Preservation of primordial follicles from lions by slow freezing and xenotransplantation of ovarian cortex into an immunodeficient mouse.

Assisted reproductive technology (ART) is considered an important tool in the conservation of endangered species, but often the most limiting factor of ART is the availability of mature oocytes. The aim of the present study was to investigate the feasibility of preserving female germ cells from ovaries of female lions (Panthera leo). Good quality cumulus-oocyte complexes (COCs) were isolated and subjected to in vitro maturation (IVM). In addition, ovarian cortex was obtained and cut into pieces for culture and cryopreservation by slow freezing. The survival of ovarian follicles was assessed by histology. Frozen-thawed samples of ovarian cortex samples were xenotransplanted under the skin of ovariectomized immunodeficient mouse for 28 days. Overall, 178 intact COCs were obtained from 13 lions, but only 28.1% were matured in vitro indicating insufficient IVM conditions. In contrast, almost all follicles within the ovarian cortex survived culture when the original sample was from a young healthy lion collected immediately after euthanasia. Within the xenotransplants, the number of primordial follicles decreased after 28 days by 20%, but the relation between primordial and growing follicles changed in favour of follicular growth. Female gamete rescue from valuable felids may be performed by slow freeze cryopreservation of ovarian cortex. Although the IVM protocol for lions is not yet optimized, mature oocytes may be obtained after long-term xenotransplantation and IVM and could potentially represent one way of salvage of endangered felid species in the future.