The dynamic expression of SOX17 in germ cells from human female foetus and adult ovaries after specification

Background

SOX17 has been identified as a critical factor in specification of human primordial germ cells, but whether SOX17 regulates development of germ cells after sex differentiation is poorly understood.

Methods

We collected specimens of gonadal ridge from an embryo (n=1), and ovaries of foetuses (n=23) and adults (n=3). Germ cells were labelled with SOX17, VASA (classic germ cells marker), phosphohistone H3 (PHH3, mitosis marker) and synaptonemal complex protein 3 (SCP3, meiosis marker).

Results

SOX17 was detected in both cytoplasm and nucleus of oogonia and oocytes of primordial and primary follicles from 15 to 28 gestational weeks (GW). However, it was exclusively expressed in cytoplasm of oogonia at 7 GW, and in nucleus of oocytes in secondary follicles. Co-expression rates of SOX17 in VASA+ germ cells ranged from 81.29% to 97.81% in foetuses. Co-staining rates of SOX17 and PHH3 or SCP3 were 0%-34% and 0%-57%, respectively. Interestingly, we distinguished a subpopulation of SOX17+VASA- germ cells in fetal ovaries. These cells clustered in the cortex and could be co-stained with the mitosis marker PHH3 but not the meiosis marker SCP3.

Conclusions

The dynamic expression of SOX17 was detected in human female germ cells. We discovered a population of SOX17+ VASA- germ cells clustering at the cortex of ovaries. We could not find a relationship between mitosis or meiosis and SOX17 or VASA staining in germ cells. Our findings provide insight into the potential role of SOX17 involving germ cells maturation after specification, although the mechanism is unclear and needs further investigation.

Msl3 promotes germline stem cell differentiation in female Drosophila

Gamete formation from germline stem cells (GSCs) is essential for sexual reproduction. However, the regulation of GSC differentiation is incompletely understood. Set2, which deposits H3K36me3 modifications, is required for GSC differentiation during Drosophila oogenesis. We discovered that the H3K36me3 reader Male-specific lethal 3 (Msl3) and histone acetyltransferase complex Ada2a-containing (ATAC) cooperate with Set2 to regulate GSC differentiation in female Drosophila. Msl3, acting independently of the rest of the male-specific lethal complex, promotes transcription of genes, including a germline-enriched ribosomal protein S19 paralog RpS19b. RpS19b upregulation is required for translation of RNA-binding Fox protein 1 (Rbfox1), a known meiotic cell cycle entry factor. Thus, Msl3 regulates GSC differentiation by modulating translation of a key factor that promotes transition to an oocyte fate.

Co-localization of DrPiwi-1 and DrPiwi-2 in the oogonial cytoplasm is essential for oocyte differentiation in sexualized planarians

P-Element-induced wimpy testis (Piwi) subfamily proteins form complexes that bind to Piwi-interacting RNA. This interaction is crucial for stem cell regulation and formation, maintenance of germline stem cells, and gametogenesis in several metazoans. Planarians are effective models for studying stem cells. In the planarian Dugesia ryukyuensis, DrPiwi-1 is essential for the development of germ cells, but not somatic cells and sexual organs. DrPiwi-2 is indispensable for regeneration. In this study, we aimed to investigate the effects of Piwi on the differentiation of germ cells using monoclonal antibodies against DrPiwi-1 and DrPiwi-2. DrPiwi-1 and DrPiwi-2 co-localized more in immature germ cells than in mature germ cells in the ovary. DrPiwi-1 was found in the cytoplasm of early oogonia as undifferentiated germ cells, whereas DrPiwi-2 was found to localize not only in the nuclei but also in the cytoplasm of early oogonia. In descendant germ cells (oocytes), DrPiwi-2 was not present in the cytoplasm, but was strongly detected in the nucleolus. Moreover, we found that DrPiwi-1 forms a complex with DrPiwi-2. The cause of DrPiwi-1 depletion may be the severe reduction in the DrPiwi-2 level in the cytoplasm of oogonia. These results suggest that the formation of the DrPiwi-1 and DrPiwi-2 complex in the cytoplasm of oogonia is essential for oocyte differentiation. Our findings support the conclusion that DrPiwi-1 forms a complex with DrPiwi-2 in the cytoplasm of undifferentiated germ cells, and it signifies the start of gametogenesis. In contrast, in the testes, Drpiwi-1 was found in undifferentiated germ cells (spermatogonia), whereas DrPiwi-2 was found in descendant germ cells (spermatocytes). The process of germ cell differentiation from adult stem cells in planarians may be regulated in different ways in female and male germ lines by the Piwi family.

Translation and codon usage regulate Argonaute slicer activity to trigger small RNA biogenesis

In the Caenorhabditis elegans germline, thousands of mRNAs are concomitantly expressed with antisense 22G-RNAs, which are loaded into the Argonaute CSR-1. Despite their essential functions for animal fertility and embryonic development, how CSR-1 22G-RNAs are produced remains unknown. Here, we show that CSR-1 slicer activity is primarily involved in triggering the synthesis of small RNAs on the coding sequences of germline mRNAs and post-transcriptionally regulates a fraction of targets. CSR-1-cleaved mRNAs prime the RNA-dependent RNA polymerase, EGO-1, to synthesize 22G-RNAs in phase with translating ribosomes, in contrast to other 22G-RNAs mostly synthesized in germ granules. Moreover, codon optimality and efficient translation antagonize CSR-1 slicing and 22G-RNAs biogenesis. We propose that codon usage differences encoded into mRNA sequences might be a conserved strategy in eukaryotes to regulate small RNA biogenesis and Argonaute targeting.

Characterization of the Polysialylation Status in Ovaries of the Salmonid Fish Coregonus maraena and the Percid Fish Sander lucioperca

In vertebrates, the carbohydrate polymer polysialic acid (polySia) is especially well known for its essential role during neuronal development, regulating the migration and proliferation of neural precursor cells, for instance. Nevertheless, sialic acid polymers seem to be regulatory elements in other physiological systems, such as the reproductive tract. Interestingly, trout fish eggs have polySia, but we know little of its cellular distribution and role during oogenesis. Therefore, we localized α2,8-linked N-acetylneuraminic acid polymers in the ovaries of Coregonus maraena by immunohistochemistry and found that prevalent clusters of oogonia showed polySia signals on their surfaces. Remarkably, the genome of this salmonid fish contains two st8sia2 genes and one st8sia4 gene, that is, three polysialyltransferases. The expression analysis revealed that for st8sia2-r2, 60 times more mRNA was present than st8sia2-r1 and st8sia4. To compare polysialylation status regarding various polySiaT configurations, we performed a comparable analysis in Sander lucioperca. The genome of this perciform fish contains only one st8sia2 and no st8sia4 gene. Here, too, clusters of oogonia showed polysialylated cell surfaces, and we detected high mRNA values for st8sia2. These results suggest that in teleosts, polySia is involved in the cellular processes of oogonia during oogenesis.

Environmentally-relevant exposure to diethylhexyl phthalate (DEHP) alters regulation of double-strand break formation and crossover designation leading to germline dysfunction in Caenorhabditis elegans

Exposure to diethylhexyl phthalate (DEHP), the most abundant plasticizer used in the production of polyvinyl-containing plastics, has been associated to adverse reproductive health outcomes in both males and females. While the effects of DEHP on reproductive health have been widely investigated, the molecular mechanisms by which exposure to environmentally-relevant levels of DEHP and its metabolites impact the female germline in the context of a multicellular organism have remained elusive. Using the Caenorhabditis elegans germline as a model for studying reprotoxicity, we show that exposure to environmentally-relevant levels of DEHP and its metabolites results in increased meiotic double-strand breaks (DSBs), altered DSB repair progression, activation of p53/CEP-1-dependent germ cell apoptosis, defects in chromosome remodeling at late prophase I, aberrant chromosome morphology in diakinesis oocytes, increased chromosome non-disjunction and defects during early embryogenesis. Exposure to DEHP results in a subset of nuclei held in a DSB permissive state in mid to late pachytene that exhibit defects in crossover (CO) designation/formation. In addition, these nuclei show reduced Polo-like kinase-1/2 (PLK-1/2)-dependent phosphorylation of SYP-4, a synaptonemal complex (SC) protein. Moreover, DEHP exposure leads to germline-specific change in the expression of prmt-5, which encodes for an arginine methyltransferase, and both increased SC length and altered CO designation levels on the X chromosome. Taken together, our data suggest a model by which impairment of a PLK-1/2-dependent negative feedback loop set in place to shut down meiotic DSBs, together with alterations in chromosome structure, contribute to the formation of an excess number of DSBs and altered CO designation levels, leading to genomic instability.

Faithful chromosome segregation during meiosis, the specialized cell division program that produces haploid gametes (i.e. eggs and sperm) from a diploid organism, is key for successful sexual reproduction. Diethylhexyl phthalate (DEHP), a commonly used plasticizer found in personal care and household products, has emerged as an endocrine disruptor that exerts reprotoxicity in mammals. In this study, we provide mechanistic insight into the modes of action by which environmentally-relevant levels of DEHP and its metabolites impair female meiosis in the C. elegans germline. Exposure to DEHP leads to defects in late prophase I chromosome remodeling, altered chromosome morphology in oocytes at diakinesis, errors in chromosome segregation, and impaired embryogenesis. Underlying these defects are higher levels of DSBs, altered DSB repair, defects in crossover (CO) designation/formation, germline-specific change in prmt-5 gene expression and altered chromosome structure. We propose that DEHP exposure induces an excess number of DSBs by interfering with mechanisms set in place to turn off DSBs once CO designation is accomplished and by altering chromosome structure resulting in increased chromatin accessibility to the DSB machinery.

Germ Cell Lineage Homeostasis in Drosophila Requires the Vasa RNA Helicase

The conserved RNA helicase Vasa is required for germ cell development in many organisms. In Drosophila melanogaster loss of PIWI-interacting RNA pathway components, including Vasa, causes Chk2-dependent oogenesis arrest. However, whether the arrest is due to Chk2 signaling at a specific stage and whether continuous Chk2 signaling is required for the arrest is unknown. Here, we show that absence of Vasa during the germarial stages causes Chk2-dependent oogenesis arrest. Additionally, we report the age-dependent decline of the ovariole number both in flies lacking Vasa expression only in the germarium and in loss-of-function vasa mutant flies. We show that Chk2 activation exclusively in the germarium is sufficient to interrupt oogenesis and to reduce ovariole number in aging flies. Once induced in the germarium, Chk2-mediated arrest of germ cell development cannot be overcome by restoration of Vasa or by downregulation of Chk2 in the arrested egg chambers. These findings, together with the identity of Vasa-associated proteins identified in this study, demonstrate an essential role of the helicase in the germ cell lineage maintenance and indicate a function of Vasa in germline stem cell homeostasis.

Separation and Loss of Centrioles From Primordidal Germ Cells To Mature Oocytes In The Mouse

Oocytes, including from mammals, lack centrioles, but neither the mechanism by which mature eggs lose their centrioles nor the exact stage at which centrioles are destroyed during oogenesis is known. To answer questions raised by centriole disappearance during oogenesis, using a transgenic mouse expressing GFP-centrin-2 (GFP CETN2), we traced their presence from e11.5 primordial germ cells (PGCs) through oogenesis and their ultimate dissolution in mature oocytes. We show tightly coupled CETN2 doublets in PGCs, oogonia, and pre-pubertal oocytes. Beginning with follicular recruitment of incompetent germinal vesicle (GV) oocytes, through full oocyte maturation, the CETN2 doublets separate within the pericentriolar material (PCM) and a rise in single CETN2 pairs is identified, mostly at meiotic metaphase-I and -II spindle poles. Partial CETN2 foci dissolution occurs even as other centriole markers, like Cep135, a protein necessary for centriole duplication, are maintained at the PCM. Furthermore, live imaging demonstrates that the link between the two centrioles breaks as meiosis resumes and that centriole association with the PCM is progressively lost. Microtubule inhibition shows that centriole dissolution is uncoupled from microtubule dynamics. Thus, centriole doublets, present in early G2-arrested meiotic prophase oocytes, begin partial reduction during follicular recruitment and meiotic resumption, later than previously thought.

tdrd1 is a germline-specific and sexually dimorphically expressed gene in Paralichthys olivaceus

Tudor domain containing protein 1 (tdrd1) is a member of the Tudor family and has shown essential functions during embryogenesis and gametogenesis. In this study, we cloned the full length cDNA of Paralichthys olivaceus tdrd1 (Potdrd1). PoTDRD1 is a multidomain protein with an N-terminal MYND zinc finger domain, followed by four tandem extended Tudor domains. Sequence comparison, genomic structure, phylogenetic analyses and synteny analyses showed that Potdrd1 was homologous to those of other teleosts. In adult individuals, the expression of Potdrd1 was higher in testis than in ovary, demonstrating a sexually dimorphic gene expression pattern. In situ hybridization (ISH) showed that Potdrd1 mRNA was detected in oogonia and oocytes of ovary as well as in spermatogonia and spermatocytes of testis. In juveniles during gonad differentiation its expression level increased rapidly from 30 dph to 100 dph and showed obvious sexual dimorphism that was in accordance with the expression of anti-Mullerian hormone (amh). Potdrd1 mRNA was consistently detected during embryogenesis, and its level was higher from unfertilzed eggs to the blastula stage and subsequently decreased until hatching. When chimeric RNA containing green fluorescent protein (GFP) and 3' untranslated regions (UTR) of Potdrd1 was microinjected into zebrafish fertilized eggs, the green fluorescence could be visualized only in putative PGCs. These results indicated that Potdrd1 is a germline specific and sexually dimorphic factor that potentially functionate in germline development and gametogenesis in Japanese flounder.

Scp3 expression in relation to the ovarian differentiation in the protogynous hermaphroditic ricefield eel Monopterus albus

Synaptonemal complex protein 3 (Scp3), which is encoded by scp3, is a meiotic marker commonly used to trace the timing of gonadal differentiation in vertebrates. In the present study, the ricefield eel scp3 cDNA was cloned, and a fragment encoding amino acids 49 to 244 was overexpressed. The recombinant Scp3 polypeptide was purified and used to generate a rabbit anti-Scp3 polyclonal antiserum. In adult ricefield eels, scp3 mRNA was predominantly detected in the gonads and faintly detected in discrete brain areas. In the gonads, Scp3 immunoreactivities were shown to be localized to the germ cells, including meiotic primary growth oocytes, spermatocytes, and pre-meiotic spermatogonia. During early ovarian differentiation, immunoreactive Scp3 was not detected in the gonads of ricefield eels at 6 days post-hatching (dph) but was found to be abundantly localized in the cytoplasm of some oogonia at 7 dph, coinciding with the appearance of the ovarian cavity and ovarian differentiation. At 14 dph, strong Scp3 immunostaining was detected on one side of the nucleus with a distinct polarity in some germ cells, presumably at the leptotene stage. Consistent with these results, the expression of scp3 mRNA was faintly detected in the gonads of ricefield eels at 6 dph, increased at 8 dph, and then remained relatively high thereafter. Taken together, these results suggest that the appearance of immunoreactive Scp3 in oogonia could be a marker for early ovarian differentiation in ricefield eels. The translocation of the Scp3 protein from the cytoplasm to the nucleus in the oogonium of ricefield eels appears to be a controlled process that warrants further study.

Nuclear Envelope Retention of LINC Complexes Is Promoted by SUN-1 Oligomerization in the Caenorhabditis elegans Germ Line

SUN (Sad1 and UNC-84) and KASH (Klarsicht, ANC-1, and Syne homology) proteins are constituents of the inner and outer nuclear membranes. They interact in the perinuclear space via C-terminal SUN-KASH domains to form the linker of nucleoskeleton and cytoskeleton (LINC) complex thereby bridging the nuclear envelope. LINC complexes mediate numerous biological processes by connecting chromatin with the cytoplasmic force-generating machinery. Here we show that the coiled-coil domains of SUN-1 are required for oligomerization and retention of the protein in the nuclear envelope, especially at later stages of female gametogenesis. Consistently, deletion of the coiled-coil domain makes SUN-1 sensitive to unilateral force exposure across the nuclear membrane. Premature loss of SUN-1 from the nuclear envelope leads to embryonic death due to loss of centrosome-nuclear envelope attachment. However, in contrast to previous notions we can show that the coiled-coil domain is dispensable for functional LINC complex formation, exemplified by successful chromosome sorting and synapsis in meiotic prophase I in its absence.

Companion Blood Cells Control Ovarian Stem Cell Niche Microenvironment and Homeostasis

The extracellular matrix plays an essential role for stem cell differentiation and niche homeostasis. Yet, the origin and mechanism of assembly of the stem cell niche microenvironment remain poorly characterized. Here, we uncover an association between the niche and blood cells, leading to the formation of the Drosophila ovarian germline stem cell niche basement membrane. We identify a distinct pool of plasmatocytes tightly associated with the developing ovaries from larval stages onward. Expressing tagged collagen IV tissue specifically, we show that the germline stem cell niche basement membrane is produced by these "companion plasmatocytes" in the larval gonad and persists throughout adulthood, including the reproductive period. Eliminating companion plasmatocytes or specifically blocking their collagen IV expression during larval stages results in abnormal adult niches with excess stem cells, a phenotype due to aberrant BMP signaling. Thus, local interactions between the niche and blood cells during gonad development are essential for adult germline stem cell niche microenvironment assembly and homeostasis.

Functions of the MRE11 complex in the development and maintenance of oocytes

The MRE11 complex (MRE11, RAD50, and NBS1) is a central component of the DNA damage response, governing both double-strand break repair and DNA damage response signaling. To determine the functions of the MRE11 complex in the development and maintenance of oocytes, we analyzed ovarian phenotypes of mice harboring the hypomorphic Mre11^ATLD1 allele. Mre11^ATLD1/ATLD1 females exhibited premature oocyte elimination attributable to defects in homologous chromosome pairing and double-strand break repair during meiotic prophase. Other aspects of meiotic progression, including attachment of telomeres to the nuclear envelope and recruitment of RAD21L, a component of the meiotic cohesin complex to the synaptonemal complex, were normal. Unlike Dmc1^−/− and Trp13^Gt/Gt mice which exhibit comparable defects in double-strand break repair and oocyte depletion by 5 days post-partum, we found that oocyte attrition occurred by 12 weeks in Mre11^ATLD1/ATLD1. Disruption of the oocyte checkpoint pathway governed by Chk2 gene further enhanced the survival of Mre11^ATLD1/ATLD1 follicles. Together our data suggest that the MRE11 complex influences the elimination of oocytes with unrepaired meiotic double-strand breaks post-natally, in addition to its previously described role in double-strand break repair and homologous synapsis during female meiosis.

Changes in ultrastructure and expression of steroidogenic factor-1 in ovaries of zebrafish Danio rerio exposed to glyphosate

Glyphosate is a broad-spectrum organophosphate (OP) herbicide, highly soluble in water, and when applied in terrestrial systems it penetrates into soil, eventually reaching the aquatic community and affecting nontarget organisms. The aim of this study was to evaluate the toxicity of glyphosate on ovaries of zebrafish (Danio rerio). Ovaries (n = 18 per triplicate) were exposed to 65 μg/L of glyphosate [N-(phosphonomethyl) glycine] for 15 d. This concentration was determined according to Resolution 357/2005/CONAMA/Brazil, which establishes the permissible concentration of glyphosate in Brazilian inland waters. Nonexposed ovaries (n = 18 per triplicate) were used as control. Subsequently, morphology and expression of steroidogenic factor-1 (SF-1) of exposed and nonexposed ovaries was determined. No apparent changes were noted in general morphology of exposed and nonexposed ovaries. However, a significant increase in diameter of oocytes was observed after exposure to glyphosate. When ovarian ultrastructure was examined the presence of concentric membranes, appearing as myelin-like structures, associated with the external membranes of mitochondria and with yolk granules was found. After glyphosate exposure, immunohistochemistry and immunoblotting revealed greater expression of SF-1 in the oocytes, which suggests a relationship between oocyte growth and SF-1 expression. These subtle adverse effects of glyphosate on oocytes raised a potential concern for fish reproduction. These results contribute to understanding glyphosate-induced toxicity to nontarget organisms, showing subcellular and molecular impairments that may affect reproduction in +female fish.

Nodal signaling regulates the entry into meiosis in fetal germ cells

The mechanisms regulating the entry into meiosis in mammalian germ cells remain incompletely understood. We investigated the involvement of the TGF-β family members in fetal germ cell meiosis initiation. Nodal, a member of the TGF-β family, and its target genes are precociously expressed in embryonic gonads and show sexual dimorphism in favor of the developing testis. Nodal receptor genes, Acvr2a and Acvr2b, Alk4, and Tdgf1/Cripto, were identified in male germ cells. Nodal itself, Tdgf1, and Lefty1 and Lefty2 are targets of Nodal signaling and were all found specifically expressed in male germ cells. To elucidate the role of this signaling pathway, activin-like kinases that mediate TGF-β/Nodal/activin signaling were inhibited in 11.5 d postconception testis in organotypic culture. Activin-like kinases inhibition disrupted normal male germ cell development and induced germ cell entry into meiosis such as that observed in female germ cells at the equivalent stage. Interestingly Stra8, the gatekeeper of the mitotic/meiotic switch, was induced independently of any change of either Cyp26b1 or Fgf9 expression, the two genes currently identified as testicular meiotic inhibitors. On the other hand, recombinant Nodal significantly dampened Stra8 expression and germ cell meiosis in cultured 11.5 d postconception ovaries. Our results allowed us to propose for the first time an autocrine role of Nodal during the development of germ cells and indicate that members of the TGB-β family may reinforce the male fate and prevent meiosis in embryonic germ cells.

RSPO1/β-catenin signaling pathway regulates oogonia differentiation and entry into meiosis in the mouse fetal ovary

Differentiation of germ cells into male gonocytes or female oocytes is a central event in sexual reproduction. Proliferation and differentiation of fetal germ cells depend on the sex of the embryo. In male mouse embryos, germ cell proliferation is regulated by the RNA helicase Mouse Vasa homolog gene and factors synthesized by the somatic Sertoli cells promote gonocyte differentiation. In the female, ovarian differentiation requires activation of the WNT/β-catenin signaling pathway in the somatic cells by the secreted protein RSPO1. Using mouse models, we now show that Rspo1 also activates the WNT/β-catenin signaling pathway in germ cells. In XX Rspo1^−/− gonads, germ cell proliferation, expression of the early meiotic marker Stra8, and entry into meiosis are all impaired. In these gonads, impaired entry into meiosis and germ cell sex reversal occur prior to detectable Sertoli cell differentiation, suggesting that β-catenin signaling acts within the germ cells to promote oogonial differentiation and entry into meiosis. Our results demonstrate that RSPO1/β-catenin signaling is involved in meiosis in fetal germ cells and contributes to the cellular decision of germ cells to differentiate into oocyte or sperm.

Ultrastructural aspects of previtellogenic oocyte growth in hermaphrodite sharpsnout seabream, Diplodus puntazzo (Teleostei, Sparidae)

This paper describes various aspects of previtellogenic oocyte growth in sharpsnout seabream, Diplodus puntazzo , is an important marine culture fish species in the Mediterranean. The ultrastructural characteristics of nuclear morphology, nuclear-cytoplasmic ratio and the starting of the follicle envelope formation were described in detail. These cells do not significantly differ from those of the other teleost species. The ultrastructural aspects provide new information on the reproductive biology of Sparidae.

[Study on the expression of zebrafish (Danio rerio) nanos1 during gametogenesis by in situ hybridization]

Using in situ hybridization on tissue sections with DIG antisense RNA probe, the distribution of zebrafish (Danio rerio) nanos1 mRNA during oogenesis and spermatogenesis was detected, and the expression and function of nanos 1 during zebrafish gametogenesis was preliminarily studied. The results were as follows: nanos 1 mRNA uniformly dispersed throughout the cytoplasm of oocytes at all stages. Strong expression of nanos1 mRNA was observed in oogonia and stage I, II oocytes, but the signal became weaker in later stage oocytes. Strong expression of nanos 1 mRNA was detected in spermatogonia and relatively weaker signal was found in primary spermatocytes but no nanos 1 mRNA expression was observed in spermatids. The results suggested that nanos 1 may play an important role in maintenance and functioning of the germline stem cell-oogonia and spermatogonia.

Establishment of oocyte population in the fetal ovary: primordial germ cell proliferation and oocyte programmed cell death

Strict control of cell proliferation and cell loss is essential for the coordinated functions of different cell populations in complex multicellular organisms. Oogenesis is characterized by a first phase occurring during embryo-fetal life and in common with spermatogenesis, during which mitotic proliferation of the germline stem cells, the primordial germ cells (PGC), prevails over germ cell death. The result is the formation of a relatively high number of germ cells depending on the species, ready to enter sex specific differentiation. In the female, PGC enter into meiosis and become oocytes, thereby ending their stem cell potential. After entering into meiosis in the fetal ovary, oocytes pass through leptotene, zygotene and pachytene stages before arresting in the last stage of meiotic prophase I, the diplotene or dictyate stage at about the time of birth. The most part of oocytes die during the fetal period or shortly after birth. It is widely accepted that in mammals a female is born with a fixed number of oocytes within the ovaries, which over the years progressively decreases without possibility for renewal. Once the oocyte reserve has been exhausted, ovarian senescence, driving what is referred to as the menopause in women, rapidly ensues. The fertile lifespan of a female depends by the size of the oocyte pool at birth and the rapidity of the oocyte pool depletion. Which mechanisms control PGC proliferation? Why do most of the oocytes die during fetal life and what are the mechanisms of such massive degeneration? Is it possible to prolong the lifespan of a female by reducing oocyte lost during the fetal life? This review reports some of the most recent results obtained in an attempt to answer these questions.

Female germ cell mutagenicity of model chemicals in Drosophila melanogaster: mechanistic information and analysis of repair systems

In spite of differences between female and male germ cells, and although both of them contribute to the gene pool of future generations, most germ cell mutagenicity studies in higher eukaryotes have been carried out on males. To study the response of female germ cells to mutagen/carcinogen exposure, the mutagenicity of two model chemicals like diethyl sulfate (DES) and hexamethylphosphoramide (HMPA), and the monofunctional methylating chemotherapeutic drug streptozotocin (STZ), has been analysed on repair efficient females of Drosophila melanogaster. Results previously obtained with N-ethyl-N-nitrosourea (ENU), another model chemical, have also been included in the analysis. The activity of bypass tolerance mechanism (BTM; represented by the mus308 locus) and nucleotide excision repair (NER) on the removal of oxygen and nitrogen ethylations was studied by determining DES mutagenicity in NER deficient females, comparing it with existing results for ENU, and by analysing both chemicals on BTM deficient females. Results indicate that (1) all chemicals are mutagenic on repair efficient females; (2) a measure of mutagenic activity ranked from the lowest DES to STZ, HMPA, and ENU as the highest. This order correlates with the repair of the respectively induced DNA damages, and with the mutagenic and carcinogenic potency of these compounds, considering the toxicity of cross-linking agents; (3) NER efficiently repairs nitrogen ethylation damage and seems to contribute to the processing of oxygen damage in female germ cells; and (4) BTM is involved on the processing of oxygen ethylation damage, whereas the results on nitrogen ethylation are not clear. Finally, these results indicate that differences between male and female germ cells affect the response to chemical exposure, and therefore demonstrate the necessity of analysing also female cells in germinal mutagenicity studies. In addition, these studies can provide important mechanistic information about germ cell chemical mutagenesis, and even when the analysis of oogonia is not possible, since all female germ cells are pre-meiotic, studies of oocytes could be a model for pre-meiotic cells.

Continuing primordial germ cell differentiation in the mouse embryo is a cell-intrinsic program sensitive to DNA methylation

The initial cohort of mammalian gametes is established by the proliferation of primordial germ cells in the early embryo. Primordial germ cells first appear in extraembyronic tissues and subsequently migrate to the developing gonad. Soon after they arrive in the gonad, the germ cells cease dividing and undertake sexually dimorphic patterns of development. Male germ cells arrest mitotically, while female germ cells directly enter meiotic prophase I. These sex-specific differentiation events are imposed upon a group of sex-common differentiation events that are shared by XX and XY germ cells. We have studied the appearance of GCNA1, a postmigratory sex-common germ cell marker, in cultures of premigratory germ cells to investigate how this differentiation program is regulated. Cultures in which proliferation was either inhibited or stimulated displayed a similar extent of differentiation as controls, suggesting that some differentiation events are the result of a cell-intrinsic program and are independent of cell proliferation. We also found that GCNA1 expression was accelerated by agents which promote DNA demethylation or histone acetylation. These results suggest that genomic demethylation of proliferative phase primordial germ cells is a mechanism by which germ cell maturation is coordinated.

Germ cell nuclear antigen (GCNA1) expression does not require a gonadal environment or steroidogenic factor 1: examination of GCNA1 in ectopic germ cells and in Ftz-F1 null mice

The germ cell lineage is first recognized as a population of mitotically proliferating primordial germ cells that migrate toward the gonadal ridge. Shortly after arriving at the gonadal ridge, the germ cells begin to initiate a commitment to gamete production in the developing gonad. The mechanisms controlling this transition are poorly understood. We recently reported that a mouse germ cell nuclear antigen 1 (GCNA1) is initially detected in both male and female germ cells as they reach the gonad at 11.5 days postcoitum (dpc). GCNA1 is continually expressed in germ cells through all stages of gametogenesis until the diplotene/dictyate stage of meiosis I. Since GCNA1 expression commences soon after primordial germ cells arrive at the gonadal ridge, we wanted to determine whether the gonadal environment was essential for induction of GCNA1 expression. By examining GCNA1 expression in germ cells that migrate ectopically into the adrenal gland, we determined that both the gonadal and adrenal gland environments allow GCNA1 expression. We also examined GCNA1 expression Ftz-F1 null mice, which were born lacking gonads and adrenal glands. During embryonic development in the Ftz-F1 null mice, the gonad and most germ cells undergo apoptotic degeneration at about 12.5 dpc. While most of the germ cells undergo apoptosis without expressing GCNA1, a few surviving germs cells, especially outside the involuting gonad clearly express GCNA1. Thus, although the Ftz-F1 gene is essential for gonadal and adrenal development, induction of GCNA1 expression in germ cells does not require Ftz-F1 gene products. The finding that germ cell GCNA1 expression is not restricted to the gonadal environment and is not dependent on the Ftz-F1 gene products suggests that GCNA1 expression may be initiated in the germ cell lineage by autonomous means.

Replication patterns of repetitive DNA sequences on the W chromosome are altered during development of the chick embryo

A novel method was developed to study developmental changes in the replication pattern of repetitive DNA sequences on the W chromosome (W-DNA) of the female chick embryo. The amount of total nuclear DNA and W-DNA as well as 5-bromodeoxyuridine (BrdU) incorporation was successively measured on the same cells using multiparametric microfluorometry. With this method we first examined the possibility of changes in replication patterns of W-DNA during development. Measurements were conducted on various heterogeneous cell populations obtained from whole embryo on Day 0.4 and Day 1, and from pectoral muscle, neural tube, liver, and oogonium on Day 9. Parameters of W-DNA replication, duration, and timing were found to vary according to the stage of embryonic development. Developmental features of these changes were further studied on specific cell types during their critical developmental processes. In scutate scale dermis, the W-DNA replication duration showed a characteristic lengthening from around 0.45C during Day 5 through Day 7.4 to 0.9C during Day 7.7 through Day 7.9 and shortening to 0.37C during Day 8.1 through Day 12. Transient lengthening in W-DNA replication duration was also observed in erythrocytes; 0.65C ->1.0C ->0.6C during Day 0.9 through Day 2.17. Timing also shifted earlier in accord with changes in the duration. Replication rate of whole genome DNA was monitored by measuring BrdU incorporation on respective cells and found, to a large extent, comparable to that of W-DNA. The data suggest that a link might be operative between replication patterns of genes and the developmental program.

Developmentally regulated expression of a mouse germ cell nuclear antigen examined from embryonic day 11 to adult in male and female mice

A rat IgM monoclonal antibody has been developed which recognized a mouse germ cell nuclear antigen (GCNA1). GCNA1 is present in prospermatogonia (gonocytes) in males and in oogonia and oocytes of females within the gonadal ridge from Embryonic Day 11.5 onward, but rarely in primordial germ cells prior to their arrival at the gonadal ridge. Immunolocalization demonstrates that GCNA1 is abundant in nuclei of spermatogonia and early spermatocytes, but decreases during subsequent spermatocyte and round spermatid development, and is not detected beyond step 10 elongating spermatids. The antigen is approximately 80-110 kDa on immunoblots of isolated pachytene spermatocytes and round spermatids. However, GCNA1 appears to be absent from sperm in the epididymis and vas deferens, Sertoli cells, TM3 cells (Leydig-like) and TM4 cells (Sertoli-like), lung, liver, kidney, spleen, heart, skin, brain, epididymis, and ovary. GCNA1 is present in prepuberal male mice (Days 2-14) in all stages of prespermatogonial and spermatogonial development. It is also present in prepuberal male mice (Days 2-14) in all stages of prespermatogonial and spermatogonial development. It is also present in oocytes of neonatal females until Postpartum Day 12. GCNA1 is first lost from oocytes in the medulla of the ovary as they arrest at the dictyate stage and gain a layer of granulosa cells. In addition, antigen is present in moderate amounts in F9 embryonal carcinoma cells and SCC-PSA1 pluripotent terato-carcinoma cells. Thus, GCNA1 serves as a common marker of the germ cell lineage in male and female mice after primordial germ cells arrive in the gonadal ridge until they reach the diplotene/dictyate stage of the first meiotic division.

Confocal microscopy of F-actin distribution in Xenopus oocytes

We have used rhodamine-conjugated phalloidin and confocal microscopy to examine the organisation of filamentous actin (F-actin) during oogenesis in Xenopus laevis. F-actin was restricted to a thin shell in the cortex of oogonia and post-mitotic oocytes less than 35 microns in diameter. In oocytes with diameters of 35-50 microns, F-actin was observed in three cellular domains: in the cortex, in the germinal vesicle (GV) and in a network of cytoplasmic cables. Initially, actin cables were sparsely distributed in the cytoplasm, with no evidence of discrete organising centres. In larger stage I oocytes, a dense network of actin cables extended throughout the cytoplasm, linking the GV and mitochondrial mass to the cortical actin shell. Apart from the F-actin associated with the mitochondrial mass, no evidence of a polarised distribution of F-actin was apparent in stage I oocytes. F-actin was observed also in the cortex and the GV of stage VI oocytes, and a network of cytoplasmic cables surrounded the GV. Cytoplasmic actin cables extended from the GV to the animal cortex, and formed a three-dimensional network surrounding clusters of yolk platelets in the vegetal cytoplasm. Finally, disruption of F-actin in stage VI oocytes with cytochalasin resulted in distortion and apparent rotation of the GV in the animal hemisphere, suggesting that actin plays a role in maintaining the polarised organisation of amphibian oocytes.

Carbohydrate and protein histochemistry during oogenesis in Halobatrachus didactylus (Schneider, 1801) from the Bay of Cadiz (Spain)

Histological and histochemical characteristics were studied in Halobatrachus didactylus (Schneider, 1801) during oogenesis. Three phases could be differentiated: previtellogenesis (oogonia and basophilic oocytes), vitellogenesis (yolk synthesis) and maturation-spawning. Glycogen, glycoproteins and proteins rich in certain amino acids were present in the previtellogenic as well as in the vitellogenic cytoplasm oocytes. No acid mucosubstances were detected. Three types of yolk (vesicles, vacuoles and granules) contained different types of organic reserves; granules were essentially proteic whereas globules were lipidic. Carbohydrates and proteins were present in vesicles.

Immunocytochemical evidence for methylation of the inactive X chromosome in human fetal oogonia

The state of DNA methylation of the X chromosomes of human interphase oogonia from a 46,XX and a 46,XX/47,XXX fetus at 17 weeks of gestation was tested immunocytochemically with an antibody to 5-methylcytosine (5MeC). Of 1637 oogonial nuclei from the 46,XX fetal ovary, 313 (19.1%) contained Barr bodies, of which 93.6% were positive for 5MeC. Of 1780 oogonia from the 46,XX/47,XXX fetus 327 (18.4%) contained Barr bodies; 175 oogonia had one Barr body and 152 had two. Of the single Barr bodies 145 (82.8%) had positive 5MeC reaction product. Of the 152 oogonia from the XXX line, 97 (63.8%) had positive 5MeC on both Barr bodies, 35 (23%) had one positive and one negative, and 20 (13.1%) had no product on either Barr body. This immunocytochemical evidence supports the hypothesis that the DNA of the inactive X-chromosome of the human 17-week gestation oogonium is methylated.

Extrachromosomal DNA and its activity in RNA synthesis in oogonia and oocytes in the pupal ovary of Creophilus maxillosus (Staphylinidae, Coleoptera-Polyphaga)

The participation of extrachromosomal DNA (extra DNA) in RNA synthesis in the nuclei of terminal oogonial cells and oocytes in the pupal ovary of Creophilus maxillosus (Staphylinidae, Coleoptera) was examined by autoradiography. It was found that extra DNA in the nuclei of terminal oogonial cells, although predominantly in a condensed and heterochromatic state, produces numerous nucleoli and incorporates 3H-uridine during the interphases between successive differential divisions. Moreover, it was shown that extra DNA is active in RNA synthesis at the same stage of pupal development in which it is synthesized and accumulated, i.e. in the nuclei of terminal oogonial cells. As soon as the oocyte forms RNA synthesis ceases in the extrachromosomal DNA body cells showed that nucleolar material does not disappear during division but remains, at least partly, connected with the extra DNA body.

Autoradiographic study of RNA synthesis during meiotic prophase in the human oocyte

The incorporation of 3H-uridine in oogonia and oocytes during meiotic prophase I was studied in three human fetuses 13, 18, and 19 weeks old. Following a 40- or 60-min pulse, intense nuclear and nucleolar labeling was observed in oogonia. During the preleptotene chromosome condensation stage, the heteropycnotic masses were unlabeled, while numerous silver grains were seen on the filaments persisting around these masses. During leptotene, chromosomal and nucleolar RNA synthesis was significant, but less than that in the oogonia. The rate of incorporation declined rapidly during zygotene and fell to a very low level at early pachytene. Throughout pachytene no nucleolar RNA synthesis was observed. Chromosomal RNA synthesis progressively recovered during middle pachytene, was of moderate intensity at late pachytene, and increased again at early diplotene. Nucleolar RNA synthesis was very intense at early diplotene, at the same time as nucleolar size and basophilia increased.

Morphological and histochemical observations on the ovarian balls of Centrorhynchus corvi (Acanthocephala)

Unlike in other acanthocephalans, the overian balls of Centrorhynchus corvi are complex and are composed of 24–30 ovarian ball units. Each ovarian ball unit consists of three structural and functional units – the oogonial syncytium, developing oogenetic cells and the supporting syncytium – complementary to the ovarian balls of other acanthocephalans. Three metamorphic stages of the nuclei in the oogonial syncytium are described, depending on the nuclear morphology, chromatin structure and appearance of the nucleolus. Third-stage nuclei bulge out at the periphery of the oogonial syncytium and are surrounded by its cytoplasm and a thin membrane. Ultimately these are separated from the oogonial syncytium to from oogonia containing small amounts of cytoplasmic components derived from the oogonial syncytium. Nuclei of the young oogonia, oognia at the budding stage, and 3rd-stage nuclei of the oogonial syncytium all possess nucleoli and are similar also in their nuclear dimensions and cytochemical affinities. Nuclear resemblances, and cytoplasmic similarities of the oogonia and oogonial syncytium with regard to the presence of lipids, RNA and proteins support the concept of the origin of oogonia from the oogonial syncytium.

The free oogonia divide mitotically in the cellular zone where they undergo a single mitotic division and the resulting oocytes enter the prophase of meiosis. I. The meiotic primary oocytes in contrast to the permeiotic primary oocytes of other acanthoscephalans enter the growth phase which is closely accompanied by the accumulation of various ooplasmic components such as basophilic yolk nucleus which is composed of RNA, proteins, lipoproteins and phospholipids. Histochemical features of the oogonial and supporting syncytium are described.

Renewal of the gonads in Styela clava (Urochordata: Ascidiacea) as revealed by autoradiography with tritiated thymidine

DNA-synthesizing cells in the gonads of the ascidian Styela clava were labeled with tritiated thymidine and detected with autoradiography. In the testis, spermatogonia and primary spermatocytes are labeled after 1 hr. Labeled spermatozoa occur in the lumen of the testis follicles after 10 days and in the sperm ducts after 20 days. In the ovary, only germ cells (oogonia and pre-leptotene primary oocytes) and follicle cells are labeled after 1 hr. By 60 days, oocytes with basophilic cytoplasm (15-65 mu in diameter) are labeled; test cells embedded in larger eosinophilic oocytes (150 mu in diameter) are also labeled. Germ cells give rise to both oocytes and follicle cells. Through continued cell division, follicle cells give rise to test cells.