Comparative Study on Activation and Fertilization of Mouse Oocytes at Different Ages

Comparisons of activation rates and fertilization rates were made among oocytes at different ages. Results showed that oocytes at different ages had different activation and fertilization rates when stimulated by sperm or ethanol. Oocytes at 15～24 h after the injection of hCG were readily activated by 8% ethanol. The activation rate of oocytes increased with the age of oocytes, up to the highest average of 81.6%, but decreased after 20 h posthCG. Oocytes at 20 h posthCG exhibited the highest immediate cleavage rate(48.0%) after being stimulated by ethanol. On the other hand, 13～15 h oocytes exhibited higher fertilization rates, and the older oocytes were more difficult to be fertilized by sperm in vitro. These suggest that oocytes can be activated in different ways; the mechanism of fertilization might be different from that of activation; and in vitro fertilization is more dependent on oocyte age.

Presence and integration of HBV DNA in mouse oocytes

AIM: Hepatitis B is a worldwide public health problem. To explore the feasibility of hepatitis B virus (HBV) vertical transmission via oocytes, the presence and integration of HBV DNA in mouse oocytes were studied. METHODS: Genomic DNA was isolated and metaphases were prepared, respectively from mouse oocytes cocultured with pBR322-HBV DNA plasmids. PCR, Southern blot, dot hybridization and fluorescence in situ hybridization (FISH) were performed to explore the existence and integration of HBV DNA in oocytes.RESULTS: PCR detected positive bands in the tested samples, and then Southern blot revealed clear hybridization signals in PCR products. Final washing solutions were collected for dot hybridization and no signal for HBV DNA was observed, which excluded the possibility that contamination of washing solutions gave rise to positive results of PCR and Southern blot. FISH demonstrated that 36 of 1 000 metaphases presented positive signals. CONCLUSION: HBV DNA sequences are able to pass through the zona and oolemma to enter into oocytes and tointegrate into their chromosomes. HBV DNA sequences might be brought into embryo via oocytes as vectors when they are fertilized with normal spermatozoa.

The fertilization-induced Ca^(2+) oscillation in mouse oocytes is cytoplasmic maturation dependent

Mature eggs (at metaphase Ⅱ stage) produce a series of Ca2+ oscillation at fertilization. To define whether the fertilization-induced Ca2+ oscillation is restrict to the metaphase Ⅱ eggs and cell cycle dependent, mouse oocytes at prophase Ⅰ (arrested at germinal vesicle stage),metaphase Ⅰ, metaphase Ⅱ, as well as the pronuclear embryos at interphase of the first mitotic division derived from fertilization or parthenogenetic activation were inseminated after removal of zona pellucida. The results show that the fertilization-induced Ca2+ oscillation is not specific to metaphase Ⅱ eggs. This is supported by the fact that immature oocytes generated the Ca2+ oscillations at fertilization regardless of their nuclear progression from prophase Ⅰ to metaphase Ⅰ (in vitro matured) stage. More interestingly, it was first found that pronuclear embryos at interphase derived from parthenogenetic activation showed Ca2+ oscillations in response to fertilization while the zygotes at interphase did not after reinsemination or intracytoplasmic injection of sperm extracts which induce Ca2+ oscillations in MII eggs. This suggests that the ability of oocytes to generate Ca2+ oscillation in response to sperm penetration is not regulated in a cell cycle dependent manner but dependent on the cytoplasmic maturation.

No effect of exogenous melatonin on development of cryopreserved metaphase Ⅱ oocytes in mouse

Background： This study was conducted to investigate effect of exogenous melatonin on the development of mouse mature oocytes after cryopreservation. Results： First, mouse metaphase II （MII） oocytes were vitrified in the open-pulled straws （OPS）. After warming, they were cultured for 1 h in M2 medium containing melatonin at different concentrations （0, 10-9, 10-7, 10-s, 10 3 mol/L）. Then the oocytes were used to detect reactive oxygen species （ROS） and glutathione （GSH） levels （fluorescence microscopy）, and the developmental potential after parthenogenetic activation. The experimental results showed that the ROS level and cleavage rate in 10-3 mol/L melatonin group was significantly lower than that in melatonin-free group （control）. The GSH levels and blastocyst rates in all melatonin-treated groups were similar to that in control. Based on the above results, we detected the expression of gene Hsp9Oaal, Hsfl, Hspalb, Nrf2 and Bcl-xl with qRT-PCR in oocytes treated with 10-7, or 10-3 mol/L melatonin and untreated control. After warming and culture for 1 h, the oocytes showed higher Hsp90oal expression in 10-7 mol/L melatonin-treated group than in the control （P 〈 0.05）; the Hsfl, Hsp9Oaal and Bcl-xl expression were significantly decreased in 10-3 mol/L melatonin-treated group when compared to the control. Based on the above results and previous research, we detected the development of vitrified-warmed oocytes treated with either 10-7 or 0 mol/L melatonin by in vitro fertilization. No difference was observed between them. Conclusions： Our results indicate that the supplementation of melatonin （10-9 to 10-3 mol/L） in culture medium and incubation for 1 h did not improve the subsequent developmental potential of vitrified-warmed mouse MII oocytes, even if there were alteration in gene expression.

Function of RanGAP1 in Mouse Oocyte Fertilization

RanGAP1 is a Ran GTPase-activating protein that plays a pivotal role in the majority of nucleocytoplasmic transport pathways. The protein is limited to somatic cells. In this study, the localization and possible functions of RanGAP1 were examined during mouse oocyte fertilization. Immunofluorescence analysis showed that after sperm penetration, RanGAP1 was found to diffuse within the cytoplasm, but concentrated in the microtuble of the reversed spindle and the constriction ring between the oocyte and the second polar body; with the expansion of sperm chromatin, RanGAP1 began to move to the region around the expanding sperm and oocyte chromatin, and gradually concentrated around the growing parents pronuclei. After the male and female pronuclei apposed, the membrane of one pronuclei broke first, numerous concentrated RanGAP1 dots were observed in the chromosome region. With the chromatin condensing into chromosome, the parents chromosomes mixed together and prepared to start the first mitosis, the condensed RanGAP1 was just the shape of the microtuble to assemble the first mitosis spindle. These showed that RanGAP1 played an important role in regulating spindle functions, chromosome alignment, PB2 extrusion and pronuclei nuclear envelope assembly/disassembly in mouse oocyte fertilization.

Cryobiological Characteristics of L-proline in Mammalian Oocyte Cryopreservation

Background： L-proline is a natural, nontoxic cryoprotectant that helps cells and tissues to tolerate freezing in a variety of plants and animals. The use of L-proline in mammalian oocyte cryopreservation is rare. In this study, we explored the cryobiological characteristics of L-proline and evaluated its protective effect in mouse oocyte cryopreservation. Methods： The freezing property of L-proline was detected by Raman spectroscopy and osmometer. Mature oocytes obtained from 8-week-old B6D2F 1 mice were vitrified in a solution consisting various concentration of L-proline with a reduced proportion ofdimethyl sulfoxide （DMSO） and ethylene glycol （EG）, comparing with the control group （15% DMSO and 15% EG without L-proline）. The survival rate, 5-methylcytosine （5-mC） expression, fertilization rate, two-cell rate, and blastocyst rate in vitro were assessed by immunofluorescence and in vitro fertilization. Data were analyzed by Chi-square test. Results： L-proline can penetrate the oocyte membrane within 1 min. The osmotic pressure of 2.00 mol/L L-proline mixture is similar to that of the control group. The survival rate of the postthawed oocyte in 2.00 mol/L L-proline combining 7.5% DMSO and 10% EG is significantly higher than that of the control group. There is no difference of 5-mC expression between the L-proline combination groups and control. The fertilization rate, two-cell rate, and blastocyst rate in vitro from oocyte vitrified in 2.00 mol/L L-proline combining 7.5% DMSO and 10% EG solution are similar to that of control. Conclusions： It indicated that an appropriate concentration of L-proline can improve the cryopreservation efficiency of mouse oocytes with low concentrations of DMSO and EG, which may be applicable to human oocyte vitrification.