Vitrification, compared to conventional or slow freezing, is an effective technique for cryopreserving oocytes and embryos; however, its comparative efficacy in ovarian tissue cryopreservation is not proven. Now, a study published in the recent issue of the journal Reproduction reports that conventional freezing is more effective in the cryopreservation of ovarian tissue, as it preserves greater developmental and proliferative potential.

Vladimir Isachenko from the University of Ulm, Germany, and coworkers performed morphological, endocrinal, and molecular biological analysis to compare the safety and efficacy of vitrification with the conventional freezing of ovarian tissue fragments (0.3-1 x 1-1.5 x 0.7-1 mm). The tissues obtained from 15 patients were randomly divided into fresh tissues (controls), and tissues cryopreserved by vitrification and slow freezing techniques. The cryopreserved tissues were thawed and tissues of all groups (including controls) were cultured for 16 days in vitro. Viability and proliferative capacity of these tissues were evaluated by examining hormone production, follicle development, and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) gene expression.

The study demonstrated the following findings:

No significant variations were noted between the conventionally frozen and vitrified tissues with regard to hormonal activity and follicle quality. Molecular biological analysis showed a drastic reduction in GAPDH gene expression in ovarian tissues after vitrification compared to those subjected to conventional freezing.

Contrary to these findings, a study by Keros et al (Human Reproduction, 2009) reported that vitrification maintains the morphological integrity of the ovarian stroma better than slow-programmed freezing (P<0.001). With the objective of improving the ovarian cryopreservation method (especially of ovarian stroma), the two techniques were systematically compared in ovarian tissues donated by 20 women during cesarean section. After freezing (vitrification or slow freezing), thawing, and culturing of the ovarian tissues, the researchers noted that both cryopreservation techniques maintained the morphological features of the ovarian tissue and follicles. However, detailed analysis (particularly with electron microscopy) revealed improved integrity of the ovarian stroma with vitrification. A similar study by Silber et al (Human Reproduction, 2009) reported an additional benefit of ovarian tissue vitrification in minimizing oocyte loss when compared to slow freezing.

Ovarian tissue banking, one of the promising strategies for preserving female fertility, is potentially indicated in several conditions such as autoimmune disorders, genetic diseases, and benign and malignant diseases. However, the viability of oocytes and functional lifespan of ovarian tissue post transplantation are often compromised due to factors such as cryoinjury.

A recent report by the Practice Committee of the American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology (Fertility and Sterility, 2008) recommends ovarian tissue cryopreservation as an experimental protocol in only selected patient population. The report emphasizes on the following with respect to the technique:
• Need for further research to solve unanswered queries such as tissue collection methods, optimal tissue size, choice of cryopreservation protocol and cryoprotectants, and the choice of appropriate patients.
• Counseling patients on its potential limitations such as the risk of reintroducing tumor cells following ovarian transplantation.
• Should not be employed to delay reproductive aging in healthy women, owing to the current risk-to-benefit ratio.

References

1. Isachenko V, Lapidus I, Isachenko E, et al. Human ovarian tissue vitrification versus conventional freezing: morphological, endocrinological, and molecular biological evaluation. Reproduction. 2009 Aug;138(2):319-27.

2. Isachenko V, Isachenko E, Woriedh M, Lapidus I, Weiss JM. Vitrification and conventional freezing of human ovarian tissue: follicles formation, hormone production and gene expression. Paper presented at: Annual Meeting of European Society of Human Reproduction and Embryology;June 29, 2009;Amsterdam, Netherlands.

3. Keros V, Xella S, Hultenby K, et al. Vitrification versus controlled-rate freezing in cryopreservation of human ovarian tissue. Hum Reprod. 2009 Jul;24(7):1670-83. Epub 2009 Apr 9.

4. Silber S, Kagawa N, Lenahan K, et al. Duration of function of fresh and frozen human ovarian grafts. Paper presented at: Annual Meeting of European Society of Human Reproduction and Embryology;June 29, 2009;Amsterdam, Netherlands.

5. Practice Committee of American Society for Reproductive Medicine; Practice Committee of Society for Assisted Reproductive Technology. Ovarian tissue and oocyte cryopreservation. Fertil Steril. 2008 Nov;90(5 Suppl):S241-6.