Biological benefits of collective swimming of sperm in a viscoelastic fluid

The Known and Unknown About Female Reproductive Tract Mucus Rheological Properties

Spermatozoa reach the fallopian tube during ovulation by traveling through the female reproductive tract mucus. This non-Newtonian viscoelastic medium facilitates spermatozoon movement to accomplish fertilization or, in some cases, blocks spermatozoon movement, leading to infertility. While rheological properties are known to affect spermatozoon motility with in vitro models using synthetic polymers, their precise effects in vivo are understudied. This paper reviews the rheological measurements of reproductive tract mucus during ovulation in humans and model animals, focusing on viscosity and its potential effect on spermatozoa. Mucus viscosity in the female reproductive tract's different compartments is poorly understood. While information on this subject is incomplete, most mammals appear to have a viscosity decrease along their female reproductive tracts. Based on this sparse information, we hypothesize that viscosity changes in female reproductive tracts may guide spermatozoa to eggs, a novel concept that could improve our understanding of reproductive biology.

Geometry of obstructed pathway regulates upstream navigational pattern of sperm population

Sperm navigation through the complex microarchitecture of the fallopian tube is essential for successful fertilization. Spatiotemporal structural alteration due to folded epithelium or muscle contractions in the fallopian tube changes the geometry of the sperm pathways. The role of structural complexity in sperm navigational patterns has been investigated for single sperm cells but has not been fully addressed at the population level. Here, we studied the dynamics of the navigation of a bull sperm population through obstructed pathways mimicking the architecture of the female reproductive tract. We observed that slightly tapered barriers enhance navigation by 20% compared to straight pathway; however, tapered barriers with a 90° angle restrict sperm passage. We demonstrated sperm cooperation while passing through a tapered pathway in a low-viscosity medium under elevated shear rates. These findings propose a fresh perspective on how sperm move through the fallopian tube, suggesting that the convoluted pathways' shape influences sperm navigation locally.

Evolutionary Aspects of Sperm Physiology and Its Assessment

Diversity in sperm morphology and performance could be explained by differences in molecular and cellular mechanisms that underlie the processes of sperm formation in the testis and maturation in the epididymis. Furthermore, there are additional differences in mechanisms that prepare the sperm for fertilization, once they are transferred to the female tract. Another level of understanding of sperm diversity requires the analysis of selective forces mainly related to post-copulatory sexual selection. Sperm competition and cryptic female choice have been identified as important agents driving the evolution of sperm morphology and function. Learning about mechanisms and selective forces is crucial for the design of tests to assess sperm quality, either in a clinical context or to probe their fertilizing capacity in assisted reproduction or reproductive biotechnologies. In addition, this will be important to understand the impact on fertility of new pharmaceuticals or environmental factors.

Prediction of sperm motion behavior in microfluidic channel using sperm swimming model

Several investigations have recently been conducted using microfluidic channels to sort highly motile sperm and thereby increase the probability of fertilization. To further enhance the efficiency of sperm sorting, predicting sperm movement in microfluidic channels through simulation techniques could be beneficial. In this study, we constructed a sperm swimming model based on the concept of an agent-based model. This model allows analysis at the same spatio-temporal scale similar to microfluidic channels. Sperm movement was simplistically modeled as a random walk, utilizing the distribution of sperm velocity and deflection angle obtained from experimental data. We have developed a thigmotaxis model to describe the phenomenon where sperm near the wall exhibit a reduced tendency to move away from it. Additionally, we created a rheotaxis model, in which sperm reorient in the direction opposite to the flow depending on the shear rate. Using these models, we investigated sperm behaviors within a microchannel featuring a tapered area. The results reveal that sperm accumulate within the tapered area, leading to a significant increase in sperm concentration for specific flow velocity ranges in the microchannel. This model provides valuable information for predicting the effects of sperm sorting in various microfluidic channels.

Self-organized vortex phases and hydrodynamic interactions in Bos taurus sperm cells

Flocking behavior is observed in biological systems from the cellular to superorganismal length scales, and the mechanisms and purposes of this behavior are objects of intense interest. In this paper, we study the collective dynamics of bovine sperm cells in a viscoelastic fluid. These cells appear not to spontaneously flock, but transition into a long-lived flocking phase after being exposed to a transient ordering pulse of fluid flow. Surprisingly, this induced flocking phase has many qualitative similarities with the spontaneous polar flocking phases predicted by Toner-Tu theory, such as anisotropic giant number fluctuations and nontrivial transverse density correlations, despite the induced nature of the phase and the clearly important role of momentum conservation between the swimmers and the surrounding fluid in these experiments. We also find a self-organized global vortex state of the sperm cells, and map out an experimental phase diagram of states of collective motion as a function of cell density and motility statistics. We compare our experiments with a parameter-matched computational model of persistently turning active particles and find that the experimental order-disorder phase boundary as a function of cell density and persistence time can be approximately predicted from measures of single-cell properties. Our results may have implications for the evaluation of sample fertility by studying the collective phase behavior of dense groups of swimming sperm.

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.

Effect of High Viscosity on Energy Metabolism and Kinematics of Spermatozoa from Three Mouse Species Incubated under Capacitating Conditions

In order to sustain motility and prepare for fertilization, sperm require energy. The characterization of sperm ATP production and usage in mouse species revealed substantial differences in metabolic pathways that can be differentially affected by capacitation. Moreover, spermatozoa encounter different environments with varying viscoelastic properties in the female reproductive tract. Here, we examine whether viscosity affects sperm ATP levels and kinematics during capacitation in vitro. Sperm from three mouse species (Mus musculus, M. spretus, M. spicilegus) were incubated under capacitating conditions in a modified Tyrode's medium containing bicarbonate, glucose, pyruvate, lactate, and bovine serum albumin (mT-BH) or in a bicarbonate-free medium as a non-capacitating control. Viscosity was increased with the inclusion of polyvinylpyrrolidone. ATP was measured with a bioluminescence kit, and kinematics were examined with a computer-aided sperm analysis system. In M. musculus sperm, ATP declined during capacitation, but no differences were found between non-capacitating and capacitating sperm. In contrast, in M. spretus and M. spicilegus, ATP levels decreased in capacitating sperm. Increasing viscosity in the medium did not modify the timing or proportion of cells undergoing capacitation but did result in additional time- and concentration-dependent decreases in ATP in M. spretus and M. spicilegus under capacitating conditions. Additionally, increased viscosity altered both velocity and trajectory descriptors. The limited impact of capacitation and higher viscosity on M. musculus sperm ATP and kinematics could be related to the low intensity of postcopulatory sexual selection in this species. Responses seen in the other two species could be linked to the ability of their sperm to perform better under enhanced selective pressures.