Role of hyaluronic acid in maturation and further early embryo development of bovine oocytes.

Hyaluronic acid (HA), an important component of the extracellular matrix, plays a crucial role for cumulus cell expansion. Genes and proteins involved in HA synthesis and its receptor CD44 are expressed in cumulus oocyte complexes (COCs) in different animal species and increase during maturation. Hyaluronidase enzymes (Hyal) degrade HA into smaller biologically active HA fragments. To investigate the effects of the molecular size and concentration of HA on oocyte maturation and further embryo development, bovine oocytes were matured in vitro in the presence or absence of HA, Hyal-2 or 4-methylumbelliferone (4-MU); an HA synthesis inhibitor. The rates of oocyte nuclear maturation to metaphase II stage and development of embryos to blastocyst stage and blastocyst quality were recorded. Hyal-2 inhibited cumulus cell expansion without affecting oocyte maturation and further embryo development. Whereas, 4-MU at 1 mm reduced cumulus cell expansion, oocyte maturation rate and further embryo development; an effect which was partially abrogated by exogenous HA supplementation. These data suggest that HA production by cumulus cells during maturation is essential not only for cumulus cell expansion, but also for oocyte maturation and further embryo development. This effect is not affected by HA-degradation by Hyal-2.

Studies on cryoprotectant toxicity to zebrafish (Danio rerio) ovarian tissue fragments.

Cryopreservation of ovarian tissue is a viable alternative to cryopreservation of oocytes and embryos in many species but it has not been studied in fish. Selection of cryoprotectant is an important step in designing cryopreservation protocols. In order to identify the optimum cryoprotectant (CPA) in a suitable concentration for zebrafish ovarian tissue cryopreservation, studies on toxicities of five commonly used cryoprotectants methanol, ethanol, dimethyl sulfoxide (DMSO), ethylene glycol (EG) and propylene glycol (PG) were carried out. Experiments were conducted on ovarian tissue fragments consisting of stage I and stage II ovarian follicles. Ovarian tissue fragments were incubated in 90% L-15 medium (pH 9) containing 1-4M cryoprotectants for 30min at 22°C. Three different tests were used to assess ovarian tissue fragment viability: trypan blue (TB) staining, fluorescein diacetate (FDA) combined with propidium iodide (PI) staining and adenosine 5´- triphosphate (ATP) assay. Results from these tests showed that ATP assay was more sensitive than FDA+PI or TB staining for assessing cryoprotectant toxicity to follicles in tissue fragments. Methanol and ethanol were the least toxic cryoprotectants tested. Cryoprotectant toxicity increased in the order of methanol/ethanol, DMSO, PG and EG. Ethanol was used for zebrafish ovarian tissue for the first time and the results showed that the effect of methanol and ethanol on ovarian tissue fragments were comparable. As methanol has been shown to be the most effective cryoprotectant for zebrafish ovarian follicles in our laboratory, the use of ethanol will also be considered in assisting future freezing protocol design. The present study also showed that stage II ovarian follicles are more sensitive to cryoprotectant treatment than stage I follicles in tissue fragments. The results obtained in this study provided useful information for ovarian tissue fragment cryopreservation protocol design in the future.

Oogenesis and cell death in human prenatal ovaries: what are the criteria for oocyte selection?

Prenatal oogenesis produces hundreds of thousands of oocytes, most of which are discarded through apoptosis before birth. Despite this large-scale selection, the survivors do not constitute a perfect population, and the factors at the cellular level that result in apoptosis or survival of any individual oocyte are largely unknown. What then are the selection criteria that determine the size and quality of the ovarian reserve in women? This review focuses on new data at the cellular level, on human prenatal oogenesis, offering clues about the importance of the timing of entry to meiotic prophase I by linking the stages and progress through MPI with the presence or absence of apoptotic markers. The characteristics and responsiveness of cultured human fetal ovarian tissue at different gestational ages to growth factor supplementation and the impact of meiotic abnormalities upon apoptotic markers are discussed. Future work will require the use of a tissue culture model of prenatal oogenesis in order to investigate the fate of individual live oocytes at different stages of development.

Influence of p53 and genetic background on prenatal oogenesis and oocyte attrition in mice.

BACKGROUND: Meiotic progression, and the number of oocytes surviving to birth, determine the ovarian reserve, yet the control of prenatal oogenesis is poorly understood. We investigated the effects of genetic background and p53 upon oogenesis in mice. METHODS: Fetal and neonatal ovaries were analysed in B6CBf1 and B6CBf2 mice from 15.5 to 21 days post-coitum (dpc) and p53 (a tumour suppressor gene) knockout, heterozygous and wild-type mice from 15.5 to 16 dpc. Oocytes in meiotic prophase I (MPI) were identified by labelling synaptonemal complex protein 3, and the specific stage of MPI was classified by the appearance of axial elements. Apoptosis and DNA breaks were assessed by cleaved poly-(ADP-ribose) polymerase (PARP-1) and terminal deoxynucleotidyltransferase-mediated dUTP nick-end labelling (TUNEL), respectively. RESULTS: The leptotene, zygotene and pachytene stages were earlier in f1 than f2 generations with significant differences at all stages (P< or =0.002). The p53(-/-) oocyte populations and those with the presence of p53 gene differed significantly in terms of: (i) the proportion of oocytes reaching specific stages of MPI on 15.5 and 16 dpc and (ii) the proportion of oocytes having observed abnormalities in synaptonemal complexes (P < 0.001). The absence of p53 resulted in faster progression of oocytes and more with compressed and abnormal axial elements. We observed significant differences between p53(-/-), p53(+/-) and p53(+/+) mice in terms of cleaved PARP-1 staining and TUNEL. CONCLUSION: Genotype has an important impact on prenatal meiosis and oocyte apoptosis. p53 affects the speed of oocyte development and may influence the oocyte selection through apoptosis during MPI.