Investigation of Fertilizing Capacity of Zebrafish (Danio rerio) Sperm Exposed to Heavy Metals

The objective of our study was to investigate the effects of heavy metals on the fertilizing capacity of exposed zebrafish sperm, on embryonic survival, and on occurrence of embryonic deformities following fertilization with exposed sperm. It is important to test heavy metals because they are well-known pollutants. Sperm of externally fertilizing species can get in contact with pollutants found in aquatic environment. Zebrafish sperm, despite its advantages, has seldom been used in in vitro toxicological studies and no reports are available regarding the fertilizing capacity of exposed sperm. Zebrafish sperm was stripped and exposed to concentrations of the tested heavy metals (Zn²⁺, Cd²⁺, Cr³⁺, Cu²⁺, Ni²⁺, Hg²⁺, As³⁺) for 30 or 120 minutes. Calculated half-maximal effective concentration (EC₅₀) values do not differ significantly from those calculated for motility for any of the tested heavy metals, which means fertilization rate can indicate the toxicity of the given substance following exposure of sperm. Thus, its application as in vitro toxicological end point is reasonable. The survival of embryos and embryonic development have not been affected by the exposure of spermatozoa, which means all alterations in spermatozoa caused by heavy metals have been expressed before 24 hours post fertilization.

In vitro toxicology test system based on common carp (Cyprinus carpio) sperm analysis

The effect of heavy metals on the motility parameters of common carp sperm was investigated. In vitro test systems are widespread in ecotoxicology, and fish sperm can be a suitable model. For this reason, studies had been carried out in this topic; however, the published methods are not standard in several aspects (donor species, measured endpoint, etc.). In this study, a previously published toxicology-aimed sperm analysis protocol was tested to examine the effect of heavy metals (arsenic, cadmium, chromium, copper, mercury, nickel, zinc,) on common carp sperm. According to our results, PMOT is the most sensitive of the investigated parameters: dose-response was observed in case of each metal at low concentrations, already after 30 min of exposure. VCL was less sensitive: lower effects were observed at the same concentrations compared to PMOT. Among the examined parameters, LIN was the least affected: a dose-response was observed only in case of arsenic and mercury. The same sensitivity of motility parameters was observed on zebrafish sperm previously. Moreover, we found that PMOT, VCL, and LIN of common carp sperm were affected at the same concentrations as it had been observed in zebrafish, when the identical analytical protocol was applied. The only exception was As³⁺, where common carp sperm proved to be more sensitive: lower concentrations already reduced its motility parameters. Consequently, PMOT of common carp sperm is an accurate and fast bioindicator of aquatic pollution.

Optimisation of sodium and potassium concentrations and pH in the artificial seminal plasma of common carp Cyprinus carpio L

The effect of sodium and potassium concentrations as well as optimal pH on the motility of common carp Cyprinus carpio L. sperm during short-term storage in artificial seminal plasma (ASP) was investigated. Sperm was collected from individual males (n = 5) and each sample diluted tenfold (1:9) in ASP (sperm:extender) containing 2 mM CaCl₂, 1 mM Mg₂SO₄ and 20 mM Tris at pH 8.0 and supplemented by the following concentrations of sodium and potassium (mM/mM): 0/150, 20/130, 40/110, 75/75, 110/40, 130/20 and 150/0. The osmolality of all ASP variants was set at 310 mOsm kg^-1. Sperm motility was measured using a CASA system during 72 h of storage. Immediately after dilution, sperm motility was high (90%) both in each variant and in the control group (fresh sperm). After 72-h storage, the highest sperm motility was noted in ASP containing 110 mM NaCl and 40 mM KCl. No differences were found in the motility of samples preserved within the pH range of 7.0-9.0. Our data suggest that for the short-term storage of common carp sperm, whereas the pH of the solution does not play a crucial role, a specific potassium concentration of around 40 mM is required.

Development of sperm vitrification protocols for two endangered salmonid species: the Adriatic grayling, Thymallus thymallus, and the marble trout, Salmo marmoratus

Vitrification was applied to the sperm of two endangered fish species of Soča River basin in Slovenia, the Adriatic grayling (Thymallus thymallus) and marble trout (Salmo marmoratus) following testing different cooling devices and vitrifying media. Sperm was collected, diluted in species-specific non-activating media containing cryoprotectants, and vitrified by plunging directly into liquid nitrogen without pre-cooling. Progressive motility, curvilinear velocity, and straightness of fresh and vitrified-warmed sperm were evaluated with computer-assisted sperm analysis (CASA). Fertilization trials were carried out to test the effectiveness of vitrification in the case of grayling. A protocol utilizing a glucose-based extender, 30% cryoprotectants (15% methanol + 15% propylene glycol), 1:1 dilution ratio, and droplets of 2 μl on a Cryotop as cooling device yielded the highest post-thaw motility values for both Adriatic grayling (7.5 ± 6.5%) and marble trout (26.6 ± 15.8%). Viable embryos were produced by fertilizing eggs with vitrified grayling sperm (hatching 13.1 ± 11.7%, control hatching 73.9 ± 10.4%). The vitrification protocol developed in this study can be utilized in the conservation efforts for the two species as an alternative to slow-rate freezing when working in field conditions or when specific equipment necessary for slow-rate freezing is not available.

Preservation of common carp germ cells under hypothermic conditions: Whole tissue vs isolated cells

The aim of this study was to optimize the conditions for hypothermic storage of spermatogonial stem cells (SSCs) and oogonial stem cells (OSCs) of common carp Cyprinus carpio. This was conducted by storing gonadal tissue or isolated cells for 24 hr under hypothermic conditions in the first experiment and by testing two different storage media (L-15 or DMEM supplemented with 10% FBS and 25 mM HEPES) and regular medium change (every 4 days) during two weeks of hypothermic storage in the second experiment. During the first 24 hr, isolated cells showed no decrease in viability, while cells obtained from hypothermically stored tissues displayed significantly lower viability after only 6 hr (Tukey's HSD, p < 0.01) indicating that hypothermic storage of isolated cells is superior to storing tissue pieces. The 2-week trial demonstrated that storage media have a profound influence, while regular medium exchange does not have a positive effect on cell viability. Viability of SSCs and OSCs after two weeks was approximately 40% and 25%, respectively; however, survival of ~70% was obtained after 10 days of storage for SSCs and 7 days for OSCs. Hypothermic storage developed in this study has many practical applications during the development of surrogate broodstock technologies for common carp, but also in carp hatcheries and for the conservation of genetic resources of closely related cyprinid species.

Cryopreservation of Zebrafish Spermatogonia by Whole Testes Needle Immersed Ultra-Rapid Cooling

Current trends in science and biotechnology lead to creation of thousands of new lines in model organisms thereby leading to the necessity for new methods for safe storage of genetic resources beyond the common practices of keeping breeding colonies. The main purpose of this study was to adapt the needle immersed vitrification (NIV) procedure to cryopreserve whole zebrafish testes. Cryopreservation of early-stage germ cells by whole testes NIV offers possibilities for the storage of zebrafish genetic resources, especially since after transplantation they can mature into both male and female gametes. Testes were excised, pinned on an acupuncture needle, equilibrated in two cryoprotective media (equilibration solution containing 1.5 M methanol and 1.5 M propylene glycol; and vitrification solution containing 3 M dimethyl sulfoxide and 3 M propylene glycol) and plunged into liquid nitrogen. Samples were warmed in a series of three consequent warming solutions. The main advantages of this technique are (1) the lack of spermatozoa after digestion of warmed testes thus facilitating downstream manipulations; (2) ultra-rapid cooling enabling the optimal exposure of tissues to liquid nitrogen therefore maximizing the cooling and reducing the required concentration of cryoprotectants, thereby reducing their toxicity; (3) synchronous exposure of several testes to cryoprotectants and liquid nitrogen; and (4) repeatability demonstrated by obtaining viability of above 50% in five different zebrafish strains.

Handling and Treatment of Male European Eels (Anguilla anguilla) for Hormonal Maturation and Sperm Cryopreservation

During the last years, several research groups have been working on the development and improvement of new protocols for the European eel handling and maturation. As of yet, weekly injections of human chorionic gonadotropin (hCG) have proved to maturate males after just 5-6 weeks of treatment, producing high volumes of high-quality sperm during several weeks. In addition, sperm cryopreservation protocols using different extenders, cryoprotectants and cooling and thawing times have been previously described for European eel. Here, we show that Tanaka´s extender solution can be directly used for fertilization or for cryopreservation, making unnecessary the usage of different types of solutions and dilutions. Furthermore, the use of methanol as a cryoprotectant makes this protocol easy to use as methanol has low toxicity and does not activate the sperm. The sperm does not need to be cryopreserved immediately after the addition of the cryoprotectant, and it can be used long after being thawed. Moreover, sperm motility is still high after thawing although it is lower than that of fresh sperm. The aim of this work is to show the best available protocol for European eel handling, maturation, and sperm cryopreservation.

Development of sperm vitrification protocols for freshwater fish (Eurasian perch, Perca fluviatilis) and marine fish (European eel, Anguilla anguilla)

Vitrification was successfully applied to the sperm of two fish species, the freshwater Eurasian perch (Perca fluviatilis) and marine European eel (Anguilla anguilla). Sperm was collected, diluted in species-specific non-activating media and cryoprotectants and vitrified by plunging directly into liquid nitrogen without pre-cooling in its vapor. Progressive motility of fresh and vitrified-thawed sperm was evaluated with computer-assisted sperm analysis (CASA). Additional sperm quality parameters such as sperm head morphometry parameters (in case of European eel) and fertilizing capacity (in case of Eurasian perch) were carried out to test the effectiveness of vitrification. The vitrification method for Eurasian perch sperm resulting the highest post-thaw motility (14±1.6%) was as follows: 1:5 dilution ratio, Tanaka extender, 30% cryoprotectant (15% methanol+15% propylene-glycol), cooling device: Cryotop, 2μl droplets, and for European eel sperm: dilution ratio 1:1, with 40% cryoprotectant (20% MeOH and 20% PG), and 10% FBS, cooling device: Cryotop, with 2μl of sperm suspension. Viable embryos were produced by fertilization with vitrified Eurasian perch sperm (neurulation: 2.54±1.67%). According to the ASMA analysis, no significant decrease in head area and perimeter of vitrified European eel spermatozoa were found when compared to fresh spermatozoa.

Cryosurvival of isolated testicular cells and testicular tissue of tench Tinca tinca and goldfish Carassius auratus following slow-rate freezing

Experiments were carried out to test the efficiency of cryopreservation of whole testicular tissue in tench Tinca tinca and goldfish Carassius auratus and compare it to cryopreservation of isolated testicular cells. Additionally, effects of three cryoprotectants (dimethyl sulphoxyde - Me₂SO, methanol - MeOH and ethylene glycol - EG) at three concentrations (1M, 2M and 3M) on post-thaw cell viability were assessed. Tissue pieces/isolated testicular cells were diluted in cryomedia and cryopreserved by slow-rate freezing (1°C/min to -80°C followed by a plunge into the liquid nitrogen). In both species Me₂SO and EG generally yielded higher cryosurvival of early-stage germ cells than MeOH, while spermatozoa of neither species displayed such a pattern. In most cases a 3M>2M>1M viability pattern emerged in both species for both sample types regardless of the cryoprotectant used. Sample type (dissociated testicular cells vs testicular tissue) did not seem to affect viability rates of tench early-stage germ cells and goldfish spermatozoa, while the opposite was observed for tench spermatozoa and goldfish early-stage germ cells. Additionally, through histological analysis we displayed that tissue structure mainly remained unaltered after thawing in goldfish. These results indicate that cryopreservation of whole testicular tissue is indeed a valid alternative method to cryopreservation of dissociated testicular cells. Early-stage germ cells obtained from cryopreserved testis can be further used in different purposes such as transplantation into suitable donors while viable sperm might be used for fertilization when feasible.

Improvement of common carp (Cyprinus carpio) sperm cryopreservation using a programable freezer

The applicability of a programmable freezer for the increased-scale cryopreservation of common carp sperm was investigated. The effect of different equilibration times, cryopreservation methods, extenders, dilution ratios, activating solutions on the post-thaw motility of common carp sperm was investigated. The suitable post-thaw storage time-interval as well as fertilizing capacity of cryopreserved sperm was also examined. The motility, curvilinear velocity (VCL) and straightness (STR) values did not decrease significantly during 60min of equilibration neither in equilibrated nor thawed groups. Motility parameters of thawed sperm were similar using a conventional cryopreservation technique using a polystyrene box [motility (33%), VCL (47μm/s) and STR (88%)] and a programmable freezer: [motility (32%), VCL (54μm/s) and STR (89%)]. The highest motility and VCL was measured with a sugar based extender (grayling extender) at a ratio 1:9 (motility: 52%, VCL: 76μm/s) and 1:20 (motility: 49%, VCL: 76μm/s). The activating solution for cyprinids (ASC) could prolong sperm movement up for 2min. A storage time of six hours following thawing did not have a significant effect on the motility parameters of thawed carp sperm. Agglutination was observed during cryopreservation of an elevated volume of sperm whereas motility 47%, VCL 62μm/s and STR 91% were measured after thawing. Fertilization rate with thawed sperm (32%) was significantly lower compared to the control group (73%). According to our results, the developed method using a programmable freezer is suitable for the cryopreservation of elevated number of straws. However, carp sperm agglutination during freezing may have a negative effect on the fertilizing capacity.