Coculturing denuded oocytes during the in vitro maturation of bovine cumulus oocyte complexes exerts a synergistic effect on embryo development

The present study examined the effect of coculturing cumulus oocyte complexes (COCs) and denuded oocytes (DOs) during in vitro maturation (IVM) on nuclear and cytoplasmic maturation, zona pellucida (ZP) hardening, the pattern of fertilization and glutathione peroxidase 1 (GPX1) gene expression in the oocyte. Furthermore, the rate of embryonic development and the quality of blastocysts were examined for both COCs and DOs. Three IVM conditions were studied: 1) the coculture of 12 COCs and 60 DOs, 2) COC control with 12 COCs, and 3) DO control with 60 DOs. The IVM was performed in a 120-μl droplet of TCM199-based IVM medium. Following IVM, in vitro fertilization (IVF) and in vitro culture (IVC) were conducted separately for the COCs and DOs (DO coculture) from the IVM coculture group. Coculturing COCs and DOs increased the percentage of oocytes reaching the blastocyst stage and the total number of cells per blastocyst in both the COC coculture (44.4 ± 8.6 vs 26.7 ± 9.7%, P < 0.01, and 137.9 ± 24.9 vs 121.7 ± 21.1, P < 0.05) and the DO coculture (20.5 ± 5.0 vs 11.1 ± 2.5%, P < 0.01, and 121.9 ± 27.5 vs 112.3 ± 33.2, P < 0.05) compared to their respective control groups. The synergistic effects of coculturing were detected as increased nuclear and cytoplasmic maturation, the prevention of ZP hardening, increased monospermic fertilization and increased expression of GPX1 in the oocytes in response to endogenous oocyte-secreted factors. In conclusion, coculturing COCs and DOs may be an effective culture system for both intact COCs and immature DOs.     

Effect of vitrification on the zona pellucida hardening and follistatin and cathepsin B genes expression and developmental competence of in vitro matured bovine oocytes

The purpose of our study was to assess the effect of vitrification with or without the presence of calcium in the vitrification solution on the: 1) diameter of oocytes and thickness of the zona pellucida, 2) zona pellucida hardening, 3) expression of mRNA follistatin (FST) and cathepsin B (CTSB) in oocytes and 4) developmental competence of embryos derived from in vitro matured and vitrified oocytes.The results of our study demonstrate, that vitrification did not alter thickness of the zona pellucida and diameter of the oocytes, however it triggered hardening of the zona pellucida. The presence of calcium in the vitrification solutions intensified hardening of zona in immature and mature oocytes (P < 0.04, P < 0.001, respectively) and provoked increased mRNA FST expression in oocytes matured in vitro compared to immature oocytes (P < 0.01) and those vitrified without calcium (P < 0.004). CTSB mRNA expression was increased in immature oocytes and oocytes vitrified with calcium compare to mature oocytes (P < 0.02). The developmental potential of vitrified oocytes was impaired compared to non-vitrified oocytes, being more evident in oocytes vitrified with calcium.In summary, vitrification did not change the oocyte diameter and thickness of the zona pellucida and expression of FST and CTSB mRNA. It diminished developmental potential of the vitrified oocytes. The presence of calcium in the vitrification solutions increased hardening of zona pellucida as well as affected the level of FST and CTSB mRNA in oocytes and developmental potential of these oocytes.     

N-glycosylation of zona glycoproteins during meiotic maturation is involved in sperm-zona pellucida interactions of porcine oocytes

The objective was to determine whether N-glycosylation of zona pellucida (ZP) glycoproteins occurred during meiotic maturation of porcine oocytes, and whether this had a role in fertilization. In the first of three experiments, carbohydrate residues in the ZP of in vitro matured porcine oocytes were blocked with various lectins and the influence of such blocking on sperm-ZP interactions was studied. The second experiment used a lectin-binding assay to determine whether the number of GlcNAc residues in ZP was changed by N-glycosylation during in vitro maturation (IVM) of porcine oocytes. The last experiment determined the effects of tunicamycin, a specific N-glycosylation inhibitor, for various intervals during IVM, on sperm-ZP interactions in porcine oocytes. The primary findings were that: 1) N-glycosylation of GlcNAc residues in porcine ZP occurred during the first 24 h of IVM; and 2) such glycosylation was indispensible for sperm-ZP interactions, e.g., number of sperm bound to ZP, acrosome-reacted sperm, sperm penetration rate, and level of polyspermy (P < 0.05). However, blocking N-glycosylation by tunicamycin treatment during IVM did not adversely influence the progression of oocytes to meiotic metaphase II and male pronucleus formation, indicating that this glycosylation was involved only in the initial stages of fertilization. We inferred that the increase in terminal GlcNAc residues in ZP glycoprotein through new N-glycosylation during the first 24 h of meiotic maturation played a critical role in porcine ZP acquiring the capacity to accept sperm.     

“Spontaneous” hardening of the zona pellucida of mouse oocytes during in vitro culture

Culture in vitro causes a slow, progressive hardening of the zona pellucida (ZP) of fully grown dictyate oocytes isolated from the mouse ovary. Hardening cannot be prevented by inhibitors of peroxidase or by a tyrosine analogue. Culture in anaerobic conditions is very effective in preventing ZP hardening. If the oocyte is cultured surrounded by its own follicle cells or in contact with cumuli oophori obtained from superovulated females, hardening is much reduced. The results suggest that the “spontaneous” hardening in cultured ovarian oocytes is not due to a cortical reaction, and that a diffusible factor is produced by follicle cells that protect the ZP from hardening.     

Time-dependent effects of heat shock on the zona pellucida ultrastructure and in vitro developmental competence of bovine oocytes

The aim of this study was to determine the effects of different exposure lenght to heat shock (HS) during in vitro maturation (IVM) on zona pellucida (ZP) ultrastructure and developmental competence of bovine oocytes. Cumulus-oocyte complexes (COCs) were matured in vitro (IVM) at 38.5 °C for 24 h (control group, CG), or incubated at 41 °C (HS) for 6 h (HS-6h), 12 h (HS-12h), 18 h (HS-18h), and 22h (HS-22h) followed by incubation at 38.5 °C to complete a full 24-h period of maturation. After IVM, oocytes were subjected to scanning electron microscopy (SEM) or in vitro fertilization and culture until the blastocyst stage. For heat-shocked oocytes, with exception of those in the HS-6h group, SEM examinations revealed that ZP surfaces were rough and characterized by a presence of spongy network. Oocytes from the HS-22h group displayed an increase in the number of pores, as well as a higher proportion of oocytes with amorphous ZPs. The proportion of oocytes that reached metaphase II (MII) stage decreased in all HS groups, regardless of the duration of exposure to 41 °C. These results provide evidence that HS during IVM for 12–22 h reduces the developmental competence of bovine oocytes, increasing the percentage of oocytes with abnormal chromosomal organization, and reducing fertilization and blastocysts formation rate. The effects of HS were more pronounced for the 22-h exposure group. The damage induced by HS on oocyte function clearly increased upon exposure to elevated temperature.     

Structure of the cumulus matrix and zona pellucida in the golden hamster: A new view of sperm interaction with oocyte-associated extracellular matrices

Summary Hamster oocyte-cumulus complexes (OCC), with and without sperm, were structurally analyzed by light- and electron microscopy using freeze substitution. This method has yielded a clear picture of the extracellular oocyte investments, the cumulus cell matrix and the zona pellucida. The cumulus matrix has an overall homogeneous fibrillar structure which appears to attach to cumulus cells at their filopodial extensions. The matrix also extends into the outer regions of the zona pellucida. The zona pellucida has a distinct porous configuration throughout its entire structure. During gamete interaction experiments, capacitated hamster sperm with ultrastructurally intact acrosomes were found throughout the matrix. Sperm had dramatic effects on the matrix, resulting in compression and stretching. Sperm found on the zona pellucida had initiated or completed the acrosome reaction. During the initial stages of the acrosome reaction, the matrix was in contact with the sperm. At later stages of the acrosome reaction, there was a complete loss of matrix material in regions near the sperm.     

“Spontaneous” hardening of the zona pellucida of mouse oocytes during in vitro culture. II. The effect of follicular fluid and glycosaminoglycans

We have previously reported that when isolated mouse oocytes are cultured in vitro their zonae pellucidae (ZP) become increasingly resistant to solubilization by chymotrypsin (“spontaneous” ZP hardening). In the present paper we report that follicular fluid contains factors that totally prevent such hardening. Furthermore, medium conditioned by granulosa cells partially prevents hardening. The protection against ZP hardening offered by follicular fluid may be ascribed, at least in part, to its content in sulfated glycosaminoglycans heparin and chondroitinsulfate B which, when added to the culture medium at physiological concentrations, show a marked antihardening effect.     

Dispersion of the cumulus oophorus and dissolution of the zona pellucida in oocytes of DDK and other strains of mice

The time needed in vitro for granulosa cell dispersal with hyaluronidase and for lysis of the zona pellucida with α-chymotrypsin was estimated for unfertilized eggs from three inbred strains of mice, DDK, C57BL, and C3H, and one outbred strain, Q. Granulosa cells were dispersed most rapidly in the DDK and most slowly in the C3H strain. The time needed for dissolution of the zona pellucida was also shortest for DDK eggs and longest for C3H eggs. The results suggest that the high sensitivity to enzymes of the granulosa layer and the zona pellucida of DDK eggs may allow more than one spermatozoon to penetrate through these barriers before the block against polyspermy is completed. The percentage of unfertilized eggs containing spermatozoa under the zona pellucida on the day of copulation was abnormally high in matings of females from the DDK inbred strain with males from DDK and C57BL strains. This may suggest some disturbances in establishing contact between the egg plasma membrane and fertilizing spermatozoa.     

Effect of the cumulus on in vitro fertilization of bovine matured oocytes

The effect of the cumulus on in vitro fertilization in bovines was examined. Follicular oocytes were cultured in medium 199 plus OCS and extra granulosa cells. Frozen-thawed bovine spermatozoa was separated by the swim-up technique, suspended in Talp medium and capacitated with heparin. Fresh sheep and goat semen was incubated for 4 h at room temperature, washed and spermatozoa were then suspended in Talp medium and capacitated by incubation at 38.5 °C and 5% CO₂ in air and heparin.

In experiment 1, cumulus-enclosed oocytes, denuded oocytes and denuded oocytes plus additional cumulus cells were incubated with a reduced concentration of bovine spermatozoa for 8 or 18 h. In Experiment 2, cumulus enclosed and denuded oocytes were incubated with bovine spermatozoa for 4, 6, 8 and 18 h using a sperm concentration adjusted to secure high fertilization rates. In Experiment 3, cumulus-enclosed and denuded bovine oocytes were incubated with either sheep or goat spermatozoa for 18 h. Fertilization rates were then calculated and compared statistically. The results showed that 1) the cumulus improved the fertilization rate only when cumulus cells were associated with the oocytes 2) the timing of sperm penetration was not modified by the cumulus and started at 4 h after sperm incubation and 3) the presence of the cumulus improved the heterologous fertilization rate only when sheep spermatozoa were used. The results suggest that the cumulus improves fertilization rate by providing a capacitation-inducing mechanism and by facilitating the interaction between capacitated spermatozoa and the zona pellucida surface.     

Cumulus cell function during bovine oocyte maturation,fertilization, and embryo development in vitro

Several contemporary micromanipulation techniques, such as sperm microinjection, nuclear transfer, and gene transfer by pronuclear injection, require removal of cumulus cells from oocytes or zygotes at various stages. In humans, the cumulus cells are often removed after 15–18 hr of sperm-oocyte coincubation to assist the identification of the fertilization status. This study was designed to evaluate the function of cumulus cells during oocyte maturation, fertilization, and in vitro development in cattle. Cumulus cells were removed before and after maturation and after fertilization for 0,7,20, and 48 hr. The cumulus-free oocytes or embryos were cultured either alone or on cumulus cell monolayers prepared on the day of maturation culture. Percentages of oocyte maturation, fertilization, and development to cleavage, morula, and blastocyst stages and to expanding or hatched blastocysts were recorded for statistical analysis by categorical data modeling (CATMOD) procedures. Cumulus cells removed before maturation significantly reduced the rate of oocyte maturation (4–26% vs. 93–96%), fertilization (0–9% vs. 91–92%), and in vitro development at all stages evaluated. Cumulus cells removed immediately prior to in vitro fertilization (IVF) or 7 hr after IVF reduced the rates of fertilization (58–60% and 71%, respectively, vs. 91–92% for controls), cleavage development (40–47% and 53–54% vs. 74–78% for controls), and morula plus blastocyst development (15% and 24% vs. 45%, P < 0.05). Cumulus cell co-culture started at various stages had no effect on fertilization and cleavage development but significantly improved rates of embryo development to morula or blastocyst stages (P < 0.05). Cumulus cell removal at 20 hr after IVF resulted in similar development to controls (P > 0.05) at all stages tested in this study. The intact state of surrounding cumulus cells of oocytes or embryos appears to be beneficial before or shortly after insemination (at or before 7 hr of IVF) but not essential at 20 hr after IVF. © 1995 Wiley-Liss, Inc.     

Comparison of the ability of progesterone and heat solubilized porcine zona pellucida to initiate the porcine sperm acrosome reaction in vitro

It has been previously shown that progesterone can initiate the acrosome reaction (AR) of capacitated human and hamster sperm in vivo. We report here that progesterone can initiate a morphologically normal AR in porcine sperm that have undergone capacitation in a Hepes-buffered medium in vitro. In addition, we have compared the abilities of progesterone and heat-solubilized porcine zona pellucida (zona) to initiate the porcine sperm AR. Capacitated porcine sperm were treated with 1 m?g/ml progesterone, 150 m?g/ml porcine zona, or solvent control for 10 min. After treatment, sperm were incubated with the supravital dye Hoechst 33258, fixed and the acrosomal status determined in the previously viable sperm by fluorescence microscopy with fluorescein isothiocyanate-labeled Pisum sativum agglutini (FITC-PSA). There was no significant difference between the percentage of AR initiated by zona compared to that initiated by progesterone. In order to determine whether there was a synergistic interaction between the two AR initiators, both were added simultaneously to capacitated porcine sperm at optimal (1 m?g/ml progesterone, 150 m?g/ml zona) and suboptimal (75 ng/ml progesterone and 75 m?g/ml zona) concentrations. Simultaneous addition of the two AR-initiators at the two concentrations stimulated an additive AR-initating response, rather than a synergistic one. Several possible explanations for the additive results are discussed. © 1994 Wiley-Liss, Inc.     

Localization of vascular endothelial growth factor in the zona pellucida of developing ovarian follicles in the rat: a possible role in destiny of follicles

There is increasing evidence that in many species angiogenic factors, such as vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF), may have important roles in folliculogenesis. The aim of this study is to determine the localization of VEGF and its receptors, Flt-1 and KDR, and bFGF expression in the rat ovary and to evaluate their distributions throughout the different follicular stages. Out of 20 virginal female rats, 10 were studied during the natural ovarian cycle without any ovulation induction. The other 10 were superovulated and their ovaries were studied by western analysis and immunohistochemistry. Granulosa cells (GC) and oocytes of primordial follicles were negative for VEGF. In early primary follicles, VEGF was present in the oocyte but its immunoreactivity was weak, while newly developing zona pellucida (ZP) of primary follicles was negative for VEGF. Subsequently, with the commencement of antral spaces between GC of the secondary follicle, ZP of some secondary follicles became strongly positive for VEGF, forming a continuous ring around the oocyte. In preovulatory mature follicles granulosa and theca interna (TI) cells showed a weak immunoreactivity for VEGF. Western blot analyses have also demonstrated that VEGF, a 26-kDa protein, was present in follicles. Moreover, in ovulated cumulus–oocyte complex we observed a halo-like immunoreactivity of VEGF around the fully mature oocyte. The immunoreactivity for Flt-1 and KDR receptors in growing follicles was mostly limited to GC and TI cells. Anti-bFGF did not exhibit any immunoreactivity in ZP of follicles at any stage. Its expression was weak in GC of the follicles at different stages, whereas, it could be localized to some extent in the blood capillaries of TI of antral follicles and in blood vessels localized in the stroma. Interestingly, VEGF immunoreactivity in the ZP of some secondary follicles is very striking. Accordingly, the possibility that VEGF may be an important regulatory molecule for the dominant follicle selection or atresia should be considered.     

Developmental ability of enucleated bovine oocytes matured in vitro after fusion with single blastomeres of eight-cell embryos matured and fertilized in vitro

Single blastomeres from eight-cell stage bovine embryos matured and fertilized in vitro were electrically fused with enucleated oocytes matured in vitro. In experiment 1, The percentage of these reconstituted embryos developed to the two- to eight-cell stage 48 hr after electrofusion was increased when both the eight-cell embryos and the enucleated oocytes were derived from oocytes cultured with granulosa cells (14% vs. 38%). In experiment 2, the relationship between activation of oocytes and developmental ability of reconstituted embryos was examined. Although both ethanol and electrical stimulation efficiently induced parthenogenetic activation of oocytes matured in vitro for 26–28 hr (ethanol, 89%; electrical stimulation, 73%), the ratio of the second polarbody extrusion differed (80% vs. 22%). Ethanol-treated enucleated oocytes, however, were not significantly different from the early cleavage of the reconstituted embryos 48 hr after electrofusion (nontreated, 38%; treated, 43%). In experiment 3, reconstituted embryos at the two- to eight-cell stage 48 hr after the electrofusion were cocultured with granulosa cells for 6–7 days. Of 69 embryos, one developed to a morula and three developed to blastocysts. © 1993 Wiley-Liss, Inc.     

Secretion of zona pellucida glycoprotein mZP2 by growing oocytes from mZP3+/+ and mZP3−/− mice

The mouse egg extracellular coat, or zona pellucida (ZP), is composed of three glycoproteins, called mZP1–3, which are synthesized and secreted concomitantly by growing oocytes. Disruption of the mZP3 gene by targeted mutagenesis yields mice that are homozygous nulls (mZP3^−/−). Growing oocytes from mZP3^−/− mice do not synthesize mZP3 mRNA or protein and, as a result, do not assemble a ZP. Here, we examined secretion of mZP2 by growing oocytes and eggs from mZP3^−/− mice, as well as incorporation of mZP2 into the ZP of oocytes from mZP3^+/+ mice. Laser scanning confocal microscopy (LSCM) of antibody‐labeled samples showed that, indeed, mZP2 was synthesized and secreted by oocytes isolated from mZP3^−/− mice and cultured in vitro. Nascent mZP2 was found in the culture medium, associated with the surface of the plasma membrane of growing oocytes, and in the oocyte cytoplasm. By contrast, mZP2 was barely detectable at any of these sites when ovulated eggs from mZP3^−/− mice were examined. Examination of oocytes from wild‐type (mZP3^+/+) mice showed that, while a portion of nascent mZP2 was assembled into the ZP (approximately 40%), here too a significant fraction was secreted into the culture medium (approximately 60%). Similar results also were obtained when intact pre‐antral follicles were isolated from mZP3^+/+ mice and cultured in vitro. Several of these observations are consistent with previous results obtained with oocytes from heterozygous null mice (mZP3^+/−). Furthermore, the results suggest that ZP assembly from nascent glycoproteins may be a stochastic process that requires the presence of both mZP2 and mZP3 and occurs completely outside the growing oocyte. Dev. Genet. 25:95–102, 1999. © 1999 Wiley‐Liss, Inc.     

Surface alterations of the bovine oocyte and its investments during and after maturation and fertilization in vitro

Surface characteristics of the bovine oocyte and its investments before, during, and after maturation, and fertilization in vitro were evaluated by scanning electron microscopy (SEM). Oocyte diameters were also measured during SEM analysis of the oocyte. The cumulus cells manifested a compact structure with minimal intercellular spaces among them in the immature oocytes. These became fully expanded with increased intercellular spaces after maturation in vitro, but contracted again after fertilization. The zona pellucida (ZP) showed a fibrous, open mesh-like structure in the maturing and matured oocytes. The size and number of meshes on the ZP decreased dramatically after fertilization. The vitelline surface of immature oocytes was characterized by distribution of tongue-shaped protrusions (TSPs) varying in density. After 10 and 22 hr of maturation incubation, oocyte surface microvilli (MV) increased to become the predominant surface structure, and TSPs decreased substantially. The vitelline surface of fertilized oocytes (at 6 and 20 hr) was similar to that of the matured oocytes, but unfertilized oocytes had less dense MV than did fertilized oocytes (at 20 hr). The diameter of the oocytes decreased from 99 to 80 μm during maturation and increased to 106 μm after insemination (P < 0.05). Membrane maturation was characterized by surface changes from a TSP-predominant pattern to a MV-predominant pattern. Thus, the bovine oocyte maturation process was found to involve the expansion of cumulus cells and the maturation of the ZP, which changes dramatically upon fertilization. Also, volumetric changes occurred in ooplasm processed for SEM following oocyte maturation and insemination. © 1994 Wiley-Liss, Inc.     

Characterization of the zona pellucida glycoproteins from bovine ovarian and fertilized eggs

Bovine zone pellucida (ZP) glycorproteins from ovarian egg emerged as three bands with molecular mass of 78 kDa, 64 kDa and 21 kDa in SDS-PAGE under reducing conditions. Endo-β-galactosidase (EβG) digestion of the glycoproteins yielded five products with molecular mass of 76 kDa (EβG-76), 68 kDa (EβG-68), 63 kDa(EβG-63), 47 kDa (EβG-47) and 21 kDa (EβG-21) under the same conditions. The N-terminal amino acid sequences of EβG-76 and EβG-21 were identical. This fact together with the results of diagonal SDS-PAGE indicated that EβG-21 (N-terminal region) is linked to EβG-63 (C-terminal region) through disulfide bond to form EβG-76. Immunoblot analysis using anti-pig ZP protein antibodies revealed that bovine EβG-76, EβG-68 and EβG-47 correspond to pig PZP2, PZP3α and PZPEβ glycoproteins, respectively. The EβG-76 and EβG-68 components were shown to be specifically cleaved during fertilization.     

Expression of the rabbit sperm protein Sp17 in cos cells and interaction of recombinant Sp17 with the rabbit zona pellucida

This study extends our analysis of rabbit recombinant Sp17 (rSp17) by examining whether rSp17 synthesized in transfected COS cells will show a particular localization within the cell and whether the COS cell will bind with zona pellucida. We show, using the cross-linking, reagent DSS that rSp17 can bind to rabbit zona glycoprotein R45 or R55. © 1995 Wiley-Liss, Inc.