Cryopreservation of female germ cells and ovarian tissues for fertility preservation

To preserve the fertility of patients who undergo chemotherapy and/or radiotherapy, procedures for cryopreservation of female germ cells have been investigated. Cyropreservation methods differ according to follicle stage because the mammalian ovary contains a large number of oocytes at different growth stages. Follicles at very early stages, for example the primordial and primary stages, are usually cryopreserved within ovarian cortical tissue because they need surrounding somatic cells for subsequent development. In contrast, fully-grown oocytes in Graafian follicles are cryopreserved without any other cells at the metaphase II stage. Recently, ultra-rapid cooling was incorporated into cryopreservation procedures for human ovaries. In this review, we describe oocyte freezing, the development of ultra-rapid cooling systems for ovarian tissues, freezing of human ovaries, and ovarian transplantation.