Perivitelline space: does it play a role in blocking polyspermy in mammals?

Lipid content and G6PDH activity in relation to ooplasm morphology and oocyte maturational competence in the domestic cat model

The aim of the study was to investigate the relationship between ooplasm morphology, lipid content, glucose-6-phosphate dehydrogenase activity (G6PDH) and maturation potential of domestic cat oocytes. Cumulus-oocyte complexes were classified according to ooplasm morphology: evenly dark (dCOC), heterogeneous/mosaic (hCOC), or light/transparent (lCOC), however only dCOCs are thought to be the best-quality, the remaining ones are usually rejected, therefore little is known about their intracellular properties. Lipid droplets (LDs) were visualized and quantified using Oil Red O. G6PDH activity was assessed before in vitro maturation (IVM), using the brilliant cresyl blue (BCB) test. IVM-control oocytes underwent IVM without BCB staining. The dCOCs and hCOCs had different patterns of LD spatial distribution, but similar amounts of lipid, although this tended towards being lower in hCOCs. Low G6PDH activity (BCB+) was observed in 74 %, 60 % and 24 % (P < 0.01) of dCOCs, hCOCs, and lCOCs, respectively. Significantly more BCB+ /oocytes than BCB-/oocytes reached the metaphase II stage in all groups. The maturation rate of BCB+ /hCOCs was higher than that of IVM/hCOC-controls (40 % v.s. 20 %, P < 0.001), and was comparable to that of BCB+ /dCOCs (54 %; P > 0.05). lCOCs were the smallest (P < 0.01), contained fewer (P < 0.01) lipids than dCOCs or hCOCs, and displayed reduced maturational potential. Overall, LD content and distribution, as well as G6PDH activity, in cat oocytes were strongly associated with ooplasm morphology and oocyte maturational competence. Deeper understanding of the intrinsic properties of oocytes with different ooplasm morphology using the domestic cat model, may be particularly important in the context of the conservation of endangered felids.

Human metaphase II oocytes with narrow perivitelline space have poor fertilization, developmental, and pregnancy potentials

PURPOSE

To analyze the fertilization, developmental, and pregnancy potentials in oocytes with narrow perivitelline space.

METHODS

Perivitelline space (PVS) of oocytes was evaluated at the time of ICSI, and those without sufficient PVS were judged as oocytes with narrow PVS (NPVS oocytes), and those with sufficient PVS formation were judged as oocytes with non-narrow PVS (non-NPVS oocytes). The analysis included 634 NPVS oocytes from 278 cycles and 12,121 non-NPVS oocytes from 1698 cycles. The fertilization and developmental potentials of NPVS and non-NPVS oocytes were compared by calculating odds ratios using a mixed-effects logistic regression model. We also compared the embryo transfer outcomes of those used for single vitrified-warmed blastocyst transfer after developing into the blastocyst stage.

RESULTS

NPVS oocytes had higher odds ratios for degeneration (adjusted odds ratio [aOR], 1.555; 95% confidence interval [CI], 1.096-2.206; p = 0.0133) and 0PN (aOR, 1.387; 95% CI, 1.083-1.775; p = 0.0095), resulting in a lower 2PN rate (aOR, 0.761; 95% CI, 0.623-0.929; p = 0.0072). Even embryos with confirmed 2PN had lower odds ratios for cleavage (aOR, 0.501; 95% CI, 0.294-0.853; p = 0.0109) and blastocyst development (Gardner criteria; CC-AA) rates (aOR, 0.612; 95% CI, 0.476-0.788; p = 0.0001). Blastocysts developed from NPVS oocytes had significantly lower odds ratios for clinical pregnancy (aOR, 0.435; 95% CI, 0.222-0.854; p = 0.0156) than those developed from non-NPVS oocytes.

CONCLUSIONS

Oocytes with NPVS have low fertilization and developmental potential, as well as low likelihood of pregnancy.

Mogroside V alleviates the heat stress-induced disruption of the porcine oocyte in vitro maturation

Heat stress (HS) is a stressor that negatively affect female reproduction. Specially, oocytes are very sensitive to HS. It has been demonstrated that some active compounds can protect oocyte from HS. We previously found that Mogroside V (MV), extracted from Siraitia grosvenorii (Luo Han Guo), can protect oocyte from many kinds of stresses. However, how MV alleviates HS-induced disruption of oocyte maturation remains unknown. In this study, we treated the HS-induced porcine oocytes with MV to examine their maturation and quality. Our findings demonstrate that MV can effectively alleviate HS-induced porcine oocyte abnormal cumulus cell expansion, decrease of first polar body extrusion rate, spindle assembly and chromosome separation abnormalities, indicating MV attenuates oocyte mature defects. We further observed that MV can effectively alleviate HS-induced cortical granule distribution abnormality and decrease of blastocyst formation rate after parthenogenesis activation. In addition, MV treatment reversed mitochondrial dysfunction and lipid droplet content decrease, reduced reactive oxygen species levels, early apoptosis and DNA damage in porcine oocytes after HS. Collectively, this study suggests that MV can effectively protect porcine oocytes from HS.

Positive selection on ADAM10 builds species recognition in the synchronous spawning coral Acropora

The reef-building coral Acropora is a broadcast spawning hermaphrodite including more than 110 species in the Indo-Pacific. In addition, many sympatric species show synchronous spawning. The released gametes need to mate with conspecifics in the mixture of the gametes of many species for their species boundaries. However, the mechanism underlying the species recognition of conspecifics at fertilization remains unknown. We hypothesized that rapid molecular evolution (positive selection) in genes encoding gamete-composing proteins generates polymorphic regions that recognize conspecifics in the mixture of gametes from many species. We identified gamete proteins of Acropora digitifera using mass spectrometry and screened the genes that support branch site models that set the "foreground" branches showing strict fertilization specificity. ADAM10, ADAM17, Integrin α9, and Tetraspanin4 supported branch-site model and had positively selected site(s) that produced polymorphic regions. Therefore, we prepared antibodies against the proteins of A. digitifera that contained positively selected site(s) to analyze their functions in fertilization. The ADAM10 antibody reacted only with egg proteins of A. digitifera, and immunohistochemistry showed ADAM10 localized around the egg surface. Moreover, the ADAM10 antibody inhibited only A. digitifera fertilization but not the relative synchronous spawning species A. papillare. This study indicates that ADAM10 has evolved to gain fertilization specificity during speciation and contributes to species boundaries in this multi-species, synchronous-spawning, and species-rich genus.

Oocyte quality and aging

It is well known that female reproduction ability decreases during the forth decade of life due to age-related changes in oocyte quality and quantity; although the number of women trying to conceive has today increased remarkably between the ages of 36 to 44. The causes of reproductive aging and physiological aspects of this phenomenon are still elusive. With increase in the women's age, during Assisted Reproductive Technologies (ART) we have perceived a significant decline in the number and quality of retrieved oocytes, as well as in ovarian follicle reserves. This is because of increased aneuploidy due to factors such as spindle apparatus disruption; oxidative stress and mitochondrial damage. The aim of this review paper is to study data on the potential role of the aging process impacting oocyte quality and female reproductive ability. We present the current evidence that show the decreased oocyte quality with age, related to reductions in female reproductive outcome. The aging process is complicated and it is caused by many factors that control cellular and organism life span. Although the factors responsible for reduced oocyte quality remain unknown, the present review focuses on the potential role of ovarian follicle environment, oocyte structure and its organelles. To find a way to optimize oocyte quality and ameliorate clinical outcomes for women with aging-related causes of infertility.

Effects of glucuronic acid and N-acetyl-D-glucosamine supplementation on the perivitelline space during the IVM of pig oocytes

The objective of this study was to minimise polyspermic penetration by increasing the perivitelline space (PVS) thickness through supplementation of the hyaluronic acid components glucuronic acid and N-acetyl-d-glucosamine (GlcNAc). Oocytes (n=4690) were supplemented during the first 24h and/or the remainder of maturation (final 16-18h) with 0.01mM glucuronic acid and 0.01mM GlcNAc and then evaluated for PVS thickness, hyaluronic acid, glutathione and glutathione peroxidase concentrations. Fertilised oocytes were evaluated for polyspermic penetration and embryo development. The PVS thickness and amount of hyaluronic acid was significantly (P<0.05) greater in oocytes supplemented with 0.01mM glucuronic acid and 0.01mM GlcNAc during the second part or all of maturation compared with the other treatments. In addition, polyspermic penetration was significantly (P<0.05) less in oocytes supplemented with 0.01mM glucuronic acid and 0.01mM GlcNAc during the second part or all of maturation compared with the other treatments. Supplementing 0.01mM glucuronic acid and GlcNAc during maturation significantly (P<0.05) increased the percentage of cleaved embryos by 48h after IVF and blastocysts formed by 144h after IVF compared those not supplemented. These results indicate that supplementing PVS components during maturation decreases polyspermic penetration by increasing PVS thickness.

Effect of D-Glucuronic Acid and N-acetyl-D-Glucosamine Treatment during In Vitro Maturation on Embryonic Development after Parthenogenesis and Somatic Cell Nuclear Transfer in Pigs

Simple Summary

Hyaluronic acid, also known as hyaluronan, is essential for the expansion of cumulus cells, the maturation of oocytes, and further embryo development. This study aimed to examine the effects of treatment with glucuronic acid and N-acetyl-D-glucosamine, which are components of hyaluronic acid, during porcine oocyte in vitro maturation and embryonic development after parthenogenetic activation and somatic cell nuclear transfer. We measured the diameter of mature oocytes, the thickness of the perivitelline space, the intracellular reactive oxygen species level, and the expression of cumulus cell expansion genes and reactive oxygen species-related genes and examined the cortical granule reaction of oocytes after electrical activation. In conclusion, the addition of 0.05 mM glucuronic acid and 0.05 mM N-acetyl-D-glucosamine and during the initial 22 h of in vitro maturation in pig oocytes has beneficial effects on cumulus expansion, perivitelline space thickness, cytoplasmic maturation, reactive oxygen species level, cortical granule exocytosis, and early embryonic development after parthenogenesis and somatic cell nuclear transfer. Glucuronic acid and N-acetyl-D-glucosamine can be applied to in vitro production technology and can be used as ingredients to produce high-quality porcine blastocysts.

Abstract

This study aimed to examine the effects of treatment with glucuronic acid (GA) and N-acetyl-D-glucosamine (AG), which are components of hyaluronic acid (HA), during porcine oocyte in vitro maturation (IVM). We measured the diameter of the oocyte, the thickness of the perivitelline space (PVS), the reactive oxygen species (ROS) level, and the expression of cumulus cell expansion and ROS-related genes and examined the cortical granule (CG) reaction of oocytes. The addition of 0.05 mM GA and 0.05 mM AG during the first 22 h of oocyte IVM significantly increased oocyte diameter and PVS size compared with the control (non-treatment). The addition of GA and AG reduced the intra-oocyte ROS content and improved the CG of the oocyte. GA and AG treatment increased the expression of CD44 and CX43 in cumulus cells and PRDX1 and TXN2 in oocytes. In both the chemically defined and the complex medium (Medium-199 + porcine follicular fluid), oocytes derived from the GA and AG treatments presented significantly higher blastocyst rates than the control after parthenogenesis (PA) and somatic cell nuclear transfer (SCNT). In conclusion, the addition of GA and AG during IVM in pig oocytes has beneficial effects on oocyte IVM and early embryonic development after PA and SCNT.

Tetraspanins in mammalian reproduction: spermatozoa, oocytes and embryos

It is known that tetraspanin proteins are involved in many physiological somatic cell mechanisms. Additionally, research has indicated they also have a role in various infectious diseases and cancers. This review focuses on the molecular interactions underlying the tetraspanin web formation in gametes. Primarily, tetraspanins act in the reproductive tract as organizers of membrane complexes, which include the proteins involved in the contact and association of sperm and oocyte membranes. In addition, recent data shows that tetraspanins are likely to be involved in these processes in a complex way. In mammalian fertilization, an important role is attributed to CD molecules belonging to the tetraspanin superfamily, particularly CD9, CD81, CD151, and also CD63; mostly as part of extracellular vesicles, the significance of which and their potential in reproduction is being intensively investigated. In this article, we reviewed the existing knowledge regarding the expression of tetraspanins CD9, CD81, CD151, and CD63 in mammalian spermatozoa, oocytes, and embryos and their involvement in reproductive processes, including pathological events.

New cellular imaging of oocytes and preimplantation embryos using Lumitein™: Evaluation of oocyte quality and new information on protein dynamics within the perivitelline space during the one-cell oocyte stage in mice

The extracellular matrix between the oocyte and zona pellucida (ZP) plays an important role in mammalian fertilization and preserves the specific environment of the perivitelline space (PVS) during the development of a preimplantation embryo after fertilization. In this study, we applied a highly sensitive luminescent protein dye, Lumitein™, to observe the hydrophobic status of proteins in oocytes and preimplantation embryos. Lumitein™ is widely used for detecting denatured proteins after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Lumitein™ fluorescence was detected primarily in the PVS and degenerated first polar body of fresh normal metaphase II (MII) oocytes but much less within the ZP and ooplasm, which suggested a hydrophobic PVS environment in the MII oocytes. Unexpectedly, abnormally-shaped fresh or aged oocytes showed stronger fluorescence in the PVS, which reflected oocyte quality. Interestingly, 10 h after fertilization, the fluorescent signal in the PVS temporarily increased in a patched pattern that appeared and then disappeared by the two-cell stage. After the two-cell stage, the decreased fluorescent signal was maintained throughout the development of the preimplantation embryo. These results suggest new protein dynamics in the PVS during the one-cell stage of the oocyte. Thus, cellular imaging of oocytes and preimplantation embryos using Lumitein™ provides new information on protein dynamics.

Preventing polyspermy in mammalian eggs-Contributions of the membrane block and other mechanisms

The egg's blocks to polyspermy (fertilization of an egg by more than one sperm) were originally identified in marine and aquatic species with external fertilization, but polyspermy matters in mammalian reproduction too. Embryonic triploidy is a noteworthy event associated with pregnancy complications and loss. Polyspermy is a major cause of triploidy with up to 80% of triploid conceptuses being the result of dispermic fertilization. The mammalian female reproductive tract regulates the number of sperm that reach the site of fertilization, but mammals also utilize egg-based blocks to polyspermy. The egg-based blocks occur on the mammalian egg coat (the zona pellucida) and the egg plasma membrane, with apparent variation between different mammalian species regarding the extent to which one or both are used. The zona pellucida block to polyspermy has some similarities to the slow block in water-dwelling species, but the mammalian membrane block to polyspermy differs substantially from the fast electrical block that has been characterized in marine and aquatic species. This review discusses what is known about the incidence of polyspermy in mammals and about the mammalian membrane block to polyspermy, as well as notes some lesser-characterized potential mechanisms contributing to polyspermy prevention in mammals.

Detection of CD9 and CD81 tetraspanins in bovine and porcine oocytes and embryos

Tetraspanins are multifunctional molecules located in specific microdomains on the plasma membrane. Thanks to their ability to form networks with other proteins they can participate in many cellular functions. Tetraspanins are part of the interactive network in gametes; however, their precise role in fertilization is not yet clear. The aim of this study was to compare the localization of CD9 and CD81 tetraspanins during oocyte maturation and early development of the embryos in bovine and porcine model. CD9 was detected on the oocyte plasma membrane and vesicles in the perivitelline space of bovine oocytes and embryos. We suggest that CD9 could be a component involved in transzonal projections. Based on the results of in vitro fertilization assay, CD9 and CD81 seem to be part of a more complex fusion network on the plasma membrane of bovine oocytes. On the other hand, both tetraspanins showed a clustered expression pattern on the plasma membrane and inner margin of zona pellucida (ZP) in porcine oocytes and embryos. We found a new species-specific pattern of CD9 and CD81 distribution in ZP which could reflect their specialized role in processes associated with cell adhesion and intercellular communication upon fertilization.

Microfluidic analysis of oocyte and embryo biomechanical properties to improve outcomes in assisted reproductive technologies

Measurement of oocyte and embryo biomechanical properties has recently emerged as an exciting new approach to obtain a quantitative, objective estimate of developmental potential. However, many traditional methods for probing cell mechanical properties are time consuming, labor intensive and require expensive equipment. Microfluidic technology is currently making its way into many aspects of assisted reproductive technologies (ART), and is particularly well suited to measure embryo biomechanics due to the potential for robust, automated single-cell analysis at a low cost. This review will highlight microfluidic approaches to measure oocyte and embryo mechanics along with their ability to predict developmental potential and find practical application in the clinic. Although these new devices must be extensively validated before they can be integrated into the existing clinical workflow, they could eventually be used to constantly monitor oocyte and embryo developmental progress and enable more optimal decision making in ART.

Mechanisms of the epithelial-to-mesenchymal transition in sea urchin embryos

Sea urchin mesenchyme is composed of the large micromere-derived spiculogenetic primary mesenchyme cells (PMC), veg2-tier macromere-derived non-spiculogenetic mesenchyme cells, the small micromere-derived germ cells, and the macro- and mesomere-derived neuronal mesenchyme cells. They are formed through the epithelial-to-mesenchymal transition (EMT) and possess multipotency, except PMCs that solely differentiate larval spicules. The process of EMT is associated with modification of epithelial cell surface property that includes loss of affinity to the apical and basal extracellular matrices, inter-epithelial cell adherens junctions and epithelial cell surface-specific proteins. These cell surface structures and molecules are endocytosed during EMT and utilized as initiators of cytoplasmic signaling pathways that often initiate protein phosphorylation to activate the gene regulatory networks. Acquisition of cell motility after EMT in these mesenchyme cells is associated with the expression of proteins such as Lefty, Snail and Seawi. Structural simplicity and genomic database of this model will further promote detailed EMT research.

Sericin accelerates the production of hyaluronan and decreases the incidence of polyspermy fertilization in bovine oocytes during in vitro maturation

Fetal bovine serum (FBS) has been widely used as a supplement in the maturation medium of bovine oocytes in vitro. However, serum contains many undefined factors and is potentially infectious to humans and animals. As a serum replacement, we evaluated the feasibility of using the silk protein, sericin, derived from the cocoons of silkworm. To examine the rates of oocyte maturation and fertilization, cumulus-oocyte complexes were cultured in TCM-199 supplemented with 0.01%, 0.05%, 0.1% or 0.15% sericin or 5% FBS. The sizes of the perivitelline space that might relate to polyspermy, the expressions of Has2 and CD44 mRNA, the amount of hyaluronan (hyaluronic acid: HA) contained in the oocytes and the rates of blastocyst formation following insemination were then compared between the oocytes cultured with 0.05% sericin and 5% FBS, because the polyspermy rates in oocytes cultured with 0.05% sericin were significantly lower than in those cultured with 5% FBS. After in vitro maturation (IVM), the mean size of the perivitelline space was significantly greater in oocytes cultured with sericin than in those cultured with FBS, although the rates of nuclear maturation, fertilization and blastocyst formation of oocytes under both IVM conditions were not significantly different. The expression of HAS2 and CD44 mRNA and the amount of HA in the denuded oocytes cultured with 0.05% sericin were significantly greater than in those cultured with FBS. These results indicate the feasibility of sericin as an alternative protein supplement for IVM in bovine oocytes.

Pregnancy with oocytes characterized by narrow perivitelline space and heterogeneous zona pellucida: is intracytoplasmic sperm injection necessary?

PURPOSE

This retrospective study analyzed fertilization protocols and pregnancy outcomes for oocytes with with narrow perivitelline space and heterogeneous zona pellucid (NPVS/HZP).

METHODS

In 63 in-vitro fertilization cycles filled with NPVS/HZP oocytes (abnormal oocytes group) and 521 cycles with normal oocytes (normal oocytes group), major clinical and laboratory parameters were recorded and compared in different fertilization cycles (conventional IVF cycles, rescue ICSI cycles, and traditional ICSI cycles).

RESULTS

NPVS/HZP oocytes meant lower MIIoocytes rates in both IVF and ICSI cycles compared with normal oocytes (p < 0.05). The 2PN rates for abnormal oocytes were significantly lower than those for normal oocytes in both conventional IVF cycles (58.8% VS 71.3%, P < 0.05) and rescue ICSI cycles (58.0% VS 78.0%, P = 0.0000). The high-quality embryo rates in normal oocytes groups were significantly higher than those in abnormal oocytes groups in different fertilization cycles (52.2% VS 35.0%, P < 0.01; 42.9% VS 23.9%, P < 0.001; 50.6% VS 31.0%, P = 0.0000, respectively). No clinical pregnancy was obtained from abnormal oocytes in 11 conventional IVF cycles. The clinical pregnancy rates in rescue ICSI and traditional ICSI cycles were comparatively lower in abnormal oocytes groups, but there was no significant difference as compared with normal oocytes groups (35.0% VS 48.1% and 26.7% VS 50.7%, P > 0.05, respectively).

CONCLUSIONS

Retrieval of oocytes characterized by NPVS/PZP from cycle to cycle was one of the reasons for obscure infertility. ICSI may be the right way to avoid fertilization failure and get pregnancy in women with NPVS/HZP oocytes.

Evolution of the egg: new findings and challenges

The evolution of the egg is dynamic, and eggs have numerous species-specific properties across vertebrates and invertebrates. Interestingly, although the structure and function of the egg have remained relatively conserved over time, some constituents of the egg's extracellular barriers are undergoing rapid evolution. In this article, we review current ideas regarding sperm-egg interactions, discuss genetic approaches used to elucidate egg gene functions, and highlight the interesting differences that have evolved across taxa. We suggest that the rapid evolution of egg components and the mechanisms behind sperm-egg interactions are integrally connected, and delve in depth into each component of the egg's extracellular matrices. Finally, we discuss the promising future of reproductive research and how high-throughput genomics and proteomics have the potential to revolutionize the field and provide new evidence that will challenge previously held views about the fertilization process.

Glycosaminoglycans improves early development of zona-free 8-cell rat embryos to blastocysts in a chemically defined medium, but not the pregnancy rate following transfer of the blastocysts

The objective of the present study was to clarify the possible role of the zona pellucida (ZP) in early development of rat embryos and to determine the effect of glycosaminoglycans on the development of ZP-free 8-cell embryos before or after embryo transfer at the blastocyst stage. Eight-cell embryos were divided into three groups comprised of, 1) intact controls, 2) embryos with the ZP was removed with acidic solution and 3) pairs of ZP-free 8-cell embryos aggregated in a small hollow. These embryos were cultured in a chemically defined mR1ECM for 24 h. Developmental ability to the blastocyst stage and mean cell number in the blastocyst was lower in ZP-free embryos than in intact controls. When these blastocysts were transferred, the farrowing rate and efficiency of embryos developed to term were also lower in ZP-free embryos, but not in the aggregated ones. Supplementation with hyaluronan (HA; 63-250 µg/ml) or heparan sulfate proteoglycan (HS; 15 µg/ml) significantly improved blastocyst formation of ZP-free embryos and the cell number in the blastocyst by reducing the incidence of apoptosis. However, there were no beneficial effects of HA or HS on farrowing and newborn rates after transfer of the blastocysts. In conclusion, the ZP plays roles in maintaining successful development of early rat embryos at least from the 8-cell stage not only to the blastocyst stage but also to posttransfer stages. Glycosaminoglycans, such as HA or HS, appear to contribute to successful cleavage during early development to the blastocyst stage but may be insufficient to maintain the posttransfer survival of ZP-free embryos.

Size of the perivitelline space and incidence of polyspermy in rabbit and hamster oocytes

Purpose

The size of the perivitelline space and the incidence of polyspermy were observed in ovulated and cultured oocytes from rabbits and hamsters with or without treatment by 4-methylumbelliferone (MU), an inhibitor of hyaluronic acid (HA) synthase, in order to examine the relationship between the incidence of polyspermy and the size of the perivitelline space. The amount of HA in the medium with MU-treated hamster oocytes was measured and compared with that in the medium with untreated oocytes.

Methods

The perivitelline space of oocytes with 1st polar bodies was measured by use of a micrometer under a microscope, and the incidence of polyspermy was observed in the oocytes after insemination. The amount of HA in the medium was examined using an enzyme-linked immunosorbent assay. The distribution of cortical granules was observed by staining with FITC-conjugated LCA.

Results

In both rabbit and hamster, the mean size of the perivitelline space was significantly smaller and the incidence of polyspermy was significantly higher in the cultured and MU-treated oocytes than in the ovulated and untreated oocytes. The mean amount of HA in the medium with MU-treated oocytes (8.96 pg) was significantly smaller than that in the medium with untreated oocytes (21.77 pg). The distribution of cortical granules did not differ among the ovulated, cultured, and MU-treated oocytes.

Conclusions

These findings suggest that the size of the perivitelline space is closely related to the incidence of polyspermy, and that the oocyte itself synthesizes and secretes the HA involved in the enlargement of the perivitelline space.

Cell volume regulation in oocytes and early embryos: connecting physiology to successful culture media

BACKGROUND

Preimplantation embryos are particularly susceptible to in vitro developmental blocks. These could be alleviated by lowering culture medium osmolarity. Because mammalian cells regulate their volumes by adjusting intracellular osmotic pressure, cell volume regulation could be critical to early embryos.

METHODS

We reviewed the literature on cell volume regulation in preimplantation embryos and the effects of increased osmolarity on embryo development, focusing also on the relation with improvements in embryo culture media.

RESULTS

Embryos failed to develop from fertilized oocytes when osmolarity is increased. This could be alleviated by decreasing osmolarity or including certain compounds such as certain amino acids. Early preimplantation mouse embryos require intracellular accumulation of glycine to provide osmotic support and thus control cell volume. The glycine-specific transporter, GLYT1, mediates osmoregulated glycine accumulation in mouse embryos and likely in human embryos. GLYT1 is activated during meiotic maturation starting at ovulation. Prior to this, oocyte size is not independently controlled but instead is determined by strong adhesion between the oocyte plasma membrane and the inner surface of the zona pellucida.

CONCLUSIONS

Early preimplantation embryos are particularly sensitive to increased osmolarity, and require the importation of glycine to regulate their cell volumes using a mechanism unique to early embryos. Cell volume regulation first appears when ovulation is triggered, oocyte zona pellucida adhesion is released, and glycine transport is activated. The requirement for supporting these physiological functions in oocytes and embryos should be taken into account when developing and improving systems for in vitro oocyte maturation and embryo culture.

Cell volume regulation is initiated in mouse oocytes after ovulation

Fertilized mouse eggs regulate their size principally by accumulating glycine as an intracellular osmolyte using the GLYT1 (SLC6A9) transporter, a mechanism of cell volume homeostasis apparently unique to early embryos before the morula stage. However, nothing was known of cell volume regulation in oocytes before fertilization. We show here that GLYT1 is quiescent in mouse germinal-vesicle-stage oocytes but becomes fully activated within hours after ovulation is triggered. This initiates accumulation of substantial amounts of intracellular glycine in oocytes during meiotic progression, reaching a maximal level in mature eggs. Measurements of endogenous free glycine showed that there were nearly undetectable levels in ovarian germinal-vesicle-stage oocytes, but high levels were present in mature ovulated eggs and in preimplantation embryos through the two-cell stage, but not in morulae. Furthermore, intracellular glycine was regulated in response to changes in external tonicity in eggs and embryos through the two-cell stage, but not in oocytes or embryos after the two-cell stage. Before activation of GLYT1,oocytes were unable to independently regulate their volume. As GLYT1 became active, however, oocyte volume decreased substantially and oocytes gained the ability to regulate their size, which required GLYT1 activity. Before ovulation, oocyte size was instead determined by a strong adhesion to the rigid extracellular matrix of the oocyte, the zona pellucida, which was released coincident with GLYT1 activation. The ability to acutely regulate cell size is thus acquired by the oocyte only after ovulation, when it first develops glycine-dependent cell volume regulation.

Oocyte-specific deletion of complex and hybrid N-glycans leads to defects in preovulatory follicle and cumulus mass development

Complex and hybrid N-glycans generated by N-acetylglucosaminyltransferase I (GlcNAcT-I), encoded by Mgat1, affect the functions of glycoproteins. We have previously shown that females with oocyte-specific deletion of a floxed Mgat1 gene using a zona pellucida protein 3 (ZP3)Cre transgene produce fewer pups primarily due to a reduction in ovulation rate. Here, we show that the ovulation rate of mutant females is decreased due to aberrant development of preovulatory follicles. After a superovulatory regime of 48 h pregnant mare's serum (PMSG) and 9 h human chorionic gonadotropin (hCG), mutant ovaries weighed less and contained approximately 60% fewer preovulatory follicles and more atretic and abnormal follicles than controls. Unlike controls, a proportion of mutant follicles underwent premature luteinization. In addition, mutant preovulatory oocytes exhibited gross abnormalities with approximately 36% being blebbed or zona-free. While 97% of wild-type oocytes had a perivitelline space at the preovulatory stage, approximately 54% of mutant oocytes did not. The cumulus mass surrounding mutant oocytes was also smaller with a decreased number of proliferating cells compared with controls, although hyaluronan around mutant oocytes was similar to controls. In addition, cumulus cells surrounding mutant eggs were resistant to removal by either hyaluronidase or incubation with capacitated sperm. Therefore, the absence of complex and hybrid N-glycans on oocyte glycoproteins leads to abnormal folliculogenesis resulting in a decreased ovulation rate.

The presence of heparan sulfate in the mammalian oocyte provides a clue to human sperm nuclear decondensation in vivo

BACKGROUND

Previous results from our laboratory have led us to propose heparan sulfate (HS) as a putative protamine acceptor during human sperm decondensation in vivo. The aim of this paper was to investigate the presence of glycosaminoglycans in the mammalian oocyte in an effort to better support this contention.

METHODS

Two experimental approaches are used: oocyte labeling to identify the presence of HS and analysis of sperm decondensing ability of fresh oocytes in the presence or absence of specific glycosidases.

RESULTS

Staining of mouse zona-intact oocytes with the fluorescent cationic dye, Rubipy, at pH 1.5 allowed for the detection of sulfate residues in the ooplasm by confocal microscopy. HS was detected in the ooplasm by immunocytochemistry. A sperm decondensation microassay using heparin and glutathione was successfully developed. The same level of sperm decondensation could be attained when heparin was replaced by mouse zona-free oocytes. Addition of heparinase to the oocyte/glutathione mixture significantly reduced sperm decondensation (P = 0.0159), while there was no effect following addition of either chondroitinase ABC or hyaluronidase.

CONCLUSIONS

The results presented in this paper demonstrate for the first time that HS is present in the mammalian oocyte and show that HS is necessary for fresh oocytes to express their sperm decondensing ability in vitro.

Free-radical crosslinking of specific proteins alters the function of the egg extracellular matrix at fertilization

All animal embryos begin development by modifying the egg extracellular matrix. This protein-rich matrix protects against polyspermy, microbes and mechanical stress via enzyme-dependent transformations that alter the organization of its constituents. Using the sea urchin fertilization envelope,a well-defined extracellular structure formed within minutes of fertilization,we examine the mechanisms whereby limited permeability is established within this matrix. We find that the fertilization envelope acquires a barrier filtration of 40,000 daltons within minutes of insemination via a peroxidase-dependent mechanism, with dynamics that parallel requisite production of hydrogen peroxide by the zygote. To identify the molecular targets of this free-radical modification, we developed an in vivo technique to label and isolate the modified matrix components for mass spectrometry. This method revealed that four of the six major extracellular matrix components are selectively crosslinked, discriminating even sibling proteins from the same gene. Thus, specific free-radical chemistry is essential for establishing the embryonic microenvironment of early development.

The distribution of transglutaminase in the rat oocytes and embryos

Transglutaminases (TGs) are calcium-dependent enzymes that catalyze the transamidation of glutamine residues of a protein substrate to form intermolecular isopeptide bonds. The zona pellucida (ZP) is an extracellular, glycoprotein matrix that surrounds the oocytes of all Eutherian mammals. We aimed to identify the immunoreactivity of tissue transglutaminase (tTG) and ultrastructural changes occuring in rat oocytes before and after fertilization. Female rats were stimulated to superovulate, then mated with males. Oocytes and embryos were collected and examined by immunohistochemistry and electron microscopy. Before fertilization, tTG was present only in the oolemma and the cortical cytoplasm. After fertilization, tTG reactivity increased in the ZP of the early zygote and the preimplantation embryos, but decreased in the cytoplasm and perivitelline space (PVS). After fertilization, the PVS ultrastructure became asymmetrical and large around the polar bodies with many cortical granule contents. In conclusion, tTG immunoreactivity was found to be spatially and temporarily heterogeneous in the rat oocytes and embryos, especially in the ZP.

The Perivitelline Space-Forming Capacity of Mouse Oocytes is Associated with Meiotic Competence

Although mouse oocytes progressively acquire meiotic competence during their growth in the ovaries, only half of full-grown oocytes can accomplish meiosis. Two types of full-grown oocytes have been reported on the basis of their chromatin configuration, the surrounded-nucleolus (SN) type and the non-surrounded-nucleolus (NSN) type. Therefore, full-grown oocytes collected from the ovaries of adult animals comprise a heterogeneous population; some oocytes are meiotically incompetent (NSN-type), and some are competent (SN-type). In the present study, we found that full-grown oocytes could be classified into two groups using the criterion of formation of the perivitelline space (PVS) after culture with 3-isobutyl-1-methylxanthine (IBMX) for 1 h. In oocytes with a PVS, actin-filled processes within zona pellucidae originating from cumulus cells were reduced, while they were rich in oocytes without a PVS, suggesting that a reduction in these processes contributes to PVS formation. PVS formation was highly correlated with meiotic competence and SN-type configuration. The results of this study demonstrate that PVS formation is a useful criterion for easily distinguishing between SN- and NSN-type oocytes, without injury to the cells.

Effect of Zona Pellucida Removal on DNA Methylation in Early Mouse Embryos1

Removal of the zona pellucida is known to affect mouse development to term. Zygotes were recovered immediately after fertilization and their zona pellucida removed by exposure to pronase before culture and comparison with zona-intact embryos. The effect of removing the zona pellucida was assessed in terms of embryo development to blastocyst, DNA methylation, histone acetylation, and expression of three developmentally regulated genes. No significant differences were seen in percentage of embryos that developed to the blastocyst stage. However, zona-free embryos showed a significant reduction in the DNA methylation level at two-cell and four-cell stages, but no differences at pronuclear, morula, and blastocyst stages, as observed by immunofluorescence. Mechanical or enzymatic removal of the zona pellucida showed similar DNA methylation staining patterns at the two-cell stage. The time when the zona pellucida was removed appears to influence the levels of DNA methylation. When zona removal was delayed for 8 h, there was no difference in DNA methylation levels between zona-free and zona-intact two-cell embryos, indicating that the critical time is early on, between 1 and 8 h postfertilization. In contrast, when immunofluorescence analysis of histone acetylation was performed, no significant differences were seen between zona-free and zona-intact embryos at any of the developmental stage. Similarly, no differences were found regarding the onset of transcription of Dnmt1s, Nanog, and Fgf4 genes.

CD9 controls the formation of clusters that contain tetraspanins and the integrin α6β1, which are involved in human and mouse gamete fusion

The process of gamete fusion has been largely studied in the mouse and has revealed the crucial role of the tetraspanin CD9. By contrast, human gamete fusion remains largely unknown. We now show that an anti-α6 integrin mAb (GoH3) strongly inhibited human sperm-egg fusion in human zona-free eggs. Furthermore, a mAb directed against CD151, a tetraspanin known to associate with α6β1, partially inhibited sperm-egg fusion. By contrast, the addition of an anti-CD9 mAb to zona free eggs had no effect. The integrin α6β1, CD151 and CD9 tetraspanins were evenly distributed on human zona-intact oocytes. On zona-free eggs, the integrin α6β1 and tetraspanin CD151 patched and co-localized but the tetraspanin CD9 remained unchanged. CD9 mAb prevented α6β1 integrin clustering and gamete fusion when added prior to, but not after, zona removal. Antibody-mediated aggregation of integrin α6β1 yielded patches that were bigger and more heterogeneous in mouse oocytes lacking CD9. Moreover, a strong labelling of α6β1 could be observed at the sperm entry point. Altogether, these data show that CD9 controls the redistribution of some membrane proteins including the α6β1 integrin into clusters that may be necessary for gamete fusion.

Ultrastructural Dynamics of Human Reproduction, from Ovulation to Fertilization and Early Embryo Development1

This study describes the updated, fine structure of human gametes, the human fertilization process, and human embryos, mainly derived from assisted reproductive technology (ART). As clearly shown, the ultrastructure of human reproduction is a peculiar multistep process, which differs in part from that of other mammalian models, having some unique features. Particular attention has been devoted to the (1) sperm ultrastructure, likely "Tygerberg (Kruger) strict morphology criteria"; (2) mature oocyte, in which the MII spindle is barrel shaped, anastral, and lacking centrioles; (3) three-dimensional microarchitecture of the zona pellucida with its unique supramolecular filamentous organization; (4) sperm-egg interactions with the peculiarity of the sperm centrosome that activates the egg and organizes the sperm aster and mitotic spindles of the embryo; and (5) presence of viable cumulus cells whose metabolic activity is closely related to egg and embryo behavior in in vitro as well as in vivo conditions, in a sort of extraovarian "microfollicular unit." Even if the ultrastructural morphodynamic features of human fertilization are well understood, our knowledge about in vivo fertilization is still very limited and the complex sequence of in vivo biological steps involved in human reproduction is only partially reproduced in current ART procedures.

Mouse SLLP1, a sperm lysozyme-like protein involved in sperm-egg binding and fertilization

This study demonstrates the retention of mouse sperm lysozyme-like protein (mSLLP1) in the equatorial segment of spermatozoa following the acrosome reaction and a role for mSLLP1 in sperm-egg binding and fertilization. Treatment of cumulus intact oocytes with either recmSLLP1 or its antiserum resulted in a significant (P < or = 0.05) inhibition of fertilization. Co-incubation of zona-free mouse oocytes with capacitated mouse spermatozoa in the presence of varying concentrations of anti-recmSLLP1 serum or recmSLLP1 also inhibited sperm-oolemma binding. A complete inhibition of binding and fusion of spermatozoa to the oocyte occurred at 12.5 muM concentration of recmSLLP1, while conventional chicken and human lysozymes did not block sperm-egg binding. mSLLP1 showed receptor sites in the perivitelline space as well as on the microvillar region of the egg plasma membrane. The retention of mSLLP1 in the equatorial segment of acrosome-reacted sperm, the inhibitory effects of both recmSLLP1 and antibodies to SLLP1 on in vitro fertilization with both cumulus intact and zona-free eggs, and the definition of complementary SLLP1-binding sites on the egg plasma membrane together support the hypothesis that a c lysozyme-like protein is involved in the binding of spermatozoa to the egg plasma membrane during fertilization.

Defending the zygote: search for the ancestral animal block to polyspermy

Fertilization is the union of a single sperm and an egg, an event that results in a diploid embryo. Animals use many mechanisms to achieve this ratio; the most prevalent involves physically blocking the fusion of subsequent sperm. Selective pressures to maintain monospermy have resulted in an elaboration of diverse egg and sperm structures. The processes employed for monospermy are as diverse as the animals that result from this process. Yet, the fundamental molecular requirements for successful monospermic fertilization are similar, implying that animals may have a common ancestral block to polyspermy. Here, we explore this hypothesis, reviewing biochemical, molecular, and genetic discoveries that lend support to a common ancestral mechanism. We also consider the evolution of alternative or radical techniques, including physiological polyspermy, with respect to our ability to describe a parsimonious guide to fertilization.

Calreticulin on the mouse egg surface mediates transmembrane signaling linked to cell cycle resumption

Calreticulin, a protein best known as an endoplasmic reticulum chaperone, also is found on the extracellular plasma membrane surface of many cell types where it serves as a mediator of adhesion and as a regulator of the immune response. In this report, we demonstrate that calreticulin is present on the extracellular surface of the mouse egg plasma membrane and is increased in the perivitelline space after egg activation. The extracellular calreticulin appears to be secreted by vesicles in the egg cortex that are distinct from cortical granules. An anticalreticulin antibody binds to extracellular calreticulin on live eggs and inhibits sperm-egg binding but not fusion. In addition, engagement of cell surface calreticulin by incubation of mouse eggs in the presence of anticalreticulin antibodies results in alterations in the localization of cortical actin and the resumption of meiosis as indicated by alterations in chromatin configuration, decreases in cdc2/cyclin B1 and MAP kinase activities, and pronuclear formation. These events occur in the absence of any observable alterations in intercellular calcium. These data demonstrate that calreticulin functionally interacts with the egg cytoskeleton and can mediate transmembrane signaling linked to cell cycle resumption. These studies suggest a role for calreticulin as a lectin that may be involved in signal transduction events during or after sperm-egg interactions at fertilization.