Unilateral ovariectomy increases loss of primordial follicles and is associated with increased metestrous concentration of follicle-stimulating hormone in old rats

Unraveling the mechanisms of chemotherapy-induced damage to human primordial follicle reserve: road to developing therapeutics for fertility preservation and reversing ovarian aging

Among the investigated mechanisms of chemotherapy-induced damage to human primordial follicle reserve are induction of DNA double-strand breaks (DSBs) and resultant apoptotic death, stromal-microvascular damage and follicle activation. Accumulating basic and translational evidence suggests that acute exposure to gonadotoxic chemotherapeutics, such as cyclophosphamide or doxorubicin, induces DNA DSBs and triggers apoptotic death of primordial follicle oocytes within 12-24 h, resulting in the massive loss of ovarian reserve. Evidence also indicates that chemotherapeutic agents can cause microvascular and stromal damage, induce hypoxia and indirectly affect ovarian reserve. While it is possible that the acute reduction of the primordial follicle reserve by massive apoptotic losses may result in delayed activation of some primordial follicles, this is unlikely to be a predominant mechanism of loss in humans. Here, we review these mechanisms of chemotherapy-induced ovarian reserve depletion and the potential reasons for the discrepancies among the studies. Based on the current literature, we propose an integrated hypothesis that explains both the acute and delayed chemotherapy-induced loss of primordial follicle reserve in the human ovary.

Chapter 13 Human Ovarian Tissue Vitrification

Ovarian freezing and transplantation has garnered increasing interest as a potential way of preserving fertility in cancer patients as well as for women who just wish to delay childbearing. This chapter spells out our techniques of ovarian cortex vitrification and results for frozen compared to fresh ovarian cortex transplantation (in one single series from one center for the sake of consistency), as well as potentially provides insight into the mechanism behind ovarian follicle recruitment. This represents an effort to simplify and popularize an approach that has yielded favorable results (all cases recovered ovulation and 75% had successful spontaneous pregnancy) in one single, disciplined study. It should be clear that this is a review for the more general reader of our original scientific papers published in Reproductive BioMedicine Online, New England Journal of Medicine, Fertility and Sterility, Human Reproduction, Molecular Human Reproduction, and Journal of Assisted Reproduction and Genetics (JARG).

Ovarian tissue cryopreservation and transplantation: scientific implications

After fresh or frozen ovary transplantation, FSH levels return to normal, and menstrual cycles resume by 150 days, coincident with anti-Müllerian hormone rising to higher than normal levels. AMH then returns to well below normal levels by 240 days, remaining as such for many years with nonetheless normal ovulation and fertility. To date, 20 babies have been born in our program from 11 fresh and 13 cryopreserved ovary transplant recipients with a live baby rate of over 70 % (11 babies from fresh and 9 from frozen). Globally, over 70 live births have been reported for both fresh and frozen ovary transplants with an approximate 30 % live birth rate. Given the rapid rise of AMH after the fall of FSH, with a subsequent AMH decrease with retention of ovarian function, it is tempting to speculate the existence of a shared mechanism controlling primordial follicle recruitment, fetal oocyte meiotic arrest, and recruitment in the adult ovary. With the massive recruitment of primordial follicles observed after human ovarian cortical tissue transplantation, which subsides to an extremely low recruitment rate, we will discuss how this phenomenon suggests a unifying theory implicating ovarian cortical tissue rigidity in the regulation of both fetal oocyte arrest and recruitment of follicles in the adult ovary. As the paper by Winkler-Crepaz et al. in this issue demonstrates, our in vivo results are consistent with the in vitro demonstration that primordial follicles in the fetal cortex are "locked" in development, resulting in meiotic arrest, which spares the oocytes from being rapidly lost all at once (Winkler-Crepaz et al., J Assist Reprod Genet, 1). Winkler-Crepaz et al. demonstrate that follicle loss after ovarian cortex transplantation is unlikely due to ischemic apoptosis, but rather from a "burst" of primordial follicle recruitment. In vivo, primordial follicles are normally resistant to further development or activation to prevent oocyte depletion. The dense fibrous ovarian cortex, through as yet unresolved mechanisms, arrests the further continuation of meiosis and also prevents a sudden depletion of all resting follicles in the adult ovary. Intrinsic tissue pressure is released after cortical tissue transplantation, temporarily resulting in a rapid follicle depletion. These results are consistent with the observation that once the ovarian reserve is reduced in the graft, the rate of recruitment diminishes and the ovarian tissue exhibits a relatively long duration of function.

The impact of passive immunisation against BMPRIB and BMP4 on follicle development and ovulation in mice

The primordial follicle reserve is the corner stone of female fertility and determines the longevity and quality of reproduction. Complete depletion of this reserve will lead to primary infertility, and the key-limiting step of follicle depletion is the transition from primordial to primary follicles. It has been reported that this process is gonadotrophin-independent, but other conflicting reports are indicated otherwise and this discrepancy needs to be unequivocally clarified. The aim of this study was to investigate the role of bone morphogenetic proteins (BMPs) in the regulation of folliculogenesis in mice passively immunised against BMP receptor 1B (BMPRIB) and BMP4. While a stereological study revealed that the numbers of primordial follicles in immunised mice were significantly higher when compared with control animals, treatment with equine chorionic gonadotrophin showed no effect. In parallel, immunofluorescence microscopy revealed the presence of BMPRIB but not FSH receptor in primordial follicles. The number of primary follicles in immunised mice were also significantly increased when compared with control animals. After puberty, the rates of depletion of primordial and primary follicles were increased with age, particularly in treated animals; however, there was no significant difference between the treatment groups of the same age. Based on these results together with our previous reports in sheep and mice, we confirm that the attenuation of BMP signalling system can be an effective approach to sustain the primordial follicle reserve while promoting the development of growing follicles, ovulation and consequently overall female fertility.

Fresh and cryopreserved ovary transplantation and resting follicle recruitment

Ovary cryopreservation and transplantation has garnered increasing interest as a possible method to preserve fertility for cancer patients and to study ovarian resting follicle recruitment. Eleven consecutive women underwent fresh donor ovary transplantation, and 11 underwent cryopreserved ovary auto-transplantation in the same centre, with the same surgeon. Of the 11 fresh transplant recipients, who were all young but menopausal, nine women had normal ovarian cortex transplanted from an identical twin sister, and two had a fresh allograft from a non-identical sister. In the second group, 11 women with cancer had ovarian tissue cryopreserved before bone marrow transplant, and then after years of therapeutically induced menopause, underwent cryopreserved ovarian cortex autotransplantation. Recovery of ovarian function and follicle recruitment was assessed in all 22 recipients, and the potential for pregnancy was further investigated in 19 (11 fresh and 8 cryopreserved) with over 1-year follow-up. In all recipients, normal FSH levels and menstruation returned by about 150 days, and anti-Müllerian hormone reached much greater than normal concentrations by about 170 days. Anti-Müllerian hormone levels then fell below normal by about 240 days and remained at that lower level. Seventeen babies have been born to these 11 fresh and eight cryopreserved ovary transplant recipients.

Fertility preservation for age-related fertility decline

Cryopreservation of eggs or ovarian tissue to preserve fertility for patients with cancer has been studied since 1994 with R G Gosden's paper describing restoration of fertility in oophorectomised sheep, and for decades previously by others in smaller mammals. Clinically this approach has shown great success. Many healthy children have been born from eggs cryopreserved with the Kuwayama egg vitrification technique for non-medical (social) indications, but until now very few patients with cancer have achieved pregnancy with cryopreserved eggs. Often, oncologists do not wish to delay cancer treatment while the patient goes through multiple ovarian stimulation cycles to retrieve eggs, and the patient can only start using the oocytes after full recovery from cancer. Ovarian stimulation and egg retrieval is not a barrier for patients without cancer who wish to delay childbearing, which makes oocyte cryopreservation increasingly popular to overcome an age-related decline in fertility. Cryopreservation of ovarian tissue is an option if egg cryopreservation is ruled out. More than 35 babies have been born so far with cryopreserved ovarian tissue in patients with cancer who have had a complete return of hormonal function, and fertility to baseline. Both egg and ovarian tissue cryopreservation might be ready for application to the preservation of fertility not only in patients with cancer but also in countering the increasing incidence of age-related decline in female fertility.

In vivo evidence of role of bone morphogenetic protein-4 in the mouse ovary

The transition of a primordial follicle to a primary follicle is an early step in folliculogenesis. All female mammals are born with a fixed stock of primordial follicles, and exhaustion of that stock leads to menopause or infertility. Recently, several in vitro studies have indicated that BMP-4, BMP-7, and several other growth factors affect the transition of primordial to primary follicles. The aim of our present study was to investigate role of BMP-4 in this process using passive immunization to investigate the role of BMP-4 in a prepubertal mouse model. After seven days of treatment, the weight of antiBMP-4 treated ovaries was significantly lower than the ovaries from mice treated with nonimmune Ig. The number of primary follicles was lower, and the numbers of primordial follicles were higher in antiBMP-4 treated ovaries compared to control ovaries. Treatment with equine chorionic gonadotrophin (eCG) showed no influence on the effects of antiBMP-4 in the mouse ovary. Thus, the results of our study indicate that in vivo BMP-4 acts as transition factor in transition of primordial to primary follicle.

Ovarian aging and menopause: current theories, hypotheses, and research models

Aging of the reproductive system has been studied in numerous vertebrate species. Although there are wide variations in reproductive strategies and hormone cycle components, many of the fundamental changes that occur during aging are similar. Evolutionary hypotheses attempt to explain why menopause occurs, whereas cellular hypotheses attempt to explain how it occurs. It is commonly believed that a disruption in the hypothalamic-pituitary-gonadal axis is responsible for the onset of menopause. Data exist to demonstrate that the first signs of menopause occur at the level of the brain or the ovary. Thus, finding an appropriate and representative animal model is especially important for the advancement of menopause research. In primates, there is a gradual decline in the function of the hypothalamic-pituitary-gonadal (HPG) axis ultimately resulting in irregularities in menstrual cycles and increasingly sporadic incidence of ovulation. Rodents also exhibit a progressive deterioration in HPG axis function; however, they also experience a period of constant estrus accompanied by intermittent ovulations, reduced progesterone levels, and elevated circulating estradiol levels. It is remarkable to observe that females of other classes also demonstrate deterioration in HPG axis function and ovarian failure. Comparisons of aging in various taxa provide insight into fundamental biological mechanisms of aging that could underlie reproductive decline.

Gonadotropin Control of Inhibin Secretion and the Relationship to Follicle Type and Number in the hpg Mouse1

Inhibin is secreted in two distinct heterodimeric forms, A and B, but the mechanism for the differential control of these two forms is unclear. To evaluate the relationship between secretion of inhibin forms and folliculogenesis, the effects of gonadotropins on inhibin concentrations were studied in parallel with stereological enumeration of ovarian follicle types in gonadotropin-deficient hypogonadal (hpg) female mice treated with recombinant human FSH (10 IU/day), hCG (1 IU/day), or both for 20 days. Treatment with FSH alone significantly increased blood concentrations of both inhibin A and inhibin B, whereas hCG alone had no effect on either inhibin. The combination of FSH and hCG further increased the concentration of inhibin A but had no effect on the concentration of inhibin B beyond that of FSH. The number of primordial follicles per ovary was significantly reduced in FSH-treated hpg mice, but was not affected by hCG treatment. Antral follicles were absent in the untreated hpg mice, present following treatment with FSH, and were present in only limited numbers following hCG treatment alone. Preovulatory follicles were observed only in the wild-type and combined FSH and hCG treatment groups. These results demonstrate that secretion of both inhibins is associated with the presence of antral follicles. Inhibin A secretion is increased by the presence of preovulatory follicles, whereas the concentration of inhibin B is not affected. The observed effects of gonadotropins on inhibin A and B secretion may be explained by corresponding gonadotropin effects on follicle development.

Effect of edifenphos on compensatory ovarian hypertrophy, follicular kinetics and estrous cycle in hemicastrated rats

Edifenphos, an organophosphate fungicide, was administered to hemicastrated albino rats intraperitoneally for 15 consecutive days in graded doses of 2 to 8 mg/kg body weight. In the oil-treated hemicastrated control group, ovarian weight and total number of healthy and atretic follicles were significantly higher than the same parameters in sham-operated control animals, and the estrous cycle was normal. Treatment with 2, 4, 6, or 8 mg/kg/d edifenphos significantly decreased ovarian weight (-6.75, -9.79, -18.71, and -34.13 percent, respectively) below that of the controls. Although treatment with 2 or 4 mg/kg/d edifenphos did not change the number of healthy and atretic follicles, a dosage of 4 mg/kg/d significantly decreased the number and duration of estrous cycles. Treatment with 6 or 8 mg/kg/d, however, significantly decreased the number and duration of different phases of the estrous cycle as well. In all treated groups, the weights of the thymus and uterus were significantly reduced when compared with those of hemicastrated oil-treated control animals.

Reduction of Primordial Follicles Caused by Maternal Treatment with Busulfan Promotes Endometrial Adenocarcinoma Development in Donryu Rats

Ovarian dysfunction leading to hormonal imbalance plays a crucial role in uterine carcinogenesis in rats as well as women. However, the effects of a reduction in primordial follicles at birth on uterine adenocarcinoma development have hitherto not been determined. The present study was therefore conducted using female Donryu rats, a high incidence rat strain of uterine adenocarcinoma. The animals were maternally exposed to 2.5 or 5.0 mg/kg of busulfan on gestation day 14 to reduce primordial follicles, and were then initiated by intrauterine treatment with N-ethyl-N'-nitro-N-nitrosoguanidine at 11 weeks of age. Both busulfan treatment doses caused earlier occurrence of persistent estrus, with dose-dependence as compared to controls. At 15 months of age, the rats were euthanized. The incidence of uterine adenocarcinomas and multiplicity of uterine neoplastic lesions were significantly increased by the 5.0 mg/kg, but not the 2.5 mg/kg busulfan treatment. Morphologically, the ovaries exposed to busulfan treatment exhibited severe atrophy, with few or no follicles and corpus lutea. Serum 17beta-estradiol (E2), progesterone, and inhibin levels were significantly decreased in the busulfan treatment groups, with a clear dose-relation. Interestingly, only the 5.0 mg/kg busulfan treatment elevated the E2/progesterone ratio. These results provide evidence that the reduction of primordial follicles promotes uterine adenocarcinoma development in rats in association with an earlier occurrence of the persistent estrus status.

Temporal effect of carbofuran on estrous cycle compensatory ovarian hypertrophy and follicles in hemiovariectomized albino mice

Carbofuran, a systemic N-methyl carbamate pesticide was administered orally with an effective dose of 1.3mg/kg per day to hemiovariectomized (HOVX) mice for 5, 10, and 15 days. Sham-operated and HOVX control mice were administered a similar quantity of olive oil. The vaginal smear and body weight of the mice were recorded daily and mice were sacrificed on day 16. The results of the present study indicate that there is a significant decrease in the duration of the estrus with a concomitant significant increase in the duration of the diestrus with carbofuran treatment for 10 days. In the HOVX mice treated with carbofuran for 15 days there was a significant decrease in the length of the estrous cycle and the duration of the estrus and metestrus with a concomitant significant increase in the diestrus. There was a significant increase in the ovarian weight in HOVX control mice when compared to that of the sham operated control mice. HOVX mice treated with carbofuran for 10 and 15 days showed a significant decrease in the relative ovarian weight with a concomitant inhibition of compensatory ovarian hypertrophy in HOVX mice. There was also a significant decrease in the ovarian growth rate in relation to the contralateral ovary of the same animal. There was a significant decrease in the number of small and total number of healthy follicles with a concomitant significant increase in the number of medium, large and total number of atretic follicles in HOVX mice treated with carbofuran for 10 days. There was a significant decrease in the number of small, medium, large, and total number of healthy follicles with a concomitant significant increase in the number of medium, large, and total number of atretic follicles in HOVX mice treated with carbofuran for 15 days. The administration of carbofuran in the present study showed that there is no significant change in the body and organs weight such as uterus, kidney, adrenals, liver, spleen, thymus and thyroid in all the carbofuran-treated mice. The above findings such as disruption in the estrous cycle, inhibition of compensatory ovarian hypertrophy, and follicular toxicity may be due to a direct effect on the ovary or through the hypothalamo-hypophysial-ovarian axis.

Anti-Müllerian hormone attenuates the effects of FSH on follicle development in the mouse ovary

Although ovarian follicle growth is under the influence of many growth factors and hormones of which FSH remains one of the most prominent regulators. Therefore, factors affecting the sensitivity of ovarian follicles to FSH are also important for follicle growth. The aim of the present study was to investigate whether anti-Müllerian hormone (AMH) has an inhibitory effect on follicle growth by decreasing the sensitivity of ovarian follicles to FSH. Furthermore, the combined action of AMH and FSH on ovarian follicle development was examined. Three different experiments were performed. Using an in vitro follicle culture system it was shown that FSH-stimulated preantral follicle growth is attenuated in the presence of AMH. This observation was confirmed by an in vivo experiment showing that in immature AMH-deficient females, more follicles start to grow under the influence of exogenous FSH than in their wild-type littermates. In a third experiment, examination of the follicle population of 4-month-old wild-type, FSH beta-, AMH-, and AMH-/FSH beta-deficient females revealed that loss of FSH expression has no impact on the number of primordial and preantral follicles, but the loss of inhibitory action of AMH on the recruitment of primordial follicles in AMH-deficient mice is increased in the absence of FSH. In conclusion, these studies show that AMH inhibits FSH-stimulated follicle growth in the mouse, suggesting that AMH is one of the factors determining the sensitivity of ovarian follicles for FSH and that AMH is a dominant regulator of early follicle growth.

Initial and cyclic recruitment of ovarian follicles

Mammalian ovaries consist of follicles as basic functional units. The total number of ovarian follicles is determined early in life, and the depletion of this pool leads to reproductive senescence. Each follicle develops to either ovulate or, more likely, to undergo degeneration. The dynamics of ovarian follicle development have interested endocrinologists and developmental biologists for many years. With the advent of assisted reproductive techniques in humans, the possibility of regulating follicle development in vivo and in vitro has gained clinical relevance. In this review, we focus upon key branching points during the development of ovarian follicles as well as factors involved in determining the eventual destiny of individual follicles. We discuss inconsistencies in the literature regarding the definitions of follicle recruitment and selection and propose to name the two major steps of follicle development as initial and cyclic recruitment, respectively. Because some of these disparities have arisen due to differences in the animal systems studied, we also compare the development of the ovarian follicles of both humans and rats. We also review the status of knowledge of several puzzling clinical issues that may provide important clues toward unlocking the mechanisms of follicle development.

Control of primordial follicle recruitment by anti-Müllerian hormone in the mouse ovary

The dimeric glycoprotein anti-Müllerian hormone (AMH) is a member of the transforming growth factor-beta superfamily of growth and differentiation factors. During male fetal sex differentiation, AMH is produced by Sertoli cells and induces degeneration of the Müllerian ducts, which form the anlagen of part of the internal female genital system. In females, AMH is produced by the ovary, but only postnatally. The function of AMH in the ovary is, however, still unknown. Female AMH null mice were reported to be fertile, with normal litter size, but this does not exclude a more subtle function for ovarian AMH. To investigate the function of AMH in the ovary, the complete follicle population was determined in AMH null mice, in mice heterozygous for the AMH null mutation, and in wild-type mice of different ages: 25 days, 4 months, and 13 months. In the present study we found that ovaries of 25-day- and 4-month-old AMH null females, compared to those of wild-type females, contain more preantral and small antral follicles. In addition, in 4- and 13-month-old AMH null females, smaller numbers of primordial follicles were found. Actually, in 13-month-old AMH null females, almost no primordial follicles could be detected, coinciding with a reduced number of preantral and small antral follicles in these females. In almost all females heterozygous for the AMH null mutation the number of follicles fell in between the numbers found in wild-type and AMH null females. In 4-month-old AMH null females serum inhibin levels were higher and FSH levels were lower compared to those in wild-type females. In contrast, inhibin levels were lower in 13-month-old AMH null females, and FSH levels were unchanged compared to those in wild-type females. Furthermore, the weight of the ovaries was twice as high in the 4-month-old AMH null females as in age-matched wild-type females. We conclude that AMH plays an important role in primordial follicle recruitment, such that more primordial follicles are recruited in AMH null mice than in wild-type mice; the mice heterozygous for the AMH null mutation take an in-between position. Consequently, the ovaries of AMH null females and those of females heterozygous for the AMH null mutation will show a relatively early depletion of their stock of primordial follicles. The female AMH null mouse may thus provide a useful model to study regulation of primordial follicle recruitment and the relation between follicular dynamics and ovarian aging.

Restricted expression of WT1 messenger ribonucleic acid in immature ovarian follicles: uniformity in mammalian and avian species and maintenance during reproductive senescence

WT1 is a zinc finger protein with transcriptional repressor activity on several growth factor and growth factor receptor genes. In the ovary, a potential role for WT1 in the suppression of the development of immature follicles has been demonstrated. Here, gel retardation assays further showed that recombinant WT1 protein interacted with consensus DNA sequences in the inhibin-alpha gene promoter. We investigated the pattern of WT1 expression in a wide variety of species and also over the reproductive life span in rats. In chicken ovaries, Northern blot analysis revealed the presence of WT1 transcript in small healthy white follicles (1-5 mm in diameter) and its absence in small yellow (6-12 mm in diameter) or larger follicles (F1-F5). In pig and monkey ovaries, WT1 expression was limited to granulosa cells of preantral follicles, as shown by in situ hybridization analysis. In rats, Northern blot analyses demonstrated the presence of WT1 transcript in the ovaries of young (3-mo-old) and middle-aged (9-mo-old) rats on the proestrous day, with a decrease in old (12-mo-old) rats in persistent estrus. In situ hybridization analysis further suggested that the decrease in WT1 expression in aging ovaries was associated with fewer immature follicles. Thus, WT1 expression is restricted to immature follicles in diverse avian and mammalian species and over the reproductive life span in rats. These data demonstrated that WT1 is a marker for immature follicles and suggested a potential role of this transcriptional repressor in the slow growth of early follicles.

Temporal effect of methyl parathion on ovarian compensatory hypertrophy, follicular dynamics and estrous cycle in hemicastrated albino rats

Methyl parathion, an organophosphorus pesticide, was administered i.p. at a dose of 5 mg/kg body weight to hemicastrated virgin rats for 1, 5, 10 and 15 days. Treatment with methyl parathion for 1 and 5 days revealed no change in the ovarian weight with -0.64% and -9.63% hypertrophy respectively and no change in healthy and atretic follicles when compared with hemicastrated oil-treated controls. However, treatment with methyl parathion for 10 and 15 days resulted in a significant decrease in ovarian weight gain with -21.36% and -31.98% hypertrophy, respectively, a significant decrease in the number of healthy follicles, and no change in the number of atretic follicles. The number of estrous cycles and duration of each phase of the estrous cycle were significantly affected with methyl parathion treatment for 5, 10 and 15 days. However, there were no significant changes in the number of estrous cycles and duration of estrus and metestrus phases in 1 day methyl parathion treatment except for a decrease and increase in proestrus and diestrus phases respectively. The weight of the uterus was significantly decreased whereas those of the liver, kidney and adrenal did not change in any of the methyl parathion treated groups.

Menopause: the aging of multiple pacemakers

Menopause signals the permanent end of menstrual cyclicity in a woman's life. Its impact reaches far beyond just the reproductive system. An understanding of the factors that interact and govern the process of aging in the reproductive system will help us to develop strategies for alleviating the negative aspects of menopause and may help us to better comprehend the process of biological aging.