Cellular transfection using rapid decrease in hydrostatic pressure

Of all methods exercised in modern molecular biology, modification of cellular properties through the introduction or removal of nucleic acids is one of the most fundamental. As such, several methods have arisen to promote this process; these include the condensation of nucleic acids with calcium, polyethylenimine or modified lipids, electroporation, viral production, biolistics, and microinjection. An ideal transfection method would be (1) low cost, (2) exhibit high levels of biological safety, (3) offer improved efficacy over existing methods, (4) lack requirements for ongoing consumables, (5) work efficiently at any scale, (6) work efficiently on cells that are difficult to transfect by other methods, and (7) be capable of utilizing the widest array of existing genetic resources to facilitate its utility in research, biotechnical and clinical settings. To address such issues, we describe here Pressure-jump-poration (PJP), a method using rapid depressurization to transfect even difficult to modify primary cell types such as embryonic stem cells. The results demonstrate that PJP can be used to introduce an array of genetic modifiers in a safe, sterile manner. Finally, PJP-induced transfection in primary versus transformed cells reveals a surprising dichotomy between these classes which may provide further insight into the process of cellular transformation.

Nitric oxide and hormesis

This present paper provides an assessment of the occurrence of nitric oxide (NO)-induced hormetic-biphasic dose/concentration relationships in biomedical research. A substantial reporting of such NO-induced hormetic effects was identified with particular focus on wound healing, tumor promotion, and sperm biology, including mechanistic assessment and potential for translational applications. Numerous other NO-induced hormetic effects have been reported, but require more development prior to translational applications. The extensive documentation of NO-induced biphasic responses, across numerous organs (e.g., bone, cardiovascular, immune, intestine, and neuronal) and cell types, suggests that NO-induced biological activities are substantially mediated via hormetic processes. These observations are particularly important because broad areas of NO biology are constrained by the quantitative features of the hormetic response. This determines the amplitude and width of the low dose stimulation, affecting numerous biomedical implications, study design features (e.g., number of doses, dose spacing, sample sizes, statistical power), and the potential success of clinical trials.

Effect of high hydrostatic pressure on the in vitro development and molecular quality of transgenic rabbit embryos derived from nano-transfected zygotes

The aim of this study was to evaluate the effect of high hydrostatic pressure (HHP) on the in vitro developmental abilities of nano-transfected rabbit zygotes, their transfection efficiency, and the molecular quality of the blastocysts generated. This quality was assessed by estimating the quantitative profiles of Oct4, Casp7, and Bcl2 mRNA transcripts. The nano-transfection efficiencies of zygotes that had been pre-treated with either 20 MPa or 40 MPa of HHP (13.5% and 13.7%, respectively) were insignificantly lower than those found in zygotes not exposed to HHP prior to their nano-transfection (20.1%; P≥0.05). Moreover, applying HHP treatment with the parameters of 20 MPa and 40 MPa followed by the nano-transfection of zygotes brought about an insignificant decrease in the rates of embryos at the blastocyst stage (30.4% and 23.0%, respectively) as compared to the control group of nano-transfected zygotes (40.4%; P≥0.05). Furthermore, analyzing the transcriptional activity of Oct4, Bcl2, and Casp7 genes revealed that HHP enhances the relative abundance (RA) of all mRNA transcripts in blastocysts derived from non-transfected rabbit zygotes. In turn, the augmented RAs found in the pro-apoptotic Casp7 and anti-apoptotic Bcl-2 transcripts confirmed the onset and progression of programmed cell death in blastocysts developed from nano-transfected zygotes that had undergone HHP pre-treatment. The conceptualization based not only on a novel nano-transfection approach used to genetically modify in vivo-fertilized rabbit zygotes but also on their HHP pre-treatment is elaborated here for the first time, with an emphasis on further investigations aimed at producing transgenic rabbit and other mammalian species embryos by somatic cell cloning.

Melatonin improves the quality of frozen bull semen and influences gene expression related to embryo genome activation

The efficiency of animal artificial breeding in vitro is still low. Oxidative damage is an important obstacle for in vitro artificial breeding of animals. Melatonin can reduce the degree of oxidative damage to both gametes and embryos caused by the external environment. However, there is still some controversy concerning the effect of melatonin on frozen semen, especially in the processes of freezing semen, IVM, IVF and IVC. Here, the effects of melatonin on the whole processes of sperm cryopreservation, oocyte maturation, and embryonic development were studied. The results demonstrated that melatonin at 10^-3 M concentration significantly improved progressive sperm viability, plasma membrane integrity, mitochondrial membrane integrity, and acrosome integrity; however, there were also individual differences between bulls, depending on the age of different individuals. The 10^-3 M melatonin treatment reduced the reactive oxygen species (ROS) level by nearly 50% in sperm during IVF. Meanwhile, during IVM, the addition of 10^-7 M melatonin significantly increased the maturation rate of oocytes and reduced the ROS levels by 58.8%. In addition, 10^-7 M melatonin improved the total cell numbers of the IVF blastocysts. Notably, treatment of IVF embryos with melatonin significantly reduced the levels of ROS and influenced the expression levels of key regulatory genes associated with embryo genome activation. This study is of significance for understanding the function of melatonin in animal artificial breeding.

Recent progress in bovine in vitro-derived embryo cryotolerance: Impact of in vitro culture systems, advances in cryopreservation and future considerations

Cryopreservation of in vitro-derived bovine embryos is a crucial step for the widespread reproduction and conservation of valuable high-merit animals. Given the current popularity of bovine in vitro embryo production (IVP), there is a demand for a highly efficient ultra-low temperature storage method in order to maximize donor ovum pickup (OPU) turn-over, recipient availability/utilization and domestic/overseas commercial trading opportunities. However, IVP bovine embryos are still very sensitive to chilling and cryopreservation, and despite recent progress, a convenient (simple and robust) protocol has not yet been developed. At the moment, there are two methods for bovine IVP embryo cryopreservation: slow programmable freezing and vitrification. Both of the aforementioned techniques have pros and cons. While controlled-rate slow cooling can easily be adapted for direct transfer (DT), ice crystal formation remains an issue. On the other hand, vitrification solved this problem but the possibility of successful DT commercial incorporation remains to be determined. Moreover, simplification of the vitrification protocol (including warming) through the use of an in-straw dilution without the use of a microscope is a prerequisite for its use under farm conditions. This review summarizes the bovine IVP embryo cryopreservation achievements, strengths and limitations of both freezing systems and prospective improvements to enhance cryosurvival, as well as perspectives on future directions of this assisted reproductive technology.

Effect of high hydrostatic pressure on mitochondrial activity, reactive oxygen species level and developmental competence of cultured pig embryos

High hydrostatic pressure (HHP) has been previously used to increase mammalian oocyte and embryo tolerance on subsequent stresses related with different assisted reproductive technologies. Nevertheless, the mechanisms for HHP-induced stress responses in early embryos have not been yet well understood. Previous studies focused mainly on HHP-modified gene expression while possible changes in cellular functions, including modification of energy metabolism and oxidative stress were neglected. Therefore, we aimed to analyze the effect of HHP treatment on the efficiency of subsequent in vitro pig embryos culture in NCSU-23 medium, on mitochondrial membrane potential (ΔΨm) and reactive oxygen species (ROS) level during their pre-implantation development. Porcine embryos were exposed to the hydrostatic pressure of 20 MPa and their quick response to such stress was analyzed 1 h later. In comparison with control embryos, we detected lower ΔΨm by ∼13% only in expanded blastocysts as well as decreased ROS level by ∼30% and ∼42% at the morula and expanded blastocyst stages, respectively. After HHP-treatment at transcriptionally inactive zygote stage and subsequent embryo culture, long-time responses were found: (1) at expanded blastocyst stage manifesting by ΔΨm decrease by ∼16%, (2) at the morula and expanded blastocyst stages in the form of ROS level reduction by ∼38% and ∼33% respectively. Following HHP stress applied at the transcriptionally active morula stage the long-time response in the expanded blastocysts as a decrease of ΔΨm by ∼19% and ROS level by ∼37% was observed. The percentage of obtained expanded blastocysts was higher after culture of HHP-treated zygotes in comparison to the control. Moreover, expanded blastocysts developed in vitro from both HHP-treated zygotes or morulae, exhibited higher total number of cells per blastocyst, higher number of cells in the inner cell mass as well as lower number of TUNEL-positive nuclei per blastocyst and lower TUNEL index, when compared to untreated embryos. Therefore, the HHP stress applied at the zygote stage, enhances developmental potential and quality of in vitro obtained porcine blastocysts due to the both decreased ΔΨm and ROS level. Our findings may contribute to better understanding of the mechanism of HHP-mediated modifications of energy metabolism and oxidative stress during in vitro development of pig embryos.

Cloning: A Sleeping Beauty Awaiting the Kiss?

The first 20 years of somatic cell nuclear transfer can hardly be described as a success story. Controversially, many factors leading to the fiasco are not intrinsic features of the technique itself. Misunderstandings and baseless accusations alongside with unsupported fears and administrative barriers hampered cloners to overcome the initial challenging period with obvious difficulties that are common features of a radically new approach. In spite of some promising results of mostly sporadic and small-scale experiments, the future of cloning is still uncertain. On the other hand, a reincarnation, just like the idea of electric cars, may result in many benefits in various areas of science and economy. One can only hope that-in contrast to electric cars-the ongoing paralyzed phase will not last for 100 years, and breakthroughs achieved in some promising areas will provide enough evidence to intensify research and large-scale application of cloning in the next decade.

Sperm cryopreservation: A review on current molecular cryobiology and advanced approaches

The cryopreservation of spermatozoa was introduced in the 1960s as a route to fertility preservation. Despite the extensive progress that has been made in this field, the biological and biochemical mechanisms involved in cryopreservation have not been thoroughly elucidated to date. Various factors during the freezing process, including sudden temperature changes, ice formation and osmotic stress, have been proposed as reasons for poor sperm quality post-thaw. Little is known regarding the new aspects of sperm cryobiology, such as epigenetic and proteomic modulation of sperm and trans-generational effects of sperm freezing. This article reviews recent reports on molecular and cellular modifications of spermatozoa during cryopreservation in order to collate the existing understanding in this field. The aim is to discuss current freezing techniques and novel strategies that have been developed for sperm protection against cryo-damage, as well as evaluating the probable effects of sperm freezing on offspring health.

Induction of Sublethal Oxidative Stress on Human Sperm before Cryopreservation: A Time-Dependent Response in Post-Thawed Sperm Parameters

Objective

A recent innovative approach, based on induction of sublethal oxidative stress to enhance sperm cryosurvival, has been applied before sperm cryopreservation. The purpose of this study was to investigate the effects of different induction times of sublethal oxidative stress before cryopreservation on human post-thawed sperm quality.

Materials and Methods

In this experimental study, we selected semen samples (n=20) from normozoospermic men according to 2010 World Health Organization (WHO) guidelines. After processing the samples by the density gradient method, we divided each sample into 5 experimental groups: fresh, control freezing, and 3 groups exposed to 0.01 μM sodium nitroprusside (SNP) [nitric oxide (NO) donor] for 30 (T30), 60 (T60), or 90 minutes (T90) at 37˚C and 5% CO₂ before cryopreservation. Motion characteristics [computer-assisted sperm analyser], viability, apoptosis [annexin V/propidium iodide (PI) assay], DNA fragmentation [sperm chromatin structure assay (SCSA)], and caspase 3 activity (FLICA Caspase Detection Kit) were assessed after thawing. The results were analysed by using one-way ANOVA and Tukey's test. The means were significantly different at P<0.05.

Results

Cryopreservation significantly decreased sperm viability and motility parameters, and increased the percentage of apoptosis, caspase 3 activity, and DNA fragmentation (P<0.01) compared to the fresh group. The T60 group had a higher significant percentage of total motility (TM) and progressive motility compared with other cryopreserved groups (P<0.05). We observed a significantly lower percentage of apoptotic rate and caspase 3 activity in the T60 group compared to the other cryopreserved groups (P<0.05). DNA integrity was not significantly affected by this time of sublethal stress induction (P>0.05).

Conclusion

Our results have demonstrated that the application of sublethal oxidative stress by using 0.01 μM NO for 60 minutes before the freezing process can be a beneficial approach to improve post-thawed human sperm quality.

Stimulus-triggered enhancement of chilling tolerance in zebrafish embryos

Background

Cryopreservation of zebrafish embryos is still an unsolved problem despite market demand and massive efforts to preserve genetic variation among numerous existing lines. Chilled storage of embryos might be a step towards developing successful cryopreservation, but no methods to date have worked.

Methods

In the present study, we applied a novel strategy to improve the chilling tolerance of zebrafish embryos by introducing a preconditioning hydrostatic pressure treatment to the embryos. In our experiments, 26-somites and Prim-5 stage zebrafish embryos were chilled at 0°C for 24 hours after preconditioning. Embryo survival rate, ability to reach maturation and fertilizing capacity were tested.

Results

Our results indicate that applied preconditioning technology made it possible for the chilled embryos to develop normally until maturity, and to produce healthy offspring as normal, thus passing on their genetic material successfully. Treated embryos had a significantly higher survival and better developmental rate, moreover the treated group had a higher ratio of normal morphology during continued development. While all controls from chilled embryos died by 30 day-post-fertilization, the treated group reached maturity (~90–120 days) and were able to reproduce, resulting in offspring in expected quantity and quality.

Conclusions

Based on our results, we conclude that the preconditioning technology represents a significant improvement in zebrafish embryo chilling tolerance, thus enabling a long-time survival. Furthermore, as embryonic development is arrested during chilled storage this technology also provides a solution to synchronize or delay the development.

Effects of High Hydrostatic Pressure on Expression Profiles of In Vitro Produced Vitrified Bovine Blastocysts

High hydrostatic pressure (HHP) has been used to pre-condition embryos before essential, yet potentially detrimental procedures such as cryopreservation. However, the mechanisms for HHP are poorly understood. We treated bovine blastocysts with three different HHP (40, 60 and 80 MPa) in combination with three recovery periods (0, 1 h, 2 h post HHP). Re-expansion rates were significantly higher at 40 and 60 but lower at 80 MPa after vitrification-warming in the treated groups than controls. Microarray analysis revealed 399 differentially expressed transcripts, representing 254 unique genes, among different groups. Gene ontology analysis indicated that HHP at 40 and 60 MPa promoted embryo competence through down-regulation of genes in cell death and apoptosis, and up-regulation of genes in RNA processing, cellular growth and proliferation. In contrast, 80 MPa up-regulated genes in apoptosis, and down-regulated protein folding and cell cycle-related genes. Moreover, gene expression was also influenced by the length of the recovery time after HHP. The significantly over-represented categories were apoptosis and cell death in the 1 h group, and protein folding, response to unfolded protein and cell cycle in the 2 h group compared to 0 h. Taken together, HHP promotes competence of vitrified bovine blastocysts through modest transcriptional changes.

Stress preconditioning of semen before cryopreservation improves fertility and increases the number of offspring born: a prospective randomised study using a porcine model

The aim of the present study was to investigate the effect of applying sublethal stress treatment at room temperature, before cryopreservation (hydrostatic pressure (HP): 40 MPa, 80 min) of 34 boar ejaculate samples, on post-thawed motility and sow fertility. Sows (n = 102) were randomly allocated into equal groups inseminated with HP-treated or untreated frozen–thawed semen. Sows were inseminated twice, 10 h apart, with 6 × 109 spermatozoa per dose without oestrus synchronisation. Rates of non-return of oestrus and pregnancy, and total numbers of piglets and live piglets were significantly higher (P < 0.05) in the HP-treated group. There was also a numerical, albeit non-significant (P > 0.05), improvement in the farrowing rate in the HP-treated group. Although the number of live piglets per litter decreased approximately 15% in both groups by 42 days after farrowing, but this remained significantly higher in the HP-treated group. Although total and progressive sperm motility were significantly (P < 0.001) higher in the HP-treated group, there were no significant differences (P > 0.05) in these parameters between pregnant and non-pregnant sows in either group; thus motility can indicate, but not predict, improved fertility. In conclusion, HP treatment, with sperm cryopreservation, increases in vitro sperm motility and improves reproductive performance without adversely affecting the health of the piglets.

Controlled hydrostatic pressure stress downregulates the expression of ribosomal genes in preimplantation embryos: a possible protection mechanism?

The efficiency of various assisted reproductive techniques can be improved by preconditioning the gametes and embryos with sublethal hydrostatic pressure treatment. However, the underlying molecular mechanism responsible for this protective effect remains unknown and requires further investigation. Here, we studied the effect of optimised hydrostatic pressure treatment on the global gene expression of mouse oocytes after embryonic genome activation. Based on a gene expression microarray analysis, a significant effect of treatment was observed in 4-cell embryos derived from treated oocytes, revealing a transcriptional footprint of hydrostatic pressure-affected genes. Functional analysis identified numerous genes involved in protein synthesis that were downregulated in 4-cell embryos in response to hydrostatic pressure treatment, suggesting that regulation of translation has a major role in optimised hydrostatic pressure-induced stress tolerance. We present a comprehensive microarray analysis and further delineate a potential mechanism responsible for the protective effect of hydrostatic pressure treatment.

Beneficial Effects of Nitric Oxide Induced Mild Oxidative Stress on Post-Thawed Bull Semen Quality

Background

Cryopreservation of semen requires optimized conditions to minimize the harmful effects of various stresses. The main approach for protection of sperm against stress is based on the use of antioxidants and cryoprotectants, which are described as defensive methods. Recently, the application of controlled mild stressors has been de- scribed for activation of a temporary response in oocyte, embryo and somatic cells. In this study a sub-lethal oxidative stress induced by precise concentrations of nitric oxide (NO) has been evaluated for sperm during cryopreservation.

Materials and Methods

In this experimental study, we used different concentrations of NO [0 µM (NO-0), 0.01 µM (NO-0.01), 0.1 µM (NO-0.1), 1 µM (NO-1), 10 µM (NO-10) and 100 µM (NO-100)] during cryopreservation of bull semen. Their effects on post-thawed sperm quality that included motility and velocity parameters, plasma mem- brane functionality, acrosome integrity, apoptosis status, mitochondrial activity and lipid peroxidation after freezing-thawing were investigated.

Results

Exposure of sperm before freezing to NO-1 significantly increased total motility (88.4 ± 2.8%), progressive motility (50.4 ± 3.2%) and average path velocity (VAP, 53.8 ± 3.1 µm/s) compared to other extenders. In addition, NO-1 significantly increased plasma mem- brane functionality (89.3 ± 2.9%) compared to NO-0 (75.3 ± 2.9%), NO-0.01 (78.3 ± 2.9%), NO-0.1 (76.4 ± 2.9%), NO-10 (64 ± 2.9%) and NO-100 (42 ± 2.9%). Sperm exposed to NO-1 produced the highest percentage of viable (85.6 ± 2.3%) and the lowest percentage of apoptotic (10.8 ± 2.4%) spermatozoa compared to the other extenders. Also, NO-100 resulted in a higher percentage of dead spermatozoa (27.1 ± 2.7%) compared to the other extenders. In terms of mitochondrial activity, there was no significant difference among NO-0 (53.4 ± 3.2), NO-0.01 (52.1 ± 3.2), NO-0.1 (50.8 ± 3.2) and NO-1 (53.1 ± 3.2). For acrosome integrity, no significant different was observed in sperm exposed to different concentrations of NO.

Conclusion

Induction of sub-lethal oxidative stress with 1 µM NO would be beneficial for cryopreservation of bull semen.

Effect of ethanol induced mild stress on post-thawed bull sperm quality

This study was performed to investigate the effect of sub-lethal exposure of bull semen to ethanol on the post-thaw spermatozoa quality. Semen samples (n=24, 6 ejaculates/bull) from 4 Holstein bulls were collected and pooled. Pooled samples were divided into 4 equal parts and each part was frozen after being diluted with Optidyl® extender containing 0 (O-E0), 0.03 (O-E3), 0.09 (O-E9) and 0.15 (O-E15) % (v/v) absolute ethanol. Sperm motility and velocity, plasma membrane integrity and functionality, mitochondrial activity, malondialdehyde concentration, and apoptosis status were evaluated after thawing. A higher percentage of total motility was observed in the O-E9 group as compared to the O-E0, O-E3 and O-E15 groups (p<0.05). Also, plasma membrane integrity was higher (p<0.05) in the O-E9 group compared to the O-E3, and O-E15 groups. However, the difference between the O-E9 and O-E0 groups was not significant (p>0.05). In terms of the proportion of sperm abnormality and plasma membrane functionality no differences (p>0.05) were observed between the groups. Our results revealed that malondialdehyde level was lower in ethanol treated (O-E3, O-E9 and O-E15) groups compared to the O-E0 group (p<0.05). Furthermore, the percentage of live spermatozoa with active mitochondria was higher in the O-E9 and O-E15 groups compared to the O-E0 and O-E3 groups (p<0.05). The O-E3 and O-E9 groups resulted in the highest and lowest percentage of apoptotic spermatozoa, respectively (p<0.05). The results of this study demonstrate that supplementation of semen extender with sub-lethal concentration of ethanol influences post-thawed bull sperm quality in a dose dependent manner.

Targeting stress responses for regenerative medicine

Some internal and external stimuli elicit stress responses on the cellular level and at the level of the organism. When the stimulus is brief and its intensity mild to moderate, it triggers adaptation changes that improve the cell's or organism's survival. This adaptation is achieved through a variety of cellular mechanisms such as induction of repair mechanisms, improved removal of damaged macromolecules, upregulation of endogenous antioxidant defenses, and prevention of apoptosis triggering by moderate stressors. The key intracellular signaling pathways involved in stress adaptation are the mTORC1 and SIRT1. Manipulating these stress adaptation signaling pathways with a variety of agents, improves the cellular adaptation to stress, prolongs cell survival, and improves the transplantation outcome in animal models and in clinical trials. The challenge for the future is to fine-tune the numerous experimental techniques to suit the needs of transplantation and regenerative medicine.

Adaptive response, evidence of cross-resistance and its potential clinical use

Organisms and their cells are constantly exposed to environmental fluctuations. Among them are stressors, which can induce macromolecular damage that exceeds a set threshold, independent of the underlying cause. Stress responses are mechanisms used by organisms to adapt to and overcome stress stimuli. Different stressors or different intensities of stress trigger different cellular responses, namely induce cell repair mechanisms, induce cell responses that result in temporary adaptation to some stressors, induce autophagy or trigger cell death. Studies have reported life-prolonging effects of a wide variety of so-called stressors, such as oxidants, heat shock, some phytochemicals, ischemia, exercise and dietary energy restriction, hypergravity, etc. These stress responses, which result in enhanced defense and repair and even cross-resistance against multiple stressors, may have clinical use and will be discussed, while the emphasis will be on the effects/cross-effects of oxidants.

Oocyte quality determines bovine embryo development after fertilisation with hydrogen peroxide-stressed spermatozoa

Exposure of gametes to specific stressors at sublethal levels can enhance the gametes’ subsequent performance in processes such as cryopreservation. In the present study, bull spermatozoa were subjected to H2O2 for 4 h at 100-, 200- and 500-μM levels; computer-assisted sperm analysis (CASA) and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL) assay were used for evaluation of subsequent sperm motility and DNA integrity, respectively. Exposure of spermatozoa to H2O2 did not affect sperm motility but DNA integrity was negatively affected by 500 μM H2O2 compared with mock-exposed spermatozoa, whereas both motility and DNA integrity were affected compared with untreated spermatozoa. Nevertheless, insemination of oocytes with spermatozoa exposed to 200 μM H2O2 increased fertilisation, cleavage and blastocyst rates (P < 0.05). Furthermore, the higher blastocyst yield after fertilisation of oocytes with spermatozoa exposed to 200 μM H2O2 was related to oocyte diameter, with large–medium oocytes yielding higher blastocyst rates, while small-diameter oocytes consistently failed to develop into blastocysts. In conclusion, the results indicate that exposure of spermatozoa to 200 μM H2O2 before sperm–oocyte interaction may enhance in vitro embryo production in cattle. However, this increased embryo production is largely dependent on the intrinsic quality of the oocytes.

Sperm surface changes and physiological consequences induced by sperm handling and storage

Spermatozoa interact with their immediate environment and this contact remodels the sperm surface in preparation for fertilisation. These fundamental membrane changes will be critically covered in this review with special emphasis on the very specific surface destabilisation event, capacitation. This process involves very subtle and intricate modifications of the sperm membrane including removal of suppression (decapacitation) factors and changes in the lateral organisation of the proteins and lipids of the sperm surface. Processing of sperm for assisted reproduction (storage, sex-sorting, etc.) subjects spermatozoa to numerous stressors, and it is possible that this processing overrides such delicate processes resulting in sperm instability and cell damage. To improve sperm quality, novel mechanisms must be used to stabilise the sperm surface during handling. In this review, different types of membrane stress are considered, as well as novel surface manipulation methods to improve sperm stability.