The hypoosmotic swelling test performed with coulter counter: a method to assay functional integrity of sperm membrane in rainbow trout

Pikeperch (Sander lucioperca) spermatozoa motility and volume regulation under different osmotic and ionic conditions

Pikeperch (Sander lucioperca) is a highly profitable commercial species whose economic value has greatly increased in the last decade. As in other species, the quality of spermatozoa in this species is a principal feature inherent in fertilization success and efficient natural and artificial reproduction. The capacity of fish spermatozoa to be activated and tolerate environmental changes (in osmolality, ion composition, external pH, temperature, etc.) during the motility period contributes to fertilization success. In this study, we investigated the effects of environmental osmolality and ion composition on spermatozoa motility. To determine if the activation mechanism is affected by sperm quality parameters, we measured semen characteristics such as semen volume, spermatozoa concentration, seminal fluid osmolality and ion composition, and spermatozoa lipid composition. An additional parameter of sperm quality reflecting spermatozoa osmoresistance, the swelling rate, was measured by the nephelometry method. We detected that sperm samples with the highest content of palmitic (C16:0) and palmitoleic (C16:1) acids showed the lowest motility activation under the studied conditions, suggesting that these fatty acids are possible markers for the determination of spermatozoa quality in fish. Our results show that pikeperch spermatozoa can be activated under different osmotic conditions and that cell swelling always accompanies motility. However, spermatozoa sustain their volume under hypotonic conditions when motility is not initiated, suggesting that pikeperch spermatozoa activation is mainly controlled by ion composition rather than the osmolarity of the surrounding medium.

Semen cryopreservation for the Mediterranean brown trout of the Biferno River (Molise-Italy): comparative study on the effects of basic extenders and cryoprotectants

This study was designed to optimize the semen freezing protocol of the native Mediterranean brown trout inhabiting the Molise rivers through two experiments: an in vitro analysis of the effects of two basic extenders combined with three cryoprotectants on post-thaw semen quality; and an in vivo test to assess the fertilization and hatching rate. Semen was diluted at a ratio of 1:3 in a freezing medium composed of a glucose extender (A) or mineral extender (B). Each basic component contained 10% dimethylsulfoxide, dimethylacetamide or methanol. The post-semen quality was evaluated considering motility, duration of motility, viability and DNA integrity. The basic extender and cryoprotectant were shown to have significant effects on these variables, and the best results were obtained using extender A or B combined with dimethylsulfoxide (P < 0.05). These freezing protocols were selected for fertilization trials in vivo. Fertilization and hatching rates were significantly higher in fresh semen. No significant differences were observed in frozen semen using extender A or B, although higher percentages of eyed eggs and hatching rates were recorded using extender A. According to our in vitro and in vivo results, the glucose-based extender and dimethylsulfoxide emerged as the best combination for an effective cryopreservation protocol for semen of this trout.

Potassium ions in extender differentially influence the post-thaw sperm motility of salmonid fish

Potassium ions are known to have an inhibitory effect on the sperm motility of salmonids. For this reason, the addition of K(+) to the extender is frequently applied. However, the effect of the addition of K(+) to the extender has not yet been tested. The aim of this study was to test the influence of potassium ion supplementation of the extender on the sperm motility parameters from five Salmonidae species (rainbow trout (Oncorhynchus mykiss), sex-reversed female rainbow trout, whitefish (Coregonus lavaretus), brown trout (Salmo trutta) and brook trout (Salvelinus fontinalis)). Semen samples were diluted in extender containing 0.18 M glucose in 9% methanol (GM) supplemented with 0, 20 or 40 mM potassium chloride. After thawing sperm were stored for 30, 60, 120 and 240 min at 4 °C. Our results demonstrated that the presence of potassium ions in the extender had a negative effect on percentage of motile sperm in four of the salmonid species. In contrast, potassium ions appeared to have a positive effect on percentage of post-thaw motile sperm in whitefish semen. However, this effect could be mimicked by changing the osmolality of the extender (which was achieved by increasing the glucose concentration to 0.22 M). The addition of potassium ions turned out to have no positive effect on post-thaw storage time. Our results suggest that osmolality, rather than potassium ions, seems to be essential for cryopreservation success of salmonids sperm. Further studies should focus on the effects of small changes in osmolality on the post-thaw quality of semen.

Review on the screening of semen by hypo-osmotic swelling test

The hypo-osmotic swelling test (HOST) is widely used as a valuable test for determining sperm quality by evaluating the membrane integrity of spermatozoa of various domestic animals including cattle, horses and swine. The HOST has also been used as an indicator of the fertilising capacity of spermatozoa. This test is based on the swelling ability when functional spermatozoa submitted to hypo-osmotic solutions. This test is commonly used as an important parameter for the evaluation of semen due to its strong correlation with semen evaluation parameters. The objective of this review was to analyse its significance in semen evaluation, swelling of spermatozoa under various osmolarities and variations in swelling percentage under different seasons.

Motility of fish spermatozoa: from external signaling to flagella response

For successful fertilization, spermatozoa must access, bind, and penetrate an egg, processes for which activation of spermatozoa motility is a prerequisite. Fish spermatozoa are stored in seminal plasma where they are immotile during transit through the genital tract of most externally fertilizing teleosts and chondrosteans. Under natural conditions, motility is induced immediately following release of spermatozoa from the male genital tract into the aqueous environment. The nature of an external trigger for the initiation of motility is highly dependent on the aquatic environment (fresh or salt water) and the species' reproductive behavior. Triggering signals include osmotic pressure, ionic and gaseous components of external media and, in some cases, egg-derived substances. Extensive study of environmental factors influencing fish spermatozoa motility has led to the proposal of several mechanisms of activation in freshwater and marine fish. However, the signal transduction pathways initiated by these mechanisms remain clear. This review presents the current knowledge with respect to (1) membrane reception of the activation signal and its transduction through the spermatozoa plasma membrane via the external membrane components, ion channels, and aquaporins; (2) cytoplasmic trafficking of the activation signal; (3) final steps of the signaling, including signal transduction to the axonemal machinery, and activation of axonemal dyneins and regulation of their activity; and (4) pathways supplying energy for flagellar motility.

Volume changes during the motility period of fish spermatozoa: interspecies differences

The aim of this study was to describe spermatozoa volume changes during the motility period of fish species with either osmotic (common carp Cyprinus carpio) or with ionic (sterlet Acipenseri ruthenus and brook trout Salvelinus fontinalis) modes of motility activation. Nephelometry, light microscopy, and spermatocrit methods were used for quantitative assessment of cell volume changes in media of different osmolalities. Significant correlation (R(2) = 0.7341; P < 0.001) between parameter of volume changes measured using nephelometry and light microscopy methods confirmed nephelometry as a sufficiently sensitive method to detect changes of spermatozoa volume. The spermatocrit alteration method resulted in a large proportion of damaged and potentially immotile spermatozoa in media of osmolality less than 150 mOsm/kg in carp and osmolalities from 10 to 300 mOsm/kg in sterlet and brook trout. Therefore, this method is not reliable for assessing spermatozoa swelling in hypotonic solutions, because the integrity of the cells is not fully preserved. Increase in carp spermatozoa (osmotic activation mode) volume occurred during the motility period in hypotonic conditions, but no indications of volume changes were found in sterlet and brook trout spermatozoa (ionic activation mode) associated with environmental osmolality alteration. Accordingly, we conclude that sperm volume changes are differentially involved in the motility activation process. Species-specific differences in spermatozoa volume changes as a response to a hypotonic environment during the motility period are discussed in relation to their potent physiological role.

Effect of cryopreservation on fish sperm subpopulations

The evaluation of the motility data obtained with a CASA system, applying a Two-Step Cluster analysis, identified in seabream sperm 3 different sperm subpopulations that correlated differently with embryo hatching rates. Hence, we designed an experiment to understand the effect of the application of different cryopreservation protocols in these sperm motility-based subpopulations. We analyzed Sparus aurata frozen/thawed semen motility 15, 30, 45 and 60s after activation, using CASA software. Different protocols were applied for cryopreservation: three different cryoprotectants (Dimethyl Sulfoxide (Me(2)SO), Ethylene Glycol (EG) and Propylene Glycol (PG)) each at two different concentrations and two packaging volumes (0.5ml straws, and 1.8ml cryovials) were tested. Different freezing rates were evaluated corresponding to 1, 2, 3, 4 and 8cm above the liquid nitrogen surface for the straws and 1, 2 and 4cm for the cryovials. Motility parameters rendered by CASA were treated with a Two-Step Cluster analysis. Three different subpopulations were obtained: SP1 - slow non-linear spermatozoa, SP2 - slow linear spermatozoa and SP3 - fast linear spermatozoa. We considered SP3 as the subpopulation of interest and focused further analyses on it. Generally, SP3 was the best represented subpopulation 15s after activation and was also the one showing a greater decrease in time, being the least represented after 60s. According to the applied univariate general linear model, samples frozen in straws with 5% Me(2)SO and in cryovials with 10% Me(2)SO at 2 and 1cm from the LN(2,) respectively, produced the best results (closer to the control). Clustering analysis allowed the detection of fish sperm subpopulations according to their motility pattern and showed that sperm composition in terms of subpopulations was differentially affected by the cryopreservation protocol depending on the cryoprotectant used, freezing rates and packaging systems.

Adaptation to long sperm in Drosophila: correlated development of the sperm roller and sperm packaging

Sperm are generally small and produced in huge numbers, but some species combine exaggerated sperm length with extremely limited numbers of sperm, an evolutionary trend that deviates from the theory of anisogamy. Sperm gigantism has arisen recurrently in various species, but insects exhibit the longest sperm, with some species of the Drosophilidae family producing sperm up to 6 cm in length. The anatomical, cytological, and physiological requirements for males to cope with these giant sperm were hitherto poorly understood. In this paper, we investigate the internal morphology of the male reproductive tract, and highlight specific features that may be linked to this increase in sperm size. We focus on species in the repleta group, within which sperm length varies by a factor of 35. An associated development of the sperm roller, a special twisting device inserted between the testis and the seminal vesicle, is demonstrated. Its length and the number of coils involved increase with sperm size, and it allows individual sperm to swell and roll into a spermatic pellet before reaching the seminal vesicle. This process occurs independently of and in addition to the sperm bundle coiling that takes place at the base of the testis. It is suggested that the emergence and development of the sperm roller may be a male adaptation to sperm gigantism.

Cryopreservation of European eel (Anguilla anguilla) spermatozoa: Effect of dilution ratio, foetal bovine serum supplementation, and cryoprotectants

The main aim of the present study was to investigate the effect of sperm freezing medium dilution ratio (1:1, 1:2, and 1:5 v/v), two cryoprotectants: dimethyl sulphoxide (Me(2)SO) and methanol (MeOH), and the addition of foetal bovine serum (FBS) on the cryopreservation of European eel sperm. The effect of these factors was evaluated comparing post-thawing viability with fluorescent staining (Hoechst bisbenzimide 33258) and the spermatozoa head morphometry, determined with computer-assisted morphology analysis (ASMA). The 1:5 (v/v) dilution ratio resulted in a lower viability in comparison with 1:1 and 1:2 (52.8+/-2.3% vs. 67.4+/-2.3% and 65.1+/-2.3%, respectively, p=0.0001), but without effects on the head morphology. Although the viability was not significantly different between Me(2)SO and MeOH (60.4+/-1.9 vs. 63.2+/-1.9%, respectively, p=0.305), a decrease of spermatozoa head area and perimeter was found when spermatozoa were frozen with methanol (6.19+/-0.01 vs. 6.36+/-0.01 microm(2) and 17.28+/-0.05 vs. 17.49+/-0.05 microm, for area and perimeter and MeOH and Me(2)SO, respectively, p=0.0001). Finally, a higher viability (75.1+/-1.7 vs. 48.5+/-1.7, with or without FBS, respectively, p=0.0001) and higher spermatozoa head size (6.40+/-0.01 vs. 6.15+/-0.01microm(2) and 17.88+/-0.05 vs. 16.89+/-0.05 microm, for area and perimeter, with or without FBS, respectively, p=0.0001) were found when cells were frozen-thawed in freezing media supplemented with FBS. Based on the above findings, dilution ratios lower than 1:5 (v/v) and the addition of serum improved the viability results after cryopreservation. Future studies are required in order to understand the spermatozoa membrane interchange mechanisms in response to the changes in spermatozoa head size caused by cryoprotectants and freezing media supplements.

Functional significance of the cell volume for detecting sperm membrane changes and predicting freezability in dog semen

Due to the similarity of plasma membrane changes induced by capacitation and cryopreservation, the parameters describing sperm response to capacitating conditions can be used for evaluating the cryopreservation response in many animal systems. In dog sperm, the response of the total sperm population to ionophore treatment has been shown to be an indication of the freezability of semen samples. Another sperm functional characteristic decisive for cryopreservability is cell volume regulation, due to the generation of essential osmotic gradients across the plasma membrane during the freeze-thaw cycles. In the present study, cryopreservation-induced changes in the membrane functional integrity were examined by monitoring the osmotically induced response of cell volume and the response to an ionophore in live cell populations. Cell volume measurements were performed on Percoll-washed suspensions of freshly diluted and frozen-thawed dog spermatozoa. The proportion of live acrosome-reacted cells was evaluated by flow cytometry after incubation under capacitating conditions in the presence of the calcium ionophore, A23187. During freezing-thawing, significant membrane changes occurred related to the disturbance of volume control ability and the loss of a proportion of live acrosome-reacted cells (P < 0.05). There were significant differences between individuals with respect to the degree of functional and structural membrane changes after thawing. Significant correlations were found between acrosomal integrity and functional membrane integrity. When assessed in freshly diluted semen, these parameters correlated with those of frozen-thawed semen samples, pointing to the similarities between mechanisms of cryopreservation-related changes and those mechanisms that mediate changes in membrane permeabilities and in cell volume regulation. The detection of changes in the sperm plasma membrane by monitoring the sperm cell volume represents a simple, rapid and sensitive method to estimate sperm quality after the cryopreservation procedure. The individual variability in response to osmotic stress or to calcium ionophore treatment appears to reflect the subtle differences in the sperm membrane functionality which are crucial for the prediction of cryopreservability.

Effect of external cryoprotectants as membrane stabilizers on cryopreserved rainbow trout sperm

The process of freezing and thawing induces certain cellular damage in rainbow trout (Oncorhynchus mykiss) spermatozoa. We have previously demonstrated that after freezing and thawing decreased fertility in rainbow trout (Oncorhynchus mykiss) spermatozoa, is related to sublethal damage to the plasma membrane. External cryoprotectants are known to stabilize the sperm cell membrane against such damage. In the current study, we used a basic freezing extender containing #6 Erdahl and Graham and 7% DMSO and added egg yolk, BSA, and a soybean-protein complex (DanPro S760) singly and in various combinations. To assess the effect of these cryoprotectants we evaluated the percentage of cells with progressive motility, permeability of cells to propidium iodide (viability) after exposure for 30 sec, 2, 5, 10 and 15 min. to hypo- and isoosmotic solutions of 10 and 300 mOsm, and the in vitro fertility rate. Fertility trials were performed using 1.87 x 10(7) spermatozoa/egg. Some of the tested stabilizers increased motility, increased viability, or reduced cell fragility after freezing and thawing. Nevertheless these quality improvements demonstrated by the "in vitro" tests do not always correlate with high fertility. The best membrane protection in terms of resistance to hypoosmotic shock was achieved when BSA and egg yolk were added to the extender. The highest fertility rates were obtained with DanPro S760 alone or in combination with BSA; the use of BSA with egg yolk did not improve this parameter. Our results demonstrated that some external cryoprotectants effectively increased membrane resistance during freezing and thawing, but some of the tested mixtures interfered with fertilization. Soybean protein concentrate provided good protection and increased fertility rates in cryopreserved trout spermatozoa.