Cryopreservation of ovine primordial follicles using dimethyl sulfoxide

Fertility preservation through gonadal cryopreservation

Fertility preservation is an area of immense interest in today's society. The most effective and established means of fertility preservation is cryopreservation of gametes (sperm and oocytes) and embryos. Gonadal cryopreservation is yet another means for fertility preservation, especially if the gonadal function is threatened by premature menopause, gonadotoxic cancer treatment, surgical castration, or diseases. It can also aid in the preservation of germplasm of animals that die before attaining sexual maturity. This is especially of significance for valuable, rare, and endangered animals whose population is affected by high neonatal/juvenile mortality because of diseases, poor management practices, or inbreeding depression. Establishing genome resource banks to conserve the genetic status of wild animals will provide a critical interface between ex-situ and in-situ conservation strategies. Cryopreservation of gonads effectively lengthens the genetic lifespan of individuals in a breeding program even after their death and contributes towards germplasm conservation of prized animals. Although the studies on domestic animals are quite promising, there are limitations for developing cryopreservation strategies in wild animals. In this review, we discuss different options for gonadal tissue cryopreservation with respect to humans and to laboratory, domestic, and wild animals. This review also covers recent developments in gonadal tissue cryopreservation and transplantation, providing a systematic view and the advances in the field with the possibility for its application in fertility preservation and for the conservation of germplasm in domestic and wild species.

Effects of neutral red assisted viability assessment on the cryotolerance of isolated bovine preantral follicles

PURPOSE

Fertility preservation strategies warrant non-invasive viability assessment of preantral follicles (PAF) such as staining with Neutral Red (NR) that is incorporated by viable follicles. To optimize the procedure, we firstly determined the lowest concentration and shortest exposure time needed for optimal viability screening of isolated bovine PAF. Secondly, we combined this protocol to a vitrification procedure to assess cryotolerance of the stained follicles.

METHODS

Isolated PAF (900, divided over 6 replicates) were cultured in DMEM/Ham's F12 (Culture Medium - Cm) for 4 days (38.5 °C, 5% CO2). On D0, D2 and D4, follicles were stained, by adding NR medium (NRm = Cm with different concentrations NR) after which viability was assessed by counting stained/non-stained PAF every 30 min for a period of 2 h.

RESULTS

Following a binary logistic regression analysis with staining as a result (yes/no) versus log-concentration, a probability model could be fitted, indicating that the proportion of stained follicles remained stable after 30 min when 15 μg/ml NR was used, without compromising follicular health and viability. Consequently, using this protocol, no significant effect of staining prior to vitrification, was found on PAF viability immediately after warming or following 4 days of culture.

CONCLUSIONS

In conclusion, we propose NR staining as a non-invasive, non-detrimental viability assessment tool for PAF, when applied at 15 μg/ml for 30 min, being perfectly compatible with PAF vitrification.

Vitreous cryopreservation of human preantral follicles encapsulated in alginate beads with mini mesh cups

To completely avoid ice crystal formation and thus get a higher survival rate, vitrification methods have been commonly used for cryopreservation of oocytes and embryos. However, currently used vitrification methods for oocytes and embryos are not suitable for the cryopreservation of preantral follicles (PFs). In the present study, stainless steel mesh was fabricated into mini mesh cups to vitrify isolated PFs. Moreover, isolated follicles were encapsulated and then subjected to vitreous cryopreservation to facilitate in vitro culture/maturation of follicles after warming. The results showed that the percentages of viable follicles did not differ significantly between the vitrification group and fresh group soon after warming (81.25% vs. 85.29%, P>0.05) and after a 7-day culture period (77.78% vs. 83.33%, P>0.05). No difference in mean follicular diameter was observed between cryopreserved and fresh follicles when cultured in vitro. Transmission electron microscopic analysis revealed that vitreous cryopreservation could maintain the ultrastructure of follicles in alginate beads. In conclusion, the present vitrification method could efficiently cryopreserve isolated human ovarian follicles encapsulated by calcium alginate, which could be put into immediate use (in vitro culture/ maturation) after warming. However, more follicles and some detailed biochemical analyses are required to further investigate the effects of vitrification on the long-term growth of human encapsulated PFs.

Cryopreservation and in vitro culture of caprine preantral follicles

Preantral follicles (PFs) form a far larger oocyte reservoir (~90% of the follicular population) than antral follicles. Several laboratories have focussed efforts on cryopreservation and in vitro culture (IVC) of PFs to obtain large numbers of fertilisable oocytes. This technology could be used to improve the reproductive potential of economically important animals, including goats, to preserve endangered species and breeds and improve fertility after chemotherapy in young women. Caprine PFs have been successfully cryopreserved using either vitrification or slow freezing. In addition, in vitro embryo production from oocytes enclosed in caprine PFs grown and matured in vitro was also achieved. The present paper selectively reviews the published studies on cryopreservation and IVC of caprine PFs to highlight advances, limitations and prospects.

Goat and sheep ovarian tissue cryopreservation: Effects on the morphology and development of primordial follicles and density of stromal cell

The effect of exposure to cryoprotectant and cryopreservation of goat and sheep ovarian cortical fragments on the morphology of primordial follicles, stromal cell density and follicular development was performed. Goat and sheep ovarian fragments were exposed to 1.0 or 1.5M ethylene glycol (EG) for 5, 10 or 20min, followed or not by conventional cryopreservation. Follicular morphology and stromal cell density were evaluated by means of classical histological analysis. In addition, ovarian fragments were cultured for 1 or 7 days after cryopreservation to evaluate follicular development. Both exposure to cryoprotectant and cryopreservation of goat and sheep ovarian tissue did affect the morphology of primordial follicles and stromal cell density, except when goat ovarian tissue was exposed to EG for 5min. Although exposure time did not influence follicular morphology in both species, increase in the exposure time from 5 to 20min did reduce goat stromal cell density. Increase in EG concentration from 1.0 to 1.5M did result in the decrease of the percentage of goat morphologically normal primordial follicles evaluated after exposure only. In vitro culture of frozen-thawed goat and sheep ovarian tissue showed that exposure to 1.0M, for 10min, before freezing of goat and sheep ovarian tissue does not impair follicular developmental capacity. In addition, stromal cell density may play a role in follicular survival and development after cryopreservation of ovarian tissue.

Advanced methods of isolation and identification of porcine primordial follicles

An effective isolation and identification method of primordial follicles would greatly benefit the animal production practice, transgenic animal production and endangered species conservation in the future. This study has not only advanced the isolation method but also developed an identification marker of primordial follicles. After enzymatic digestion, Percoll gradient centrifugation and mesh filtrations, the obtained follicular separations were then subjected to a cell sorter in order to collect primordial follicles. The study greatly improved the yield of primordial follicles (from about 1.85 x 10(5) to 7.79 x 10(5) per prepubertal ovary) by means of increasing cell layer number from 1 ml to 2.5 ml after Percoll gradient centrifugation. Based on traditional morphological criteria, the purity of recovered primordial follicles was averagely about 82.43+/-9.41% (n=5) because of their similar size and appearance with somatic cells. To further exactly appreciate the purity of sorted primordial follicles, a germ cell-specific protein, MSY2, was used to recognize the oocytes of primordial follicles. The results of repeat experiments showed that about 98.31+/-0.73% (n=4) of the primordial follicles cluster was MSY2-positive, which indicated the identification method of primordial follicles was more effective and high-yielded than the previous methods because of its higher purity.

Effect of cryopreservation on viability, activation and growth of in situ and isolated ovine early-stage follicles

Isolated or cortical tissue-enclosed (in situ) sheep early-stage follicles were exposed to 1.5 M dimethyl sulfoxide (DMSO), ethylene glycol (EG) or unexposed, or frozen/thawed in the presence of these cryoprotectants and then cultured for 5 days in enriched minimal essential medium (MEM) or not cultured. Cultured and uncultured follicles were classified as non-viable/viable when they were stained/not stained with trypan blue, respectively. Follicular diameter was measured and the percentages of primordial and developing follicles calculated. Exposure of isolated or in situ follicles to DMSO or EG led to a marked decrease in the percentage of viable follicles. The percentage of viable isolated and in situ follicles further decreased when they were in vitro-cultured for 5 days, EG-exposed follicles generally showing a more damaging effect than DMSO-exposed follicles. Cultured follicles, both isolated and in situ, which were exposed to EG and DMSO, as well as in situ follicles, which had been frozen/thawed in the presence of one of these cryoprotectants, showed similar growth rates as cultured, untreated follicles, while in these groups significantly lower percentages of primordial follicles and higher percentages of more advanced follicular stages were observed. Among the treated groups, the highest percentage (71-75%) of developing follicles was observed after culturing cryoprotectant-exposed isolated follicles. In contrast, when cryopreserved, isolated follicles were cultured, they did not increase in diameter and did not develop into more advanced stages. In conclusion, exposure to or cryopreservation in the presence of EG and DMSO, as well as their further in vitro culture, negatively affected the viability of ovine isolated and in situ early-stage follicles. In vitro growth of early-stage follicles and activation of primordial follicles were better maintained when follicles had been frozen/thawed and cultured in situ.

Cryopreservation of Sheep Primordial Follicles

The aim of this study was to evaluate the efficiency of 1 M dimethylsulphoxide (DMSO), ethylene glycol (EG), propylene glycol (PROH) and glycerol (GLY) to cryopreserve primordial follicles. The first evaluation was performed soon after cryopreservation and the second evaluation after 4 days of in vitro culture, using the cryoprotectants that allowed the higher results (higher follicular survival rate) after cryopreservation. The results after follicular isolation (control) and cryopreservation using 1 M DMSO, EG, PROH and GLY showed that the mean number (+/- SEM) of live follicles per millilitre was 3204 (100%) +/- 319.27, 2798 (87%) +/- 239.14, 2492 (78%) +/- 345.8, 448 (14%) +/- 46.3 and 208 (7%) +/- 75.26, respectively. Higher follicular survival was reported when DMSO and EG were used. Control follicles and follicles cryopreserved with these two cryoprotectants were cultured and the percentage of follicular survival was 55% (control), 42% (EG) and 34% (DMSO). Similar results were found between control and follicles cryopreserved with EG. In conclusion, 1 M EG is the most effective cryoprotectant to preserve primordial follicles isolated from ovaries of sheep.

Cryopreservation of preantral ovarian follicles in situ from domestic cats (Felis catus) using different cryoprotective agents

The objective of this investigation was to verify the structural characteristics of preantral follicles (PAF) of cat ovarian tissue after cryopreservation in 1.5 M glycerol or ethylene glycol, using a slow-freezing procedure. Ovaries (n = 10) from domestic cats were divided into fragments. One fragment was immediately preserved for classical histology (fresh control), and additional fragments were immersed in minimum essential medium plus 10% bovine fetal serum (MEM+BFS), or MEM+BFS supplemented with 1.5 M glycerol or ethylene glycol. The samples were frozen and plunged into liquid nitrogen. After 1 wk, the samples were thawed. A total of 600 PAF were evaluated. In the fresh control, there were 71.3% normal PAF. After thawing, the rates of normal PAF were 26.0, 39.3 and 58.0% for samples without cryoprotectant or with glycerol or ethylene glycol, respectively. We concluded that ethylene glycol was useful for the cryopreservation of feline PAF in situ.

Preservation of caprine preantral follicle viability after cryopreservation in sucrose and ethylene glycol

Caprine preantral follicles within ovarian fragments were cryopreserved in the absence or presence of 0.5 M sucrose with or without 1 M dimethyl sulfoxide and/or 1 M ethylene glycol (EG). After being thawed, they were washed in minimum essential medium with or without 0.3 M sucrose. Histological analysis of follicle integrity immediately after cryopreservation showed consistent beneficial effects of including sucrose in the three cryoprotectant solutions analyzed when tissue was thawed without sucrose (53.9+/-14.8-82.4+/-3.2% normal vs 27.6+/-1.6-36.6+/-6.5%, P<0.05). However, in further studies, the addition of sucrose to the thaw solutions proved detrimental or of no benefit. An analysis of the cryopreserved material with calcein-AM and ethidium homodimer (markers for living and dead cells, respectively) gave comparable results to those obtained by histology. Follicles cryopreserved in EG, EG plus sucrose, or sucrose alone were cultured in vitro for 24 h following warming. During this culture period, viability fell most rapidly in material cryopreserved in sucrose alone and was no longer correlated with either the viability or integrity estimates made immediately after warming. By contrast, the viability of follicles cryopreserved in EG with sucrose and then cultured for 24 h was not significantly different from the cultured non-frozen controls. These results indicate that cryopreservation in 1 M EG plus 0.5 M sucrose combined with thawing without sucrose is effective for caprine ovarian tissue.

Update on reproductive biotechnologies in small ruminants and camelids

Recent advances in reproductive biotechnologies in small ruminants include improvement of methods for in vitro production of embryos and attempts at spermatogonial stem cell transplantation. In vitro production of embryos by IVM/IVF, intra-cytoplasmic sperm injection (ICSI), or nuclear transfer (NT) has been made possible by improvements in oocyte collection and maturation techniques, and early embryo culture systems. However, in vitro embryo production still is not very efficient due to several limiting factors affecting the outcome of each step of the process. This paper discusses factors affecting in vitro embryo production in small ruminants and camelids, as well as preliminary results with the technique of spermatogonial stem cell transplantation.

Cryopreservation of isolated ovine primordial follicles with propylene glycol and glycerol

OBJECTIVE

To verify the viability of isolated primordial follicles to different propylene glycol (PROH) and glycerol (GLY) concentrations before and after cryopreservation.

DESIGN

Isolated primordial follicles were stained with trypan blue to evaluate the effect of different PROH and GLY concentrations before and after cryopreservation.

SETTING

Laboratorio Renzo Giuliani, University of Florence, Italy.

PATIENT(S)

Thirty- to forty-day-old lambs.

INTERVENTION(S)

: Isolation of primordial follicles with subsequent exposure to cryoprotectant and freezing.

MAIN OUTCOME MEASURE(S)

Histologic structure and follicular mortality.

RESULT(S)

After the isolation procedure (control), the mean number of live primordial follicles/mL was 2,688 and 4,452 in the GLY and PROH groups, respectively. When GLY was used, the number of live follicles before cryopreservation was 820, 756, 640, 524, 564, and 460 follicles/mL with concentrations of 0, 0.5, 1.0, 1.5, 2.0, and 2.5 mol/L, respectively. After cryopreservation, this number decreased to 0, 12, 36, 100, 84, and 68 follicles/mL, respectively, with the same concentrations. When PROH was used, the number of live follicles before cryopreservation was 4,216, 3,880, 3,560, 1,812, 704, and 568 follicles/mL with concentrations of 0, 0.5, 1.0, 1.5, 2.0, and 2.5 mol/L, respectively. After cryopreservation, this number decreased to 0, 116, 336, 472, 360, and 244 follicles/mL, respectively, with the same concentrations.

CONCLUSION(S)

Both cryoprotectants were shown to preserve isolated primordial follicles after cryopreservation.

Isolated ovine primordial follicles cryopreserved in different concentrations of ethylene glycol

Cryopreservation of primordial follicles represents an opportunity to preserve female gametes, and consequently to protect the reproductive capacity of humans and animals, as well as to safeguard genetic material from endangered animal species or rare breeds. The aim of this work was to assess the toxicity of different concentrations of ethylene glycol (EG) to primordial follicles, and verify the viability of these follicles after the freezing-thawing procedure. Primordial follicles were isolated from ovine ovaries and exposed to different EG concentrations to evaluate the cryoprotectant (CPA) toxicity before and after cryopreservation. After isolation of primordial follicle (control), the number (mean+/-S.E.M.) of viable primordial follicles/ml was 3764+/-795.21. The number of viable follicles in the toxicity test using EG at 0, 0.5, 1.0, 1.5, 2.0, and 2.5 M was 1096+/-447.9, 960+/-446.67, 948+/-366.14, 832+/-313.59, 856+/-280.67, and 700+/-255.02, respectively. The number of viable follicles at concentrations of 2.5 M was less than for controls. After cryopreservation, the numbers decreased to 0+/-0, 148+/-85.46, 764+/-246.69, 824+/-291.9, 844+/-296.27, and 588+/-200.65, respectively for 0, 0.5, 1.0, 1.5, 2.0, and 2.5 M EG. The number of viable follicles at 0, 0.5, and 2.5 M was less than for controls. In conclusion, after the freezing and thawing procedure, concentrations of 1.0, 1.5, and 2.0 M EG can be successfully used for the cryopreservation of isolated follicles in sheep.