Use of confocal microscopy and intracytoplasmic sperm injection (ICSI) to assess viability of equine oocytes from young and old mares after vitrification

BACKGROUND

The impact of vitrification on oocyte developmental competence as a function of donor age remains an important issue in assisted reproductive technologies (ARTs).

METHODS

Equine germinal vesicle (GV) or metaphase II (M(II) oocytes were vitrified using the Cryotop® method. Spindle organization and chromosome alignment were evaluated from confocal imaging data sets of in vivo (IVO) or in vitro (IVM) matured oocytes subjected to vitrification or not. Intracytoplasmic sperm injection (ICSI) from the same groups was used to assess developmental potential.

RESULTS

An increase in chromosome misalignment was observed in spindles from older mares when compared to those of younger mares (P < 0.05). When MII oocytes subjected to vitrification were examined following warming, there was no difference in the percentage of oocytes displaying chromosome misalignment. Next, GV oocytes, collected from the ovaries of younger and older mares, were compared between fresh IVM and IVM following vitrification and warming. For nonvitrified samples, an age difference was again noted for spindle organization and chromosome alignment, with a higher (P < 0.05) percentage of normal bipolar meiotic spindles with aligned chromosomes observed in nonvitrified oocytes from young versus older mares. Vitrification led to a reduction of spindle length (P < 0.05) for oocytes from old mares, whether vitrified at GV or MII stages, whereas this effect was not observed in oocytes from young mares except those vitrified at GV and subjected to IVM. Oocyte developmental potential after vitrification was evaluated after ICSI of vitrified and warmed MII or GV oocytes from young mares. From 25 MII oocytes, 18 oocytes were injected with sperm, and six blastocysts were produced, which, upon transfer to mares' uteri, resulted in four pregnancies. Immature (GV) oocytes collected from live mares were also vitrified, warmed, and matured in vitro before ICSI. In this group, nonvitrified, control, and vitrified oocytes did not differ (P > 0.05) with respect to the incidence of maturation to MII, cleavage after ICSI, or blastocyst development.

CONCLUSION

These findings demonstrate an effect of maternal age in an equine model at the level of meiotic spindle integrity and chromosome positioning that is influenced by both the meiotic stage at which oocytes are vitrified and whether meiotic maturation occurred in vivo or in vitro.

Sperm centriolar factors and genetic defects that can predict pregnancy

The human sperm centrosome, comprising the two morphologically distinct centrioles and associated pericentriolar materials, plays a crucial role in fertilization and early embryonic development after fertilization. Once inside the oocyte, the sperm centrosome serves as a microtubule-organizing center, orchestrating mitotic spindle formation, chromosome segregation, and syngamy. Abnormalities of the sperm centrosome can lead to abnormal embryonic development and embryonic chromosomal instability, and are associated with pregnancy loss. Recent research has shed light on the molecular composition, regulation, and function of this vital organelle. Understanding the intricacies of the sperm centrosome is crucial for elucidating the mechanisms underlying successful fertilization and early embryonic development, as well as addressing infertility and developmental disorders associated with centrosomal defects.

Changing clinical significance of oocyte maturity grades with advancing female age advances precision medicine in IVF

In current IVF practice, metaphase-2 (M2) oocytes are considered most efficient in producing good quality embryos. Maximizing their number at all ages is standard clinical practice, while immature germinal vesicle (GV) oocytes are mostly automatically discarded. We present preliminary evidence that oocyte maturity grades with advancing age significantly change in their abilities to produce good quality embryos, with M2 oocytes significantly declining, GV oocytes improving, and M1 oocytes staying the same. These data contradict the over-40-year-old dogma that oocyte grades functionally do not change with advancing age, supporting potential changes to current IVF practice: (1) Stimulation protocols and timing of oocyte retrieval can be adjusted to a patient's age and ovarian function. (2) In older and younger women with prematurely aging ovaries, GV oocytes may no longer be automatically discarded. (3) In some infertile women, rescue in vitro maturation of immature oocytes may delay the need for third-party egg donation.

Preliminary report of intraovarian injections of autologous platelet-rich plasma (PRP) in extremely poor prognosis patients with only oocyte donation as alternative: a prospective cohort study

STUDY QUESTION

Does intraovarian injection of platelet-rich plasma (PRP) change ovarian function in patients with extremely low functional ovarian reserve (LFOR) who, otherwise, would likely only have a chance of pregnancy through third-party oocyte donation?

SUMMARY ANSWER

No clinically significant effects of PRP treatment on ovarian function were observed over 1 year of follow-up.

WHAT IS KNOWN ALREADY

Several investigators have reported improved responses to ovulation induction after treatment with PRP. However, previous published reports have involved, at most, only small case series. Whether PRP actually improves ovarian performance is, therefore, still unknown. PRP is nevertheless widely offered as an ‘established’ fertility treatment, often under the term ‘ovarian rejuvenation’.

STUDY DESIGN, SIZE, DURATION

We are reporting a prospective cohort study of 80 consecutive patients at ages 28–54 with LFOR, defined by anti-Müllerian hormone &lt;1.1 ng/ml, FSH &gt;12 mIU/ml or at least one prior IVF cycle with ≤3 oocytes within 1 year. The women were followed for 1 year after an intraovarian PRP procedure.

PARTICIPANTS/MATERIALS, SETTING, METHODS

PRP (1.5 ml) was injected into the cortex of ovaries with an average of 12 injections per ovary. Study participants were followed every 3 days for 2 weeks after PRP treatment with estradiol and FSH measurements and vaginal ultrasound to observe follicle growth and thereafter followed weekly. Beginning 1 month after their PRP treatment, participants underwent one or more cycles of ovarian stimulation for IVF. Outcome measures were endocrine response, and numbers of oocytes and embryos produced in response to a maximal gonadotropin stimulation before and after PRP treatment.

MAIN RESULTS AND THE ROLE OF CHANCE

In this study, women failed to demonstrate statistically significant outcome benefits from intraovarian PRP. However, two 40-year-old very poor-prognosis patients, with prior failed IVF cycles that never reached embryo transfer at other centers, achieved pregnancy, resulting in an ongoing pregnancy rate of 4.7% among patients who, following PRP, produced at least one oocyte (n = 42).

LIMITATIONS, REASONS FOR CAUTION

As an observational study of patients who performed poorly in past ovarian stimulation cycles, the improvement may be accounted for by regression to the mean. Similar considerations may also explain the occurrence of the two pregnancies.

WIDER IMPLICATIONS OF THE FINDINGS

This study demonstrates that, even in extremely poor prognosis patients due to LFOR, sporadic pregnancies are possible. The study, however, does not allow for the conclusion that those pregnancies were the consequence of PRP treatments. A case series, indeed, does not allow for such conclusions, even if results are more suggestive than here. This registered study, therefore, must be viewed as a preliminary report, with further data expected from this study but also from two other prospectively randomized ongoing registered studies with more controlled patient selection.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by intramural funds from The Center for Human Reproduction and the not-for-profit research Foundation for Reproductive Medicine, both in New York, NY, USA. N.G. and D.H.B. are listed as co-inventors on several US patents. Some of these patents relate to pre-supplementation of hypo-androgenic infertile women with androgens, such as dehydroepiandrosterone and testosterone and, therefore, at least peripherally relate to the subject of this manuscript. They, as well as D.F.A., have also received research support, travel funds and speaker honoraria from several pharmaceutical and medical device companies, though none related to the here presented subject and manuscript. N.G. is a shareholder in Fertility Nutraceuticals and he and D.H.B. receive royalty payments from Fertility Nutraceuticals LLC. E.M. has no conflicts of interest to declare.

TRIAL REGISTRATION NUMBER

NCT04275700

The changing world of IVF: the pros and cons of new business models offering assisted reproductive technologies

This analysis contrasts traditional not-for-profit academic with new corporate practices of reproductive medicine and offers an assessment of risks to quality of patient care with investors entering the for-profit reproductive medicine market. Large corporate enterprises may have a global impact on access to care while at the same time is putting at risk the training of the next generation of reproductive medicine specialists.

O-176 Secondary ovarian insufficiency (SOI) - a new infertility diagnosis

Study question

Can, in contrast to primary ovarian insufficiency (POI), ovarian insufficiency also be caused as a secondary event (SOI)?

Summary answer

Adrenal hypo-androgenism, caused by insufficiency in androgen production by the zona reticularis, may mimic POI in clinical presentation.

What is known already

A variety of small and large animal models have, especially at small growing follicle stages, conclusively demonstrated the dependence of normal follicle maturation on adequate testosterone levels, a process mediated via the androgen receptor on granulosa cells. Increasing clinical evidence, moreover, has demonstrated clinical improvements in oocyte numbers and oocyte quality following androgen supplementation at these stages in hypo-androgenic women with premature ovarian aging (POA). Whether adrenal hypo-androgenism in extreme cases can, however, also lead to complete ovarian insufficiency mimicking POI, is unknown.

Study design, size, duration

Based on FSH levels &gt;40.0 mIU/mL and amenorrhea, we searched our center’s electronic research data bank for patients who between 2016-2018 presented with a diagnosis of presumed POI.

Participants/materials, setting, methods

Among 78 POIs, 13 demonstrated low DHEAS (&lt; 100 ug/dL), i.e., adrenal hypo-androgenism, 6 rejected egg donation and received DHEA (Fertinatal®, 25mg TID, Fertility Nutraceuticals, LLC, New York, N.Y., USA) and Coq10 (Ovoenergen®, 333mg TID,same ­manufacturer) before stimulation with 450IU FSH and 150 IU hMG (different ­manufacturers). All women were &lt; age 41, demonstrated menopausal FSH (&gt;40.0 mIU/mL), undetectable AMH, amenorrhea, &gt; prior cycle cancellation, elevated SHBG, low total testosterone and low DHEAS (high DHEA/DHEAS).

Main results and the role of chance

Out of the 6 patients so stimulated, 5 demonstrated follicular responses following DHEA supplementation and 2/6 conceived spontaneously and delivered healthy offspring. One of these two is currently in treatment for another child.

Limitations, reasons for caution

To better understand adrenal control of ovaries via androgen production, further elucidation of endocrine signaling between adrenals and ovaries is required, including detection of unknown, though increasingly likely, feedback loops, considering that adrenals and ovaries share a primordium.

Wider implications of the findings

We confirmed existence of SOI due to adrenal hypoandrogenism, defined its phenotype, demonstrating that some patients with alleged POI actually exhibit SOI. Better pregnancy chances with adrenal SOI than POI, reemphasize the importance of correct differential diagnoses to avoid premature referrals of women into third-party egg donations

Trial registration number

N/A

O-170 Predictive value of cytoplasmic granulation patterns during IVF in MII oocytes from young donors

Study question

Do ooplasm granulation patterns of donor MII oocytes have similar predictive values for in vitro fertilization (IVF) outcomes as they have in older infertile women?

Summary answer

Ooplasm granulation patterns of donor MII oocytes are predictive for IVF outcomes in young oocyte donors even more pronounced than in older poor prognosis patients.

What is known already

Cytoplasmic granules had been noticed for years, with data mostly focused on central granulation. Dispersed granulations were mentioned but lacked analysis.

Study design, size, duration

A retrospective cohort study during 2017-2020.

Participants/materials, setting, methods

We investigated 776 fresh and 381 vitrified-thawed MII oocytes from carefully selected young donors (mean, 26.7±2.7; range, 21-35 years) and determined cytoplasmic granulation patterns during intracytoplasmic sperm injection as fine, central, uneven, dispersed and peripheral (see only in thawed oocytes). Fertilization, pregnancy and live-birth rates in fresh and thawed donor oocytes were analyzed

Main results and the role of chance

In fresh donor oocytes: 2PN rates significantly trended down from 96.3% to 90.7%, 89.2%, 66.7% from fine to central, uneven, dispersed granulations; overall pregnancy rates trended down from 48.8% to 29.0%, 19.0% and 6.4%, as did live birth rates (42.1%, 21.6%, 12.5%, 6.4%), from fine to uneven, central and dispersed granulations. Known-pregnancy and known-live-birth analyses showed similar findings. Thawed donor oocytes demonstrated similar trends, though with significantly worse outcomes than fresh oocytes. Peripheral granulation, unique to vitrification and thawing, always demonstrated the worst IVF outcomes. Interestingly, granulation patterns were mostly disassociated from morphologic embryo grades in fresh and thawed donor oocytes.

Limitations, reasons for caution

As a retrospective cohort study, some cases had to be excluded for lack of information. The scoring system may have diluted the real contribution of an oocyte when two or more embryos were transferred.

Wider implications of the findings

Ooplasm granulation patterns have predictive values for fertilization, pregnancy and live birth in IVF cycles, supporting integration of them into embryo selection, and suggesting that ooplasm granulation patterns reflect intrinsic features of oocytes that relate to oocyte quality, cytoplasmic maturity and developmental competence, but are largely independent of clinical co-variables.

Trial registration number

NA

P–666 Validating the hypo-androgenic PCOS-like phenotype (H-PCOS), derived from the “lean” PCOS phenotype at younger ages

Study question

Is the resistance to standard infertility treatments of the H-PCOS-like phenotype reversed through reconstitution of androgen levels and can principle diagnostic markers of H-PCOS be validated?

Summary answer

Pre-supplementation with dehydroepiandrosterone (DHEA) eliminated treatment resistance of H-PCOS in comparison to matched infertile controls, also validating previously reported diagnostic features of this condition.

What is known already

H-PCOS evolves at older ages from a hyper-androgenic “lean” PCOS phenotype at young ages. Its ontogeny diverts from other PCOS phenotype between 20s and mid–30s by going from being hyper- to being hypo-androgenic due to insufficiency in adrenal androgen production, believed to represent an autoimmune process. In contrast to other PCOS phenotypes, the “lean” PCOS phenotype appears highly treatment resistant to standard fertility treatments.

Study design, size, duration

Retrospective case control study.

Participants/materials, setting, methods

We investigated 54 H-PCOS patients with qualifying diagnostic criteria1,2 and 50 matched infertility patients without diagnostic H-PCOS criteria as controls. Both study groups underwent routine in vitro fertilization (IVF) cycles, including androgen pre-supplementation in both groups via dehydroepiandrosterone (DHEA) for women diagnosed as hypo-androgenic. Main outcome measures were clinical pregnancy and live birth rates.

1Gleicher et al., J Sterodi Biochem Mol Biol 2017;167:144–152; 2Gleicher N, et al., Endocrine 2018;59(3):661–676

Main results and the role of chance

Study groups were similar in age, number of prior IVF cycles and previous live births. H-PCOS patients in contrast to controls, however, demonstrated previously reported characteristics of H-PCOS diagnosis, including a significantly higher DHEA/DHEAS ratio, significantly higher AMH, confirming higher functional ovarian reserve, significantly lower free testosterone and significantly higher sex hormone binding globulin (SHBG), further confirming lower androgens. Finally, H-PCOS patients also demonstrated significantly increased evidence for immune system hyperactivity. Clinical pregnancy and live birth rates were separately assessed in first IVF cycles and cumulatively. Both analyses demonstrated, even after age-adjustments, absolutely no outcome differences in cycle cancellations, numbers of oocytes retrieved, first and cumulative pregnancy and live birth rates. At least one pregnancy was achieved in 12 women in both groups (22.2% and 24.0%) and at least one live birth in 11 (20.4%) vs. 8 (14.8%), respectively.

Limitations, reasons for caution

As a retrospective case control study, here presented data must be interpreted with caution. The close match between H-PCOS and control patients and the very clear differentiation in patient characteristics between the two groups, however, support the credibility of this study.

Wider implications of the findings: This study demonstrated that androgen reconstitution in H-PCOS patients completely reversed treatment resistance compared to well-matched infertile control patients. It also validated previously defined diagnostic criteria of H-PCOS, hopefully facilitating a timelier diagnosis of a, still, widely overlooked condition in female infertility.

Trial registration number

NA

P–199 A case report to suggest that there must be other mutations than PATL2 or TUBB8 to cause oocyte maturation arrest

Study question

What causes our patient’s repeated almost complete oocyte maturation arrest (OMA)?

Summary answer

Since we did not detect PATL2 and TUBB8 mutations, both known to cause OMA, this case was likely caused by mutations in HUS1 and ITGB3

What is known already

OMA has been associated with loss-of-function in key genes, such as PATL2 and TUBB8. Such patients have, however, uniformly have been unable to conceive with IVF

Study design, size, duration

We here report the case of repeatedly presenting patient between 2009 until 2020 (age 30 at 1st and 41 at last visit).

Participants/materials, setting, methods

The couple underwent 7 IVF treatments under several ovarian stimulation protocols at different gonadotropin dosages and in different preparations to try to recruit mature eggs. She conceived in her 2nd IVF cycle in 2009 and delivered uneventfully in 2010. She then conceived spontaneously and delivered a healthy boy in 2014. The couple since then has been attempting another pregnancy. Remarkably, in all IVF cycles all eggs but one arrested at prophase.

Main results and the role of chance

The female demonstrates abnormally high ovarian reserve for age (AMH=5.9 ng/mL in 2019) (mean, 10.6 oocytes). In all cycles, all but one retrieved were immature. In vitro maturation rate for the GV oocytes was 28%. Resultant M2s, however, demonstrated morphological abnormalities, such as giant polar bodies. In vivo M2s, in contrast, were always morphologically unremarkable, and their fertilization rate was 85%. Embryo morphology deteriorated appreciatively with advancing age. Sanger sequencing for TUBB8 and PATL2 genes were unremarkable. Whole genome sequencing of her and her sister (who had no fertility problems) revealed mutations of genes belonging to the integrin family (ITGB3) and DNA repair checkpoint (HUS1), both of which could be determinants in the observed maturation arrest.

Limitations, reasons for caution

A functional study, coupled with imaging of the discarded material, will likely offer further information regarding the mechanisms leading to OMA in this female.

Wider implications of the findings: This case report represents a new phenotype of female infertility, characterized by almost complete maturation arrest which, however, still offers opportunity for pregnancy. Further isolation of underlying mutation(s) may offer additional insights about checkpoints required for the transition of prophase to metaphase in human oocytes.

Trial registration number

NA

Perspectives on the development and future of oocyte IVM in clinical practice

Oocyte in vitro maturation (IVM) is an assisted reproductive technology designed to obtain mature oocytes following culture of immature cumulus–oocyte complexes collected from antral follicles. Although IVM has been practiced for decades and is no longer considered experimental, the uptake of IVM in clinical practice is currently limited. The purpose of this review is to ensure reproductive medicine professionals understand the appropriate use of IVM drawn from the best available evidence supporting its clinical potential and safety in selected patient groups. This group of scientists and fertility specialists, with expertise in IVM in the ART laboratory and/or clinic, explore here the development of IVM towards acquisition of a non-experimental status and, in addition, critically appraise the current and future role of IVM in human ART.

The subcortical maternal complex: emerging roles and novel perspectives

Since its recent discovery, the subcortical maternal complex (SCMC) is emerging as a maternally inherited and crucial biological structure for the initial stages of embryogenesis in mammals. Uniquely expressed in oocytes and preimplantation embryos, where it localizes to the cell subcortex, this multiprotein complex is essential for early embryo development in the mouse and is functionally conserved across mammalian species, including humans. The complex has been linked to key processes leading the transition from oocyte to embryo, including meiotic spindle formation and positioning, regulation of translation, organelle redistribution, and epigenetic reprogramming. Yet, the underlying molecular mechanisms for these diverse functions are just beginning to be understood, hindered by unresolved interplay of SCMC components and variations in early lethal phenotypes. Here we review recent advances confirming involvement of the SCMC in human infertility, revealing an unexpected relationship with offspring health. Moreover, SCMC organization is being further revealed in terms of novel components and interactions with additional cell constituents. Collectively, this evidence prompts new avenues of investigation into possible roles during the process of oogenesis and the regulation of maternal transcript turnover during the oocyte to embryo transition.

A Nuclear and Cytoplasmic Characterization of Bovine Oocytes Reveals That Cysteamine Partially Rescues the Embryo Development in a Model of Low Ovarian Reserve

Simple Summary

Women’s reproductive performance starts declining in the mid-30s, and by age 40–45, the possibility of becoming pregnant becomes very small. Reproductive aging is a physiological process of fertility decline characterized by a decrease in quality and stockpile of eggs (also called ovarian reserve) in most mammals. However, young individuals too can show an accelerated reproductive aging that similarly results in a low ovarian reserve and hypofertility. This syndrome, called premature ovarian failure (POF), is becoming a relevant problem due to the general tendency to postpone the first pregnancy. In this study, we used bovine ovaries that were classified in two categories, according to the number of follicles visible on the ovarian surface, and analyzed some parameters of egg maturation. We observed that eggs from the ‘aging-like’ ovaries carry several defects that impair maturation. However, one of the parameters was improved upon supplementation with a scavenger of free radicals, providing a proof of concept that in-depth knowledge of the cellular mechanisms is essential to find solutions to everyday-life problems.

Abstract

Decreased oocyte quality is a major determinant of age-associated fertility decline. Similarly, individuals affected by early ovarian aging carry low-quality oocytes. Using an established bovine model of early ovarian aging, we investigated key features of ‘quality’ oocyte maturation, associated with the onset of egg aneuploidy and reproductive aging, such as histone modifications, mitochondria distribution and activity, reduced glutathione (GSH) content, and gap junction functionality. Bovine ovaries were classified according to the antral follicle count (AFC), and the retrieved oocytes were processed immediately or matured in vitro. We observed alterations in several cellular processes, suggesting a multifactorial etiology of the reduced oocyte quality. Furthermore, we performed a rescue experiment for one of the parameters considered. By adding cysteamine to the maturation medium, we experimentally increased the free radical scavenger ability of the ‘low competence’ oocytes and obtained a higher embryo development. Our findings show that adopting culture conditions that counteract the free radicals has a positive impact on the quality of ‘compromised’ oocytes. Specifically, cysteamine treatment seems to be a promising option for treating aging-related deficiencies in embryo development.

A form of secondary ovarian insufficiency (SOI) due to adrenal hypoandrogenism as new infertility diagnosis

BACKGROUND

Mediated via the androgen receptor on granulosa cells, models of small growing follicle stages demonstrate dependence on testosterone. Androgen deficiency reduces ovarian response to follicle stimulation hormone (FSH), granulosa cell mass and estradiol (E2) production falls and FSH, therefore, rises. Though potentially of adrenal and/or ovarian origin, androgen deficiency in association with female infertility is almost universally primarily of adrenal origin, raising the possibility that women with presumptive diagnosis of primary ovarian insufficiency (POI), also called primary ovarian failure (POF) may actually suffer from secondary ovarian insufficiency (SOI) due to adrenal hypoandrogenism that leads to follicular arrest at small-growing follicle stages.

METHODS

This retrospective cohort study was performed in a private, academically affiliated infertility center in New York City. We searched the center's anonymized electronic research data bank for consecutive patients who presented with a diagnosis of POI, defined by age <41 year, FSH > 40.0 mIU/mL, amenorrhea for at least 6 month, and low testosterone (T), defined as total T (TT) in the lowest age-specific quartile of normal range. This study did not include patients with oophoritis. Since dehydroepiandrosterone sulfate (DHEAS) is the only androgen almost exclusively produce by adrenals, adrenal hypoandrogenism was defined by DHEAS < 100ug/dL. Thirteen of 78 presumed POI women (16.67%) qualified and represented the original study population. POI patients are usually treated with third-party egg donation; 6/13, however, rejected egg donation for personal or religious reasons and insisted on undergoing at least one last IVF cycle attempt (final study population). In preparation, they were supplemented with DHEA 25 mg TID and CoQ10 333 mg TID for at least 6 weeks prior to ovarian stimulation for IVF with FSH and human menopausal gonadotropins (hMG). Since POI patients are expected to be resistant to ovarian stimulation, primary outcome for the study was ovarian response, while secondary outcome was pregnancy/delivery.

RESULTS

Though POI/POF patients usually are completely unresponsive to ovarian stimulation, to our surprise, 5/6 (83.3%) patients demonstrated an objective follicle response. In addition, 2/6 (33.3%) conceived spontaneously between IVF cycles, while on DHEA and CoQ10 supplementation and delivered healthy offspring. One of those is currently in treatment for a second child.

CONCLUSIONS

This preliminary report suggests that a surprising portion of young women below age 41, tagged with a diagnosis of POI/POF, due to adrenal hypoandrogenism actually suffer from a form of SOI, at least in some cases amenable to treatment by androgen supplementation. Since true POI/POF usually requires third-party egg donation, correct differentiation between POI and SOI in such women appears of great importance and may warrant a trial stimulation after androgen pre-supplementation for at least 6 weeks.

Germ cell nests in adult ovaries and an unusually large ovarian reserve in the naked mole-rat

The naked mole-rat (NMR, Heterocephalus glaber) is renowned for its eusociality and exceptionally long lifespan (> 30 y) relative to its small body size (35-40 g). A NMR phenomenon that has received far less attention is that females show no decline in fertility or fecundity into their third decade of life. The age of onset of reproductive decline in many mammalian species is closely associated with the number of germ cells remaining at the age of sexual maturity. We quantified ovarian reserve size in NMRs at the youngest age (6 months) when subordinate females can begin to ovulate after removal from the queen's suppression. We then compared the NMR ovarian reserve size to values for 19 other mammalian species that were previously reported. The NMR ovarian reserve at 6 months of age is exceptionally large at 108,588 ± 69,890 primordial follicles, which is more than 10-fold larger than in mammals of a comparable size. We also observed germ cell nests in ovaries from 6-month-old NMRs, which is highly unusual since breakdown of germ cell nests and the formation of primordial follicles is generally complete by early postnatal life in other mammals. Additionally, we found germ cell nests in young adult NMRs between 1.25 and 3.75 years of age, in both reproductively activated and suppressed females. The unusually large NMR ovarian reserve provides one mechanism to account for this species' protracted fertility. Whether germ cell nests in adult ovaries contribute to the NMR's long reproductive lifespan remains to be determined.