Whole Mount Immunofluorescence and Follicle Quantification of Cultured Mouse Ovaries

Culture of the Intact Postnatal Naked Mole-Rat Ovary: From Meiotic Prophase to Single-Cell RNASeq

Recently, we reported that, in the naked mole-rat (Heterocephalus glaber) ovary, there is mitotic expansion of the primordial germ cells (PGCs), and the initiation of the meiotic program occurs postnatally. This is opposite to almost all other mammals, including humans and mice, whose reproductive cycle begins very early in development. In both mouse and human, the ovaries become populated with PGCs in utero; these PGCs will later generate the oogonia. After mitotic proliferation, these cells will trigger the meiotic program and initiate meiotic prophase I. Given that all these processes happen in utero, their analysis has been very challenging; so the ability to study them postnatally and to manipulate them with inhibitors or other substances, in the naked mole-rat, opens new possibilities in the field. In this chapter, we present a comprehensive collection of protocols that permit the culture of whole naked mole-rat ovaries, followed by analysis of germ cells, from PGCs to oocytes, in meiotic prophase I, as well the obtention of single-cell suspension or single-nuclei suspension for RNASeq.

Comparative analysis of vertebrates reveals that mouse primordial oocytes do not contain a Balbiani body

Oocytes spend the majority of their lifetime in a primordial state. The cellular and molecular biology of primordial oocytes is largely unexplored; yet, studying these is necessary to understand the mechanisms through which oocytes maintain cellular fitness for decades, and why they eventually fail with age. Here, we develop enabling methods for live-imaging based comparative characterization of Xenopus, mouse and human primordial oocytes. We show that primordial oocytes in all three vertebrate species contain active mitochondria, Golgi apparatus and lysosomes. We further demonstrate that human and Xenopus oocytes have a Balbiani body characterized by a dense accumulation of mitochondria in their cytoplasm. However, despite previous reports, we did not find a Balbiani body in mouse oocytes. Instead, we demonstrate what was previously used as a marker for the Balbiani body in mouse primordial oocytes is in fact a ring-shaped Golgi apparatus that is not functionally associated with oocyte dormancy. Our work provides the first insights into the organisation of the cytoplasm in mammalian primordial oocytes, and clarifies relative advantages and limitations of choosing different model organisms for studying oocyte dormancy.

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.

Comparison of methods for quantifying primordial follicles in the mouse ovary

Background

Accurate evaluation of primordial follicle numbers in mouse ovaries is an essential endpoint for studies investigating how endogenous and exogenous insults, such as maternal aging and chemotherapy, impact the ovarian reserve. In this study, we compared and contrasted two methods for counting healthy primordial follicles following exposure to cyclophosphamide (75 mg/kg), a well-established model of follicle depletion. The first was the fractionator/optical dissector technique, an unbiased, assumption-free stereological approach for quantification of primordial follicle numbers. While accurate, highly reproducible and sensitive, this method relies on specialist microscopy equipment and software, requires specific fixation, embedding and sectioning parameters to be followed, and is largely a manual process that is tedious and time-consuming. The second method was the more widely used serial section and direct count approach, which is relatively quick and easy. We also compared the impacts of different fixatives, embedding material and section thickness on the overall results for each method.

Results

Direct counts resulted in primordial follicle numbers that were significantly lower than those obtained by stereology, irrespective of fixation and embedding material. When applied to formalin fixed tissue, the direct count method did not detect differences in follicle numbers between saline and cyclophosphamide treated groups to the same degree of sensitivity as the gold standard stereology method (referred to as the Reference standard). However, when Bouin’s fixative was used, direct counts and stereology were comparable in their ability to detect follicle depletion caused by cyclophosphamide.

Conclusions

This work indicates that the direct count method can produce similar results to stereology when Bouin’s fixative is used instead of formalin. The findings presented here will assist others to select the most appropriate experimental approach for accurate follicle enumeration, depending on whether the primary objective of the study is to determine absolute primordial follicle numbers or relative differences between groups.

A reporter mouse for in vivo detection of DNA damage in embryonic germ cells

Maintaining genome integrity in the germline is essential for survival and propagation of a species. In both mouse and human, germ cells originate during fetal development and are hypersensitive to both endogenous and exogenous DNA damaging agents. Currently, mechanistic understanding of how primordial germ cells respond to DNA damage is limited in part by the tools available to study these cells. We developed a mouse transgenic reporter strain expressing a 53BP1-mCherry fusion protein under the control of the Oct4ΔPE embryonic germ cell-specific promoter. This reporter binds sites of DNA double strand breaks (DSBs) on chromatin, forming foci. Using ionizing radiation as a DNA DSB-inducing agent, we show that the transgenic reporter expresses specifically in the embryonic germ cells of both sexes and forms DNA damage induced foci in both a dose- and time-dependent manner. The dynamic time-sensitive and dose-sensitive DNA damage detection ability of this transgenic reporter, in combination with its specific expression in embryonic germ cells, makes it a versatile and valuable tool for increasing our understanding of DNA damage responses in these unique cells.

Female-biased embryonic death from inflammation induced by genomic instability

Genomic instability (GIN) can trigger cellular responses including checkpoint activation, senescence, and inflammation ^1,2. Though extensively studied in cell culture and cancer paradigms, little is known about the impact of GIN during embryonic development, a period of rapid cellular proliferation. We report that GIN-causing mutations in the MCM2–7 DNA replicative helicase ^3,4 render female mouse embryos to be dramatically more susceptible than males to embryonic lethality. This bias was not attributable to X-inactivation defects, differential replication licensing, or X vs Y chromosome size, but rather “maleness,” since XX embryos could be rescued by transgene-mediated sex reversal or testosterone (T) administration. The ability of exogenous or endogenous T to protect embryos was related to its anti-inflammatory properties ⁵. The NSAID ibuprofen rescued female embryos containing mutations not only in MCM genes but also Fancm, which like MCM mutants have elevated GIN (micronuclei) from compromised replication fork repair ⁶. Additionally, deficiency for the anti-inflammatory IL10 receptor was synthetically lethal with the Mcm4^Chaos3 helicase mutant. Our experiments indicate that DNA replication-associated DNA damage during development induces inflammation that is preferentially lethal to female embryos, whereas male embryos are protected by high levels of intrinsic T.