|Year : 2018 | Volume
| Issue : 1 | Page : 3-8
Ovarian tissue cryopreservation and transplantation using thawed ovarian cortex for fertility preservation
Department of Obstetrics and Gynecology, St. Marianna University School of Medicine, Kawasaki, Japan
|Date of Web Publication||13-Feb-2018|
Department of Obstetrics and Gynecology, St. Marianna University School of Medicine, 2-16-1 Sugao, Miyamae, Kawasaki, Kanagawa 216-8511
Source of Support: None, Conflict of Interest: None
In the late 1990s, ovarian tissue cryopreservation was first employed clinically to preserve fertility in female children, adolescents, and young adults with cancer in Europe and the United States. In 2004, Donnez reported the first live birth after ovarian tissue cryopreservation and transplantation. Ovarian tissue cryopreservation can be employed when ova cannot be collected by intravaginal procedures, when induction of ovulation is impossible in girls before menarche, and when cancer therapy must be initiated promptly and there is insufficient time to induce ovulation. In patients with some cancers (e.g., ovarian cancer and leukemia), tumor cells can potentially infiltrate the ovaries and could be transferred by transplanting thawed ovarian tissue so ovarian tissue cryopreservation is contraindicated. Recently, live birth has been achieved up to 30% of women undergoing transplantation of cryopreserved and thawed ovarian tissue. If ovarian tissue contains more primordial follicles (as in children/adolescents), the likelihood of live birth after transplantation is higher. Therefore, the patient's age should also be considered. However, even a woman who underwent ovarian tissue cryopreservation in her late 30s has achieved live birth. Since initial clinical application of ovarian tissue cryopreservation and transplantation in 1997, approximately 100 live births have been reported, including 3 in Japan. This article reviews the current status of ovarian tissue cryopreservation and transplantation of thawed ovarian cortex for fertility preservation.
Keywords: Adolescent and young adult, cancer survivorship, childhood, fertility preservation, ovarian tissue cryopreservation, ovarian tissue transplantation
|How to cite this article:|
Suzuki N. Ovarian tissue cryopreservation and transplantation using thawed ovarian cortex for fertility preservation. Onco Fertil J 2018;1:3-8
|How to cite this URL:|
Suzuki N. Ovarian tissue cryopreservation and transplantation using thawed ovarian cortex for fertility preservation. Onco Fertil J [serial online] 2018 [cited 2018 Nov 20];1:3-8. Available from: http://www.tofjonline.org/text.asp?2018/1/1/3/225413
| Introduction|| |
Current status of ovarian tissue cryopreservation and transplantation in Europe and the United States
In Europe and the United States, ovarian tissue cryopreservation has been used clinically for fertility preservation in female children, adolescents, and young adults with cancer since the late 1990s, with Donnez et al. reporting the world'sfirst live birth after ovarian tissue cryopreservation and transplantation in 2004. In recent years, clinical use of this technique has been accepted in an increasing number of countries and over 100 live births have been achieved following ovarian tissue cryopreservation and transplantation. Nevertheless, the American Society for Reproductive Medicine published the following opinion in 2014: "Ovarian tissue cryopreservation is an experimental technique and should not be offered to women with benign disease or women who wish to delay childbearing. It is a technique that should not be used for a social reason." Despite this, live birth rates of 25%–31% have recently been reported by some researchers after ovarian tissue cryopreservation and transplantation.,, In Europe and the United States, ovarian tissue cryopreservation has been recognized as a technique that should be used in appropriate clinical situations, although it is still thought to be innovative. In particular, ovarian tissue cryopreservation and transplantation is the best (and only) option for fertility preservation in female children or young adolescents with cancer and in female patients who require urgent anticancer therapy and have insufficient time for the induction of ovulation. In 2015, von Wolff et al. reported that more than 1000 patients had undergone ovarian tissue cryopreservation in Europe. According to the 2017 report of the ESHRE Working Group on Oocytes in Europe, ovarian tissue cryopreservation was performed in 12 of 17 EU countries investigated during the 5 years from 2010 and ovarian tissue transplantation was carried out in 7 countries. Ovarian tissue cryopreservation was performed in a total of 5529 patients, being most frequent in Germany (1,895 patients), followed by France (1,373 patients), and then Belgium, (727 patients). Ovarian tissue transplantation was carried out in 237 patients, and was again most frequent in Germany (85 patients), followed by Denmark (727 patients) and France (39 patients). In Japan, according to the Survey for Children and Child Care Promotion of the Ministry of Health, Labour and Welfare, ovarian tissue cryopreservation was performed in 201 patients from 2006 to 2016 and transplantation of cryopreserved ovarian tissue was carried out in 157 cases. Approved indications for ovarian tissue cryopreservation and transplantation in Japan include primary ovarian insufficiency, and ovarian tissue transplantation has mainly been performed in patients with this condition. As of October 2017, transplantation of cryopreserved ovarian tissue has only been carried out in 3 cancer patients, with no live births. However, 3 Japanese patients with primary ovarian insufficiency have achieved live birth after transplantation of ovarian tissue following cryopreservation and in vitro activation (unpublished data).,
To perform ovarian tissue cryopreservation, all or a part of an ovary is excised laparoscopically, after which the ovarian cortex is sectioned and frozen for preservation. Compared with cryopreservation of fertilized or unfertilized ova, ovarian tissue cryopreservation has the following merits: (1) a large number of ova can be preserved (2), the procedure can be performed in rapidly without considering the menstrual cycle (3), it can be used for children with cancer who are intolerant of transvaginal procedures, and (4) it has the potential to provide hormone replacement through secretion of estrogen by the implanted ovarian tissue. Thus, not only the preservation of fertility but also improvement of ovarian deficiency symptoms could be possible, which may prevent the adverse effects of estrogen lack on the cardiovascular system and bone.
According to a review, there are 3 approaches to oophorectomy for ovarian tissue cryopreservation, which are (1) ovarian cortex biopsy, (2) partial oophorectomy, and (3) complete oophorectomy. Resection of the right ovary appears to be more common for anatomical reasons. The advantages and disadvantages of various devices for oophorectomy have not been validated, and these devices have not been standardized. The incidence of complications associated with oophorectomy is reported to be 0.4%–1.1% and serious complications are rare, although caution is required in patients with active systemic lupus erythematosus., The FeriPROTEKT Network (von Wolf's group) reported that 50% resection of the ovary is frequently performed since this provides sufficient ovarian tissue for cryopreservation.
After remission of cancer is achieved, cryopreserved ovarian sections are thawed and laparoscopically implanted on the surface of the residual ovary or on the peritoneum. According to Pacheco et al., among 246 women who underwent transplantation of thawed ovarian tissue, 81 have delivered offspring and 8 were pregnant at the time of reporting. They also reported that 62.3% of their patients achieved spontaneous pregnancy while in vitro fertilization was employed in 37.6%. Ovarian tissue transplantation was usually performed laparoscopically, and the thawed ovarian tissue was transplanted orthotopically in 80% of the patients. Only 3 patients receiving heterotopic transplantation, and a combined orthotopic/heterotopic approach was employed in the remaining patients. Orthotopic transplantation usually involved implantation of tissue on the ovarian surface or the pelvic floor near the ovary after making an incision or freshening the site. In Denmark, ovarian tissue is commonly transplanted to the residual ovary (orthotopic transplantation), whereas the standard approach in Germany (FeriPROTEKT) is transplantation to the peritoneum near the fimbriae of the fallopian tube (heterotopic transplantation). A multinational, multicenter, randomized controlled study of ovarian tissue transplantation sites (residual ovary vs, the peritoneum near the tubal fimbriae) (OVATRANS: NCT02780791) is currently ongoing (unpublished letter from von Wolf).
| Risk of Minimal Residual Disease in Cryopreserved Ovarian Tissue|| |
When cryopreserved and thawed ovarian tissue is transplanted after remission has been achieved, whether or not minimal residual disease (MRD) exists in the cryopreserved tissue is critically important. If tumor cells have infiltrated the cryopreserved ovarian tissue, there is a possibility of transferring these cells to the patient. Therefore, the characteristics of the cancer must be considered carefully and the risks of ovarian tissue cryopreservation should be fully explained to the patient when necessary. Currently, the risk of MRD existing in cryopreserved ovarian tissue and the possibility of transferring viable cancer cells by grafting thawed ovarian tissue are still uncertain. Based on a systematic review of 289 patients undergoing ovarian tissue transplantation, hematopoietic tumors such as leukemia were associated with a high risk of tumor cell transfer after ovarian tissue transplantation, but the risk was low with other cancers. In particular, no tumor cells were found in ovarian tissues harvested from patients with lymphoma or breast cancer. At present, ovarian tissue cryopreservation is not recommended for patients with hematopoietic malignancies, especially for ALL, ovarian cancer and patients with metastatic ovarian cancer.
There have been several reports about MRD in cryopreserved ovarian tissue.,,
Ellen's group investigated the risk of MRD in ovarian cortical tissues obtained from 47 cancer patients for cryopreservation using immunohistochemical analysis to detect tumor-related antibodies and gene mutations in the resected ovary and appendage. As a result, they identified tumor cells in the fallopian tube of only one of the 47 patients, who had peritoneal metastasis of esophageal cancer. In addition, when DNA analysis was performed in patients with ovarian cancer, rectal cancer, Ewing's sarcoma, breast cancer, and uterine cervical cancer, no evidence of MRD was detected in ovarian tissue.,
Because it is impossible to evaluate MRD in cryopreserved ovarian tissue that will be used for autotransplantation, it is very difficult to assess the risk of tumor cell transfer associated with this procedure. Therefore, it is important for reproductive medicine physicians to carefully assess the presence of MRD before and after ovarian tissue transplantation through collaboration with oncologists. One way to eliminate the risk of MRD associated with transplantation is to isolate mature ova from ovarian tissue. There have also been reports about cryopreservation of ova or embryos after in vitro maturation of oocytes collected during surgery., However, this approach requires intensive collaboration between a surgeon and trained laboratory staff. At the research level, follicles of all stages have been induced from cryopreserved ovarian tissue, followed by isolation and culture of mature ova, and this technique has achieved a live birth in animal models.,, Moreover, Morohaku et al. reported successful induction of ova using primordial germ cells obtained from cryopreserved and thawed ovarian tissue in mice, obtaining up to 7 offspring. It is expected that this technique may be applied clinically in the future.
| Current Status of Ovarian Tissue Cryopreservation|| |
Cryopreservation of ovarian tissue is performed by 2 methods, which are slow freezing or extremely rapid freezing (vitrification). In routine clinical treatment of infertility, the main method for cryopreservation of unfertilized ova or embryos has shifted from slow freezing to vitrification, which is currently regarded as superior in terms of both viability of thawed ova or embryos and the live birth rate.
In contrast, slow freezing is still the standard method for ovarian tissue cryopreservation, but which method is actually better remains open to question. Ovarian tissue cryopreservation by vitrification has been reported by several groups. In 2005, Yeoman's group compared the slow freezing and vitrification methods with respect to follicular viability using simian ovaries. They reported no notable differences between the two methods, suggesting that vitrification could be an option for ovarian tissue cryopreservation in patients requiring fertility preservation. According to Gandolfi et al., the cow ovary is the optimum animal model for evaluation of the vitrification method. In 2011, based on electron microscopic analysis and immunohistochemical staining, Ting et al. reported that vitrification achieved comparable results to control unfrozen ovarian tissue. In 2017, Ting's group produced an ovarian tissue cryopreservation kit for use with the vitrification method. In 2010, the author's team performed a comparison of cryoprotectant solutions for use with vitrification of ovarian tissue. In cynomolgus monkeys, the ovarian medulla was resected and cut into cubes (1 cm × 1 cm × 0.1–0.2 cm). Then, we compared the cryopreservation solution reported by Ishimori et al. (H199 supplemented with 20% synthetic serum substitute, 3.22 mol/L ethylene glycol, 2.56 mol/L dimethyl sulfoxide, and 0.5 mol/L sucrose), and the other cryopreservation solution (H199 supplemented with 20% synthetic serum substitute, 5.64 mol/l ethylene glycol, 5% polyvinylpyrrolidone, and 0.5 mol/l sucrose). Based on the results of light and electron microscopy, we concluded that the optimum method was equilibration for 5 min in the latter one which does not contain diemethyl sulfoxide. The author's team also evaluated the graft site for thawed ovarian tissue after cryopreservation, revealing that grafting ovaries into the greater omentum, mesosalpinx, iliac fossa, or uterine serosa led to restoration of the menstrual cycle and release of ova. When heterotopic transplantation is adopted for clinical application, the best site for ovarian tissue grafting is considered to be the mesosalpinx or greater omentum so that follicles can easily be harvested after transplantation.
In Japan, 3 patients have achieved live birth after transplantation of ovarian tissue that had been cryopreserved by the vitrification method (unpublished data)., Kikuchi et al. reported that cryopreservation of ovarian tissue can be safely performed by vitrification., Amorim et al. stated that which of the two methods is superior for ovarian tissue cryopreservation remains controversial, despite a number of studies performed in recent years to compare slow freezing with vitrification. They pointed out that the published reports varied with regard to the animal species used, as well as the concentration and composition of the cryoprotectant (freezing solution), equilibration time, devices, thawing solution, subsequent culture time, and method of evaluating the cryopreserved ovarian tissue, making it very difficult to draw conclusions about the relative merits of the two techniques. We also previously reviewed studies comparing slow freezing and vitrification, but we could not find a significant difference between the two methods. Nakamura et al. used gas chromatography to compare the residual cryoprotectant content of frozen and thawed ovarian tissue between slow freezing and vitrification, demonstrating that the slow freezing method was associated with a lower tissue content of cryoprotectant. Because their research was carried out on a small number of specimens, further assessment seems to be necessary.
In 2017, Shi et al. conducted a meta-analysis of 14 studies that compared ovarian tissue cryopreservation by slow freezing and vitrification [Table 1]. While there was no significant difference between the two methods with regard to the percentage of intact primordial follicles, they reported that primordial follicles showed significantly less DNA fragmentation after vitrification. The percentage of normal stromal cells in the ovarian tissue specimens was also significantly larger after vitrification, although there was no significant difference of primordial follicle density between the two methods. These results suggested that vitrification is possibly superior to slow freezing for cryopreservation of ovarian tissue. However, various cryoprotectants have been used for vitrification in previous studies, and this technique has not been standardized. Moreover, vitrification has a short history of clinical application and there have been only 3 live births after transplantation of vitrified ovarian tissue in Japan. Thus, further validation is required before routine use of vitrification will be possible.
|Table 1: Meta-analysis of 14 studies that compared ovarian tissue cryopreservation by slow freezing and vitrifiction (Shi et al 2017)|
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| Conclusions|| |
Ovarian tissue cryopreservation is indicated for women up to the age of about 35 years. Because the likelihood of achieving live birth becomes higher as the number of primordial follicles increases, prepubertal/adolescent girls or young women aged up to 30 years with cancer are probably the best candidates for ovarian tissue cryopreservation. However, if this method is performed in children with cancer, the cryopreserved ovarian tissue will need to be retained for many years, and this raises the issue of who will be responsible for its management. Moreover, informed consent needs to be obtained for ovarian tissue cryopreservation, as well as for subsequent long-term storage and eventual transplantation, so its performance should be carefully discussed. Technical advances have allowed ovarian tissue cryopreservation and transplantation to become a feasible option for young female cancer patients, but it is essential for reproductive medicine physicians and oncologists to collaborate when determining the indications for this fertility preservation technique, taking the type of malignancy into account.
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Conflicts of interest
There are no conflicts of interest.
| References|| |
Donnez J, Dolmans MM, Demylle D, Jadoul P, Pirard C, Squifflet J, et al.
Livebirth after orthotopic transplantation of cryopreserved ovarian tissue. Lancet 2004;364:1405-10.
Anderson RA, Hamish W, Wallace B, Telfer EE. Ovarian tissue cryopreservation for fertility preservation: Clinical and research perspectives. Hum Reprod Open 2017;1:1-9.
Practice Committee of American Society for Reproductive Medicine. Ovarian tissue cryopreservation: A committee opinion. Fertil Steril 2014;101:1237-43.
Donnez J, Dolmans MM, Pellicer A, Diaz-Garcia C, Ernst E, Macklon KT, et al.
Fertility preservation for age-related fertility decline. Lancet 2015;385:506-7.
Jensen AK, Kristensen SG, Macklon KT, Jeppesen JV, Fedder J, Ernst E, et al.
Outcomes of transplantations of cryopreserved ovarian tissue to 41 women in Denmark. Hum Reprod 2015;30:2838-45.
Oktay K, Bedoschi G, Pacheco F, Turan V, Emirdar V. First pregnancies, live birth, and in vitro
fertilization outcomes after transplantation of frozen-banked ovarian tissue with a human extracellular matrix scaffold using robot-assisted minimally invasive surgery. Am J Obstet Gynecol 2016;214:94.e1-9.
Chung K, Donnez J, Ginsburg E, Meirow D. Emergency IVF versus ovarian tissue cryopreservation: Decision making in fertility preservation for female cancer patients. Fertil Steril 2013;99:1534-42.
von Wolff M, Dittrich R, Liebenthron J, Nawroth F, Schüring AN, Bruckner T, et al.
Fertility-preservation counselling and treatment for medical reasons: Data from a multinational network of over 5000 women. Reprod Biomed Online 2015;31:605-12.
The ESHRE Working Group on Oocyte Cryopreservation in Europe, Shenfield F, Mouzon JD, Scaravelli G, Kupka M, Ferratti AP, et al
. Oocyte and ovarian tissue cryopreservation in European countries: Statutory background, practice, storage and use. Hum Reprod Open 2017;1:1-9.
Kawamura K, Cheng Y, Suzuki N, Deguchi M, Sato Y, Takae S, et al.
Hippo signaling disruption and Akt stimulation of ovarian follicles for infertility treatment. Proc Natl Acad Sci U S A 2013;110:17474-9.
Suzuki N, Yoshioka N, Takae S, Sugishita Y, Tamura M, Hashimoto S, et al.
Successful fertility preservation following ovarian tissue vitrification in patients with primary ovarian insufficiency. Hum Reprod 2015;30:608-15.
Corkum KS, Laronda MM, Rowell EE. A review of reported surgical techniques in fertility preservation for prepubertal and adolescent females facing a fertility threatening diagnosis or treatment. Am J Surg 2017;214:695-700.
Beckmann MW, Dittrich R, Lotz L, Oppelt PG, Findeklee S, Hildebrandt T, et al.
Operative techniques and complications of extraction and transplantation of ovarian tissue: The Erlangen experience. Arch Gynecol Obstet 2017;295:1033-9.
Lawrenz B, Jauckus J, Kupka MS, Strowitzki T, von Wolff M. Fertility preservation in >1,000 patients: Patient's characteristics, spectrum, efficacy and risks of applied preservation techniques. Arch Gynecol Obstet 2011;283:651-6.
Pacheco F, Oktay K. Current success and efficiency of autologous ovarian transplantation: A meta-analysis. Reprod Sci 2017;24:1111-20.
Dolmans MM, Jadoul P, Gilliaux S, Amorim CA, Luyckx V, Squifflet J, et al.
Areview of 15 years of ovarian tissue bank activities. J Assist Reprod Genet 2013;30:305-14.
Luyckx V, Durant JF, Camboni A, Gilliaux S, Amorim CA, Van Langendonckt A, et al.
Is transplantation of cryopreserved ovarian tissue from patients with advanced-stage breast cancer safe? A pilot study. J Assist Reprod Genet 2013;30:1289-99.
Seshadri T, Gook D, Lade S, Spencer A, Grigg A, Tiedemann K, et al.
Lack of evidence of disease contamination in ovarian tissue harvested for cryopreservation from patients with Hodgkin lymphoma and analysis of factors predictive of oocyte yield. Br J Cancer 2006;94:1007-10.
Dolmans MM, Marinescu C, Saussoy P, Van Langendonckt A, Amorim C, Donnez J, et al.
Reimplantation of cryopreserved ovarian tissue from patients with acute lymphoblastic leukemia is potentially unsafe. Blood 2010;116:2908-14.
Hoekman EJ, Smit VT, Fleming TP, Louwe LA, Fleuren GJ, Hilders CG, et al.
Searching for metastases in ovarian tissue before autotransplantation: A tailor-made approach. Fertil Steril 2015;103:469-77.
Fadini R, Dal Canto M, Mignini Renzini M, Milani R, Fruscio R, Cantù MG, et al.
Embryo transfer following in vitro
maturation and cryopreservation of oocytes recovered from antral follicles during conservative surgery for ovarian cancer. J Assist Reprod Genet 2012;29:779-81.
Huang JY, Tulandi T, Holzer H, Tan SL, Chian RC. Combining ovarian tissue cryobanking with retrieval of immature oocytes followed by in vitro
maturation and vitrification: An additional strategy of fertility preservation. Fertil Steril 2008;89:567-72.
Xu M, Kreeger PK, Shea LD, Woodruff TK. Tissue-engineered follicles produce live, fertile offspring. Tissue Eng 2006;12:2739-46.
Smitz J, Dolmans MM, Donnez J, Fortune JE, Hovatta O, Jewgenow K, et al.
Current achievements and future research directions in ovarian tissue culture,in vitro
follicle development and transplantation: Implications for fertility preservation. Hum Reprod Update 2010;16:395-414.
Morohaku K, Tanimoto R, Sasaki K, Kawahara-Miki R, Kono T, Hayashi K, et al.
Complete in vitro
generation of fertile oocytes from mouse primordial germ cells. Proc Natl Acad Sci U S A 2016;113:9021-6.
Martínez-Burgos M, Herrero L, Megías D, Salvanes R, Montoya MC, Cobo AC, et al.
Vitrification versus slow freezing of oocytes: Effects on morphologic appearance, meiotic spindle configuration, and DNA damage. Fertil Steril 2011;95:374-7.
Yeoman RR, Wolf DP, Lee DM. Coculture of monkey ovarian tissue increases survival after vitrification and slow-rate freezing. Fertil Steril 2005;83 Suppl 1:1248-54.
Gandolfi F, Paffoni A, Papasso Brambilla E, Bonetti S, Brevini TA, Ragni G, et al.
Efficiency of equilibrium cooling and vitrification procedures for the cryopreservation of ovarian tissue: Comparative analysis between human and animal models. Fertil Steril 2006;85 Suppl 1:1150-6.
Ting AY, Yeoman RR, Lawson MS, Zelinski MB.In vitro
development of secondary follicles from cryopreserved rhesus macaque ovarian tissue after slow-rate freeze or vitrification. Hum Reprod 2011;26:2461-72.
Laronda MM, McKinnon KE, Ting AY, Le Fever AV, Zelinski MB, Woodruff TK, et al.
Good manufacturing practice requirements for the production of tissue vitrification and warming and recovery kits for clinical research. J Assist Reprod Genet 2017;34:291-300.
Hashimoto S, Suzuki N, Yamanaka M, Hosoi Y, Ishizuka B, Morimoto Y, et al.
Effects of vitrification solutions and equilibration times on the morphology of cynomolgus ovarian tissues. Reprod Biomed Online 2010;21:501-9.
Ishimori H, Takahashi Y, Kanagawa H. Factors affecting survival of mouse blastocysts vitrified by a mixture of ethylene glycol and dimethyl sulfoxide. Theriogenology 1992;38:1175-85.
Suzuki N, Hashimoto S, Igarashi S, Takae S, Yamanaka M, Yamochi T, et al.
Assessment of long-term function of heterotopic transplants of vitrified ovarian tissue in cynomolgus monkeys. Hum Reprod 2012;27:2420-9.
Kikuchi I, Kagawa N, Silber S, Kuwayama M, Takehara Y, Aono F, et al.
Oophorectomy for fertility preservation via reduced-port laparoscopic surgery. Surg Innov 2013;20:219-24.
Kagawa N, Kikuchi I, Kuwayama M. Ovarian tissue vitrification for fertility preservation. Vitrification in Assisted Reproduction. India: Springer; 2015. p. 43-9.
Amorim CA, Curaba M, Van Langendonckt A, Dolmans MM, Donnez J. Vitrification as an alternative means of cryopreserving ovarian tissue. Reprod Biomed Online 2011;23:160-86.
Sugishita Y, Hashimoto S, Yamochi T, Igarashi S, Nakajima M, Nishijima C, et al
. Ovarian tissue cryopreservation cortical tissue vitrification. In: Evers JL, editors. Gonadal Tissue Cryopreservation in Fertility. India: Springer; 2016. p. 79-94.
Nakamura Y, Obata R, Okuyama N, Aono N, Hashimoto T, Kyono K, et al.
Residual ethylene glycol and dimethyl sulphoxide concentration in human ovarian tissue during warming/thawing steps following cryopreservation. Reprod Biomed Online 2017;35:311-3.
Shi Q, Xie Y, Wang Y, Li S. Vitrification versus slow freezing for human ovarian tissue cryopreservation: A systematic review and meta-anlaysis. Sci Rep 2017;7:8538.