Cryopreservation of ovarian tissue: An emerging technology for female germline preservation of endangered species and breeds

Many hundreds of exotic species and domestic animal breeds have been lost over the course of the last few decades. In order to avoid a similar fate to other animals threatened with extinction, it is crucial to develop and apply rescue strategies to ensure their survival for the future. One option as a safeguard measure is the cryopreservation of the main source of female gametes enclosed within the ovary: the primordial follicles. So far, there are three options to cryopreserve small ovarian follicles: whole ovary, ovarian cortical tissue or isolated follicles, with the use of slow freezing or vitrification methods. After cryopreservation, the harvested material can be transplanted or cultured, with the aim to produce mature fertilizable oocytes. The objective of this review is to summarize the current status of the cryopreservation of ovarian tissue in domestic species and non-endangered wild mammals as model for threatened and endangered species and breeds, and to provide new insights into techniques that can be applied in the future.