Oogenesis as a foundation for embryogenesis.

The majority of oocytes in postnatal ovaries are small, non-growing and reside in primordial follicles. They have to undergo a prolonged phase of growth and differentiation before nuclear and cytoplasmic maturation enables them to resume meiosis and undergo fertilization. A better knowledge of this phase of oogenesis is essential for understanding causes of oocyte pathology and optimizing methods for growing oocytes in vitro and for cryopreservation. There could also be spin-off discoveries for contraceptive strategies and pharmacologically controlling oocyte maturation. During oocyte growth, a molecular programme for development is assembled for the timely expression of mRNAs, some of which are expressed throughout oogenesis while others are 'masked' until or after meiotic maturation. Masking and stability in storage are largely due to a truncated poly(A) tail, controlled by regulatory sequences on the 3'-untranslated region (UTR) of the mRNA. Most maternal RNAs are degraded early in cleavage, there being a narrow overlap between persisting maternal mRNAs and activation of the embryonic genome. Accumulation of RNAs and proteins are not, however, the only major changes taking place during oogenesis. Cytoplasmic organelles multiply and redistribute, and there are epigenetic modifications of DNA for monoallelic expression of imprinted genes. The granulosa cells are obligatory for they provide physical support, nutrients and mediate the regulatory influences of gonadotrophic hormones. On the other hand, the oocyte actively influences the growth and differentiation of its granulosa cells. Thus, healthy embryos reflect the quality of both the oocyte and the granulosa cells.