Metabolic characteristics and gene expression of in vitro and in vivo matured mammalian oocytes

Abstract

Current in vitro maturation (IVM) systems for oocytes impair cellular function, leading to compromised developmental potential. Identification of components in IVM systems that affect cell function requires development of a defined maturation medium capable of supporting both nuclear and cytoplasmic maturation. The extent to which factors such as medium supplementation, oxygen tension, and priming oocyte donors with exogenous gonadotropins affect oocyte viability is unclear. Characterization of these aspects will facilitate optimization of a defined maturation system capable of producing oocytes of equivalent developmental competence to in vivo matured counterparts. The main objective of this thesis was to examine these factors in IVM systems and subsequent developmental rates after fertilization. Metabolic and gene expression patterns of in vitro matured oocytes were studied, and characteristics of in vivo matured oocytes were used as a baseline. A defined maturation system was developed, yielding equivalent blastocyst development to systems including blood serum. In the defined system, metabolism of individual mouse oocytes was positively correlated with subsequent developmental potential. Inclusion of EGF during maturation of mouse oocytes was correlated with increased developmental competence, while maturation under 20% O2 instead of 5% O2 resulted in reduced cell numbers of resulting embryos, as well as decreased oxygen metabolism and mitochondrial membrane potential. Variations in medium supplementation during bovine in vitro maturation altered the quantity and quality of blastocysts produced. Gene expression of bovine oocytes matured in vitro at 20% O2 differed from that of in vivo matured oocytes. Reduced O2 (5%) during bovine oocyte maturation resulted in similar carbohydrate metabolism and gene expression; however, oxygen metabolism was lower for oocytes matured at 5% rather than 20% O2. These studies indicate that maturation conditions profoundly affect the quality of oocytes, as well as the quantity and quality of resulting embryos. Metabolic and gene markers will aid in development of optimal IVM systems and will serve as viability markers of oocyte quality and subsequent developmental potential.