study mammalian spermatozoa × FRT interactions and their relevance to fertility and the advancement of assisted reproductive technologies.

Biological benefits of collective swimming of sperm in a viscoelastic fluid.

Collective swimming is evident in the sperm of several mammalian species. In bull (Bos taurus) sperm, high viscoelasticity of the surrounding fluid induces the sperm to form dynamic clusters. Sperm within the clusters swim closely together and align in the same direction, yet the clusters are dynamic because individual sperm swim into and out of them over time. As the fluid in part of the mammalian female reproductive tract contains mucus and, consequently, is highly viscoelastic, this mechanistic clustering likely happens in vivo. Nevertheless, it has been unclear whether clustering could provide any biological benefit. Here, using a microfluidic in vitro model with viscoelastic fluid, we found that the collective swimming of bull sperm in dynamic clusters provides specific biological benefits. In static viscoelastic fluid, clustering allowed sperm to swim in a more progressive manner. When the fluid was made to flow in the range of 2.43-4.05 1/sec shear rate, clustering enhanced the ability of sperm to swim upstream. We also found that the swimming characteristics of sperm in our viscoelastic fluid could not be fully explained by the hydrodynamic model that has been developed for sperm swimming in a low-viscosity, Newtonian fluid. Overall, we found that clustered sperm swam more oriented with each other in the absence of flow, were able to swim upstream under intermediate flows, and better withstood a strong flow than individual sperm. Our results indicate that the clustering of sperm can be beneficial to sperm migrating against an opposing flow of viscoelastic fluid within the female reproductive tract.

Assessment of sperm motility in livestock: Perspectives based on sperm swimming conditions in vivo.

Evaluation of sperm motility is well-established in farm animals for quickly selecting ejaculates for semen processing into insemination doses and for evaluating the quality of preserved semen. Likewise, sperm motility is a fundamental parameter used by spermatologists in basic and applied science. Motility is commonly assessed using computer-assisted semen analysis (CASA). Recent increases in computational power, as well as utilization of mobile CASA systems and open-source CASA programs, broaden the possibilities for motility evaluation. Despite this technological progress, the potential of computer-generated motility data to assess male fertility remains challenging and may be limited. Relevance for fertility assessment could be improved if measurement conditions would more closely mimic the in vivo situation. Hence, this review is focused on the current trends of automated semen assessment in livestock and explores perspectives for future use with respect to the physiological and physical conditions encountered by sperm in the female reproductive tract. Validation of current CASA systems with more complex, microfluidic-based devices mimicking the female reproductive tract environment could improve the value of sperm kinematic data for assessing the fertilizing capacity of semen samples, not only for application in livestock but also for use in conducting assisted reproduction techniques in other species.

Mammalian sperm hyperactivation regulates navigation via physical boundaries and promotes pseudo-chemotaxis.

Mammalian sperm migration within the complex and dynamic environment of the female reproductive tract toward the fertilization site requires navigational mechanisms, through which sperm respond to the tract environment and maintain the appropriate swimming behavior. In the oviduct (fallopian tube), sperm undergo a process called "hyperactivation," which involves switching from a nearly symmetrical, low-amplitude, and flagellar beating pattern to an asymmetrical, high-amplitude beating pattern that is required for fertilization in vivo. Here, exploring bovine sperm motion in high-aspect ratio microfluidic reservoirs as well as theoretical and computational modeling, we demonstrate that sperm hyperactivation, in response to pharmacological agonists, modulates sperm-sidewall interactions and thus navigation via physical boundaries. Prior to hyperactivation, sperm remained swimming along the sidewalls of the reservoirs; however, once hyperactivation caused the intrinsic curvature of sperm to exceed a critical value, swimming along the sidewalls was reduced. We further studied the effect of noise in the intrinsic curvature near the critical value and found that these nonthermal fluctuations yielded an interesting "Run-Stop" motion on the sidewall. Finally, we observed that hyperactivation produced a "pseudo-chemotaxis" behavior, in that sperm stayed longer within microfluidic chambers containing higher concentrations of hyperactivation agonists.

Co-Adaptation of Physical Attributes of the Mammalian Female Reproductive Tract and Sperm to Facilitate Fertilization.

The functions of the female reproductive tract not only encompass sperm migration, storage, and fertilization, but also support the transport and development of the fertilized egg through to the birth of offspring. Further, because the tract is open to the external environment, it must also provide protection against invasive pathogens. In biophysics, sperm are considered "pusher microswimmers", because they are propelled by pushing fluid behind them. This type of swimming by motile microorganisms promotes the tendency to swim along walls and upstream in gentle fluid flows. Thus, the architecture of the walls of the female tract, and the gentle flows created by cilia, can guide sperm migration. The viscoelasticity of the fluids in the tract, such as mucus secretions, also promotes the cooperative swimming of sperm that can improve fertilization success; at the same time, the mucus can also impede the invasion of pathogens. This review is focused on how the mammalian female reproductive tract and sperm interact physically to facilitate the movement of sperm to the site of fertilization. Knowledge of female/sperm interactions can not only explain how the female tract can physically guide sperm to the fertilization site, but can also be applied for the improvement of in vitro fertilization devices.

Sperm interaction with the uterine innate immune system: toll-like receptor 2 (TLR2) is a main sensor in cattle.

During the passage through the female reproductive tract, sperm interact with various compartments and their immune systems. The immune system that protects the female against pathogens also could destroy sperm or prevent them from reaching the site of fertilisation. In particular, the uterine innate immune response is crucial from the perspectives of both the sperm and the uterus. Following insemination, sperm immediately start to trigger inflammation in the uterus by entering uterine glands and activating an innate immune response. In cattle, the activation occurs mainly via TLR2 signalling, if not the only one, between sperm and the uterine epithelium lining the glands. This acute immune response is manifested as the upregulation of mRNA expression of IL8, TNFA, IL1B , and PGES . As a consequence, many sperm are trapped by polymorphonuclear neutrophils, the first and major component of innate immunity. The sperm-induced uterine innate immune responses apparently serve to clear the uterus of excess sperm and, importantly, prepare the endometrium for implantation. Pathophysiological conditions in the uterus seriously disrupt this phenomenon, and thus could directly decrease fertility.

Toll-like receptor 2 mediates the immune response of the bovine oviductal ampulla to sperm binding.

We previously reported that sperm binding to cultured bovine oviduct epithelial cells induces an anti-inflammatory immune response. Now we have developed a differentiated explant model to focus on the oviductal ampulla, where fertilization occurs, and to study the effect of sperm capacitation on the immune response. We used heparin to stimulate bovine sperm capacitation. Fluorescence imaging showed that 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolyl-carbocyanine iodide-labeled sperm pretreated with (Hep(+) ) or without (Hep(-) ) heparin rapidly attached to the explant ciliated epithelium in similar numbers. However, only Hep(+) sperm upregulated explant messenger RNA (mRNA) transcription of TLR2, IL8, TGFB1, and PGES, without changes in TNFA and IL-10 expression, while Hep(-) sperm only upregulated PGES. The responses were primarily anti-inflammatory, with a greater response produced by Hep(+) sperm, which also produced a substantial increase in TLR2 protein expression in the epithelium. The addition of TLR1/2 (toll-like receptor 1/2) antagonist to the Hep(+) and (Hep(-) ) sperm-explant coincubations reduced sperm attachment to the epithelium and inhibited TLR2 protein expression and some of the Hep(+) sperm-induced mRNA transcription. Our observations suggest that the ampullar epithelium immunologically reacts more strongly to sperm that have undergone heparin stimulation of capacitation. This anti-inflammatory response could serve to protect capacitated sperm as they approach the oocyte in the ampulla.

Sperm enter glands of preovulatory bovine endometrial explants and initiate inflammation.

We previously reported that sperm binding to cultured monolayers of bovine uterine epithelial cells induces an acute inflammatory response involving the Toll-like receptor (TLR2) signaling pathway. This response serves to clear the uterus of sperm and thereby prepares the endometrium for implantation. The endometrium is lined by surface epithelial cells; however, epithelial cells also line uterine glands. To investigate the source of the immune response, we used an explant model. Explants of bovine endometrium were incubated with bull sperm illuminated by JC1 fluorescent labeling in their mitochondria. The sperm glided over the surface epithelium until they encountered and entered uterine glands, where they remained. Scanning electron microscopy of explants revealed polymorphonuclear neutrophils (PMNs) in uterine glands along with sperm. In the absence of sperm, PMNs were not seen in glands. The incubation of sperm with explants resulted in an acute inflammatory response, seen as the upregulation of mRNA expression of IL8, TNFA, IL1B, PGES and TLR2 in whole explants, as well as increased TNFA protein expression in uterine glands. TLR1/2 antagonist reduced sperm numbers in the glands and inhibited the increase of TNFA. Our observations suggest that uterine glands serve as a site where sperm interact with the uterine epithelium to trigger the innate immune response to clear excess sperm from the uterus.

Sperm success and immunity.

The moment of the fertilization of an egg by a spermatozoon-the point of "sperm success"-is a key milestone in the biology of sexually reproducing species and is a fundamental requirement for offspring production. Fertilization also represents the culmination of a suite of sexually selected processes in both sexes and is commonly used as a landmark to measure reproductive success. Sperm success is heavily dependent upon interactions with other key aspects of male and female biology, with the immune system among the most important. The immune system is vital to maintaining health in both sexes; however, immune reactions can also have antagonistic effects on sperm success. The effects of immunity on sperm success are diverse, and may include trade-offs in the male between investment in the production or protection of sperm, as well as more direct, hostile, immune responses to sperm within the female, and potentially the male, reproductive tract. Here, we review current understanding of where the biology of immunity and sperm meet, and identify the gaps in our knowledge.

Oviduct epithelium induces interferon-tau in bovine Day-4 embryos, which generates an anti-inflammatory response in immune cells.

Recent studies indicate that communication between the bovine embryo and the mother begins in the oviduct. Here, we aimed to investigate the effect of embryos on bovine oviducts for their immune responses using an in vitro model. First, zygotes were cultured with or without bovine oviduct epithelial cells (BOECs) for 4 days, when embryos had reached the 16-cell stage. At that time, we detected interferon-tau (IFNT) in embryos co-cultured with BOECs, but not in embryos cultured alone. Next, peripheral blood mononuclear cells (PBMCs) were incubated either in media from embryo alone cultures or from co-cultures of embryos with BOECs. The medium from embryo alone cultures did not modulate PBMCs gene expression; whereas the embryo-BOEC co-culture medium increased interferon-stimulated genes (ISGs: ISG15, OAS1, MX2), STAT1, PTGES and TGFB1 but suppressed IL17 expression in PBMCs. Both IFNT-treated BOEC culture medium and IFNT-supplemented fresh medium alone without BOEC, modulated PBMCs gene expressions similar to those by the embryo-BOEC co-culture medium. Further, specific antibody to IFNT neutralized the effect of embryo-BOEC co-culture medium on PBMCs gene expression. Our results indicate that BOECs stimulate embryos to produce IFNT, which then acts on immune cells to promote an anti-inflammatory response in the oviduct.

Fluid viscoelasticity promotes collective swimming of sperm.

From flocking birds to swarming insects, interactions of organisms large and small lead to the emergence of collective dynamics. Here, we report striking collective swimming of bovine sperm in dynamic clusters, enabled by the viscoelasticity of the fluid. Sperm oriented in the same direction within each cluster, and cluster size and cell-cell alignment strength increased with viscoelasticity of the fluid. In contrast, sperm swam randomly and individually in Newtonian (nonelastic) fluids of low and high viscosity. Analysis of the fluid motion surrounding individual swimming sperm indicated that sperm-fluid interaction was facilitated by the elastic component of the fluid. In humans, as well as cattle, sperm are naturally deposited at the entrance to the cervix and must swim through viscoelastic cervical mucus and other mucoid secretions to reach the site of fertilization. Collective swimming induced by elasticity may thus facilitate sperm migration and contribute to successful fertilization. We note that almost all biological fluids (e.g. mucus and blood) are viscoelastic in nature, and this finding highlights the importance of fluid elasticity in biological function.

Overactivation of hedgehog signaling in the developing Müllerian duct interferes with duct regression in males and causes subfertility.

The influence of the hedgehog signaling pathway on reproduction was studied in transgenic mice in which a dominant active allele of the hedgehog signal transducer, smoothened (Smo), was conditionally expressed in the developing Müllerian duct and gonads through recombination mediated by anti-Müllerian hormone receptor 2-cre (Amhr2cre ). Previous studies showed that development of the oviduct and uterus are abnormal in female Amhr2cre/+SmoM2 mice. In the current study, focusing on mutant males, litter size was reduced 53% in crosses with wild-type females. An extra band of undifferentiated tissue extended along each epididymis and vas deferens, a position suggesting derivation from Müllerian ducts that failed to regress fully. Hedgehog signaling was elevated in this tissue, based on mRNA levels of target genes. Amhr2 mRNA was dramatically reduced in the uterus of mutant females and in the extra tissue in the tract of mutant males, suggesting that AMHR2 signaling was inadequate for complete Müllerian duct regression. Spermatogenesis and sperm motility were normal, but testis weight was reduced 37% and epididymal sperm number was reduced 36%. The number of sperm recovered from the uteri of wild-type females after mating with mutant males was reduced 78%. This suggested that sperm transport through the male tract was reduced, resulting in fewer sperm in the ejaculate. Consistent with this, mutant males had unusually tortuous vas deferentia with constrictions within the lumen. We concluded that persistence of a relatively undifferentiated remnant of Müllerian tissue is sufficient to cause subtle changes in the male reproductive tract that reduce fertility.

Dynamics of Bovine Sperm Interaction with Epithelium Differ Between Oviductal Isthmus and Ampulla.

In mammals, many sperm that reach the oviduct are held in a reservoir by binding to epithelium. To leave the reservoir, sperm detach from the epithelium; however, they may bind and detach again as they ascend into the ampulla toward oocytes. In order to elucidate the nature of binding interactions along the oviduct, we compared the effects of bursts of strong fluid flow (as would be caused by oviductal contractions), heparin, and hyperactivation on detachment of bovine sperm bound in vitro to epithelium on intact folds of isthmic and ampullar mucosa. Intact folds of oviductal mucosa were used to represent the strong attachments of epithelial cells to each other and to underlying connective tissue that exist in vivo. Effects of heparin on binding were tested because heparin binds to the Binder of SPerm (BSP) proteins that attach sperm to oviductal epithelium. Sperm bound by their heads to beating cilia on both isthmic and ampullar epithelia and could not be detached by strong bursts of fluid flow. Addition of heparin immediately detached sperm from isthmic epithelium but not ampullar epithelium. Addition of 4-aminopyridine immediately stimulated hyperactivation of sperm but did not detach them from isthmic or ampullar epithelium unless added with heparin. These observations indicate that the nature of binding of sperm to ampullar epithelium differs from that of binding to isthmic epithelium; specifically, sperm bound to isthmic epithelium can be detached by heparin alone, while sperm bound to ampullar epithelium requires both heparin and hyperactivation to detach from the epithelium.

Mammalian sperm interactions with the female reproductive tract.

The mammalian female reproductive tract interacts with sperm in various ways in order to facilitate sperm migration to the egg while impeding migrations of pathogens into the tract, to keep sperm alive during the time between mating and ovulation, and to select the fittest sperm for fertilization. The two main types of interactions are physical and molecular. Physical interactions include the swimming responses of sperm to the microarchitecture of walls, to fluid flows, and to fluid viscoelasticity. When sperm encounter walls, they have a strong tendency to remain swimming along them. Sperm will also orient their swimming into gentle fluid flows. The female tract seems to use these tendencies of sperm to guide them to the site of fertilization. When sperm hyperactivate, they are better able to penetrate highly viscoelastic media, such as the cumulus matrix surrounding eggs. Molecular interactions include communications of sperm surface molecules with receptors on the epithelial lining of the tract. There is evidence that specific sperm surface molecules are required to enable sperm to pass through the uterotubal junction into the oviduct. When sperm reach the oviduct, most bind to the oviductal epithelium. This interaction holds sperm in a storage reservoir until ovulation and serves to maintain the fertilization competence of stored sperm. When sperm are released from the reservoir, they detach from and re-attach to the epithelium repeatedly while ascending to the site of fertilization. We are only beginning to understand the communications that may pass between sperm and epithelium during these interactions.

Ejaculated mouse sperm enter cumulus-oocyte complexes more efficiently in vitro than epididymal sperm.

The mouse is an established and popular animal model for studying reproductive biology. Epididymal mouse sperm, which lack exposure to secretions of male accessory glands and do not precisely represent ejaculated sperm for the study of sperm functions, have been almost exclusively used in studies. We compared ejaculated and epididymal sperm in an in vitro fertilization setting to examine whether ejaculated sperm enter cumulus-oocyte complexes more efficiently. In order to prepare sperm for fertilization, they were incubated under capacitating conditions. At the outset of incubation, ejaculated sperm stuck to the glass surfaces of slides and the incidences of sticking decreased with time; whereas, very few epididymal sperm stuck to glass at any time point, indicating differences in surface charge. At the end of the capacitating incubation, when sperm were added to cumulus-oocyte complexes, the form of flagellar movement differed dramatically; specifically, ejaculated sperm predominantly exhibited increased bending on one side of the flagellum (a process termed pro-hook hyperactivation), while epididymal sperm equally exhibited increased bending on one or the other side of the flagellum (pro-hook or anti-hook hyperactivation). This indicates that accessory sex gland secretions might have modified Ca2+ signaling activities in sperm, because the two forms of hyperactivation are reported to be triggered by different Ca2+ signaling patterns. Lastly, over time, more ejaculated than epididymal sperm entered the cumulus oocyte complexes. We concluded that modification of sperm by male accessory gland secretions affects the behavior of ejaculated sperm, possibly providing them with an advantage over epididymal sperm for reaching the eggs in vivo.

Microgrooves and fluid flows provide preferential passageways for sperm over pathogen Tritrichomonas foetus.

Successful mammalian reproduction requires that sperm migrate through a long and convoluted female reproductive tract before reaching oocytes. For many years, fertility studies have focused on biochemical and physiological requirements of sperm. Here we show that the biophysical environment of the female reproductive tract critically guides sperm migration, while at the same time preventing the invasion of sexually transmitted pathogens. Using a microfluidic model, we demonstrate that a gentle fluid flow and microgrooves, typically found in the female reproductive tract, synergistically facilitate bull sperm migration toward the site of fertilization. In contrast, a flagellated sexually transmitted bovine pathogen, Tritrichomonas foetus, is swept downstream under the same conditions. We attribute the differential ability of sperm and T. foetus to swim against flow to the distinct motility types of sperm and T. foetus; specifically, sperm swim using a posterior flagellum and are near-surface swimmers, whereas T. foetus swims primarily via three anterior flagella and demonstrates much lower attraction to surfaces. This work highlights the importance of biophysical cues within the female reproductive tract in the reproductive process and provides insight into coevolution of males and females to promote fertilization while suppressing infection. Furthermore, the results provide previously unidentified directions for the development of in vitro fertilization devices and contraceptives.