Autotransplantation of cryopreserved ovarian tissue--effective method of fertility preservation in cancer patients

Investigating the impact of vitrification on bovine ovarian tissue morphology, follicle survival, and transcriptomic signature

PURPOSE

Ovarian tissue cryopreservation is vital for fertility preservation, yet its effect on ovarian tissue follicle survival and transcriptomic signature requires further investigation. This study delves into the effects of vitrification on tissue morphology, function, and transcriptomic changes, helping to find possibilities for vitrification protocol improvements.

METHODS

Ovarian cortex from 19 bovine animals were used to conduct pre- and post-vitrification culture followed by histological assessment, immunohistochemistry, and TUNEL assay. Follicles' functionality was assessed for viability and growth within the tissue and in isolated cultures. RNA-sequencing of ovarian tissue was used to explore the transcriptomic alterations caused by vitrification.

RESULTS

Follicle density, cell proliferation, and DNA damage in ovarian stroma were unaffected by vitrification. However, vitrified cultured tissue exhibited reduced follicle density of primordial/primary and antral follicles, while freshly cultured tissue manifested reduction of antral follicles. Increased stromal cell proliferation and DNA damage occurred in both groups post-culture. Isolated follicles from vitrified tissue exhibited similar viability to fresh follicles until day 4, after which the survival dropped. RNA-sequencing revealed minor effects of vitrification on transcriptomic signatures, while culture induced significant gene expression changes in both groups. The altered expression of WNT and hormonal regulation pathway genes post-vitrification suggests the molecular targets for vitrification protocol refinement.

CONCLUSION

Vitrification minimally affects tissue morphology, follicle density, and transcriptomic signature post-thawing. However, culture revealed notable changes in vitrified tissue samples, including reduced follicle density, decreased isolated follicle survival, and alteration in WNT signalling and ovarian hormonal regulation pathways, highlighted them as possible limitations of the current vitrification protocol.

Current Trends on Bioengineering Approaches for Ovarian Microenvironment Reconstruction

Ovarian tissue has a unique microarchitecture and a complex cellular and molecular dynamics that are essential for follicular survival and development. Due to this great complexity, several factors may lead to ovarian insufficiency, and therefore to systemic metabolic disorders and female infertility. Techniques currently used in the reproductive clinic such as oocyte cryopreservation or even ovarian tissue transplant, although effective, have several limitations, which impair their wide application. In this scenario, mimetic ovarian tissue reconstruction comes as an innovative alternative to develop new methodologies for germ cells preservation and ovarian functions restoration. The ovarian extracellular matrix (ECM) is crucial for oocyte viability maintenance, once it acts actively in folliculogenesis. One of the key components of ovarian bioengineering is biomaterials application that mimics ECM and provides conditions for cell anchorage, proliferation, and differentiation. Therefore, this review aims at describing ovarian tissue engineering approaches and listing the main limitations of current methods for preservation and reestablishment of ovarian fertility. In addition, we describe the main elements that structure this study field, highlighting the main advances and the challenges to overcome to develop innovative methodologies to be applied in reproductive medicine. Impact Statement This review presents the main advances in the application of tissue bioengineering in the ovarian tissue reconstruction to develop innovative solutions for ovarian fertility reestablishment.

Ovarian Tissue Cryopreservation in Children and Adolescents

Cancer during childhood and adolescence remains a major public health issue, affecting a significant portion of this age group. Although newer anti-cancer treatments have improved survival rates, this comes at a cost in terms of gonadotoxic effects. As a result, the preservation of fertility is important. Ovarian tissue cryopreservation, one of the newest methods, has some advantages, especially for prepubertal patients: no need for ovarian stimulation, thus, no further risk for estrogen-sensitive cancer types, and preservation of more and better-quality primordial follicles of the ovarian cortex. The most frequent indications include treatment with alkylating agents, ovarian-focused radiotherapy, leukemias, lymphomas, brain and neurological tumors, as well as Turner syndrome and benign hemoglobinopathies. An expected survival exceeding 5 years, the absence of systematic disease and an overall risk of premature ovarian insufficiency over 50% are among the criteria that need to be fulfilled in order for a patient to undertake this method. Orthotopic transplantation is more frequently used, since it can allow both live birth and the recovery of endocrine function. Reimplantation of malignant cells is always a major risk and should always be taken into consideration. Histological analysis, as well as immunohistochemical and molecular methods, are needed in order to improve the search for malignant cells before transplantation. Ovarian tissue cryopreservation appears to be a method with specific benefits, indications and risks which can be an important tool in terms of preserving fertility in younger women.

The role of Hippo pathway signaling and A-kinase anchoring protein 13 in primordial follicle activation and inhibition

OBJECTIVE

To determine whether the mechanotransduction and pharmacomanipulation of A-kinase anchoring protein 13 (AKAP13) altered Hippo signaling pathway transcription and growth factors in granulosa cells. Primary ovarian insufficiency is the depletion or dysfunction of primordial ovarian follicles. In vitro activation of ovarian tissue in patients with primary ovarian insufficiency alters the Hippo and phosphatase and tensin homolog/phosphatidylinositol 3-kinase/protein kinase B/forkhead box O3 pathways. A-kinase anchoring protein 13 is found in granulosa cells and may regulate the Hippo pathway via F-actin polymerization resulting in altered nuclear yes-associated protein (YAP)/transcriptional coactivator with PDZ-binding motif coactivators and Tea domain family (TEAD) transcription factors.

DESIGN

Laboratory studies.

SETTING

Translational science laboratory.

PATIENT(S)

None.

INTERVENTION(S)

COV434 cells, derived from a primary human granulosa tumor cell line, were studied under different cell density and well stiffness conditions. Cells were transfected with a TEAD-luciferase (TEAD-luc) reporter as well as expression constructs for AKAP13 or AKAP13 mutants and then treated with AKAP13 activators, inhibitors, and follicle-stimulating hormone.

MAIN OUTCOME MEASURE(S)

TEAD gene activation or inhibition was measured by TEAD-luciferase assays. The messenger ribonucleic acid levels of Hippo pathway signaling molecules, including connective tissue growth factor (CTGF), baculoviral inhibitors of apoptosis repeat-containing 5, Ankyrin repeat domain-containing protein 1, YAP1, and TEAD1, were measured by quantitative real-time polymerase chain reaction. Protein expressions for AKAP13, CTGF, YAP1, and TEAD1 were measured using Western blot.

RESULT(S)

Increased TEAD-luciferase activity and expression of markers for cellular growth were associated with decreased cell density, increased well stiffness, and AKAP13 activator (A02) treatment. Additionally, decreased TEAD-luc activity and expression of markers for cellular growth were associated with AKAP13 inhibitor (A13) treatment, including a reduced expression of the BIRC5 and ANKRD1 (YAP-responsive genes) transcript levels and CTGF protein levels. There were no changes in TEAD-luc with follicle-stimulating hormone treatment, supporting Hippo pathway involvement in the gonadotropin-independent portion of folliculogenesis.

CONCLUSION(S)

These findings suggest that AKAP13 mediates Hippo-regulated changes in granulosa cell growth via mechanotransduction and pharmacomanipulation. The AKAP13 regulation of the Hippo pathway may represent a potential target for regulation of follicle activation.

Fertility preservation in ovarian cancer patients

Ovarian cancer is more frequent in post-menopausal women, however it can also occur in young premenopausal women. After diagnosis and cancer staging, health care providers should address the possibility of infertility and might offer fertility preservation options. Chemotherapy, frequently used when treating ovarian cancer, has proven to cause extensive ovarian damage. Standard surgery may be aggressive and the recurrence risk may not be relevant enough to opt for these approaches. Fertility sparing surgery has been progressively accepted and many alternative surgical approaches have proven to be successful in both cancer treatment, fertility preservation and low recurrence rates. Though there are several techniques available for fertility preservation in cancer patients, when dealing with ovarian cancer patients, oocyte and embryo cryopreservation are the only suitable. Ovarian tissue cryopreservation has been largely studied, but no data on ovarian cancer patients exist, due to the risk of reimplanting cancer cells.

Methods of Ovarian Tissue Cryopreservation: Is Vitrification Superior to Slow Freezing?-Ovarian Tissue Freezing Methods

After cancer treatment, female survivors often develop ovarian insufficiency or failure. Oocyte and embryo freezing are well-established fertility preservation options, but cannot be applied in pre-pubescent girls, in women with hormone-sensitive malignancies, or when gonadotoxic treatment cannot be delayed. Although ovarian tissue cryopreservation (OTC) has been used to restore fertility and endocrine function, the relative efficacy of its two major protocols, slow freezing and vitrification, remains controversial. This literature review evaluates clinical and lab-based studies published between January 2012 and June 2020 to determine whether vitrification, the optimal technique for oocyte and embryo cryopreservation, preserves ovarian tissue more effectively than slow freezing. Due to limited clinical data involving ovarian tissue vitrification, most clinical studies focus on slow freezing. Only 9 biochemical studies that directly compare the effects of slow freezing and vitrification of human ovarian tissue were noted. Most studies report no significant difference in follicular morphology and distribution between cryopreservation methods, but these findings must be interpreted in the context of high methodological variability. Discrepant findings regarding the effects of cryopreservation method on follicle viability, gene expression, and hormone production require further evaluation. Early clinical outcomes appear favorable for vitrification, but additional studies and longer term follow-up are needed to establish its efficacy. Sharing data through national or international registries would expedite this analysis. However, even if research corroborates conclusions of no clinical or biochemical difference between cryopreservation methods, the decreased costs and increased efficiency associated with vitrification make this method more accessible and cost-effective.

Current and Future Treatments for Classic Galactosemia

Type I (classic) galactosemia, galactose 1-phosphate uridylyltransferase (GALT)-deficiency is a hereditary disorder of galactose metabolism. The current therapeutic standard of care, a galactose-restricted diet, is effective in treating neonatal complications but is inadequate in preventing burdensome complications. The development of several animal models of classic galactosemia that (partly) mimic the biochemical and clinical phenotypes and the resolution of the crystal structure of GALT have provided important insights; however, precise pathophysiology remains to be elucidated. Novel therapeutic approaches currently being explored focus on several of the pathogenic factors that have been described, aiming to (i) restore GALT activity, (ii) influence the cascade of events and (iii) address the clinical picture. This review attempts to provide an overview on the latest advancements in therapy approaches.

Effects of vitrification and transplantation on follicular development and expression of EphrinB1 and PDGFA in mouse ovaries

The aim of this study was to assess the follicular development and the patterns of EphrinB1 and PDGFA immunostaining in vitrified mouse ovarian tissue (OT) with and without transplantation. Histological evaluation was performed on fresh and vitrified OTs, whether transplanted or not. RT-PCR was performed on fresh and vitrified ovarian samples (OSs) and vitrified OS graft. Vitrification alone did not significantly reduce the normal primordial, primary, and secondary follicles except antral ones (p > 0.05). However, transplantation decreased all the follicle types. The EphrinB1 immunoexpression showed high intensity in all follicular types in vitrified OT and the significant increased was detected in secondary and antral follicles (p < 0.05). PDGFA protein immunoexpression of primordial and primary follicles was decreased in vitrified OT (p < 0.05). However, the lowest immunoexpression of EphrinB1 and PDGFA was detected after transplantation (p < 0.05). The levels of ephrinb1 and pdgfa mRNA expressions in vitrified OS and vitrified OS grafts were found as comparable to the fresh OS. In conclusion, vitrification has no detrimental effect on the follicles at the different developmental stages, majority of ovarian follicular loss takes place after transplantation rather than vitrification. Overall, vitrification and grafting do not change the ephrinb1 and pdgfa gene expressions. In addition, EphrinB1 and PDGFA are expressed during different stages of folliculogenesis in a different manner in fresh, vitrified, or grafted OTs. Vitrification and/or grafting appear to affect the follicular expression of EphrinB1 and PDGFA. These findings suggest that these proteins could have several functions related to the development of follicles and angiogenesis after transplantation.

Prevalence and Risk Factors of Ovarian Metastases in Breast Cancer Patients < 41 Years of Age in the Netherlands: A Nationwide Retrospective Cohort Study

Purpose

Breast cancer is one of the primary indications for cryopreservation and subsequent autotransplantation of ovarian tissue. The safety of this fertility preservation method remains questionable, as the presence of disseminated breast tumor cells cannot yet be excluded in the ovarian autografts. We explored the prevalence of ovarian metastases among young breast cancer patients and determined risk factors for the development of ovarian metastases.

Methods

Using the nationwide database of the Dutch Pathology Registry (PALGA), we identified a cohort of 2648 women with primary invasive breast cancer at age < 41 years in the period 2000–2010 in the Netherlands who subsequently underwent an oophorectomy. From this source population, all cases who had histologically confirmed ovarian metastases were included. For each case of whom clinical data were available, one control without ovarian metastases who matched the time interval between breast cancer diagnosis and oophorectomy was selected. Data were collected on patient characteristics, diagnosis, treatment and follow-up.

Results

Ovarian metastases were found in 63 out of 2648 patients who met the inclusion criteria. The risk of developing ovarian metastases increased with time passed since breast cancer diagnosis. Multivariate logistic regression analyses showed significant association between tumor stage and the development of ovarian metastases (p = 0.024).

Conclusions

The prevalence of ovarian metastases was 2.4% among young breast cancer patients. Early ovary removal may reduce the risk of developing ovarian metastases. In breast cancer patients with tumors > 5 cm and/or inflammatory carcinoma, we recommend a cautious approach to ovarian tissue autotransplantation.

New advances in ovarian autotransplantation to restore fertility in cancer patients

Human ovary autotransplantation is a promising option for fertility preservation of young women and girls undergoing gonadotoxic treatments for cancer or some autoimmune diseases. Although experimental, it resulted in at least 42 healthy babies worldwide. According to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, a systematic literature review was performed for all relevant full-text articles published in English from 1 January 2000 to 01 October 2015 in PubMed to explore the latest clinical and research advances of human ovary autotransplantation. Human ovary autotransplantation involves ovarian tissue extraction, freezing/thawing, and transplantation back into the same patient. Three major forms of human ovary autotransplantation exist including (a) transplantation of cortical ovarian tissue, (b) transplantation of whole ovary, and (c) transplantation of ovarian follicles (artificial ovary). According to the recent guidelines, human ovary autotransplantation is still considered experimental; however, it has unique advantages in comparison to other options of female fertility preservation. Human ovary autotransplantation (i) does not need prior ovarian stimulation, (ii) allows immediate initiation of cancer therapy, (iii) can restore both endocrine and reproductive ovarian functions, and (iv) may be the only fertility preservation option suitable for prepubertal girls or for young women with estrogen-sensitive malignancies. As any other fertility preservation option, human ovary autotransplantation has both advantages and disadvantages and may not be feasible for all cases. The major challenges facing this option are how to avoid the risk of reintroducing malignant cells and how to prolong the lifespan of ovarian transplant as well as how to improve artificial ovary results.

Ovarian tissue transport to expand access to fertility preservation: from animals to clinical practice

Primordial follicles dictate a female's reproductive life span and therefore are central to fertility preservation for both endangered species and individuals with fertility-threatening conditions. Ovarian tissue containing primordial follicles can be cryopreserved and later thawed and transplanted back into individuals to restore both endocrine function and fertility. Importantly, increasing numbers of human live births have been reported following ovarian tissue cryopreservation and transplantation. A current limitation of this technology is patient access to sites that are approved or equipped to process and cryopreserve ovarian tissue - especially in larger countries or low resource settings. Here, we review empirical evidence from both animal models and human studies that suggest that ovarian tissue can be transported at cold temperatures for several hours while still maintaining the integrity and reproductive potential of the primordial follicles within the tissue. In fact, several human live births have been reported in European countries using tissue that was transported at cold temperatures for up to 20 h before cryopreservation and transplantation. Ovarian tissue transport, if implemented widely in clinical practice, could therefore expand both patient and provider access to emerging fertility preservation options.

Updates in preserving reproductive potential of prepubertal girls with cancer: Systematic review

INTRODUCTION

With increasing numbers of adult female survivors of childhood cancers due to advances in early diagnosis and treatment, the issue of preserving the reproductive potential of prepubertal girls undergoing gonadotoxic treatments has gained greater attention.

METHODS

According to PRISMA guidelines, a systematic review of the literature was performed for all relevant full-text articles published in PubMed in English throughout the past 15 years to explore the significant updates in preserving the reproductive potential of prepubertal girls with cancer.

RESULTS

The two established fertility preservation options, embryo freezing and egg freezing, cannot be offered routinely to prepubertal girls as these options necessitate prior ovarian stimulation and subsequent mature oocytes retrieval that are contraindicated or infeasible before puberty. Therefore, the most suitable fertility preservation options to prepubertal girls are (1) ovarian tissue freezing and autotransplantation, (2) in vitro maturation, and (3) ovarian protection techniques. In this review, we discuss in detail those options as well as their success rates, advantages, disadvantages and future directions. We also suggest a new integrated strategy to preserve the reproductive potential of prepubertal girls with cancer.

CONCLUSION

Although experimental, ovarian tissue slow freezing and orthotopic autotransplantation may be the most feasible option to preserve the reproductive potential of prepubertal girls with cancer. However, this technique has two major and serious disadvantages: (1) the risk of reintroducing malignant cells, and (2) the relatively short lifespan of ovarian tissue transplants. Several medical and ethical considerations should be taken into account before applying this technique to prepubertal girls with cancer.

Ovarian tissue cryopreservation and retransplantation--what do patients think about it?

Cryopreservation of ovarian tissue has been successfully applied clinically, with over 60 live births to date. The aim of the present study was to perform a survey of patients who have had ovarian tissue cryopreserved in the Department of Obstetrics and Gynecology, Erlangen University Hospital, in order to obtain information about: why patients opt for fertility preservation; their current fertility; pregnancy attempts and outcomes; and their intended plans for the cryopreserved ovarian tissue. In total, 147 women took part in the survey (average age 25.0 ± 7.0 years; response rate 48%; mean follow-up period 6 years). Sixty-six reported regular menstrual cycles; 48 were amenorrhoeic. Sixty-two women had tried to conceive; 33 reported pregnancies. Twenty-five had delivered healthy children after conceiving naturally; eight had conceived with assisted reproduction. Five patients had had their ovarian tissue retransplanted. Although many patients continued to have ovarian function, none of them regretted choosing cryopreservation of ovarian tissue. Cryopreservation of ovarian tissue is an effective option and is very important for women diagnosed with cancer. Analyses of the clinical outcomes in these patients are essential in order to identify those patients capable of benefiting most from the procedure and in order to improve the technique.

Surveillance and Care of the Gynecologic Cancer Survivor

BACKGROUND

Care of the gynecologic cancer survivor extends beyond cancer treatment to encompass promotion of sexual, cardiovascular, bone, and brain health; management of fertility, contraception, and vasomotor symptoms; and genetic counseling.

METHODS

This is a narrative review of the data and guidelines regarding care and surveillance of the gynecologic cancer survivor. We searched databases including PubMed, Cochrane, and Scopus using the search terms gynecologic cancer, cancer surveillance, and cancer survivor and reached a consensus for articles chosen for inclusion in the review based on availability in the English language and publication since 2001, as well as key older articles, consensus statements, and practice guidelines from professional societies. However, we did not undertake an extensive systematic search of the literature to identify all potentially relevant studies, nor did we utilize statistical methods to summarize data. We offer clinical recommendations for the management of gynecologic cancer survivors based on review of evidence and our collective clinical experience.

RESULTS

Key messages include the limitations of laboratory studies, including CA-125, and imaging in the setting of gynecologic cancer surveillance, hormonal and non-hormonal management of treatment-related vasomotor symptoms and genitourinary syndrome of menopause, as well as recommendations for general health screening, fertility preservation, and contraception.

CONCLUSIONS

A holistic approach to care extending beyond cancer treatment alone benefits gynecologic cancer survivors. In addition to surveillance for cancer recurrence and late treatment side effects, survivors benefit from guidance on hormonal, contraceptive, and fertility management and promotion of cardiovascular, bone, brain, and sexual health.

Basic fibroblast growth factor promotes macaque follicle development in vitro

Fertility preservation is an important type of frontier scientific research in the field of reproductive health. The culture of ovarian cortices to i) initiate primordial follicle growth and ii) procure developing follicles for later oocyte maturation is a promising fertility preservation strategy, especially for older women or cancer patients. At present, this goal remains largely unsubstantiated in primates because of the difficulty in attaining relatively large follicles via ovarian cortex culture. To overcome this hurdle, we cultured macaque monkey ovarian cortices with FSH, kit ligand (KL), basic fibroblast growth factor (bFGF), and/or epidermal growth factor (EGF). The various factors and factor combinations promoted primordial follicle development to different extents. Notably, both bFF (bFGF, 100 ng/ml and FSH, 50 ng/ml) and KF (KL, 100 ng/ml and FSH, 50 ng/ml) contributed to the activation of primordial follicles at day 12 (D12) of culture, whereas at D18, the proportions of developing follicles were significantly higher in the bFF and KF groups relative to the other treatment groups, particularly in the bFF group. Estradiol and progesterone production were also highest in the bFF group, and primary follicle diameters were the largest. Up until D24, the bFF group still exhibited the highest proportion of developing follicles. In conclusion, the bFGF–FSH combination promotes nonhuman primate primordial follicle developmentin vitro, with the optimal experimental window within 18 days. These results provide evidence for the future success of human ovarian cortex culture and the eventual acquisition of mature human follicles or oocytes for fertility restoration.

Successful fertility preservation following ovarian tissue vitrification in patients with primary ovarian insufficiency

STUDY QUESTION

Is ovarian tissue cryopreservation using vitrification followed by in vitro activation (IVA) of dormant follicles a potential approach for infertility treatment of patients with primary ovarian insufficiency (POI)?

SUMMARY ANSWER

Our vitrification approach followed by IVA treatment is a potential infertility therapy for POI patients whose ovaries contain residual follicles.

WHAT IS KNOWN ALREADY

Akt (protein kinase B) stimulators [PTEN (phosphatase with TENsin homology deleted in chromosome 10) inhibitor and phosphatidyinositol-3-kinase (PI3 kinase) stimulator] activate dormant primordial follicles in vitro and ovarian fragmentation disrupts the Hippo signaling pathway, leading to the promotion of follicle growth. We treated POI patients with a combination of ovarian vitrification, fragmentation and drug treatment, followed by auto-transplantation, and reported successful follicle growth and pregnancies.

STUDY DESIGN, SIZE, DURATION

Prospective clinical study of 37 infertile women with POI between 12 August 2011 and 1 November 2013. We enrolled 10 new patients since the previous publication.

PARTICIPANTS/MATERIALS, SETTING, METHODS

POI patients were originally selected based on a history of amenorrhea for more than 1 year and elevated serum FSH levels of >40 mIU/ml (n = 31) but this was later changed to >4 months, age <40 years and serum FSH levels of >35 mIU/ml (n = 6) (mean 71.8 ± 30.8, range 35.5-197.6) so as to include patients with a shorter duration of amenorrhea. Under laparoscopic surgery, ovariectomy was performed and ovarian cortices were dissected into strips for vitrification. Some pieces were examined histologically. After warming, two to three strips were fragmented into smaller cubes before culturing with Akt stimulators for 2 days. After washing, ovarian cubes were transplanted beneath the serosa of Fallopian tubes under laparoscopic surgery. Follicle growth was monitored by ultrasound and serum estrogen levels. After oocyte retrieval from mature follicles, IVF was performed.

MAIN RESULTS AND THE ROLE OF CHANCE

Among 37 patients, 54% had residual follicles based on histology. Among patients with follicles, 9 out of 20 showed follicle growth in auto-grafts with 24 oocytes retrieved from six patients. Following IVF and embryo transfer into four patients, three pregnancies were detected based on serum hCG, followed by one miscarriage and two successful deliveries. For predicting IVA success, we found that routine histological analyses of ovarian cortices and shorter duration from initial POI diagnosis to ovariectomy are valid parameters.

LIMITATIONS, REASONS FOR CAUTION

Although our findings suggest that the present vitrification protocol is effective for ovarian tissue cryopreservation, we have not compared the potential of vitrification and slow freezing in follicle growth after grafting. We chose the serosa of Fallopian tubes as the auto-grating site due to its high vascularity and the ease to monitor follicle growth. Future studies are needed to evaluate the best auto-grafting sites for ovarian tissues. Also, future studies are needed to identify biological markers to indicate the presence of residual follicles in POI to predict IVA treatment outcome.

WIDER IMPLICATIONS OF THE FINDINGS

In POI patients, ovarian reserve, namely the pool of residual follicles, continues to diminish with age. If one ovary is cryopreserved at an earlier stage of POI, patients could undergo additional non-invasive infertility treatments before the final decision for the IVA treatment. Furthermore, in the cases of unmarried POI patients, cryopreservation of ovarian tissues allows their fertility preservation until they desire to bear children.

STUDY FUNDING/COMPETING INTERESTS

This work was supported by Grant-In-Aid for Scientific Research (Research B: 24390376, Challenging Exploratory Research: 24659722, and Innovative Areas, Mechanisms regulating gamete formation in animals: 26114510) and by research funds from the Smoking Research Foundation, and the Takeda Science Foundation. None of the authors has a conflict of interest.

TRIAL REGISTRATION NUMBER

UMIN000010828.