Subcellular distribution of ZP1, ZP2, and ZP3 glycoproteins during folliculogenesis and demonstration of their topographical disposition within the zona matrix of mouse ovarian oocytes

Effect of blastocyst development on hatching and embryo implantation

The mammalian zygote, formed after a sperm fertilizes an egg, undergoes several rounds of mitosis and morphogenesis to form the blastocyst. During the peri-implantation period, the blastocyst hatches out of the zona pellucida (ZP) and invades the receptive uterine endometrium. This process promotes maternal-fetal dialogue at the physiological and molecular level, thereby initiating the implantation process. Blastocyst hatching is a consequence of elevated osmotic pressure due to active Na+/K+ ion transporter in the blastocyst cavity, as well as proteases produced by trophectoderm (TE) that hydrolyze the ZP. This review summarizes the process underpinning blastocyst hatching, such as the hatching schedule, the location of TEs during initial hatching out of the ZP, the molecules involved in blastocyst hatching, and how these processes affect implantation events. Additionally, we focus on identifying crucial molecules that may influence the quality of implantation and predict the outcome of embryo implantation. Further understanding the mechanism of these molecules may help us to improve the efficiency of Assisted reproductive technology (ART) in livestock breeding. This review provides insight into embryonic development, specifically during the short-term process of blastocyst hatching and its effects on the following implantation.

Zona Pellucida Genes and Proteins: Essential Players in Mammalian Oogenesis and Fertility

All mammalian oocytes and eggs are surrounded by a relatively thick extracellular matrix (ECM), the zona pellucida (ZP), that plays vital roles during oogenesis, fertilization, and preimplantation development. Unlike ECM surrounding somatic cells, the ZP is composed of only a few glycosylated proteins, ZP1–4, that are unique to oocytes and eggs. ZP1–4 have a large region of polypeptide, the ZP domain (ZPD), consisting of two subdomains, ZP-N and ZP-C, separated by a short linker region, that plays an essential role in polymerization of nascent ZP proteins into crosslinked fibrils. Both subdomains adopt immunoglobulin (Ig)-like folds for their 3-dimensional structure. Mouse and human ZP genes are encoded by single-copy genes located on different chromosomes and are highly expressed in the ovary by growing oocytes during late stages of oogenesis. Genes encoding ZP proteins are conserved among mammals, and their expression is regulated by cis-acting sequences located close to the transcription start-site and by the same/similar trans-acting factors. Nascent ZP proteins are synthesized, packaged into vesicles, secreted into the extracellular space, and assembled into long, crosslinked fibrils that have a structural repeat, a ZP2-ZP3 dimer, and constitute the ZP matrix. Fibrils are oriented differently with respect to the oolemma in the inner and outer layers of the ZP. Sequence elements in the ZPD and the carboxy-terminal propeptide of ZP1–4 regulate secretion and assembly of nascent ZP proteins. The presence of both ZP2 and ZP3 is required to assemble ZP fibrils and ZP1 and ZP4 are used to crosslink the fibrils. Inactivation of mouse ZP genes by gene targeting has a detrimental effect on ZP formation around growing oocytes and female fertility. Gene sequence variations in human ZP genes due to point, missense, or frameshift mutations also have a detrimental effect on ZP formation and female fertility. The latter mutations provide additional support for the role of ZPD subdomains and other regions of ZP polypeptide in polymerization of human ZP proteins into fibrils and matrix.

The inhibition of WIP1 phosphatase accelerates the depletion of primordial follicles

RESEARCH QUESTION

What role does wild-type p53-induced phosphatase 1 (WIP1) play in the regulation of primordial follicle development?

DESIGN

WIP1 expression was detected in the ovaries of mice of different ages by western blotting and immunohistochemical staining. Three-day-old neonatal mouse ovaries were cultured in vitro with or without the WIP1 inhibitor GSK2830371 (10 μM) for 4 days. Ovarian morphology, follicle growth and follicle classification were analysed and the PI3K-AKT-mTOR signal pathway and the WIP1-p53-related mitochondrial apoptosis pathway evaluated.

RESULTS

WIP1 expression was downregulated with age. Primordial follicles were significantly decreased in the GSK2830371-treated group, without a significant increase in growing follicles. The ratio of growing follicles to primordial follicles was not significantly different between the control and GSK2830371 groups, and no significant variation was observed in the PI3K-AKT-mTOR signal pathway. The inhibition of WIP1 phosphatase accelerated primordial follicle atresia by activating the p53-BAX-caspase-3 pathway.

CONCLUSIONS

These findings reveal that WIP1 participates in regulating primordial follicle development and that inhibiting WIP1 phosphatase leads to massive primordial follicle loss via interaction with the p53-BAX-caspase-3 pathway. This might also provide valuable information for understanding decreased ovarian reserve during ovarian ageing.

New Insights into the Mammalian Egg Zona Pellucida

Mammalian oocytes are surrounded by an extracellular coat called the zona pellucida (ZP), which, from an evolutionary point of view, is the most ancient of the coats that envelope vertebrate oocytes and conceptuses. This matrix separates the oocyte from cumulus cells and is responsible for species-specific recognition between gametes, preventing polyspermy and protecting the preimplantation embryo. The ZP is a dynamic structure that shows different properties before and after fertilization. Until very recently, mammalian ZP was believed to be composed of only three glycoproteins, ZP1, ZP2 and ZP3, as first described in mouse. However, studies have revealed that this composition is not necessarily applicable to other mammals. Such differences can be explained by an analysis of the molecular evolution of the ZP gene family, during which ZP genes have suffered pseudogenization and duplication events that have resulted in differing models of ZP protein composition. The many discoveries made in recent years related to ZP composition and evolution suggest that a compilation would be useful. Moreover, this review analyses ZP biosynthesis, the role of each ZP protein in different mammalian species and how these proteins may interact among themselves and with other proteins present in the oviductal lumen.

Mutations in ZP4 are associated with abnormal zona pellucida and female infertility

BACKGROUND

The zona pellucida (ZP) of human oocytes plays essential protective roles in sperm-egg interactions during fertilisation and embryo development. ZP4-null female rabbits exhibit a thin and irregular ZP, which severely impairs embryo development and fertility. However, the effects of ZP4 defect on human female reproduction remain unknown.

METHODS AND RESULTS

We performed whole-exome sequencing in 26 female patients with abnormal (thin and irregular) ZP and identified heterozygous variants in ZP4 (OMIM: 613514) from 3 patients (approximately 11%). No ZP4 variant was found in the 30 control women with proven fertility. We constructed ZP4-mutated plasmids and found that the variants reduced the secretion of ZP4 in vitro. Lower suction pressure facilitated egg retrieval, and intracytoplasmic sperm injection (ICSI) was a desirable treatment for ZP4-mutated patients with abnormal ZP.

CONCLUSIONS

We identified ZP4 as a novel gene for human abnormal ZP and found that lower suction pressure and ICSI are efficient treatment strategies.

Role of diagnostic intracytoplasmic sperm injection (ICSI) in the management of genetically determined zona pellucida-free oocytes during in vitro fertilization: a case report

PURPOSE

To report the utilization of diagnostic intracytoplasmic sperm injection (D-ICSI), an ICSI cycle performed in the natural cycle, to obtain information about embryo development potential after sperm injection into zona pellucida (ZP)-free oocytes.

MATERIALS AND METHODS

We report the case of a couple with primary unexplained infertility with a history of previous failed, in vitro fertilization intracytoplasmic sperm injection (IVF-ICSI) cycles characterized by the presence of ZP-free oocytes. Whole exome sequencing (WES) was carried out to analyse the possible genetic basis of oocyte abnormality.

RESULTS

Diagnostic ICSI provided information about the embryo development potential from ZP-free oocytes and allowed better planning of the subsequent ICSI cycle. WES revealed that the absence of ZP was likely to be due to a new (ZP1) mutation. The subsequent ICSI cycle resulted in the delivery of a healthy baby.

DISCUSSION

To the best of our knowledge, our report is the first to describe the use of D-ICSI to determine the feasibility of embryo development and implantation in a patient with ZP1 mutation, resulting in the subsequent delivery of a healthy baby. We used 'diagnostic' ICSI in the normal menstrual cycle to explore the feasibility of embryo development after sperm injection into ZP-free oocytes. Our results may expand the spectrum of diagnostic procedures associated with unexplained infertility.

Incorporation of arginine, glutamine or leucine in culture medium accelerates in vitro activation of primordial follicles in 1-day-old mouse ovary

In vitro activation of primordial follicles provides cancer patients subjected to oncotherapy with a safe therapeutic strategy for fertility preservation, however a successful protocol for activation of primordial follicles in prepubertal patients has not yet been defined comprehensively. There is evidence that amino acids such as leucine, arginine and glutamine could stimulate the mammalian target of rapamycin (mTOR) pathway, which plays a pivotal role in primordial follicle activation. Nevertheless, there has been no report that elucidates the effect of these amino acids on in vitro development of ovarian follicles. Therefore, the present study was conducted to evaluate the effects of these amino acids and their combination on the formation and activation of primordial follicles in 1-day-old murine ovaries during an 11-day culture period. The experimental groups consisted of base medium (BM), base medium + arginine (ARG), base medium + glutamine (GLU), base medium + leucine (LEU) and base medium + a combination of arginine, glutamine and leucine (AGL). The proportions of different stages of ovarian follicles and gene expression of regulatory factors were assessed using histology and quantitative real-time PCR on days 5 and 11 of culture. The proportion of transitional and primary follicles was greater in all amino acid-treated groups compared with the BM group (P < 0.05). Moreover, leucine resulted in elevated expression of Gdf9 and Bmp15, and glutamine augmented the expression of Pi3k on day 11 of culture. In conclusion, the present study showed that inclusion of leucine, glutamine, arginine or their combination in the culture medium for murine ovarian tissue could accelerate the activation of primordial follicles and alter the expression of the corresponding factors.

Proteomic analysis of mouse ovaries during the prepubertal stages

Postnatal folliculogenesis, primordial follicle activation and follicular development at early stage are important for normal ovarian function and fertility, and a comprehensive understanding of this process under physiological condition is necessary. To observe the regulation and mechanism of ovarian follicle development during the prepubertal stages, we collected the mouse ovaries from three time points, including 1 day, 7 days, and 4 weeks after birth. We then performed a proteomic analysis using tandem mass tags (TMT) labeling combined with a two-dimensional liquid chromatography-tandem mass spectrometry (2D LC-MS/MS) technique. A total of 706 proteins were determined to be significant differential abundance (P-SDA). Sixty upregulated proteins and 12 downregulated proteins that were P-SDA and 3 significant KEGG pathways (P < 0.05) were found at 7 days vs. 1 day after birth, while 237 upregulated proteins, 271 downregulated proteins and 42 significant KEGG pathways were found for 4 weeks vs. 7 days after birth. Some vital genes (Figla, Ooep, Padi6, Zp3, Hsd3b1, cyp11a1), key pathways (ECM-receptor interaction, focal adhesion, ovarian steroidogenesis, complement and coagulation cascades, PI3K/Akt/mTOR), and metabolic regulation (energy metabolism, lipid metabolism, metal ion metabolism) were found to be related to the postnatal folliculogenesis, primordial follicle activation and follicular development. Finally, qRT-PCR and western blotting verified some vital genes and further elucidated the developmental process of follicles, and the results may contribute to the understanding of the formation and activation of primordial follicle and follicular development. Significance: This study offers the first proteomic insights into mechanisms of follicle development under physiological condition during the prepubertal stages. By comparing P-SDA of mouse ovaries during various period of age, our data reveals that the regulation of primordial follicle formation and activation is significantly different from that of follicular development. These findings demonstrate that many unique molecular mechanisms underlie ovarian development could be used for ovarian disease research.

Low expression of SEMA6C accelerates the primordial follicle activation in the neonatal mouse ovary

The primordial follicle assembly, activation and the subsequent development are critical processes for female reproduction. A limited number of primordial follicles are activated to enter the growing follicle pool each wave, and the primordial follicle pool progressively diminishes over a woman's life‐time. The number of remaining primordial follicles represents the ovarian reserve. Identification and functional investigation of the factors involved in follicular initial recruitment will be of great significance to the understanding of the female reproduction process and ovarian ageing. In this study, we aimed to study whether and how semaphorin 6C (Sema6c) regulated the primordial follicle activation in the neonatal mouse ovary. The attenuation of SEMA6C expression by SiRNA accelerated the primordial follicle activation in the in vitro ovary culture system. PI3K‐AKT‐rpS6 pathway was activated when SEMA6C expression was down‐regulated. And the LY294002 could reverse the effect of low SEMA6C expression on primordial follicle activation. Our findings revealed that Sema6c was involved in the activation of primordial follicles, and the down‐regulation of SEMA6C led to massive primordial follicle activation by interacting with the PI3K‐AKT‐rpS6 pathway, which might also provide valuable information for understanding premature ovarian failure and ovarian ageing.

Expression and cellular distribution of zona pellucida glycoproteins in canine oocytes before and after in vitro maturation

This study was aimed at investigating zona pellucida glycoproteins (ZP) ZP2, ZP3 mRNA expression as well as ZP3, ZP4 (ZPB) protein distribution before and after in vitro maturation (IVM) in canine oocytes. The cumulus–oocyte complexes (COCs) were recovered from 27 anoestrous mongrel bitches and matured for 72 h in TCM199 medium. The canine COCs were analysed before and after IVM. Using real-time quantitative polymerase chain reaction (RQ-PCR), both groups of oocytes were analysed for detection of ZP2 and ZP3 mRNA profiles as well as using confocal microscopic analysis for observation of ZP3 and ZP4 protein distribution. In post-IVM canine oocytes an increase in transcript content of ZP2 and ZP3 genes as well as a decrease in ZP3 and ZP4 protein levels were observed when compared with pre-IVM oocytes. Moreover, the ZP4 protein before IVM was significantly distributed in the peripheral area of cytoplasm, whereas after IVM it was localized rather than in the entire cytoplasm. In contrast, the ZP3 protein was found both before and after IVM was distributed in the peripheral area of the cytoplasm. In conclusion, we suggest that the expression of ZP2 and ZP3 genes is associated with the maturation stage of canine oocytes, as higher mRNAs levels were found after IVM. However, a decreased expression of ZP3 and ZP4 proteins after IVM suggests maturation-dependent down-regulation of these protein translations, which may result in disturbed fertilization.

Rabbit zona pellucida composition: a molecular, proteomic and phylogenetic approach

The zona pellucida (ZP) participates in sperm-egg interactions during the first steps of fertilization. Recent studies have shown that the ZP matrix of oocytes in several species is composed of four glycoproteins, designated as ZP1, ZP2, ZP3 and ZP4, rather than the three described in mouse, pig and cow. In this study, investigations were carried out to unveil a fourth glycoprotein in the rabbit (Oryctolagus cuniculus) ZP. Using total RNA isolated from rabbit ovaries, the complementary deoxyribonucleic acid (cDNA) encoding rabbit ZP1 was amplified by reverse transcribed polymerase chain reaction (RT-PCR). The ZP1 cDNA contains an open reading frame of 1825 nucleotides encoding a polypeptide of 608 amino acid residues. The deduced amino acid sequence of rabbit ZP1 showed high identity with other species: 70% identity with human and horse ZP1, and 67% identity with mouse and rat ZP1. At the proteomic level, peptides corresponding to the four proteins were detected by mass spectrometry. In addition, a molecular phylogenetic analysis of ZP1 showed that pseudogenization of this gene has occurred at least four times during the evolution of mammals. The data presented in this manuscript provide evidence, for the first time, that the rabbit ZP is composed of four glycoproteins.

Sperm arylsulfatase A binds to mZP2 and mZP3 glycoproteins in a nonenzymatic manner

We have shown previously that sperm surface arylsulfatase A (ASA) of mouse, pig, and human is involved in sperm-egg zona pellucida (ZP) binding. By treating capacitated mouse sperm with A23187 to induce the acrosome reaction, we demonstrated by immunoblotting that ASA also existed in the acrosomal content and on the inner acrosomal membrane. Since mZP2 and mZP3 are known as sperm receptors, whereas mZP1 as a cross-linker of mZP2/mZP3, we determined whether purified ASA bound to mZP2 and mZP3 selectively. The three mZP glycoproteins were purified from solubilized ovarian ZP by size exclusion column chromatography. Immuno-dot blot analyses revealed that purified sperm ASA bound to mZP2 at the highest level followed by mZP3, whereas the binding of ASA to mZP1 was minimal. The results confirmed the physiological significance of sperm ASA in the ZP binding process. The binding of ASA to mZP2 and mZP3 was, however, not dependent on the active site pocket amino acids, Cys69, Lys123, and Lys302, which are pertinent to the capturing of an arylsulfate substrate, since ASA mutant with Ala substitution at these three residues still bound to mZP2 and mZP3. The availability of the active site pocket of ASA bound to the ZP suggested that ASA would still retain enzymatic activity, which might be important for subsequent sperm penetration through the ZP.

Polarity and asymmetry during mouse oogenesis and oocyte maturation

Cell polarity and asymmetry play a fundamental role in embryo development. The unequal segregation of determinants, cues, and activities is the major event in the differentiation of cell fate and function in all multicellular organisms. In oocytes, polarity and asymmetry in the distribution of different molecules are prerequisites for the progression and proper outcome of embryonic development. The mouse oocyte, like the oocytes of other mammals, seems to apply a less stringent strategy of polarization than other vertebrates. The mouse embryo undergoes a regulative type of development, which permits the full rectification of development even if the embryo loses up to half of its cells or its size is experimentally doubled during the early stages of embryogenesis. Such pliability is strongly related to the proper oocyte polarization before fertilization. Thus, the molecular mechanisms leading to the development and maintenance of oocyte polarity must be included in any fundamental understanding of the principles of embryo development. In this chapter, we provide an overview of current knowledge regarding the development and maintenance of polarity and asymmetry in the distribution of organelles and molecules in the mouse oocyte. Curiously, the mouse oocyte becomes polarized at least twice during ontogenesis; the question of how this phenomenon is achieved and what role it might play is addressed in this chapter.

ZP2 and ZP3 cytoplasmic tails prevent premature interactions and ensure incorporation into the zona pellucida

The zona pellucida contains three proteins (ZP1, ZP2, ZP3), the precursors of which possess signal peptides, 'zona' domains and short (9-15 residue) cytoplasmic tails downstream of a transmembrane domain. The ectodomains of ZP2 and ZP3 are sufficient to form the insoluble zona matrix and yet each protein traffics through oocytes without oligomerization. ZP2 and ZP3 were fluorescently tagged and molecular interactions were assayed by fluorescent complementation in CHO cells and growing oocytes. ZP2 and ZP3 traffic independently, but colocalize at the plasma membrane. However, protein-protein interactions were observed only after release and incorporation of ZP2 and ZP3 into the extracellular matrix surrounding mouse oocytes. In the absence of their hydrophilic cytoplasmic tails, ZP2 and ZP3 interacted within the cell and did not participate in the zona pellucida. A heterologous GPI-anchored 'zona' domain protein fused with the cytoplasmic tails was integrated into the zona matrix. We conclude that the cytoplasmic tails are sufficient and necessary to prevent intracellular oligomerization while ensuring incorporation of processed ZP2 and ZP3 into the zona pellucida.

Biosynthesis of hamster zona pellucida is restricted to the oocyte

The zona pellucida (ZP) is an extracellular coat that surrounds the mammalian oocyte and the early embryo until implantation. This coat mediates several critical aspects of fertilization, including species-selective sperm recognition, the blocking of polyspermy and protection of the oocyte and the preimplantation embryo. Depending on the species, the ZP is composed of three to four different glycoproteins encoded by three or four genes. These genes have been cloned and sequenced for different species. However, controversy exists about the cell type specificity of the ZP glycoproteins, for which several models have been proposed. Different groups have reported that ZP is produced only by the oocytes, by the granulosa cells or by both cell types, depending on the species under study. We recently described the expression of four ZP proteins in the hamster ovary. By means of the complete set of the hamster ZP cDNAs, we undertook the study of the origin and expression pattern of the four ZP genes. In the present work, the expression of ZP1, ZP2, ZP3 and ZP4 is carefully analyzed by in situ hybridization (ISH) in hamster ovaries. Our data suggest that the four hamster ZP genes are expressed in a coordinate and oocyte-specific manner during folliculogenesis. Furthermore, this expression is maximal during the first stages of the oocyte development and declines in oocytes from later development stages, particularly within large antral follicles.

In humans, zona pellucida glycoprotein-1 binds to spermatozoa and induces acrosomal exocytosis

BACKGROUND

It has been suggested that the zona pellucida (ZP) may mediate species-specific fertilization. In human the ZP is composed of four glycoproteins: ZP1, ZP2, ZP3 and ZP4. In the present study, the expression profile of ZP1 in human oocytes and ovaries, and its role during fertilization, is presented.

METHODS

Human ZP1 (amino acid residues 26-551) was cloned and expressed in both non-glycosylated and glycosylated forms and its ability to bind to the capacitated human spermatozoa and to induce acrosomal exocytosis was studied. Monoclonal antibodies (MAbs), specific for human ZP1 and devoid of reactivity with ZP2, ZP3 and ZP4 were generated and used to localize native ZP1 in oocytes and ovarian tissues.

RESULTS

The MAbs generated against ZP1 recognized specifically the zona matrix of secondary and antral follicles, ovulated oocytes, atretic follicles and degenerating intravascular oocytes, but failed to react with the Fallopian tube, endometrium, ectocervix and kidney. Escherichia coli and baculovirus-expressed recombinant human ZP1 revealed bands of approximately 75 and approximately 85 kDa, respectively, in western blot. Lectin binding studies revealed the presence of both N- and O-linked glycosylation in baculovirus-expressed ZP1. Fluorescein isothiocyanate-labelled E. coli- and baculovirus-expressed recombinant ZP1 bound to the anterior head of capacitated spermatozoa, however, only baculovirus-expressed ZP1 induced acrosomal exocytosis in capacitated sperm suggesting the importance of glycosylation in mediating the acrosome reaction. The human ZP1-mediated acrosome reaction involved the activation of both T- and L-type voltage-operated calcium channels, but does not activate the G(i)-coupled receptor pathway. Inhibition of protein kinase A and C significantly also reduced the ZP1-mediated induction of the acrosome reaction.

CONCLUSION

These studies revealed for the first time that in humans ZP1, in addition to ZP3 and ZP4, binds to capacitated spermatozoa and induces acrosomal exocytosis.

Positive effect of partial zona pellucida digestion on in vitro fertilization of mouse oocytes with cryopreserved spermatozoa

Cryopreservation of mouse spermatozoa has been widely used; however, fertility of frozen spermatozoa in some strains, especially when inseminating cryopreserved oocytes, is low and may be improved by assisted fertilization techniques. The present study was performed to investigate the effect of partial zona pellucida (ZP) digestion on the in vitro fertilization (IVF) capacity of frozen mouse spermatozoa. Mouse oocytes were subjected to partial ZP digestion using acidic Tyrode's solution (pH 3.1). Fertilization rates in digestion groups (30 or 45 s) were higher (P < 0.05) than that of zona-intact control (78.3% or 86.3% vs. 52.5%). The recovery rate at 45 s was lower (P < 0.05) than that at 30 s (84.2% vs. 97.3%). Among vitrified oocytes, the fertilization rate in treatment group (digested for 30 s) was higher (P < 0.05) than that of zona-intact group (50.8% vs. 22.1%). After embryo transfer at the two-cell stage, 17.7% and 11.8% of transferred embryos derived from fresh and vitrified digested oocytes developed to term and showed no significant difference as compared with that from zona-intact oocytes (24.1%, P > 0.05). These results indicate that partial ZP digestion improves IVF efficiency of fresh and vitrified oocytes with frozen mouse spermatozoa, which can provide valuable information for in vitro assisted fertilization using cryopreserved gametes in the re-establishment of mouse colonies.

Mouse early oocytes are transiently polar: three-dimensional and ultrastructural analysis

The oocytes of many invertebrate and non-mammalian vertebrate species are not only asymmetrical but also polar in the distribution of organelles, localized RNAs and proteins, and the oocyte polarity dictates the patterning of the future embryo. Polarily located within the oocytes of many species is the Balbiani body (Bb), which in Xenopus is known to be associated with the germinal granules responsible for the determination of germ cell fate. In contrast, in mammals, it is widely believed that the patterning of the embryo does not occur before implantation, and that oocytes are non-polar and symmetrical. Although the oocytes of many mammals, including mice and humans, contain Bbs, it remains unknown how and if the presence of Bbs relates to mouse oocyte and egg polarity. Using three-dimensional reconstruction of mouse neonatal oocytes, we showed that mouse early oocytes are both asymmetrical and transiently polar. In addition, the specifics of polarity in mouse oocytes are highly reminiscent of those in Xenopus early oocytes. Based on these findings, we conclude that the polarity of early oocytes imposed by the position of the centrioles at the cytoplasmic bridges is a fundamental and ancestral feature across the animal kingdom.

Production of monoclonal antibodies against recombinant human zona pellucida glycoproteins: utility in immunolocalization of respective zona proteins in ovarian follicles

The zona pellucida (ZP) glycoproteins play an important role in oocyte development and gamete biology. To analyze their expression in follicles during various developmental stages, murine monoclonal antibodies (MAbs) were generated against the baculovirus-expressed recombinant human ZP2, ZP3 and ZP4. A panel of MAbs specific for the respective zona protein in ELISA and Western blot, and devoid of cross-reaction with other zona proteins was selected. Immunohistochemistry has shown that ZP2 MAb, MA-1620, did not react with oocytes in resting primordial follicles but showed reactivity with degenerating oocytes in primordial follicles undergoing atresia, and with oocytes in growing and antral follicles. Three MAbs against ZP3 did not react with oocytes in primordial follicles, but reacted only with oocytes in growing and antral follicles. Out of four MAbs against ZP4, three MAbs reacted with oocytes in primordial, growing and antral follicles. No reactivity of these MAbs with other ovarian cell types and other tissues studied (endometrium, uterine cervix, fallopian tubes and kidney) was detected except for a strong reactivity of ZP2 MA-1620 with epithelial cells of the uterine ectocervix or endometrium in some samples investigated. Altogether, these studies document generation of MAbs exhibiting high specificity for human zona proteins, which will be useful reagents to study their immunobiology.

Stereo-electron microscopy of the ovarian follicles of cat and mouse

Three dimensional structure of developing ovarian follicles in the cat and the mouse were examined to clarify the cellular and the extra cellular components during follicular maturation by scanning electron microscopy. Epithelial cells of the membrana granulosa (MG) and the cumulus oophorus (CO) show variable morphology, which depends on the location of the cells in the follicle. In the small antral follicle, there is no morphological difference between the surface structure of the MG and the CO lining cells. In the large antral follicle, however, the lumenal lining cells of the MG change their spherical shape to a rather flattened one showing numerous pseudopodial processes creeping over the surrounding lumenal cells. The CO cells remain rounded with fine cytoplasmic protrusions on their outer surfaces which seem to make a network. The outer surface view of the zona appears as composed of labyrinthine canals with relatively large pores of various sizes, however, the inner surface is rather compact with a number of small pores.

ZP2 and ZP3 traffic independently within oocytes prior to assembly into the extracellular zona pellucida

The extracellular zona pellucida surrounds mammalian eggs and mediates taxon-specific sperm-egg recognition at fertilization. In mice, the zona pellucida is composed of three glycoproteins, but the presence of ZP2 and ZP3 is sufficient to form a biologically functional structure. Each zona pellucida glycoprotein is synthesized in growing oocytes and traffics through the endomembrane system to the cell surface, where it is released from a transmembrane domain and assembled into the insoluble zona pellucida matrix. ZP2 and ZP3 colocalize in the endoplasmic reticulum and in 1- to 5-microm post-Golgi structures comprising multivesicular aggregates (MVA), but a coimmunoprecipitation assay does not detect physical interactions. In addition, ZP2 traffics normally in growing oocytes in the absence of ZP3 or if ZP3 has been mutated to prevent incorporation into the zona pellucida matrix, complementing earlier studies indicating the independence of ZP3 secretion in Zp2 null mice. N glycosylation has been implicated in correct protein folding and intracellular trafficking of secreted proteins. Although ZP3 contain five N-glycans, enhanced green fluorescent protein-tagged ZP3 lacking N glycosylation sites is present in MVA and is incorporated into the zona pellucida matrix of transgenic mice. Thus, ZP2 secretion is seemingly unaffected by ZP3 lacking N-glycans. Taken together, these observations indicate that ZP2 and ZP3 traffic independently through the oocyte prior to assembly into the zona pellucida.

Three-dimensional structure of the zona pellucida at ovulation

The mammalian zona pellucida (ZP) is an extracellular matrix surrounding oocytes and early embryos, which is critical for normal fertilization and preimplantation development. It is made up of three/four glycoproteins arranged in a delicate filamentous matrix. Scanning electron microscopy (SEM) studies have shown that ZP has a porous, net-like structure and/or nearly smooth and compact aspect. In this study, the fine 3-D structure of the human and mouse ZP is reviewed with the aim to integrate ultrastructural and molecular data, considering that the mouse is still used as a good model for human fertilization. By conventional SEM observations, numerous evidences support that the spongy ZP appearance well correlates with mature oocytes. When observed through more sophisticated techniques at high resolution SEM, ZP showed a delicate meshwork of thin interconnected filaments, in a regular alternating pattern of wide and tight meshes. In mature oocytes, the wide meshes correspond to "pores" of the "spongy" ZP, whereas the tight meshes correspond to the compact parts of the ZP surrounding the pores. In conclusion, the traditional "spongy" or "compact" appearance of the ZP at conventional SEM appears to be only the consequence of a prevalence of different arrangements of microfilament networks, according to the maturation stage of the oocyte, and in agreement with the modern supramolecular model of the ZP at the basis of egg-sperm recognition. Despite great differences in molecular characterization of ZP glycoproteins between human and mouse ZP, there are no differences in the 3-D organization of glycoproteic microfilaments in these species.

High magnitude of light retardation by the zona pellucida is associated with conception cycles

BACKGROUND

Failures in expression of zona proteins correlate to subfertility in animals. Low expression of the zona proteins by the growing human oocyte may indicate reduced developmental potential. Therefore, we non-invasively analysed the thickness and the structure of the zona pellucida (ZP) of human oocytes with respect to embryo fate after ICSI.

METHODS

Retardance magnitude and thickness of the inner, middle and outer layers of the ZP were quantitatively analysed by a Polscope in 166 oocytes selected for transfer after ICSI (63 patients; 32.8 +/- 4.4 years) on the basis of pronuclear score at day 1. Blastomere number was determined at day 2. Data were compared between conception cycles (CC; 65 oocytes/23 patients) and non-conception cycles (NCC; 101 oocytes/40 patients) and with respect to maternal age.

RESULTS

The thickness was slightly elevated (P < 0.001), and the mean magnitude of light retardance was nearly 30% higher (P < 0.001) in the inner layer of the zona pellucida of oocytes contributing to CC compared to NCC. Embryos in the CC group tended to develop faster.

CONCLUSIONS

The magnitude of light retardance by the zona pellucida inner layer appears to present a unique non-invasive marker for oocyte developmental potential.

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.

The extracellular matrix of porcine mature oocytes: origin, composition and presumptive roles

The extracellular matrix (ECM) of porcine mature oocytes was revealed by transmission electron microscopy (TEM) after treatment with tannic acid and ruthenium red. Present in the perivitelline space (PVS) and on the surface of the zona pellucida (ZP), it appeared to be composed of thin filaments and granules at the interconnections of the filaments, which were interpreted respectively as hyaluronic acid chains and bound proteoglycans. In order to determine whether this material is produced by the corona cells (the same ECM was found also on the surface of the zona pellucida and between cumulus cells) or by the oocyte itself, the synthesis of glycoproteins and glycosaminoglycans was checked by autoradiography on semi-thin and thin sections observed by light and electron microscopy. Immature oocytes within or without cumulus cells, were incubated with L [3H-] fucose or L [3H-] glucosamine--precursors respectively of glycoproteins and hyaluronic acid or hyaluronan (HA) bound to proteoglycans--for various times (with or without chase) and at different stages during in vitro maturation. In the first case, incorporation was found in both cumulus cells and ooplasm (notably in the Golgi area for 3H-fucose) and labeled material accumulated in the ECM of the PVS and of the ZP surface. Labeling in the PVS with both precursors was maximum between metaphase I (MI) and metaphase II (MII) and was partially extracted by hyaluronidase but not by neuraminidase. Tunicamycin, an inhibitor of glycoprotein synthesis, significantly decreased the amount of 3H-fucose labeled molecules in the PVS and increased the incidence of polyspermic penetration during subsequent in vivo fertilization. Since cumulus-free oocytes also secreted 3H-glucosamine containing compounds, both oocyte and cumulus cells probably contribute to the production of the ECM found in the PVS of mature oocytes. ECM and particularly its HA moiety present on both sides of the ZP may constitute a favourable factor for sperm penetration.

Morphodynamics of the follicular-luteal complex during early ovarian development and reproductive life

Female reproductive activity depends upon cyclic morphofunctional changes of the ovarian tissue during the female's fertile period, but the primum movens of an active gonadal rearrangement can be found from early phases of embryo development. To offer a basic account of the main steps of ovarian dynamics, we review the morphofunctional behavior of the follicular-luteal complex in an integrated study using light microscopy and transmission and scanning electron microscopy as well as through the use of numerous drawings. Particular emphasis is given to some reproductive aspects including (1) germ-somatic cell relationships and onset of folliculogenesis during early gonadal development; (2) follicular development and oocyte-follicle cell associations through adult folliculogenesis, finally leading to ovulation; (3) morphodynamics of corpus luteum formation, development, and regression, and (4) degenerative processes involving germ and somatic cells. The results reported, many of which originated in our laboratory, arise from some experiments on laboratory mammals but mostly from a large selection of human specimens. The data obtained are integrated and correlated with classic reports as well as with current views. Crucial biochemical, histophysiological, and clinical aspects are also emphasized.

Infertility observed in female rats treated with N-acetyl-L-cysteine: Histopathological examination of ovarian follicles and recovery of fertility

We previously reported infertility in female rats that received N-acetyl-L-cysteine (NAC) intravenously at a dosage of 1000 mg/kg/day. Unfertilized oocytes and gestation day 1 and 2 embryos were assessed morphologically, and the results suggested that absence or thinning of the zona pellucida (ZP) is related to infertility. However, the morphological characteristics of oocytes before ovulation and recovery from the effects of NAC were not clarified. In the present study, the ovarian follicles were histopathologically examined and the recovery of reproductive function was evaluated to investigate the effects of NAC. Female Sprague-Dawley rats at 10 weeks of age received NAC intravenously at 1000 mg/kg/day for more than 1 week. Thinning of the ZP was observed in the ovarian follicles in all stages of growth by light microscopy. Outflow of the components of the ZP between the corona radiata and disarrangement of the corona radiata were more pronounced in growing follicles than in large secondary follicles. Similar findings were observed by electron microscopy, and the effects of NAC were limited to the ZP. Infertility and thinning of the ZP were observed in the no-recovery NAC group, but not in the recovery NAC group, in which animals recovered within four estrous cycles after NAC administration. It has been reported that the ZP is expressed by oocytes or by both oocytes and granulosa cells, but no changes were noted in these cells. The present findings suggest that NAC affects the ZP directly and that reproductive function may recover from the effects of NAC.

Unresolved Issues in Mammalian Fertilization

This review considers the role of the sperm in fertilization, addressing areas of misunderstanding and unfounded assumptions and taking particular advantage of the large body of data resulting from work with rodent species in vitro. Considerable attention is given to the appropriate use and interpretation of assays for capacitation, acrosomal exocytosis, hyperactivation, and sperm protein phosphorylation, as well as tests for sperm-zona and sperm-oocyte membrane interactions. The lack of general agreement on the means of sperm adhesion to and penetration of the zona pellucida is addressed, and the need for new approaches to this problem is pointed out. Some molecular advances in our understanding of specific steps in the process of fertilization are discussed in the context of intact cell-matrix and cell-cell interaction. This review should provide practical information for researchers just beginning the study of fertilization and interesting but not widely known observations to stimulate new ideas in experienced scientists.