Cryopreservation of caprine ovarian tissue using glycerol and ethylene glycol

Effects of erythropoietin on ischaemia-reperfusion when administered before and after ovarian tissue transplantation in mice

RESEARCH QUESTION

Is the optimal timing for administering erythropoietin to minimize ischaemic injury in ovarian tissue transplantation before ovary removal for cryopreservation and subsequent transplantation or after transplantation?

DESIGN

Thirty Swiss mice (nu/nu) were divided into three groups: treatment control group (n = 10); erythropoietin before harvesting group (EPO-BH) (n = 10) and erythropoietin after transplantation group (EPO-AT) (n = 10). Animals underwent bilateral ovariohysterectomy and their hemiovaries were cryopreserved by slow freezing. At the same time, previously cryopreserved hemiovaries were transplanted subcutaneously in the dorsal region. Erythropoietin (250 IU/kg) and sterile 0.9% saline solution were administered every 12/12 h over 5 consecutive days in the EPO-AT and EPO-BH groups, respectively.

RESULTS

Administration of erythropoietin in the EPO-AT group improved the viability of ovarian follicles, reducing degeneration and increasing the number of morphologically normal growing follicles at 14 days after transplantation compared with the EPO-BH group (P = 0.002). This group also showed higher percentages of proliferative follicles at 7 days after transplantation (P ≤ 0.03), increased blood vessel count (P ≤ 0.03) and greater tissue area occupied by blood vessels at days 7 and 14 after transplantation (P ≤ 0.03), compared with hormone administration before cryopreservation (EPO-BH group) and the treatment control group. Additionally, treatment with erythropoietin before or after transplantation reduced fibrotic areas at 7 days after transplantation (P = 0.004).

CONCLUSION

Erythropoietin treatment after transplantation reduced ischaemic damage in transplanted ovarian tissue, increased angiogenesis, maintenance of ovarian follicle proliferation and reduced fibrosis areas in the grafted tissue.

Quantification of residual cryoprotectants and cytotoxicity in thawed bovine ovarian tissues after slow freezing or vitrification

STUDY QUESTION

How much residual cryoprotectant remains in thawed/warmed ovarian tissues after slow freezing or vitrification?

SUMMARY ANSWER

After thawing/warming, at least 60 min of diffusion washing in media was necessary to significantly reduce the residual cryoprotectants in ovarian tissues frozen by slow freezing or vitrification.

WHAT IS KNOWN ALREADY

Ovarian tissue cryopreservation (OTC) by slow freezing has been the conventional method; while the vitrification method has gained popularity for its practicality. The main concern about vitrification is how much potentially toxic residual cryoprotectant remains in the warmed tissues at the time of transplantation.

STUDY DESIGN, SIZE, DURATION

This was an animal study using the ovarian tissues from 20 bovine ovaries. The duration of this study was from 2018 to 2020.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Ovarian cortex tissues were prepared from 20 bovine ovaries and assigned randomly to groups of fresh (non-frozen) control, slow freezing with 1.5 M dimethyl sulfoxide (DMSO), 1.5 M 1,2-propanediol (PROH) and vitrification with 35% ethylene glycol (EG). The residual cryoprotectant concentrations in thawed/warmed tissues were measured by gas chromatography at the following time points: frozen (before thawing/warming), 0 min (immediately after thawing/warming), 30, 60 and 120 min after diffusion washing in media. Next, the ultrastructural changes of primordial follicles, granulosa cells, organelles and stromal cells in the ovarian tissues (1 mm × 1 mm × 1 mm) were examined in fresh (non-frozen) control, slow freezing with DMSO or PROH and vitrification with EG groups. Real-time quantitative PCR was carried out to examine the expressions of poly (ADP-ribose) polymerase-1 (PARP1), a DNA damage sensor and caspase-3 (CASP3), an apoptosis precursor, in thawed/warmed ovarian tissues that were washed for either 0 or 120 min and subsequently in tissues that were ex vivo cultured for 24 or 48 h. The same set of tissues were also used to analyze the protein expressions of gamma H2A histone family member X (γH2AX) for DNA double-strand breaks and activated caspase-3 (AC3) for apoptosis by immunohistochemistry.

MAIN RESULTS AND THE ROLE OF CHANCE

The residual cryoprotectant concentrations decreased with the extension of diffusion washing time. After 60 min washing, the differences of residual cryoprotectant between DMSO, PROH and EG were negligible (P > 0.05). This washing did not affect the tissue integrity or significantly elevate the percentage of AC3 and γH2AX positive cells, indicating that tissues are safe and of good quality for transplantation.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

Since the study was performed with ovarian tissues from bovines, generalizability to humans may be limited. Potential changes in ovarian tissue beyond 120 min were not investigated.

WIDER IMPLICATIONS OF THE FINDINGS

This study addresses concerns about the cytotoxicity of EG in warmed ovarian tissues and could provide insights when devising a standard vitrification protocol for OTC.

STUDY FUNDING/COMPETING INTEREST(S)

The study was funded by a Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science to N.S.

Metabolic activity in cryopreserved and grafted ovarian tissue using high-resolution respirometry

Cryopreservation of ovarian tissue followed by transplantation represents a strategy to restore ovarian function and fertility. Stress from cryopreservation-thawing processes can lead to alterations and/or damage to mitochondrial structure and functionality. High resolution respirometry and histological analysis were used to evaluate the effect of cryopreservation and transplantation on ovarian tissue. Four different conditions were performed: Fresh non-transplanted tissue, Fresh transplanted tissue, Cryopreserved non-transplanted tissue and Cryopreserved transplanted tissue. All groups were able to respond to the substrates-uncoupler-inhibitor protocol. We found a dramatic decrease in general oxygen consumption in hemi-ovaries submitted to cryopreservation and/or transplantation. The effect of cryopreservation on mitochondrial metabolism was less intense than effect of transplantation, since the transplantation affected all of the mitochondrial states. A total of 2644 follicles were analyzed. Of these, 2198 were classified as morphologically normal. The percentage of morphologically normal follicles was significantly lower in the Cryopreserved transplanted group when compared to the Cryopreserved non-transplanted group and the Fresh transplanted group (p-value < 0.05). Despite decreased follicular viability and mitochondrial activity, the cryopreservation followed by transplantation of ovarian tissue proved feasible for attempts to restore ovarian function.

Cat ovarian follicle ultrastructure after cryopreservation with ethylene glycol and dimethyl sulfoxide

Ovarian tissue cryopreservation is a promising technique for fertility maintenance. The aim of this study was to compare the morphology of domestic cat ovarian follicles after tissue cryopreservation with ethylene glycol (EG) and dimethyl sulfoxide (Me₂SO). Ovaries from healthy adult cats undergoing elective ovariohysterectomy were used. Eight fragments were obtained from each pair of ovaries: two were used as fresh controls; three were submitted to fresh perfusion toxicity test and perfused with M199, 10% fetal calf serum and 0.4% sucrose containing Me₂SO 1.5 M, EG 1.5 M or Me₂SO 0.75 M + EG 0.75 M; and the remaining three fragments were perfused as described and submitted to slow freezing. After 45 days of cryopreservation, the samples were thawed, fixed and processed for light and transmission electron microscopy (TEM). The percentages of morphologically normal follicles identified by light microscopy were higher in the control group (94.45%) in comparison to the frozen groups (80.56% with EG, 78.7% with Me₂SO and 75.87% with EG + Me₂SO). The fresh perfused tissue showed no statistical difference compared to control or frozen samples. The TEM analysis showed less damage in the ultrastructure of follicles from the Me₂SO group in comparison with the EG and Me₂SO + EG groups. According to the morphological analysis, 1.5 M Me₂SO is the best cryoprotectant for cryopreservation of domestic cat ovarian tissue regarding the morphology of preantral follicles after thawing. Further studies regarding the viability of these follicles should be performed.

Vitrification of bovine embryos followed by in vitro hatching and expansion

The objective of this study was to assess the effects of bovine embryo vitrification by applying three different vitrification solutions containing ethylene glycol (EG) and dimethylsulphoxide (DMSO) at different concentrations (10, 20 or 25% each) combined with 1.0 M glucose or 1.0 M sucrose, on the in vitro hatching and expansion rates. Healthy oocytes were selected for in vitro maturation and fertilization from 200 bovine ovaries, and subsequently cultured up to the blastocyst stage (n = 800). Control (n = 200) and vitrified cells (n = 100 per treatment; 600 in total) were cultured for an extra 24 or 48 h to evaluate hatching and expansion, respectively. Vitrification significantly decreased embryonic re-expansion and hatching rates independently of the tested solution when compared with control embryos, but solutions with 25% EG + 25% DMSO resulted in the highest re-expansion (75%) and hatching (70%) rates, independently of the added sugar. The addition of sucrose resulted in higher rates of re-expanded and hatched embryos when compared with glucose addition. We concluded that the combination of 25% EG + 25% DMSO and 1.0 M sucrose allowed hatching and expansion of vitrified-warmed bovine embryos produced in vitro.

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.

Restoring fertility after ovarian tissue cryopreservation: a half century of research

Tissue transplantation and in vitro ovarian follicle culture have been investigated as alternative techniques to restore fertility in young women who are facing fertility-threatening diseases or treatments following ovarian tissue cryopreservation. Although transplants of fresh or frozen ovarian tissue have successfully yielded healthy live births in different species including humans, the risks of reintroducing cancer cells back into the patient, post treatment, have limited its clinical purpose. The in vitro ovarian follicle culture minimizes these risks and provides a way to harvest more mature oocytes, however its clinical translation has yet to be determined. Not only is it possible for tissue cryopreservation to safeguard fertility in cancer patients, this technique also allows the maintenance of germplasm banks for animals of high commercial value or for those animals that are at risk of extinction. Given the importance of managing female genetic material, this paper reviews the progress of the methods used to preserve and restore female fertility in different species to demonstrate the results obtained in the past 50 years of research, the current achievements and the future directions on this field.

Cryopreservation and fertility: current and prospective possibilities for female cancer patients

With the evolution of the treatment of malignant neoplasms, the survival rates of patients undergoing chemo- or radiotherapy are increasing. The continuous development of techniques of assisted human reproduction has led to important strategies in an attempt to maintain reproductive function in patients subjected to treatment of neoplastic diseases, among them cryopreservation of embryos, gametes, and ovarian cortical tissue. The freezing of ovarian tissue is currently being proposed with the primary purpose of preserving ovarian function in these patients. Currently, the major challenge of groups working with preservation of fertility is the use of cryopreserved ovarian tissue after disease remission. The main alternatives presented today are the implantation of hetero- or orthotopic tissue and isolation of immature follicles from ovarian tissue followed by in vitro maturation and assisted reproduction procedures.

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.

Preservation of fertility in young cancer patients: contribution of transmission electron microscopy

During the last decade, new technologies in reproductive medicine have emerged to preserve the fertility of women whose gonadal function is threatened by premature menopause or gonadotoxic treatments. To offer an individualized approach to these patients, different experimental procedures are under investigation, including oocyte cryopreservation and cryopreservation and transplantation of ovarian tissue in the form of cortical fragments, whole ovary or isolated follicles. This review shows that transmission electron microscopy (TEM), combined with other in-vivo and in-vitro analysis techniques, is a valuable tool in the establishment of new experimental protocols to preserve female fertility. Ultrastructural studies allow in-depth evaluation of the oocyte's unique morpho-functional characteristics, which explain its low cryotolerance, and provide essential information on follicular, stromal and endothelial cell integrity, as well as cellular interactions crucial for normal folliculogenesis. In order to be able to offer appropriate and efficient options in every clinical situation, oocyte in-vitro maturation and ovarian tissue transplantation need to be optimized. Further development of new approaches, such as follicular isolation and whole ovary transplantation, should be encouraged. Fine ultrastructural details highlighted by TEM studies will be useful for the further optimization of these emerging technologies.

Osmotic tolerance and freezability of isolated caprine early-staged follicles

Isolated caprine early-staged follicles were submitted to osmotic tolerance tests in the presence of sucrose, ethylene glycol (EG), or NaCl solutions and were exposed to and cryopreserved (by slow or rapid cooling) in MEM alone or MEM supplemented with sucrose, EG (1.0 or 4.0 M), or both. When follicles were exposed to 1.5 M NaCl, only 2% of the follicles were viable, whereas 87% of the follicles were viable after exposure to 4.0 M EG. Regarding exposure time, the highest percentage of viable follicles was obtained when follicles were exposed for 10 min to 1.0 M EG + 0.5 M sucrose; exposure for 60 s to 4.0 M EG + 0.5 M sucrose also maintained high percentage viability in follicles. Slow cooling in the presence of 1.0 M EG + 0.5 M sucrose (75%) or rapid cooling in the presence of 4.0 M EG + 0.5 M sucrose (71%) resulted in a significantly higher proportion of viable follicles than all other treatments (P < 0.05). A 24-h culture of frozen-thawed follicles was used to assess survival; only slow-frozen follicles showed viability rates similar to control follicles (64% vs. 69% respectively; P > 0.05). Interestingly, the percentage of viable rapid-cooled follicles (59%) was similar to that obtained after in vitro culture of conventional slow-cooled follicles but was significantly lower than that in controls. Thus, in addition to determining improved procedures for the exposure of follicles to EG and sucrose before and after freezing of caprine early-staged follicles, we report the development of rapid- and slow-cooling protocols.

Preservation of bovine preantral follicle viability and ultra-structure after cooling and freezing of ovarian tissue

Bovine preantral follicles within ovarian fragments were exposed and cryopreserved in absence or presence of 1.5M glycerol (GLY), ethylene glycol (EG), propanediol (PROH) or dimethyl sulfoxide (DMSO), undergoing a previous cooling at 20 degrees C for 1h (protocol 1) or at 4 degrees C for 24h (protocol 2) in 0.9% saline solution. At the end of each treatment, preantral follicles were classified as non-viable/viable when they were stained/not stained with trypan blue, respectively. To confirm viability staining, ultra-structure of the follicles was evaluated by transmission electronic microscopy (TEM). Data were compared by Chi-square test (P<0.05). The storage of the ovaries at 20 degrees C for 1h (78%) and 4 degrees C for 24h (80%) did not reduce significantly the percentage of viable preantral follicles when compared to the control (75%). Similar results were obtained when ovarian fragments, respectively, for protocols 1 and 2, were exposed to MEM (78 and 77%), 1.5M EG (78 and 71%), as well as frozen in 1.5M EG (74 and 77%). Percentages of viable follicles in control were similar to those observed after exposure (75%) and freezing (76%) in presence of 1.5M DMSO only when protocol 1 was used. The increase of the concentration from 1.5 to 3.0M, for all cryoprotectants, reduced significantly the percentage of viable preantral follicles after freezing. Ultra-structural analysis has confirmed trypan blue results, showing that not only basement membrane, but also organelles, were intact in viable preantral follicles. In conclusion, ovarian tissue cooling at 4 degrees C for 24h before cryopreservation (protocol 2) does not affect the viability of bovine preantral follicles when 1.5M EG is present in the cryopreservation medium.

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.

Survival rate of preantral follicles derived from vitrified neonate mouse ovarian tissue by Cryotop and conventional methods

The aim of this study was to investigate the growth and survival rate of preantral follicles isolated from vitrified ovarian tissue by Cryotop and conventional methods. The ovaries of 14-day-old mice were separated and divided into four groups as following: Cryotop group, vitrified by Cryotop; CV (Conventional; CV) group, vitrified by conventional straw; toxicity test group and control group. After warming the vitrified ovaries, isolated preantral follicles from four groups were cultured for 4 days to compare survival rate and follicular growth between above-mentioned groups. Survival rate (97.3%) in toxicity test group alike the control group (98.7%) were significantly higher (P<0.05) than the Cryotop (92.7%) and CV (47.7%) groups. Increase in follicle diameters after 4 days in Cryotop and CV groups was significantly lower (P<0.05) than the control and toxicity test groups, but growth and survival rate of follicles in Cryotop group was significantly higher (P<0.05) than the CV group. These results demonstrated that ovarian tissue vitrification by Cryotop highly preserves the viability rate of preantral follicles.

Comparing ethylene glycol with glycerol for cryopreservation of canine semen in egg-yolk TRIS extenders

The objective of this work was to evaluate the possibility of substituting glycerol (G) for ethylene glycol (EG) when cryopreserving dog semen. A total of 15 ejaculates from 13 dogs was pooled into five samples and frozen in egg-yolk Tris extenders with variable ethylene glycol and glycerol concentrations, with or without Equex STM Paste. Two widely used glycerol extenders (Uppsala Equex II and Norwegian) were utilized as controls. Semen quality parameters assessed after thawing were total subjective motility (TSM), computer assisted sperm analysis (CASA), eosin-nigrosin staining, and flow cytometry (FC) after staining with the PI/Fitc-PSA (fluorescein isotiocianate conjugated with the agglutinin of Pisum sativum, PSA) fluorochromes. No advantages were seen in using EG to replace G when freezing dog semen or combining EG and G in the freezing medium. The Uppsala Equex II provided the best overall post-thaw parameters, followed by the egg-yolk Tris experimental extender with 5% EG and Equex STM Paste. The extender with 4% EG produced similar results to the Norwegian extender. High correlations (r>0.98) were obtained between eosin-nigrosin staining and FC, as well as between subjective and computerized motility assessment (r>0.90).

Vitrification of goat preantral follicles enclosed in ovarian tissue by using conventional and solid-surface vitrification methods

Caprine preantral follicles within ovarian fragments were exposed to or vitrified in the presence of sucrose, dimethyl sulfoxide (DMSO), ethylene glycol (EG), or various combinations thereof. The fragments were cryopreserved by using either a conventional (CV) or a solid-surface vitrification (SSV) protocol, and the cryoprotectants were removed by equilibrating vitrified ovarian fragments in "warming solution" consisting of minimum essential medium and heat-inactivated fetal calf serum (MEM(+)) followed by washes in MEM(+) with or without sucrose. Histological analysis of follicle integrity showed that the percentages of normal follicles in ovarian fragments vitrified in sucrose mixed with EG and/or DMSO (CV method) or mixed with EG or DMSO (SSV method) followed by washes in MEM(+) plus sucrose were similar to those of controls (ovarian fragments fixed without previous vitrification). Unlike for MEM(+) (supplemented or unsupplemented by sucrose) and DMSO followed by washes in the absence of sucrose, the percentages of normal follicles found after exposure to cryoprotectant did not significantly differ from that found after vitrification, indicating that follicular degeneration was attributable to a toxic effect of cryoprotectants and not to the vitrification procedure. The viability of preantral follicles after the CV and SSV procedures was investigated by using calcein-AM and the ethidium-homodimer as "live" and "dead" markers, respectively. In both tested vitrification procedures, the highest percentages of viable follicles were observed when a mixture of sucrose and EG (70.3% for CV and 72.4% for SSV) was used. Preantral follicles were also vitrified (either by CV or SSV) in sucrose and EG and then cultured for 24 h, after which their viability was compared with that of cultured fresh and uncultured vitrified follicles. The viability of these follicles was maintained after SSV, but not after CV. Thus, the viability of caprine preantral follicles can be best preserved after SSV in a mixture of sucrose and EG, followed by washes in medium containing sucrose.

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.

Histological and ultrastructural analysis of cryopreserved sheep preantral follicles

The aim of this study was to verify the histological and ultrastructural characteristics of sheep preantral follicles after exposure of ovarian tissue to cryopreservation in glycerol (GLY), ethylene glycol (EG), propanediol (PROH) or dimethyl sulfoxide (DMSO) in order to determine the optimum method to store sheep ovarian tissue for later experimental or clinical use. Each ovarian pair from five mixed-breed ewes was divided into 17 fragments. One (control) fragment was immediately fixed for routine histological and ultrastructural studies and the remaining (test) fragments were randomly distributed in cryotubes, equilibrated at 20 degrees C/20 min in 1.8 mL of minimal essential medium (MEM) containing 1.5 or 3 M GLY, EG, PROH or DMSO and then either fixed for morphological studies to determine their possible toxic effect or frozen/thawed and then fixed to test the effect of cryopreservation on preantral follicles. Histological analysis showed that, compared to control fragments, all cryoprotectants at both concentrations significantly reduced the percentage of normal preantral follicles in ovarian fragments prior to or after cryopreservation. PROH 3.0 M appeared to exert a more toxic effect (P<0.05) than the other cryoprotectants in noncryopreserved tissues. After freezing/thawing, the highest (P<0.05) percentages of lightmicroscopical normal preantral follicles were observed in ovarian fragments cryopreserved in EG (1.5 and 3 M) or DMSO (1.5 M). However, transmission electronic microscopical (TEM) examination showed that only the DMSO-cryopreserved preantral follicles had normal ultrastructure. The data suggest that sheep preantral follicles should be cryopreserved with 1.5 M DMSO for later clinical or experimental application.