Cryopreservation of mammalian ovaries and oocytes

Of Elephants and Other Mammals: A Comparative Review of Reproductive Tumors and Potential Impact on Conservation

Reproductive tumors can impact conception, pregnancy, and birth in mammals. These impacts are well documented in humans, while data in other mammals are limited. An urgent need exists to understand the reproductive impact of these lesions in endangered species, because some endangered species have a documented high prevalence of reproductive tumors. This article documents that the prevalence of both benign and malignant neoplasia differs between African and Asian elephants, with Asian elephants more frequently diagnosed and negatively affected by both. The prevalence of these tumors across mammalian species is compared, and impact plus treatment options in human medicine are reviewed to inform decision making in elephants. Evidence suggests that reproductive tumors can negatively impact elephant conservation. Future studies that document reproductive outcomes, including the success of various treatment approaches in elephants with tumors will benefit conservation efforts.

Preservation of female genetic resources in feline species

The development of assisted reproduction techniques (ART) specifically for felids has been propagated for two main reasons: (i) most felids are threatened and faced with extinction in all or part of their native habitats (IUCN Red List of Threatened Species, www.catsg.org), and (ii) the domestic cat (Felis catus) can serve as a research model for the implementation of advanced assisted reproductive techniques (ART) to be applied in exotic cats. Domestic cat ovaries can be freshly obtained from veterinary clinics and are frequently used for research on preservation of genetic resources in feline species. The presented review will summarize recent advances and obstacles in biobanking of female genetic resources and discuss alternative approaches which are under investigation.

Function of Cryopreserved Cat Ovarian Tissue after Autotransplantation

Simple Summary

Assisted reproduction techniques are potentially important tools for the creation of gene banks largely focused on preserving female germ cells and tissues, cryopreservation being one of the most important. Since there is not yet a protocol established for the preservation of cat ovarian tissue, we decided to assess our cryopreservation protocol with autotransplantation of the ovary. Our study showed that even though follicular survival was low, follicles were able to survive up to 28 days of transplantation and develop up to the antral stage, which helps elucidate the path for preservation of felid ovaries. Once this technique is improved, it may contribute to the preservation of wild feline species.

Abstract

The aim of this study was to assess a slow-freezing protocol of cat ovarian tissue cryopreservation using autotransplantation. Four adult queens were ovariohysterectomized and the ovaries were fragmented and cryopreserved. After one week, the grafts were thawed and autografted to the subcutaneous tissue of the dorsal neck of each queen, then randomly removed after 7, 14, 28, 49, and 63 days after transplantation. Percentages of morphologically normal primordial and growing follicles (MNFs) were 88% and 97%, respectively, in fresh tissue samples (fresh controls), and 74% and 100%, respectively, immediately after thawing (cryo D0). No MNFs were found after 49 days of transplantation. In both fresh control and cryo D0 fragments, granulosa cells were frequently in proliferation. Two morphologically normal antral follicles were detected in one queen on Day 28 post-transplantation. Connective tissue fibers increased, suggesting replacement of active ovarian cortex by fibrous tissue. Tissue vascularization was observed at 7 days after grafting, and wide blood vessels were clearly visible on Days 49 and 63. In conclusion, although follicular survival was low after cryopreservation and grafting of cat ovarian tissue, follicles were able to develop up to the antral stage, which is an encouraging outcome.

Brilliant cresyl blue staining allows the selection for developmentally competent immature feline oocytes

The outcome of in-vitro-maturation and in-vitro-fertilization of feline oocytes depends on the selection of high quality oocytes, and is often restricted to morphological criteria. The aim of this study was to test whether the Brilliant cresyl blue (BCB) staining is suitable for pre-selection of feline oocytes before in-vitro-maturation. Cumulus-oocytes-complexes (COC) were released from domestic cat ovaries obtained after ovariectomy and were subjected to BCB staining. BCB+ stained oocytes were characterized by a violet/pale blue staining of the ooplasma, BCB- oocytes remained unstained. Transmission electron microscopy indicated for a slightly advanced stage of BCB- oocytes within the maturation process. After 24 h in-vitro-maturation, almost 75% of BCB+ and 21.5% of BCB- oocytes were able to reach metaphase II. Also, after in-vitro-fertilization, significantly more oocytes developed to morulae (19.2%) if oocytes were preselected for BCB staining, although 8% of unstained COC still reached advanced embryo stages. Prolonged storage of ovaries before COC retrieval for 16-20 h at 4 °C was accompanied by reduced number of BCB+ oocytes (96 of 210, 45.7%) in comparison to freshly isolated COC (151 of 225, 67.1%), and impaired cleavage rate (19.8%) and morula rate (9.4%) of BCB+ oocytes but the rate of embryos which developed to advanced stages remained unchanged (∼50%). To conclude, BCB staining is a very useful tool to preselect immature COC of feline species ensuring higher developmental rates.

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.

Cat preantral follicle survival after prolonged cooled storage followed by vitrification

The aim of this study was to investigate the impact of prolonged storage at 4 °C on survival of cat preantral follicles (PAFs) pre- and post-vitrification. Ovaries were obtained from 12 queens and transported at 4 ºC within 2-6 h. Parts of the ovaries were stored for an additional 24 h or 72 h. The ovarian cortex was dissected, analyzed for viability (neutral red - NR) and morphology (histology - HE and ultrastructural analysis by TEM) and vitrified. We used 2 mm biopsy punches to obtain equal size pieces as the experimental units. After NR assessment, each sample was fixed and embedded in paraffin for HE staining to determine the number of morphologically intact follicles. Another 2 mm piece of ovary was subjected to TEM. NR viability assessment and HE results showed a similar tendency with PAF survival postvitrification even after prolonged cooling at 24 h and 72 h. With TEM, integrity of mitochondria, plasma and basal membranes as well as the presence of pre-granulose cells of PAFs were documented postvitrification for the control group and 24 h prolonged storage group, but not after 72 h storage. Our results showed that cat PAFs can survive prolonged storage followed by vitrification. The described set of techniques are applicable towards creating a gamete bank for endangered feline species.

Mammalian fertility preservation through cryobiology: value of classical comparative studies and the need for new preservation options

Human-related fertility preservation strategies have enormous potential for helping sustain and protect other species, especially to assist managing or 'rescuing' the genomes of genetically valuable individuals, including endangered species. However, wider-scale applications are limited by significant physiological variations among species, as well as a lack of fundamental knowledge of basic reproductive traits and cryosensitivity. Systematic and comparative cryopreservation studies (e.g. on membrane biophysical properties and resilience to freezing temperatures) are required to successfully recover gametes and gonadal tissues after thawing and eventually produce healthy offspring. Such data are currently available for humans and a few laboratory and livestock animals, with virtually all other species, including wildlife, having gone unstudied. Interestingly, there also are commonalities among taxa that allow a protocol developed for one species to provide useful information or guidance for another. However, when a rare animal unexpectedly dies there is no time for a prospective understanding of that species' biophysical traits. Because the odds of success will be much lower in such instances, it is essential that more fundamental studies be directed at more species. But also worthwhile is thinking beyond these systematic characterisations to consider the potential of a 'universal preservation protocol' for animal biomaterials.

Recent advances and prospects in germplasm preservation of rare and endangered species

Fertility preservation strategies using cryopreservation have enormous potential for helping sustain and protect rare and endangered species, especially to assist managing or 'rescuing' the genomes of genetically valuable individuals. However, wide-scale applications are still limited by significant physiological variations among species and a sheer lack of fundamental knowledge about basic reproductive traits as well as in germplasm cryobiology. Cryo-studies have been conducted in more species (mainly vertebrates) in the recent years but a vast majority still remains un-studied. Semen cryopreservation represents the most extensive effort with live births reported in more and more species after artificial insemination. Oocyte freezing remains challenging and unsuccessful in wild species and will require more research before becoming a standard procedure. As an alternative to fully grown gametes, gonadal tissue preservation has become a promising option in vertebrates. Yet, more fertility preservation options are necessary to save species so a change in strategy might be required. It is worthwhile thinking beyond systematic characterizations and considering the application of cutting edge approaches to universally preserve the fertility of a vast array of species.

Short-term culture of ovarian cortex pieces to assess the cryopreservation outcome in wild felids for genome conservation

Background

Cryopreservation of ovarian tissue has the potential to preserve female germ cells of endangered mammals. In the present study, a freezing protocol successfully used for human tissue, was adapted for preserving ovarian tissue of domestic and non-domestic felids. Ovaries from non-domestic felid species were obtained from seven freshly euthanized and two recently deceased wild felids kept in different European Zoos. In addition, ovaries from domestic cats were obtained after ovariectomy from local veterinary clinics for methological adaptations.

Ovarian cortex was dissected and uniform sized pieces of 2 mm diameter were obtained. Using a slow freezing protocol (-0.3°C per min) in 1.5 mol/L ethylene glycol, 0.1 mol/L sucrose, the pieces were cultured for up to 14 days both before and after cryopreservation. The integrity of primordial follicles was assessed by histology, and the impact of different protein sources (FCS or BSA) and Vitamin C was determined during two weeks of culture.

Results and conclusion

During culture the number of primordial follicles decreased within the ovarian pieces (p < 0.05). This effect was less pronounced when FCS was used as the protein source instead of BSA. Supplementation with Vitamin C had a detrimental effect on follicle survival. Since the procedure of cryopreservation had no effect on the follicle survival after one week of culture we conclude that the freezing protocol was suitable for felids. This is the first report of preserving a huge amount of follicles within ovarian tissue by slow freezing performed in several wild feline species.

Comparative cryobiological traits and requirements for gametes and gonadal tissues collected from wildlife species

A major challenge to retaining viability of frozen gametes and reproductive tissues is to understand and overcome species-specificities, especially because there is substantial diversity in cryobiological properties and requirements among cell types and tissues. Systematic studies can lead to successful post-thaw recovery, especially after determining: 1) membrane permeability to water and cryoprotectant, 2) cryoprotectant toxicity, 3) tolerance to osmotic changes, and 4) resistance to cooling and freezing temperatures. Although species-dependency ultimately dictates the ability of specific cells and tissues to survive freeze-thawing, there are commonalities between taxa that allow a protocol developed for one species to be useful information for another. This is the reason for performing comparative cryopreservation studies among diverse species. Our laboratory has compared cellular cryotolerance, especially in spermatozoa, in a diverse group of animals-from corals to elephants-for more than 30 yrs. Characterizing the biophysical traits of gametes and tissues is the most efficient way to develop successful storage and recovery protocols, but, such data are only available for a few laboratory, livestock, and fish species, with virtually all others (wild mammals, birds, reptiles, and amphibians) having gone unstudied. Nonetheless, when a rare animal unexpectedly dies, there is no time to understand the fundamentals of biophysics. In these emergencies, it is necessary to rely on experience and the best data from taxonomically-related species. Fortunately, there are some general similarities among most species, which, for example, allow adequate post-thaw viability. Regardless, there is a priority for more information on biophysical traits and freezing tolerance of distinctive biomaterials, especially for oocytes and gonadal tissues, and even for common, domesticated animals. Our colleague, Dr John Critser was a pioneer in cryobiology, earning that moniker because of his advocacy and devotion to understanding the differences (and similarities) among species to better store living genetic material.