Ovarian Tissue Cryopreservation: Limitations and Perspectives for Female Fertility Preservation

Background: The ovarian tissue cryopreservation has been achieved a great notoriety in the Reproductive Biology area, due to its potential in preserving female fertility through the protection of exocrine and endocrine functions of the ovary. The association of this technique with in vitro culture and/or transplant in adult or young individuals who has not initiated its reproductive activities represents not only the conservation and perpetuation of the genetic material of economic valuable animals, but also the preservation of female gametes from endangered species, or even from young women who may have ovarian dysfunctions caused by gonadotoxic treatments. Studies with some species (human, mice and ovine) have demonstrated the recovery of the ovarian function and the birth of healthy offspring after transplant of ovarian tissue which has been previously cryopreserved. However, most studies have shown that ovarian cryopreservation process offer risks to different structures (follicle and stroma) as well as to the different cell types (oocyte, granulosa, thecal and stromal cells), which constitute this tissue. Review: Extreme cold, intracellular ice crystallization, osmotic shock and the toxicity of the cryoprotectant agents are factors that are usually associated with the injuries caused by the cryopreservation process. As a direct or indirect consequence, those factors limit the success of the cryopreservation of ovarian tissue, since they affect the survival or alter the tissue functionality or cellular structure, like the ovarian follicles, for example, after the thawing/warming procedure. Among the injuries that may take place as a consequence of those factors, we can mention: cell death by the necrotic or apoptotic pathways; alterations in normal levels of genic expression; ischemia and changes of communication and interaction between the oocyte and follicular cells. As a result, many authors have studied and developed protocols of cryopreservation that may prevent or minimize the cryoinjuries, since the cryopreservation per si or combined to other techniques (in vitro culture and/or transplant) can compromise the ovarian integrity, leading consequently to a significant loss of follicles. In this regard, the present review seeks to approach the advantages of the cryopreservation of ovarian tissue; indicating the difficulties and challenges that encompass this procedure, with purpose of pointing out solutions to overcome the damages of ovarian tissue cryopreservation, through of convenient cryopreservation protocols that avoid those follicular losses. For this purpose, it is necessary the preservation of the follicular viability as well as the preservation of the tissue integrity and the contact between reproductive (oocytes) or somatic cells, which are essentials to the follicle development, and, consequently, to the embryo production. Conclusion: The use of cryopreserved ovarian tissue is an important strategy to the preservation of female fertility. This tool has being pointed as an alternative way to the cryopreservation of mature oocytes and embryos. However, additional studies are necessary to diminishing the cellular damages inherent to this procedure, especially those related to the comprehension of the obstacles and mechanisms associated to the exposition to extreme cold.