Mammalian cumulus-oocyte complex communication: a dialog through long and short distance messaging

Transcriptomic profiling of miRNA-mRNA interactions in canine oocytes and cumulus cells during in vitro maturation: a sequencing analysis

Oocyte maturation is a critical process for successful fertilization and early embryonic development. In this study, we investigated the molecular mechanisms underlying oocyte maturation in dogs by analyzing the transcriptomic profiles of miRNAs and mRNAs in canine cumulus cells (CCs) and oocytes during in vitro maturation (IVM). RNA sequencing identified 285 miRNAs expressed in oocytes and 310 in CCs, with 282 miRNAs shared between the two cell types, highlighting the role of intercellular communication in maintaining miRNA expression equilibrium. Differential expression analysis revealed 222 mRNAs with significant differences between CCs and oocytes, including genes involved in transcriptional regulation and nuclear structure. Enrichment analyses identified pathways such as actin cytoskeleton regulation, mTOR signaling, cAMP signaling, and calcium signaling, all critical to oocyte maturation. Network analysis revealed 643 significant miRNA-mRNA coexpression relationships, suggesting miRNAs play pivotal roles in regulating mRNA expression during oocyte maturation. Notably, key miRNAs such as miR-30b, miR-375, and miR-503 were implicated in regulating genes involved in oocyte maturation pathways, while others like miR-378 and miR-21 aligned with known roles in suppressing cumulus expansion and influencing maturation. The absence of differential miRNA expression between CCs and oocytes suggests the miRNA transfer through gap junctions. These findings provide new insights into the transcriptional and post-transcriptional regulation of oocyte maturation in dogs, offering valuable knowledge to improve reproductive biotechnologies such as in vitro fertilization and embryo development in this species.

New approaches in the analysis of spent embryo culture media in the IVF process

PURPOSE

In vitro fertilization occurs in a controlled laboratory setting, where oocytes are fertilized by sperm, and the resulting embryos are cultured to the blastocyst stage before transfer to the uterus. The secreted/consumed substances by the embryo in the extracellular environment (secretome) contain a variety of molecules that may provide insights into embryo quality. This study presents new perspectives on the non-invasive and cost-effective assessment and evaluation of embryos during the IVF process, utilizing a spent embryo culture medium (SECM).

METHODS

The SECM was used from blastocysts prepared for a single blastocyst transfer and was analyzed in two groups-the SECM with successful (F) (n = 30) and unsuccessful (N) (n = 36) embryo implantation in the woman's uterus. Building on our previous next-generation sequencing results, we decided to validate the expression levels of specific miRNAs, particularly hsa-miR-16-5p and hsa-miR-92a-3p, to assess their potential to predict embryo implantation success.

RESULTS

Our results demonstrate different expression levels of miRNA molecules in the monitored groups, which could lead to their use in non-invasive analysis of the implantation potential of embryos in the IVF process. In this study, we employed a metabolomics approach using 3D fluorescence analysis of SECM to identify differences between the studied groups, F and N. Our preliminary results indicate a slightly increased metabolic activity in the group with unsuccessful embryo implantation group.

CONCLUSIONS

This is our pilot study where we demonstrated the use of two approaches in analyzing the SECM to predict the implantation potential of embryos in the IVF process which promises further development.

Folliculogenesis: A Cellular Crosstalk Mechanism

In vitro embryo production has accelerated in the cattle industry in recent years. Because women are similar to cows, this represents an opportunity to improve women's reproductive protocols. This review focuses on crosstalk communication during folliculogenesis for an in-depth understanding of the events involved in developing the oocyte competence necessary to generate an embryo after fertilization. This knowledge can be used to improve oocytes in in vitro maturation cultures, which would allow us to obtain oocytes of high quality and competence, resulting in successful pregnancies in both women and cows. The first part of this review covers the concepts of cellular crosstalk before puberty in the primordial, primary, and secondary follicles. The next part involves cellular crosstalk after puberty, when gonadotropin hormones act on the ovary, promoting oocyte maturation. The final part comprises a perspective on using cow models to study human ovary physiology.

The mammalian egg's zona pellucida, fertilization, and fertility

The zona pellucida (ZP) is a relatively thick extracellular matrix (ECM) that surrounds all mammalian eggs and plays vital roles during oogenesis, fertilization, and preimplantation development. The ZP is a semi-permeable, viscous ECM that consists of three or four glycosylated proteins, called ZP1-4, that differ from proteoglycans and proteins of somatic cell ECM. Mammalian ZP proteins are encoded by single-copy genes on different chromosomes and synthesized and secreted by growing oocytes arrested in meiosis. Secreted ZP proteins assemble in the extracellular space into long fibrils that are crosslinked polymers of ZP proteins and exhibit a structural repeat. Several regions of nascent ZP proteins, the signal-sequence, ZP domain, internal and external hydrophobic patches, transmembrane domain, and consensus furin cleavage-site regulate secretion and assembly of the proteins. The ZP domain is required for assembly of ZP fibrils, as well as for assembly of other kinds of ZP domain-containing proteins. ZP proteins adopt immunoglobulin (Ig)-like folds that resemble C- and V-type Ig-like domains, but represent new immunoglobulin-superfamily subtype structures. Interference with synthesis, processing, or secretion of ZP proteins by either gene-targeting in mice or mutations in human ZP genes can result in failure to assemble a ZP and female infertility. ZP2 and ZP3 must be present to assemble a ZP during oocyte growth and both serve as receptors for binding of free-swimming sperm to ovulated eggs. Acrosome-reacted sperm bind to ZP2 polypeptide by inner-acrosomal membrane and acrosome-intact sperm bind to ZP3 oligosaccharides by plasma membrane overlying the sperm head. Binding of acrosome-intact sperm to ZP3 induces them to undergo cellular exocytosis, the acrosome reaction. Only acrosome-reacted sperm can penetrate the ZP, bind to, and then fuse with the egg's plasma membrane to produce a zygote. Following sperm-egg fusion (fertilization) the ZP undergoes structural and functional changes (zona reaction) induced by cortical granule components (cortical reaction) deposited into the ZP. The latter include zinc and ovastacin, a metalloendoprotease that cleaves ZP2 near its amino-terminus and hardens the egg's ZP. The changes prevent penetration of bound sperm through and binding of supernumerary sperm to the ZP of fertilized eggs as part of a secondary or slow block to polyspermy. Therefore, ZP proteins act as structural proteins and sperm receptors, and help to prevent fertilization by more than one sperm. Here we review some of this information and provide details about several key features of ZP proteins, ZP matrix, and mammalian fertilization.

Transcriptomics of bovine sperm and oocytes

Traditionally, sperm and embryos were studied using microscopy to assess morphology and motility. However, OMICS technologies, especially transcriptomic analysis, are now being used to screen the molecular dynamics of fertility markers at cellular and molecular levels, with high sensitivity. Transcriptomics is the study of the transcriptome - RNA transcripts produced by the genome - using high-throughput methods to understand how the RNAs are expressed. In this review, we have discussed gene contributions to sperm structure and function and their role in fertilization and early embryo development. Further, we identified miRNAs shared by sperm, oocytes, and early embryos and their roles in fertilization and early embryo development.

Female fertility and the mammalian egg's zona pellucida

All mammalian eggs are surrounded by a relatively thick extracellular matrix (ECM) or zona pellucida (ZP) to which free-swimming sperm bind in a species-restricted manner during fertilization. The ZP consists of either three (e.g., Mus musculus) or four (e.g., Homo sapiens) glycosylated proteins, called ZP1-4. These proteins are unlike those found in somatic cell ECM, are encoded by single-copy genes on different chromosomes, and are well conserved among different mammals. Mammalian ZP proteins are synthesized as polypeptide precursors by growing oocytes that will become ovulated, unfertilized eggs. These precursors are processed to remove a signal-sequence and carboxy-terminal propeptide and are secreted into the extracellular space. Secreted ZP proteins assemble into long, crosslinked fibrils that exhibit a structural repeat due to the presence of ZP2-ZP3 dimers every 140 Å or so along fibrils. Fibrils are crosslinked by ZP1 and are oriented either perpendicular, parallel, or randomly to the plasma membrane of eggs depending on their position in the ZP. Free-swimming mouse sperm recognize and bind to ZP2 or ZP3 that serve as sperm receptors. Acrosome-intact sperm bind to ZP3 oligosaccharides and acrosome-reacted sperm bind to ZP2 polypeptide. ZP fibrils fail to assemble in the absence of either nascent ZP2 or ZP3 and results in mouse eggs that lack a ZP and female infertility. Gene sequence variations due to point, missense, or frameshift mutations in genes encoding ZP1-4 result in human eggs that lack a ZP or have an abnormal ZP and female infertility. These and other features of the mouse and human egg's ZP are discussed here.

SMAD4 promotes somatic-germline contact during murine oocyte growth

Development of the mammalian oocyte requires physical contact with the surrounding granulosa cells of the follicle, which provide it with essential nutrients and regulatory signals. This contact is achieved through specialized filopodia, termed transzonal projections (TZPs), that extend from the granulosa cells to the oocyte surface. Transforming growth factor (TGFβ) family ligands produced by the oocyte increase the number of TZPs, but how they do so is unknown. Using an inducible Cre recombinase strategy together with expression of green fluorescent protein to verify Cre activity in individual cells, we examined the effect of depleting the canonical TGFβ mediator, SMAD4, in mouse granulosa cells. We observed a 20-50% decrease in the total number of TZPs in SMAD4-depleted granulosa cell-oocyte complexes, and a 50% decrease in the number of newly generated TZPs when the granulosa cells were reaggregated with wild-type oocytes. Three-dimensional image analysis revealed that TZPs of SMAD4-depleted cells were longer than controls and more frequently oriented towards the oocyte. Strikingly, the transmembrane proteins, N-cadherin and Notch2, were reduced by 50% in SMAD4-depleted cells. SMAD4 may thus modulate a network of cell adhesion proteins that stabilize the attachment of TZPs to the oocyte, thereby amplifying signalling between the two cell types.

Jiawei Buzhong Yiqi decoction ameliorates polycystic ovary syndrome via oocyte-granulosa cell communication

ETHNOPHARMACOLOGICAL RELEVANCE

Jiawei Buzhong Yiqi Decoction (JWBZYQ), from records of FuqingzhuNvke, is a classical formula for treating obese women related infertility. JWBZYQ has been shown to be effective in treating polycystic ovary syndrome (PCOS) in both clinical studies and practical practice, with the pharmacological mechanism remaining unknown.

AIM OF THE STUDY

To explore the potential therapeutic effects and mechanistic insights of JWBZYQ in PCOS.

MATERIALS AND METHODS

An overweight PCOS rat model was established via testosterone propionate (TP) injection and 45% high-fat diet (HFD). Then they were categorized into five distinct groups: Control group, Model group, low-dose of JWBZYQ (JWBZYQ1) group, high-dose of JWBZYQ (JWBZYQ2) group, and metformin (Met) group. Body weight, estrous cycle, and sex hormone levels were observed. Hematoxylin-Eosin staining was employed to investigate the histological characteristics of the ovaries. To identify the pathways that changed significantly, transcriptome analysis was performed. The protein and mRNA levels of key molecules in ovarian zona pellucida (ZP) organization, transzonal projections (TZPs) assembly, steroid hormone receptors, and steroidogenesis were assessed using phalloidin staining, immunohistochemistry, Western blot, and polymerase chain reaction.

RESULTS

RNA-seq analysis demonstrated that regulation of hormone secretion, cilium assembly, cell projection assembly, and ZP production may all have crucial impact on the etiology of PCOS and therapeutic effect of JWBZYQ. In particular, PCOS rats exhibited elevated expressions of ZP1-3, which can be reversed by JWBZYQ2 particularly. Simultaneously, TZPs assembly was totally disrupted in PCOS rats, evidenced by the phalloidin staining, upregulated calcium-/calmodulin-dependent protein kinase II beta (CaMKIIβ), and deficient p-CaMKIIβ, myosin X (MYO10), proline-rich tyrosine kinase 2 (PTK2), and Fascin. Nonetheless, JWBZYQ or metformin treatment revived the disturbance, repairing the oocyte-granulosa cell communication, regulating steroidogenesis in PCOS rats. In this way, JWBZYQ and metformin exerted remarkable effects in alleviating altered ovarian morphology and function in PCOS rats, with JWBZYQ2 revealing the best effect.

CONCLUSIONS

JWBZYQ restored the altered ovarian morphology and function by regulating the oocyte-granulosa cell communication, which was related with ZP organization and TZPs assembly in the ovary.

Testosterone Supplementation Promotes Estrogen Synthesis of Buffalo Cumulus Cells Surrounding In Vitro-Matured Oocytes

Cumulus cells (CCs) synthesize estrogens that are essential for follicular development. However, the effects of androgen on estrogen production in buffalo CCs remain unknown. In the present study, the impacts of testosterone on estrogen synthesis of buffalo CCs surrounding in vitro-matured oocytes were investigated. The results showed that testosterone supplementation improved both the expression levels of estrogen synthesis-related genes (CYP11A1, CYP19A1, and 17β-HSD) and the secretion levels of estradiol in buffalo CCs surrounding in vitro-matured oocytes. Furthermore, testosterone treatment enhanced the sensitivity of buffalo CCs surrounding in vitro-matured oocytes to follicle-stimulating hormone (FSH). This study indicated that testosterone supplementation promoted the estrogen synthesis of buffalo CCs surrounding in vitro-matured oocytes mainly through strengthening the responsiveness of CCs to FSH. The present study serves as a foundation of acquiring high-quality recipient oocytes for buffalo somatic cell nuclear transfer.

Unravelling the role of HAS2, GREM1, and PTGS2 gene expression in cumulus cells: implications for human oocyte development competency - a systematic review and integrated bioinformatic analysis

The leading indicator for successful outcomes in in-vitro fertilization (IVF) is the quality of gametes in oocytes and sperm. Thus, advanced research aims to highlight the parameter in assessing these qualities - DNA fragmentation in sperm and oocyte development capacity (ODC) via evaluation of microenvironments involving its maturation process. Regarding oocytes, most evidence reveals the role of cumulus cells as non-invasive methods in assessing their development competency, mainly via gene expression evaluation. Our review aims to consolidate the evidence of GDF-9 derivatives, the HAS2, GREM1, and PTGS2 gene expression in cumulus cells used as ODC markers in relevant publications and tailored to current IVF outcomes. In addition to that, we also added the bioinformatic analysis in our review to strengthen the evidence aiming for a better understanding of the pathways and cluster of the genes of interest - HAS2, GREM1, and PTGS2 in cumulus cell level. Otherwise, the current non-invasive method can be used in exploring various causes of infertility that may affect these gene expressions at the cumulus cell level. Nevertheless, this method can also be used in assessing the ODC in various cohorts of women or as an improvement of markers following targeted tools or procedures by evaluating the advancement of these gene expressions following the targeted intervention.

The relationship between CYP19A1 gene expression in luteinized granulosa cells and follicular estradiol output in women with endometriosis

Endometriosis was claimed to negatively affect the intrafollicular environment, hindering oocyte competence. Previous studies evaluated expression levels of cytochrome P450 aromatase (CYP19A) in granulosa and cumulus oophorus cells collected from endometriosis women, but results are controversial. To further investigate the intrafollicular environment whose alteration may potentially disturb ovarian steroidogenesis in endometriosis, gene expression of CYP19A and of its upstream enzymes, StAR and 3βHSD was assessed in luteinized granulosa cells isolated from follicular fluids (FF) collected during Assisted Reproduction Technology (ART) procedures in women with stage III-IV disease and from subjects without the condition. In a subgroup of patients, cumulus oophorus cells (COCs) were also assessed for CYP19A, StAR and 3βHSD gene expression. No difference in mRNA expression of CYP19A1, StAR and 3βHSD in both granulosa cells and COCs was observed between the two groups of patients. No significant difference was also found between estradiol FF levels detected in endometriosis patients (median=873, IQR=522-1221 ng/ml)) and control patients (median=878, IQR=609-1137 ng/ml). To gain more insight into the intrafollicular regulation of CYP19A in patients with endometriosis, associations between expression of the analyzed genes, systemic and follicular 17β-estradiol levels and ART outcomes were assessed. While in the control group, levels of CYP19A1, StAR and 3βHSD transcripts significantly correlated with follicular estradiol levels (adjusted R² of 0.60), no significant association was detected in affected women (adjusted R² of 0.23). After stratification of the populations based on the presence of the disease, CYP19A1 expression was shown to correlate with the number of oocytes retrieved [β:- 1.214;95%CI: - 2.085 - (-0.343); p = 0.007] in the control group while this association was not present in patients with endometriosis [β:- 0.003; 95%CI:- 0.468-0.461; p = 0.988)]. These results do not support data from the literature indicating a reduced aromatase expression in granulosa cells of affected women, but they highlight a potential subtle mechanism affecting the ovulation process in these women.

The association of RBX1 and BAMBI gene expression with oocyte maturation in PCOS women

BACKGROUND

Polycystic ovarian syndrome (PCOS) is a common endocrine disorder that affects 6-20% of women of reproductive age. One of the symptoms of PCOS is hyperandrogenism, which can impair follicular development. This disruption can cause issues with the development of oocytes and the growth of embryos. Although the exact cause of PCOS is not yet fully understood, studying the gene expression pattern of cumulus cells, which play a crucial role in the maturation and quality of oocytes, could help identify the genes associated with oocyte maturation in PCOS women. Through indirect activation of APC/Cdc20, RBX1 enables oocytes to bypass the GV (germinal vesicles) stage and advance to the MII (metaphase II) stage. our other gene is the BAMBI gene which stimulates WNT signaling, that is a crucial pathway for healthy ovarian function. This study aims to explore the expression level of the RBX1 and BAMBI genes between GV and MII oocytes of PCOS and non-PCOS groups.

METHODS

In this experiment, we gathered the cumulus cells of MII (38 cases and 33 control) and GV (38 cases and 33 control) oocytes from women with/without PCOS. Besides, quantitative RT-PCR was used to assess the semi-quantitative expression of BAMBI and RBX1.

RESULTS

According to our research, the expression level of RBX1 and BAMBI in MII and GV cumulus cells of PCOS patients was significantly lower than that in non-PCOS ones.

CONCLUSION

This research raises the possibility of RBX1 and BAMBI involvement in oocyte quality in PCOS women.

Growth factors and female reproduction in vertebrates

Female reproductive efficiency is influenced by the outcomes of various processes, including folliculogenesis, apoptosis, response to gonadotropin signaling, oocyte maturation, and ovulation. The role of hormones in regulating these processes and other reproductive activities has been well established. It is becoming increasingly evident that in addition to well-characterized hormones, growth factors play vital roles in regulating some of these reproductive activities. Growth factors and their receptors are widely distributed in vertebrate ovaries at different stages of ovarian development, indicating their involvement in intraovarian reproductive functions. In the ovary, cell surface receptors allow growth factors to regulate intraovarian reproductive activities. Understanding these actions in the reproductive axis would provide a tool to target growth factors and/or their receptors to yield desirable reproductive outcomes. These include enrichment of in vitro maturation and fertilization culture media, and management of infertility. This review discusses some widely characterized growth factors belonging to the TGF, EGF, IGF, FGF, and BDNF family of peptides and their role in female reproduction in vertebrates, with a focus on mammals.

C-Type Natriuretic Peptide Pre-Treatment Improves Maturation Rate of Goat Oocytes by Maintaining Transzonal Projections, Spindle Morphology, and Mitochondrial Function

C-type natriuretic peptide (CNP) is a peptide molecule naturally found in follicles and can be used to extend meiotic resumption and enhance the potential for oocytes to develop. However, the mechanism by which CNP improves goat oocyte quality remains unclear. In this study, cumulus-oocyte complexes (COCs) from goats were pre-treated with CNP prior to IVM, and the results showed that pre-treatment with CNP enhanced goat oocyte maturation. First, we discovered that CNP maintained communication between cumulus cells and oocytes by regulating the transzonal projections (TZPs). We then found that CNP treatment reduced abnormal spindle formation and increased the expression of genes associated with spindle assembly and the spindle assembly checkpoint. Moreover, further analysis showed that oocytes exhibited better antioxidant ability in the CNP treatment group, which mainly manifested in higher glutathione (GSH) and lower reactive oxygen species (ROS) concentrations. Enhanced mitochondrial activity was signified via the augmented expression of mitochondrial oxidative metabolism and fusion and fission-related genes, thus diminishing the apoptosis of the oocytes. Overall, these results provide novel insights into the potential mechanism by which CNP treatment before IVM can improve oocyte quality.

The function of exosomes in ovarian granulosa cells

Granulosa cells (GCs), as the basic components of ovarian tissue, play an indispensable role in maintaining normal ovarian functions such as hormone synthesis and ovulation. The abnormality of GCs often leads to ovarian endocrine disorders, which exert a negative effect on life quality and life expectancy. However, the pathogenesis and treatment of diseases are still poorly understood. Exosomes contain regulatory molecules and can transmit biological information in cell interaction. The role of exosomes in GCs has been studied extensively. This review summarizes the regulatory function of exosomes in GCs, as well as their participation in etiopathogenesis and their promising application in treatment when it comes to ovarian endocrine diseases, which can help us better understand ovarian diseases from the perspective of GCs.

Molecular cloning and characterization of STC1 gene and its functional analyses in yak (Bos grunniens) cumulus granulosa cells

Cumulus granulosa cells (CGCs), an important type of ovarian somatic cells, carries out various functions related to oogenesis, follicular development, and steroidogenesis. Studying the biological mechanisms involved in the development and function of CGCs makes a great contribution to understanding the reproductive regulation in female animals. Stanniocalcin-1 (STC1) is an important Ca2+-regulated glycoprotein hormone that exhibits high expression levels in ovaries. In this study, we cloned the coding sequence of the yak STC1, predicted the structure of STC1 protein, detected the expression and localization of STC1 in yak ovaries, and analyzed the functions of STC1 in yak CGCs. The CDS (coding sequence) region of yak STC1 gene was found to be 744 bp and encoded 247 amino acids. Homology comparison revealed that STC1 protein was highly conserved among mammals. The STC1 mRNA displayed dynamic expression profiles in different stages of yak ovaries, and the highest expression was found in the follicular phase. Regarding localization, STC1 protein was widely distributed in various kinds of yak ovarian cells, including oocytes, mural granulosa cells, CGCs, and thecal cells. Repressing the expression of STC1 resulted in defective proliferation and survival of yak CGCs. In addition, knockdown the expression of STC1 repressed the secretion of progesterone and promoted the secretion of estrogen. Overexpression of STC1 partially rescued the proliferation of CGCs and resulted in opposite effects on the secretion of progesterone and estrogen. Several apoptosis and steroidogenesis-related genes, including BAX, BCL2, HSD3B1, HSD17B1, CYP11A1 and CYP17A1 showed altered expressions after repressing or increasing the expression of STC1 in yak CGCs. To the best of our knowledge, this study is the first to focus on the role of STC1 in yak CGCs, and the outcomes offer fresh insights into the mechanism governing yak reproduction.

Oogenesis in Caenorhabditis elegans

BACKGROUND

The nematode, Caenorhabditis elegans has proven itself as a valuable model for investigating metazoan biology. C. elegans have a transparent body, an invariant cell lineage, and a high level of genetic conservation which makes it a desirable model organism. Although used to elucidate many aspects of somatic biology, a distinct advantage of C. elegans is its well annotated germline which allows all aspects of oogenesis to be observed in real time within a single animal. C. elegans hermaphrodites have two U-shaped gonad arms which produce their own sperm that is later stored to fertilise their own oocytes. These two germlines take up much of the internal space of each animal and germ cells are therefore the most abundant cell present within each animal. This feature and the genetic phenotypes observed for mutant worm gonads have allowed many novel findings that established our early understanding of germ cell dynamics. The mutant phenotypes also allowed key features of meiosis and germ cell maturation to be unveiled.

SUMMARY

This review will focus on the key aspects that make C. elegans an outstanding model for exploring each feature of oogenesis. This will include the fundamental steps associated with germline function and germ cell maturation and will be of use for those interested in exploring reproductive metazoan biology.

KEY MESSAGES

Since germ cell biology is highly conserved in animals, much can be gained from study of a simple metazoan like C. elegans. Past findings have enhanced understanding on topics that would be more laborious or challenging in more complex animal models.

Review: Environment of the ovulatory follicle: modifications and use of biotechnologies to enhance oocyte competence and increase fertility in cattle

The oocyte is the basis of life, supporting development from a fertilized cell to an independent multicellular organism. The oocyte's competence to drive the first cell cycles postfertilization are critical to embryonic survival and subsequent successful pregnancy. Coupled with the complex processes of follicle assembly, activation, differentiation, growth, and terminal maturation, oocyte developmental competence is gradually acquired during oocyte growth and meiotic maturation. Most reproduction management technologies and interventions are centered around these highly coordinated processes, targeting the ovarian follicle and the oocyte within. Thus, our objective was to highlight key aspects of oocyte and follicle development in cattle, and to discuss recent advances in oocyte and follicle-centered reproductive biotechnologies.

Intercellular communication in the cumulus-oocyte complex during folliculogenesis: A review

During folliculogenesis, the oocyte and surrounding cumulus cells form an ensemble called the cumulus-oocyte complex (COC). Due to their interdependence, research on the COC has been a hot issue in the past few decades. A growing body of literature has revealed that intercellular communication is critical in determining oocyte quality and ovulation. This review provides an update on the current knowledge of COC intercellular communication, morphology, and functions. Transzonal projections (TZPs) and gap junctions are the most described structures of the COC. They provide basic metabolic and nutrient support, and abundant molecules for signaling pathways and regulations. Oocyte-secreted factors (OSFs) such as growth differentiation factor 9 and bone morphogenetic protein 15 have been linked with follicular homeostasis, suggesting that the communications are bidirectional. Using advanced techniques, new evidence has highlighted the existence of other structures that participate in intercellular communication. Extracellular vesicles can carry transcripts and signaling molecules. Microvilli on the oocyte can induce the formation of TZPs and secrete OSFs. Cell membrane fusion between the oocyte and cumulus cells can lead to sharing of cytoplasm, in a way making the COC a true whole. These findings give us new insights into related reproductive diseases like polycystic ovary syndrome and primary ovarian insufficiency and how to improve the outcomes of assisted reproduction.

Maternal metabolic health and fertility: we should not only care about but also for the oocyte!

Metabolic disorders due to obesity and unhealthy lifestyle directly alter the oocyte's microenvironment and impact oocyte quality. Oxidative stress and mitochondrial dysfunction play key roles in the pathogenesis. Acute effects on the fully grown oocytes are evident, but early follicular stages are also sensitive to metabolic stress leading to a long-term impact on follicular cells and oocytes. Improving the preconception health is therefore of capital importance but research in animal models has demonstrated that oocyte quality is not fully recovered. In the in vitro fertilisation clinic, maternal metabolic disorders are linked with disappointing assisted reproductive technology results. Embryos derived from metabolically compromised oocytes exhibit persistently high intracellular stress levels due to weak cellular homeostatic mechanisms. The assisted reproductive technology procedures themselves form an extra burden for these defective embryos. Minimising cellular stress during culture using mitochondrial-targeted therapy could rescue compromised embryos in a bovine model. However, translating such applications to human in vitro fertilisation clinics is not simple. It is crucial to consider the sensitive epigenetic programming during early development. Research in humans and relevant animal models should result in preconception care interventions and in vitro strategies not only aiming at improving fertility but also safeguarding offspring health.

Inhibition of Hsp90 during in vitro maturation under thermoneutral or heat shock conditions compromises the developmental competence of bovine oocytes

Heat shock protein 90 (Hsp90) is critical for cell homeostasis but its role on bovine oocyte maturation is not well known. We investigated the importance of Hsp90 for competence of bovine oocyte using 17-(allylamino)-17-demethoxygeldanamycin (17AAG), an inhibitor of Hsp90, during in vitro maturation (IVM). Three experiments evaluated the effect of 17AAG on developmental competence of oocytes matured in vitro under thermoneutral (38.5ºC) or heat shock (HS; 41.5ºC) temperatures. The first experiment found that the blastocyst rates were lower (P < 0.05) with 2 µM 17AAG compared with the untreated control (0 µM). The abundance of HSF1 transcripts was higher in oocytes matured with 2 µM than with 0 and 1 µM 17AAG, whereas the abundance of HSP90AA1 and HSPA1A transcripts was lower (P < 0.05) with 1 and 2 µM than with 0 µM. The second experiment found that 2 µM 17AAG for 12 or 24 h during IVM decreased (P < 0.05) the blastocysts rates. In the third experiment, the association of 2 μM 17AAG with HS for 12 h during IVM resulted in lower (P < 0.05) blastocysts rates than 17AAG, HS or untreated control. In conclusion, inhibition of Hsp90 during in vitro maturation compromises further embryo development; the association of Hsp90 inhibition with HS aggravates the deleterious effect of both on oocyte developmental competence.

Significance of the association between early embryonic development and endocytosis

Fertilization triggers a process called maternal-to-zygotic transition, in which the oocyte undergoes oocyte-to-embryo transition, leading to massive intracellular remodeling toward early embryogenesis. This transition requires the degradation of oocyte-derived components; however, the significance and mechanism of degradation of cell surface components remain unknown. In this review, we focused on the dynamics of plasma membrane proteins and investigated the relationship between embryonic development and endocytosis. Our survey of the extant literature on the topic led to the conclusion that clathrin-mediated endocytosis is essential for the progression of early embryogenesis and selective degradation of oocyte-derived plasma membrane proteins in mouse embryos, as reported by studies analyzing maternal cellular surface proteins, including a glycine transporter, GlyT1a. Evaluation of such endocytic activity in individual embryos may allow the selection of embryos with higher viability in assisted reproductive technologies, and it is important to select viable embryos to increase the rates of successful pregnancy and live birth. Although the early embryonic developmental abnormalities are mainly accompanied with chromosomal aneuploidy, other causes and mechanisms remain unclear. This review summarizes molecular biological approaches to early embryonic developmental abnormalities and their future prospects.