Investigation of methods and factors influencing mouse oocyte quality, in vitro fertilization and embryo development
Mice are one of the most commonly used rodent species as a model for biomedical research to better understand molecular, genetic, and cellular causes of human disease and disorders. The production of good quality oocytes is one of the important determinant factors for successful assisted reproductive technologies (ARTs) clinical outcome as well as reproductive biology research. Mouse oocyte quality, morphology and functions are influenced by a variety of the factors such as euthanasia methods of female donors, superovulation regimes and cryopreservation. The objectives of these studies were mainly to investigate the methods and factors that are influencing oocyte quality, in-vitro fertilization (IVF) and embryonic development. First, how different euthanasia methods including cervical dislocation (CD), high flow rate CO (H CO) and low flow CO2 (L CO) would affect the quality and integrity of the metaphase II (MII) oocytes have been investigated. Cumulus oocyte complexes (COCs) were collected from female donors that were euthanized by three different methods and then the oocytes' subcellular structures including microtubules, F-actin, cortical granules (CGs) and mitochondria integrities were detected by specific fluorescence dyes. The results showed that L CO caused significant increase in the incidence of premature cortical granule exocytosis (PCGE) which might be responsible for significantly reducing the in-vitro fertilization (IVF) and embryonic development rate compared to CD and H CO. Secondly, how the superovulation methods would affect the resulting oocyte morphology, quality, IVF competence and embryonic development was investigated. The anti-inhibin serum (AIS) superovulation method produced a significantly higher number of oocytes compared to the pregnant mare serum gonadotrophin (PMSG). Overall, both methods yielded oocytes with similar sizes and comparable subcellular structures including microtubules, F-actin, cortical granules and mitochondria. However, superovulation with AIS produced significantly thinner zona pellucide than PMSG and the perivitelline space of the oocytes generated from AIS were significantly larger than PMSG. There were no differences in terms of two-cell embryo development, or morula and blastocyst formation rates between AIS and PMSG when the oocytes from two methods were in-vitro fertilized with fresh sperm. Morula and blastocyst development rates were significantly higher for AIS compared to PMSG when oocytes were fertilized with frozen-thawed sperm. Thirdly, clutches of mouse cumulus oocytes complexes (COCs) were cryopreserved by the cryoloop vitrification method after PMSG superovulation. The cryo-survival rate and integrity and distribution of subcellular structures including the meiotic spindles, F-actin, cortical granules and mitochondria were examined and compared with fresh MII oocytes. The vitrified-warmed oocytes maintained their subcellular structures to a high degree and resulted in acceptable IVF and embryonic development. In conclusion, for optimal research and clinical outcome, considerations should be given regard to euthanasia methods of oocyte donor mice and type of superovulation regimes. Despite of its high oocyte yield, superovulation of mice with AIS provides comparable quality oocytes to the PMSG method. Cryopreservation of the clutches of mouse COCs via cryo-loop vitrification should be considered for genome banking of genetically modified mice and biomedical research.
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