The use of immature oocytes in the fertility preservation of cancer patients: current promises and challenges.

Improved oncological treatments permit increased survival rates, although cancer patients remain at risk of losing ovarian function. An attractive option for fertility preservation includes the use of immature oocytes, a strategy which can occur on a rapid timeline and without hormonal stimulation. As a result, cancer therapy can proceed promptly even in patients with hormone-sensitive tumors. Following retrieval, immature oocytes can be cryopreserved at either the immature germinal vesicle or the mature metaphase-II stage, i.e. either before or after in vitro maturation (IVM). We present a critical review of previous human studies on the cryopreservation of immature oocytes. Evaluations include in vitro developmental competence upon thawing/warming, or organization of the spindle and chromosomes. Reported successes vary, perhaps in relation to the source of the oocytes and protocols for cryopreservation and IVM. Weaknesses exist with the experimental designs implemented to date, so caution must be exercised before considering the use of immature oocytes to be a safe and reliable practice in the fertility preservation of cancer patients. To date, results indicate that with current protocols, it may be best to cryopreserve immature oocytes after IVM at the metaphase-II stage. Nonetheless, efficacy remains very low. Future efforts should tailor and optimize not only cryo-preservation, but also IVM protocols for use in either germinal vesicle or metaphase-II oocytes, together with a comprehensive assessment of oocyte function and developmental competence to term. Despite current challenges, the burgeoning field of immature oocyte cryopreservation constitutes a promising option for cancer patients with impaired ovarian function.