UC Davis

Assisted reproductive technologies (ART) like in vitro production (IVP) of embryos in the horse is an important tool in the equine industry. In particular, the aspiration of antral follicles and the collection of immature oocytes is the most common and widely practiced procedure to obtain female gametes for IVP. However, in vitro maturation (IVM) of these immature oocytes remains a significant limitation for efficient IVP. This is mainly due to suboptimal maturation media formulations and overall suboptimal maturation culture conditions. Maturation of the oocyte is a critical and delicate phase where the oocyte acquires its capacity to be fertilized by one spermatozoon and then undergoes embryogenesis. In Chapter 1, a review of the pertinent literature regarding oocyte maturation and associated processes is presented.In the studies presented in Chapter 2 of this dissertation, it is described for the first time, that the gene expression profile of oocytes and surrounding cumulus cells (CC) and the associated changes that occur during maturation. Oocytes expressed 13,918 genes, with 68.46% being protein-coding genes. 749 differentially expressed genes (DEG) were observed between the immature (GV) and Mature (MII) groups. Within these DEG, 330 presented a higher expression in the OC-MII group and were associated with pathways of the cell cycle and chromosome segregation. 419 genes showed a lower expression in the OC-MII group and were associated with ribonucleoprotein complex biogenesis and nucleolus. In CC samples, it was established 927 DEG with 609 upregulated and 318 downregulated genes. It was determined that many pathways related to GTPase activity were part of the upregulated genes. Downregulated genes were mainly related to the extracellular matrix organization, collagen trimer, and collagen fibril. A weighted gene co-expression network analysis (WGCNA) of maturation revealed that important pathways are associated with nuclear maturation and the meiotic cycle. Moreover, the expression of the follicle-stimulating hormone receptor (FSHR) increased while the luteinizing hormone receptor (LHCGR) decreased during maturation. The studies presented in Chapter 3 of this dissertation were designed to compare IVM to in vivo maturation (IVV) by evaluating the gene expression profile of the CC. There were over six thousand DEG between the two groups with 2180 genes upregulated and 4481 downregulated in the IVM group. Genes associated with extracellular matrix and cumulus expansion were downregulated in the IVM group. Also, the expression of FSHR was higher, while LHCGR expression was lower in the IVM compared with the IVV group. Additionally, cumulus expansion during IVM exhibited an increasing linear rate of expansion. The results from Chapters 2 and 3 combined suggested that gonadotropins (FSH and LH) might have an important role during oocyte maturation in the horse, thus a re-evaluation of the maturation media formulations, like the concentration of FSH used in the formulation, could result in the improvement of maturation outcomes. Additionally, the reduced activation of genes and pathways associated with cumulus expansion is another important factor that should be addressed and investigated in future studies.