The use of fetal bovine serum for cryopreservation of stage III zebrafish (Danio rerio) ovarian follicles.

Although several studies on fish ovarian follicles cryopreservation have been carried out, their cryopreservation still remains unsuccessful. In this study, for the first time the effect of Fetal Bovine Serum (FBS) in combination with several concentrations of methanol (1, 2 or 4M) as cryoprotectant on stage III ovarian follicles viability was investigated and cryopreservation using controlled slow cooling was undertaken. Membrane integrity assessed by trypan blue (TB) staining and ATP level of stage III ovarian follicles were evaluated following cryoprotectant treatment and cryopreservation. The results showed that the survival of ovarian follicles after cryopreservation, assessed by TB staining, was significantly decreased using all three concentrations of CPAs solutions when compared to room temperature controls. The results showed a further decrease of viability when the TB test was performed 2h post-thaw. 2M methanol+10 percent FBS resulted in higher survival rate compared with the other methanol concentrations in combination with 10 percent FBS, viability results obtained 10 min and 2 hours post-thaw were 70.2 +/- 4.2 percent and 46.2 +/- 10.4 percent respectively. Whilst results obtained with 1M methanol supplemented with 10 percent FBS 10min and 2 hours post-thaw were 51.8 +/- 2.4 and 3.4 +/- 1.5 and those obtained with 4M supplemented with 10 percent FBS were 36.0 +/- 5.3 and 6.4 +/- P content was observed with all three cryoprotectant solutions indicating a compromised metabolic status. The addition of 10 percent fetal bovine serum to media used for cryopreservation zebrafish ovarian follicles did not allow the achievement of successful cryopreservation.

Effect of methanol on mitochondrial organization in zebrafish (Danio rerio) ovarian follicles.

Successful cryopreservation is usually measured in terms of cell survival. However, there may also be more subtle effects within cells that survive. Previous studies on zebrafish have produced evidence of mitochondrial DNA (mtDNA) damage in cryopreserved embryonic blastomeres and, after exposure to cryoprotectants, alterations in mtDNA replication in embryos and decreased mitochondrial membrane potential, mtDNA and ATP production in ovarian follicles. This study shows that the decreased ATP levels previously observed in stage III zebrafish ovarian follicles exposed to ≥3 M methanol persisted in those follicles that subsequently developed to stage IV. However, the decreased mtDNA levels were restored in those follicles. In order to determine whether mitochondrial distribution and/or their transport network was affected by the methanol exposure, immunocytochemistry analysis of tubulin and mitochondrial cytochrome c oxidase I (COX-I) was performed, along with phalloidin staining of polymerized actin. Neat arrangements of all proteins were observed in control follicles, with COX-I and tubulin being colocalized near granulosa cell nuclei, while actin formed hexagonal and/or polygonal structures nearer granulosa cell membranes and projected into the oocyte surface. Exposure to methanol (2 to 4 M) disrupted the COX-I and tubulin arrangements and the hexagonal and/or polygonal actin distribution and actin projections into the oocyte. These effects were still observed in those follicles that developed to stage IV, although the severity was reduced. In summary, the disruption to function and distribution of mitochondria in ovarian follicles exposed to >2 M methanol may be mediated via disruption of the mitochondrial transport system. Some recovery of this disruption may take place after methanol removal and subsequent follicle maturation.

Cytoskeleton proteins F-actin and tubulin distribution and interaction with mitochondria in the granulosa cells surrounding stage III zebrafish (Danio rerio) oocytes.

The distributional arrangement of mitochondria in the granulosa cells surrounding stage III zebrafish oocyte has been reported as a contiguous aggregation of mitochondria at the margin of the each granulosa cell. The aim of the present study was to further investigate the mitochondrial distribution in the granulosa cell layer in stage III ovarian follicles and the interaction between mitochondria and cytoskeleton elements actin and tubulin. To determine mitochondrial distribution/transport, immunocytochemistry analysis of tubulin and mitochondrial COX-I was carried out along with phalloidin staining of polymerised F-actin. The follicles were also exposed to a range of conditions that are known to affect mitochondria and the cytoskeleton proteins actin and tubulin. The mitochondrial inhibitor FCCP, the anti-mitotic drug nocodazole, and actin polymerisation inhibitor cytochalasin B were used. Levels of ATP, mtDNA copy number, and viability assessed by Trypan blue were also studied after exposure to inhibitors in order to determine the relationship between mitochondrial distribution/activity and ATP production. F-actin showed a hexagonal-polygonal distribution surrounding the mitochondria in granulosa cells, with the F-actin network adjacent to the plasma membrane of each granulosa cell. Tubulin structure presented a less organised distribution than F-actin, it was sparse in the cytosol. Interaction between mitochondria and tubulin was found indicating that mitochondria and tubulin are colocalised in zebrafish ovarian follicles. The exposure of ovarian follicles to inhibitors induced the loss of mitochondrial structural integrity showing that mitochondria distribution in granulosa cells of stage III zebrafish ovarian follicles is determined by the microtubules network.

Studies on cryoprotectant toxicity to zebrafish (Danio rerio) ovarian tissue fragments.

Cryopreservation of ovarian tissue is a viable alternative to cryopreservation of oocytes and embryos in many species but it has not been studied in fish. Selection of cryoprotectant is an important step in designing cryopreservation protocols. In order to identify the optimum cryoprotectant (CPA) in a suitable concentration for zebrafish ovarian tissue cryopreservation, studies on toxicities of five commonly used cryoprotectants methanol, ethanol, dimethyl sulfoxide (DMSO), ethylene glycol (EG) and propylene glycol (PG) were carried out. Experiments were conducted on ovarian tissue fragments consisting of stage I and stage II ovarian follicles. Ovarian tissue fragments were incubated in 90% L-15 medium (pH 9) containing 1-4M cryoprotectants for 30min at 22°C. Three different tests were used to assess ovarian tissue fragment viability: trypan blue (TB) staining, fluorescein diacetate (FDA) combined with propidium iodide (PI) staining and adenosine 5´- triphosphate (ATP) assay. Results from these tests showed that ATP assay was more sensitive than FDA+PI or TB staining for assessing cryoprotectant toxicity to follicles in tissue fragments. Methanol and ethanol were the least toxic cryoprotectants tested. Cryoprotectant toxicity increased in the order of methanol/ethanol, DMSO, PG and EG. Ethanol was used for zebrafish ovarian tissue for the first time and the results showed that the effect of methanol and ethanol on ovarian tissue fragments were comparable. As methanol has been shown to be the most effective cryoprotectant for zebrafish ovarian follicles in our laboratory, the use of ethanol will also be considered in assisting future freezing protocol design. The present study also showed that stage II ovarian follicles are more sensitive to cryoprotectant treatment than stage I follicles in tissue fragments. The results obtained in this study provided useful information for ovarian tissue fragment cryopreservation protocol design in the future.

Effect of methanol and Me2SO exposure on mitochondrial activity and distribution in stage III ovarian follicles of zebrafish (Danio rerio).

In this study the effect of cryoprotectants that have been shown to be the least toxic to zebrafish ovarian follicles (methanol and Me(2)SO), on mitochondria of stage III ovarian follicles was evaluated. The mitochondrial distributional arrangement, mitochondrial membrane potential, mtDNA copy number, ATP levels and ADP/ATP ratios were assessed following exposure to cryoprotectants for 30 min at room temperature. Results obtained by confocal microscopy showed that 30 min exposure to 2M methanol induced a loss of membrane potential, although viability tests showed no decrease in survival even after 5h post-exposure incubation. Higher concentrations of methanol (3 and 4M) induced not only a decrease in mitochondrial membrane potential but also the loss of mitochondrial distributional arrangement, which suggested a compromised mitochondrial function. Furthermore 3 and 4M treatments resulted in a decrease in viability assessed by Fluorescein diacetate-Propidium iodide (FDA-PI) and in a decrease in mtDNA copy number and ADP/ATP ratio after 5h incubation following methanol exposure, indicating a delayed effect. The use of Me(2)SO, which is considered to be a more toxic CPA to zebrafish ovarian follicles than methanol, caused a decrease in viability and a sustained decrease in ATP levels accompanied by failure to maintain mtDNA copy number within 1h post-exposure incubation. These results indicated that even CPAs that are considered to have no toxicity as determined by Trypan blue (TB) and FDA-PI tests can have a deleterious effect on mitochondrial activity, potentially compromising oocyte growth and embryo development.

Distributional arrangement of mitochondria in the granulosa cells surrounding stage III zebrafish (Danio rerio) oocytes.

Mitochondria play a vital role during oocyte maturation, fertilization, and embryo development. In this study, confocal microscopy with the mitochondrial membrane potential-sensitive dye JC-1 (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolyl-carbocyanine iodide) was used to investigate mitochondria distribution and activity of stage III zebrafish ovarian follicles. To support the mitochondrial origin of the fluorescence obtained by JC-1, a second mitochondrial probe, MitoTracker Green FM, was used. Cryo-scanning and transmission electron microscopy were also used to validate the distribution and localization of mitochondria obtained by mitochondrial staining. The mitochondrial probes were unable to penetrate the oocyte, and as a result it was not possible to observe stained mitochondria in the oocyte cytoplasm. However, mitochondrial staining of the granulosa cell layer surrounding the stage III zebrafish oocyte exhibited a contiguous aggregation pattern of mitochondria. Cryo-scanning electron microscopy studies also showed the oocyte surface to be covered by polygonal patterns of ridges of the same dimensions as the distributional arrangement of mitochondria in the granulosa cells. Though the results suggested the need for defolliculation to assess mitochondrial distribution and activity in the stage III zebrafish oocyte cytoplasm, the findings of this study will contribute to our understanding of oogenesis and folliculogenesis processes in fish.

Evaluation of zebrafish (Danio rerio) ovarian follicle viability by simultaneous staining with fluorescein diacetate and propidium iodide.

Reliable fish oocyte quality assessment methods are essential in developing protocols for cryopreservation as well as their in vitro maturation and fertilisation. Current ovarian follicle viability assessment methods either lack sensitivity (e.g. Trypan Blue staining-TB) or are stage dependent (e.g. in vitro maturation and observation of germinal vesicle breakdown-GVBD). The aim of the present study was to develop a new viability assessment method for zebrafish ovarian follicles that is reliable, sensitive and not-stage specific. Fluorescein Diacetate (FDA) and Propidium Iodide (PI) were used for the first time to assess viability of zebrafish ovarian follicles. After preliminary studies to evaluate the efficacy of FDA and PI, a combination of these two fluorochromes was subsequently chosen and compared with TB staining and GVBD test in a series of cryoprotectant toxicity studies and following cryopreservation using stage III ovarian follicles. In all cases the FDA-PI test proved to be more sensitive than TB staining but less sensitive than the GVBD test. Ovarian follicle survivals after 4M Methanol treatment for 30 min at 22 degree C were 67.4 +/- 4.4%, 43.9 +/- 3.8% and 19.6 +/- 1.9% using TB, FDA-PI and GVBD test respectively. Survivals after cryopreservation procedure were 38.9 +/- 4.0% and 28.9 +/- 2.5% using TB and FDA-PI respectively when Hank's solution was used as medium and 45.2 +/- 4.3% and 35.2 +/- 3.5% when KCl buffer was used. The results showed the method to be promising, and it may offer a new approach for viability assessment of fish ovarian follicles.