Maturation of pig oocytes in vivo and in vitro.

In this review the concept that the origins of embryonic failure occur during oocyte development is explored. The four factors that determine oocyte viability, namely a normal growth phase, adequate follicle cell support during maturation, the completion of intracellular reprogramming before fertilization and the functioning of oocyte surveillance mechanisms, form the four sections of this review. The viability of pig oocytes at the end of the growth phase is compromised by presumptive spontaneous meiotic progression and by morphological heterogeneity. Determining the percentage and identity of viable dictyate oocytes, and identifying the reasons for the loss of viability, are key areas of future investigation. Although the requirement for follicle cell support during maturation is already established, little is yet known about the underlying signals and their transmission to the oocyte. The analysis of the action and nature of somatic signals will provide the foundation for further advances in the maturation of oocytes in vitro. Signalling cascades in oocytes control both the translation of masked mRNA and the modification and spatial localization of resultant proteins. The interdependent nature of this control system explains why inappropriate signals during maturation lead to subsequent embryonic mortality. Chromosomal errors during meiosis and early mitosis accumulate because of the leaky nature of the checkpoint system during the maternally regulated part of development: effective cell cycle surveillance is established only after the activation of the embryonic genome. In summary, we emphasize that the quality of the dictyate oocyte and the provision of appropriate signals in vitro are the principal determinants of maturational success.