MATERNAL DETERMINANTS OF OOCYTE AND EMBRYO QUALITY

Abstract

Oocyte quality plays a critical role in establishment of pregnancies, embryo development, implantation and the health of offspring. The oocyte provides maternal factors necessary for the initial development of its embryo during the period of transcriptional silence. Despite the consistent increase in number of couples seeking assisted reproductive treatments, oocyte quality still remains as an obstacle in successful fertility treatments and the mechanisms governing the quality of oocyte are poorly understood. Among various factors that may potentially affect the quality of oocyte, the acquisition of oocyte developmental competence seems to mainly occur during the final stage of oocyte maturation. The correct temporal regulation of series of molecular events and the proper exchange of signals with surrounding follicular environment during this critical period will ensure the developmental competence of oocyte and its subsequent embryo. In order to identify molecular factors affecting oocyte quality, I have compared oocytes and cumulus cells of different qualities at a molecular level. I present in this thesis the pathways and molecules that may determine the developmental competence of oocyte as well as candidate molecular markers of oocyte and embryo quality. A cDNA microarray analysis was performed, comparing the transcriptomes of rhesus monkey MII oocytes of different qualities, high quality VVM oocytes and poor quality IVM oocytes. A small set of 59 Oocyte quality plays a critical role in establishment of pregnancies, embryo development, implantation and the health of offspring. The oocyte provides maternal factors necessary for the initial development of its embryo during the period of transcriptional silence. Despite the consistent increase in number of couples seeking assisted reproductive treatments, oocyte quality still remains as an obstacle in successful fertility treatments and the mechanisms governing the quality of oocyte are poorly understood. Among various factors that may potentially affect the quality of oocyte, the acquisition of oocyte developmental competence seems to mainly occur during the final stage of oocyte maturation. The correct temporal regulation of series of molecular events and the proper exchange of signals with surrounding follicular environment during this critical period will ensure the developmental competence of oocyte and its subsequent embryo. In order to identify molecular factors affecting oo was identified as differentially expressed between the two types of oocytes. These mRNAs are involved in steroid metabolism, cell-cell interactions, cellular homeostasis, cell adhesion, mRNA stability and translation. In addition, the overexpression of several imprinted genes in IVM oocytes were detected, indicating a possible loss of correct epigenetic programming during IVM. These results indicate that normal oocyte-somatic cell interactions may be disrupted during IVM and the interruptions of these interactions during the final phase of oocyte maturation may be the prime cause of reduced developmental competence of IVM oocytes. To elucidate oocyte quality factors linked to the cumulus cell phenotype, the transcriptomes of two types of rhesus monkey cumulus cells, IVM and VVM, were compared using a cDNA microarray analysis. In contrast to a relatively small difference between IVM and VVM oocytes, a large number of genes were differentially expressed between IVM and VVM rhesus cumulus cells. Moreover, a much larger number of differential mRNA expressions were observed comparing the transitions from pre-maturation cumulus cells to the IVM and VVM cumulus cells. The results from these array comparisons indicated that the cumulus cells may fail to achieve successfully normal gene regulation during IVM and thus make a remarkable amount of changes in gene expression to compensate for the loss. Numerous genes involved in lipid metabolism are incorrectly regulated during IVM, and the synthesis of sex hormones appears not suppressed during IVM. In addition, a panel of 24 cumulus cell markers of oocyte quality was identified. Genetic effects on oocyte quality were explored by comparing transcriptomes of oocytes obtained from two different inbred mouse strains, B6 and D2, and F1 hybrid. A clustering analysis and statistical tests showed that the transcriptome of F1 oocytes is more similar to the B6 transcriptome than to the D2 at both GV and MII stages. Also, comparison analyses of GV stage oocyte transcriptomes with MII oocyte transcriptomes from three different mouse strains indicated that the number of overdominance genes at the MII stage is bigger than the number of overdominance genes at the GV stage. In order to investigate how the genes gain the overdominance during the GV to MII transition, overdominance genes were categorized according to their mRNA expression patterns at GV and MII stages. The results showed that more than 80% of overdominance genes belong to one of the four major transition groups. The further evaluation of changes in array intensities from GV to MII stage transition revealed that F1 oocytes and inbred strain oocytes differentially regulate the mRNA abundance during oocyte maturation and that the differential regulation of mRNA abundance by the F1 genotype is responsible for the increase of the number of overdominance genes during maturation from GV stage to MII stage. A mRNA sequence analysis indicated that the gain of overdominant low in F1 mRNA expression pattern during maturation may be regulated by 3'UTR motif elements. The number of dominance genes also increase during GV to MII transition. At both GV and MII stages, there are more genes with B6 dominant mRNA expression pattern than those with D2 dominance pattern. Lipid metabolism, small molecule biochemistry and cell death are biofunctions overrepresented in both dominance and overdominance genes. In addition, `blebbing' was identified as a biofunction significantly downregulated in the F1 and B6 MII eggs, indicating that the cellular function may be involved in oocyte maturation.