Responses of oocytes and embryos to the culture environment.

Embryo development is strongly influenced by events occurring during oocyte maturation. Although many immature oocytes are capable of completing meiosis in vitro, only a small percentage of the original pool of immature oocytes is competent to continue development to the blastocyst stage and subsequently result in a pregnancy. This indicates that maturation of oocytes in vitro may not be occurring in an entirely normal manner. Cytoplasmic changes occurring during maturation, collectively termed cytoplasmic maturation, are essential for embryonic development. The cytoplasm of the oocyte may play a crucial role in assembling the correct metabolic machinery for production of sufficient energy for cellular functions during maturation, cleavage and blastocyst formation. A better understanding of the structural, functional and metabolic characteristics of the oocyte during maturation, and the consequence of changes in these parameters on developmental competence is needed. Understanding the role of cytoplasmic changes during oocyte maturation will help increase the efficiency of in vitro embryo production. Better embryo production strategies will facilitate basic research into the control of early development, improve implementation in endangered species, provide a source of high quality oocytes for nuclear transfer and transgenic technologies and benefit the commercial embryo transfer industry.