High-Resolution 3D Reconstruction of Human Oocytes Using Focused Ion Beam Scanning Electron Microscopy

The egg plays a pivotal role in the reproduction of our species. Nevertheless, its fundamental biology remains elusive. Transmission electron microscopy is traditionally used to inspect the ultrastructure of female gametes. However, two-dimensional micrographs contain only fragmentary information about the spatial organization of the complex oocyte cytoplasm. Here, we employed the Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) to explore human oocyte intracellular morphology in three dimensions (3D). Volume reconstruction of generated image stacks provided an unprecedented view of ooplasmic architecture. Organelle distribution patterns observed in nine donor oocytes, representing three maturational stages, documented structural changes underlying the process by which the egg acquires developmental competence. 3D image segmentation was performed to extract information about distinct organelle populations, and the following quantitative analysis revealed that the mitochondrion occupies ∼ 4.26% of the maturing oocyte cytoplasm. In summary, this proof-of-concept study demonstrates the potential of large volume electron microscopy to study rare samples of delicate female gametes and paves the way for applying the FIB-SEM technique in human oocyte research.

Effect of nucleocytoplasmic ratio on the in vitro porcine embryo development after in vitro fertilization or parthenogenetic activation

Summary This study was conducted to examine whether the nuclear to cytoplasmic (N/C) ratio had any influence on the timing of embryo compaction and blastocoel formation, as well as formation rate and quality of blastocyst. First, we produced embryos with increased N/C ratio by removal of approximately one-third of the cytoplasm and with decreased N/C ratio by doubling the oocyte cytoplasm with an enucleated oocyte. The initiation of compaction and cavitation in reduced cytoplasm group was significantly earlier (P < 0.05) compared with the control and doubled cytoplasm groups. The rate of blastocysts in the reduced cytoplasm and doubled cytoplasm groups was significantly lower (P < 0.05) compared with the control group. Blastocyst quality in terms of total cell number in the reduced cytoplasm group was significantly lower (P < 0.05) compared with the doubled cytoplasm group, but not different from the control group. Next, we produced embryos with various N/C ratios by oocyte fusion combined with cytochalasin D treatment. The onset of compaction and cavitation in the 2N/2C group (decreased N/C ratio) was significantly delayed (P < 0.05) or had the tendency to be delayed (P = 0.064), respectively, compared with the control group (2N/1C). A significantly higher rate of blastocyst was observed in the 4N/2C group compared with the 1N/1C group (P < 0.05) but not different from the remaining groups. These results demonstrated that an increase in N/C ratio caused an earlier occurrence of morula compaction and blastocyst formation in both in vitro fertilization (IVF) and parthenogenetically activated pig embryos.

The vitellogenin receptor functionality of the migratory locust depends on its phosphorylation by juvenile hormone

Vitellogenin receptor (VgR) plays a pivotal role in ovarian vitellogenin (Vg) uptake and vertical transmission of pathogenic microbes and Wolbachia symbionts. However, the regulatory mechanisms of VgR action as an endocytic receptor and translocation from oocyte cytoplasm to the membrane remain poorly understood. Here, by using the migratory locust Locusta migratoria as a model system, we report that juvenile hormone (JH) promotes VgR phosphorylation at Ser1361 in the second EGF-precursor homology domain. A signaling cascade including GPCR, PLC, extracellular calcium, and PKC-ι is involved in JH-stimulated VgR phosphorylation. This posttranslational regulation is a prerequisite for VgR binding to Vg on the external surface of the oocyte membrane and subsequent VgR/Vg endocytosis. Acidification, a condition in endosomes, induces VgR dephosphorylation along with the dissociation of Vg from VgR. Phosphorylation modification is also required for VgR recycling from oocyte cytoplasm to the membrane. Additionally, VgR phosphorylation and its requirement for Vg uptake and VgR recycling are evolutionarily conserved in other representative insects including the cockroach Periplaneta americana and the cotton bollworm Helicoverpa armigera. This study fills an important knowledge gap of low-density lipoprotein receptors in posttranslational regulation, endocytosis, and intracellular recycling.

Nuclear-to-cytoplasmic ratios of 1PN and 2PN zygotes after in vitro fertilization of mouse oocytes

Summary Numerous studies have reported comparisons of the nuclear-to-cytoplasmic (NC) ratio during mitosis. However, little information is known about how the pronuclear size is regulated and determined at the end of meiosis II in mammalian zygotes. The present study aims to analyze the NC ratio of female and male pronuclei, and also to compare the size of single pronuclei using photographs that were obtained during experiments to create chimeric hermaphrodites from 2-cell oocytes. The volume of both the female and the male pronucleus was found to correlate with the volume of the oocyte cytoplasm. The NC ratio of the male pronucleus was greater than that of the female pronucleus. The NC ratio of the average volume of the female and male pronuclei was greater than the NC ratio of the mononucleate oocytes. The occurrence of 1PN oocytes was significantly higher when the volume of cytoplasm was lower than the cut-off value. These results indicated that the NC ratio is retained during pronuclear formation. A higher NC ratio in male compared with the female pronucleus indicated structural and/or molecular difference between the two pronuclei. 1PN formation may occur when sperm enters close to the MII spindle.

High-resolution 3D reconstruction of human oocytes using FIB-SEM.

The oocyte plays a pivotal role in the reproduction of our species. Nevertheless, its biology remains poorly understood. Electron microscopy is traditionally used to inspect the ultrastructure of female gametes. However, two-dimensional micrographs contain only fragmentary information about the spatial organization of the complex oocyte cytoplasm. Here, we employed the Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) to explore human oocyte intracellular morphology in three dimensions (3D). Volume reconstruction from high-resolution image stacks provided an unprecedented view of ooplasmic architecture. Organelle distribution patterns observed in 9 donor oocytes, representing 3 maturational stages, documented structural changes underlying the process by which the egg acquires developmental competence. 3D image segmentation was performed to extract information about distinct organelle populations. The quantitative analysis of the organelle abundance revealed that mitochondrion occupies ~ 4.26 % of the maturing oocyte cytoplasm. This proof-of-concept study demonstrates the potential of FIB-SEM imaging to study human oocyte morphology.

Novel Biallelic TLE6 Variants Induce Preimplantation Embryonic Lethality That Cannot be Rescued by IVF or ICSI

Background: Preimplantation embryonic lethality is a rare cause of primary female infertility. Transducin-like enhancer of split 6 (TLE6) is a maternal effect gene which encodes a component of the subcortical maternal complex located in the oocytes and early embryos. It has been reported that biallelic variants in TLE6 can lead to preimplantation embryonic lethality. However, the incidence of TLE6 variants in patients with preimplantation embryonic lethality is not fully understood.Methods: Whole-exome sequencing and bioinformatics analyses were used to analyze a cohort of 28 patients with preimplantation embryonic lethality. We retrospectively analyzed the process and outcome of their attempts at in vitro fertilization and intracytoplasmic sperm injection. Results: In this study, four patients (14.29 %, 4/28) from three unrelated families in a cohort of 28 individuals with preimplantation embryonic lethality were identified as carrying biallelic TLE6 variants, including two homozygous variants and one compound heterozygous variant. These novel frameshift variants in TLE6 caused truncation of the TLE6 protein likely impairing its function. Immunofluorescence staining of oocytes for TLE6 indicated that it is localized in the oocyte cytoplasm, and that TLE6 was almost absent in the oocytes of the patients carrying biallelic TLE6 variants compared with normal control oocytes. A retrospective analysis showed that the four affected individuals underwent a total of nine in vitro fertilization and intracytoplasmic sperm injection attempts, in which a total of 119 metaphase II oocytes were retrieved, but none of them obtained high-quality blastocysts or obtained a successful pregnancy. However, one of these patients became pregnant on the first attempt using donated oocytes.Conclusions: Biallelic TLE6 variants cause preimplantation embryonic lethality that cannot be rescued by in vitro fertilization or intracytoplasmic sperm injection. Thus, oocyte donation may be the preferred treatment for patients harboring biallelic TLE6 variants.

Cysteamine in Maturation Medium Enhances Nuclear Maturation and Fertilization Rate of Sheep Oocytes In Vitro

Low nuclear maturation and fertilization rate is one obstacle in the in vitro embryo production which decrease embryo yield. This problem is presumable related with high production of reactive oxygen species (ROS) during maturation process. Glutathione (GSH) as an antioxidant is well known to overcome effect of ROS production. GSH synthesis in the cytosol part of the oocyte cytoplasm is influenced by cysteine availability. It is therefore, this research was conducted to evaluate the ability of cysteamine to provide cysteine availability as GSH precursor on the nuclear maturation and fertilization rate of sheep oocytes. Results of this experiment revealed additional cysteamine at 150 µm and 200 µm could significantly improve nuclear maturation rate. On the other side, although additional of cysteamine at 50 µm could not improve nuclear maturation rate, however 50 µm cysteamine in the maturation medium could significantly improve the fertilization rate. Based on those experiment results, it seems that the additional cysteamine might be improve not only GSH availability but also the oocyte quality which characterized the ability of pronuclear formation. This finding strongly suggested that additional cysteamine in the maturation medium could improve nuclear maturation and fertilization rate of sheep oocytes.

Oogenesis in Crustaceans: Ultrastructural Aspects and Selected Regulating Factors

This chapter explores ultrastructural aspects of crustacean oogenesis. It focuses on various cellular processes associated with female germline development in selected crustacean groups. Oogenesis in crustaceans comprises four stages: proliferation of germline cells, previtellogenesis, vitellogenesis, and formation of egg coverings. The greater part of oogenesis occurs in the ovary. In Crustacea, two structurally and functionally distinct types of ovary are recognized: panoistic and meroistic. In panoistic ovaries, all germline cells differentiate into oocytes, and this type of ovarian organization occurs in a great majority of crustaceans, including Malacostraca. In contrast, in the meroistic ovaries, oogonial cells are connected by intercellular bridges and form characteristic linear cysts. Within each cyst, only one cell becomes an oocyte, and the remaining cells differentiate into nurse cells. Meroistic ovaries are typical for Branchiopoda and Ostracoda: Podocopida. Ultrastructural studies reveal that the nucleus and cytoplasmic organelles of the oocyte are highly synthetically active in the panoistic ovary, whereas in the meroistic type, oocyte development is supported, to some extent, by accompanying nurse cells. During previtellogenesis, oocytes accumulate large numbers of various organelles, e.g. ribosomes, mitochondria, and cisternae of endoplasmic reticulum. The oocyte cytoplasm also contains characteristic disc-shaped bodies and cortical granules. A comparative analysis of the proteinaceous yolk formation in different crustaceans reveals two distinct types of vitellogenesis (autosynthesis and heterosynthesis), and indicates that a mixed type prevails in these arthropods. In most crustacean species, germline cells associate with somatic follicle cells that may fulfill several functions during oogenesis.

The mitochondrial DNA copy number of cumulus granulosa cells may be related to the maturity of oocyte cytoplasm

STUDY QUESTION Is the mitochondrial DNA (mtDNA) copy number of cumulus granulosa cells (CGCs) related to the maturation of oocyte cytoplasm? SUMMARY ANSWER Compared with the mtDNA copy number of CGCs from germinal vesicles (GV), CGCs from Metaphase I (MI) oocytes appear to have a lower mtDNA copy number. WHAT IS KNOWN ALREADY The growth and development of CGCs and oocyte are synchronised. The interaction between CGCs and the oocyte provides the appropriate balance of energy, which is necessary for mammalian oocyte development. Moreover, in the oocyte–cumulus complex (OCC), mature oocytes with higher mtDNA copy numbers tend to have corresponding CGCs with higher mtDNA copy numbers. STUDY DESIGN, SIZE, DURATION This is a prospective study of 302 OCCs obtained from 70 women undergoing in vitro fertilisation with intracytoplasmic sperm injection (ICSI) at the Reproductive and Genetic Hospital of CITIC-Xiangya, between 24 February 2018 and 21 December 2019. The CGCs were divided into three groups (GV, MI and MII stages) based on the maturation status of their corresponding oocyte. The sample sizes (n = 302) of CGCs in the three stages were 63 (CGCGV), 70 (CGCMI) and 169 (CGCMII), respectively. Some of the samples (n = 257) was used to quantify the mtDNA copy number, while the rest (n = 45) were used to analyse the expression level of mitochondrial genes. Furthermore, we retrieved 82 immature oocytes from among the 257 OCCs used for mtDNA copy numbers, including 36 GV oocytes and 46 MI oocytes, for analysis of oocyte mtDNA. PARTICIPANTS/MATERIALS, SETTING, METHODS We selected genes with high consistency of real-time PCR results to accurately measure the mtDNA copy number by testing the efficacy and the reproducibility in whole genome amplification (WGA) samples from a human embryonic stem cell line. The CGCs of each oocyte were individually isolated. The mtDNA copy number and gene expression of the CGCs were assessed using real-time PCR techniques. Mitochondrial DNA copy number of the corresponding immature oocytes was also evaluated. MAIN RESULTS AND THE ROLE OF CHANCE MT-ND1, MT-CO1 and β-globin genes were chosen for the assessment of mtDNA content, and mRNA expressions of MT-ND1, MT-CO1, PGC-1α and TFAM were also measured. The genome of 257 CGCs and 82 immature oocytes were amplified according to the multiple displacement amplification (MDA) protocol, and RNA was extracted from 45 CGCs. Compared with CGCGV, CGCMI had a significantly lower mtDNA copy number. In the MT-ND1 assay, the CGCGV: CGCMI was [270 ± 302]: [134 ± 201], P = 0.015. In the MT-CO1 assay, CGCGV: CGCMI was [205 ± 228]: [92 ± 112], P = 0.026. There was no statistical difference in mtDNA between CGCGV and CGCMII. In the MT-ND1 assay, CGCGV: CGCMII was [270 ± 302]: [175 ± 223], P = 0.074. In the MT-CO1 assay, CGCGV: CGCMII was [205 ± 228]: [119 ± 192], P = 0.077. No statistical difference of mtDNA copy number was observed between CGCMI and CGCMII. In the MT-ND1 assay, CGCMI: CGCMII was [134 ± 201]: [175 ± 223], P = 0.422. In the MT-CO1 assay, CGCMI: CGCMII was [92 ± 112]: [119 ± 192], P = 0.478. To verify the reliability of the above results, we further analysed the mtDNA copy number of CGCs of 14 patients with GV, MI and MII oocytes, and the results showed that the mtDNA copy number of CGCMI may be lower. The mtDNA copy number of CGCGV and CGCMI was statistically different in the MT-ND1 assay where CGCGV: CGCMI was [249 ± 173]: [118 ± 113], P = 0.016, but in the MT-CO1 assay, CGCGV: CGCMI was [208 ± 199]: [83 ± 98], P = 0.109. There was no significant difference in mtDNA between CGCGV and CGCMII. In the MT-ND1 assay, CGCGV: CGCMII was [249 ± 173]: [185 ± 200], P = 0.096. In the MT-CO1 assay, CGCGV: CGCMII was [208 ± 199]: [114 ± 139], P = 0.096. There was also no significant difference in mtDNA between CGCMI and CGCMII. In the MT-ND1 assay, CGCMI: CGCMII was [118 ± 113]: [185 ± 200], P = 0.198. In the MT-CO1 assay, CGCMI: CGCMII was [83 ± 98]: [114 ± 139], P = 0.470. Moreover, there were no statistical differences in the expression levels of MT-ND1, MT-CO1, PGC-1α and TFAM between CGCGV, CGCMI and CGCMII (P &gt; 0.05). LARGE SCALE DATA N/A LIMITATIONS, REASONS FOR CAUTION Due to the ethical issues, the study did not quantify the mtDNA content of MII oocytes. Thus, whether the change in mtDNA copy number in CGCs is related to the different developmental stages of oocytes has not been further confirmed. Moreover, the sample size was relatively small. WIDER IMPLICATIONS OF THE FINDINGS The mtDNA copy number of CGCs decreases from the GV phase to the MI phase and stays steady from the MI to MII stage. At different stages of oocyte maturation, the mtDNA of CGCs may undergo self-degradation and replication to meet the energy requirements of the corresponding oocyte and the maturation of the oocyte cytoplasm. STUDY FUNDING/COMPETING INTEREST(S) Funding was provided by the National Key R&D Program of China (Grant 2018YFC1003100, to L.H.), the science and technology major project of the Ministry of Science and Technology of Hunan Province, China (grant 2017SK1030, to G.L.), the National Natural Science Foundation of China (grant 81873478, to L.H.), and Merck Serono China Research Fund for Fertility Experts (to L.H.). There is no conflict of interest.