Fertility of mice following receipt of ovaries slow cooled in dimethyl sulphoxide or ethylene glycol is largely independent of cryopreservation equilibration time and temperature

Influence of graft size, histocompatibility,and cryopreservation on reproductive outcome following ovary transplantation in mice

Purpose

Transplantation of ovarian tissue is a valuable method to rescue mouse strains with fertility problems and to revitalize archived strains. The purpose of this study was to investigate the effect of (i) different sizes of transplanted ovary pieces on reproductive outcome, (ii) use of immunodeficient recipients to overcome the limitation of histocompatibility, and (iii) to compare different protocols for cryopreservation of ovarian tissue.

Methods

Halves, quarters, and eights of mouse ovaries were transplanted. Half ovaries from B6 donors were transferred into immunodeficient mice. Halves of ovaries were frozen according to four different protocols, thawed and transferred.

Results

Pregnancy rate after transplantation of ovarian tissue was high (90–100%) independent of the transplant size. Although, the average litter size was significantly lower for recipients of quarters and eights (4.4 and 4.6 vs. 6.5), the total number of offspring produced per donor ovary was higher compared with recipients of halves. Pregnancy rate of immunodeficient recipients was 40% (mean 4.7 offspring per litter). All four cryopreservation protocols used were able to preserve functionality of the ovarian tissue.

Conclusions

Transplantation of ovarian tissue smaller than halves resulted in reduced litter sizes. The distribution of ovarian tissue of one donor female to 4 or 8 recipients will therefore yield in a higher total number of offspring in a certain time period. The use of immunodeficient recipients is an option for non-histocompatible donors. Cryopreservation of ovarian tissue is generally feasible but the function of frozen-thawed ovary halves after transplantation differs depending on the freezing protocol used.

Mouse Ovary Cryopreservation

Ovary cryopreservation in combination with orthotopic ovary transplantation can produce live offspring and complement embryo and sperm cryopreservation programs. One method for ovary cryopreservation is described here.

Oocytes recovered after ovarian tissue slow freezing have impaired H2AX phosphorylation and functional competence

It has been shown that oocytes isolated from ovarian tissue cryopreservation acquire DNA damage during the process of freeze-thawing. Using a mouse model, here we have investigated the functional competence and phosphorylation of H2AX (γ-H2AX) in germinal vesicle (GV) and parthenogenetically activated oocytes derived from conventional ovarian tissue slow freezing and vitrification techniques. The number of GV-stage oocytes with γ-H2AX foci was not significantly different between the slow-freezing and vitrification groups. Although the in vitro maturation (IVM) potential of GV oocytes in the slow-freezing group showed a significant delay (P<0.0001) in the process of germinal vesicle breakdown, no difference in the maturation rate was observed between the two protocols. Nevertheless, parthenogenetic activation of IVM oocytes using strontium chloride showed a significantly lower activation rate in the slow-freezing group compared with the vitrification (P<0.05) and control (P<0.01) groups. Importantly, H2AX phosphorylation was significantly perturbed in the slow-freezing group in comparison to the control (P<0.05). Therefore, we conclude that impaired sensing of DNA strand breaks and repair processes are associated with the reduced functional competence of the oocytes recovered from the slow-freezing group, which may have a significant impact on the reproductive outcome.

Ovarian tissue vitrification is more efficient than slow freezing in protecting oocyte and granulosa cell DNA integrity

Ovarian tissue cryopreservation is the primary treatment modality currently available to women at risk of losing their ovarian function due to cytotoxic therapy. However, the impact of these techniques on the oocyte DNA integrity is not elucidated. Here we have investigated the effect of vitrification and conventional slow freezing of eight week old Swiss albino mouse ovarian tissues on the oocyte and granulosa cell DNA integrity using the comet assay. The intracellular levels of reactive oxygen species in oocytes was measured by 2',7'-dichlorodihydrofluorescein diacetate fluorescence. The cryopreservation of ovarian tissue by the slow freezing technique resulted in a significantly higher level of DNA fragmentation in oocytes in comparison to vitrification (p < 0.05) whereas DNA fragmentation in granulosa cells was significantly higher than the control (p < 0.01). Further, reactive oxygen species were significantly elevated in oocytes derived from slow freezing when compared to vitrification (p < 0.05). Therefore, we conclude that the ovarian tissue slow freeze-thawing makes the oocyte and granulosa cells more vulnerable to DNA damage whereas vitrification appears to be a safer method than slow freezing for ovarian tissue cryopreservation.

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.

Applications of emerging technologies to the study and conservation of threatened and endangered species

Sustaining viable populations of all wildlife species requires the maintenance of habitat, as well as an understanding of the behaviour and physiology of individual species. Despite substantial efforts, there are thousands of species threatened by extinction, often because of complex factors related to politics, social and environmental conditions and economic needs. When species become critically endangered, ex situ recovery programmes that include reproductive scientists are the usual first line of defence. Despite the potential of reproductive technologies for rapidly increasing numbers in such small populations, there are few examples of success. This is not the result of a failure on the part of the technologies per se, but rather is due to a lack of knowledge about the fundamental biology of the species in question, information essential for allowing reproductive technologies to be effective in the production of offspring. In addition, modern conservation concepts correctly emphasise the importance of maintaining heterozygosity to sustain genetic vigour, thereby limiting the practical usefulness of some procedures (such as nuclear transfer). However, because of the goal of maintaining all extant gene diversity and because, inevitably, many species are (or will become) 'critically endangered', it is necessary to explore every avenue for a potential contributory role. There are many 'emerging technologies' emanating from the study of livestock and laboratory animals. We predict that a subset of these may have application to the rescue of valuable genes from individual endangered species and eventually to the genetic management of entire populations or species. The present paper reviews the potential candidate techniques and their potential value (and limitations) to the study and conservation of rare wildlife species.