OOCYTE MATURATION AND EMBRYONIC FAILURE

Embryonic development is readily compromised by imperfections introduc ed during the process of oocyte maturation. We discuss the nature and causes of these imperfections, particularly in oocytes exposed to inap propriate hormonal regimes in vivo or to culture systems designed to i nduce the maturation of oocytes in vitro, The acquisition of developme ntal competence involves the synthesis and storage of a wide range of molecules during oocyte growth followed by the reprogramming and order ed utilization of these stored products during maturation, fertilizati on and early embryogenesis. The regulatory signals for these molecular changes are produced by the follicle cells in response to circulating levels of gonadotrophins; we report that some ovarian stimulation pro tocols distort these signals thereby disrupting molecular reprogrammin g of the oocyte and reducing subsequent developmental competence. The aspiration of immature oocytes from antral follicles followed by their maturation in vitro is a potential alternative to hormonal stimulatio n of patients in IVF treatment. Although relatively successful in a va riety of animals, the production of fully viable human embryos by in-v itro maturation is still unsatisfactory despite the use of a wide vari ety of culture protocols, Our data suggests that the key to maturation and embryo viability in vitro resides in the follicle cell compartmen t rather than the oocyte. Because of rapid luteinization changes, foll icle cells in culture probably fail to provide the maturing oocyte wit h the necessary ordered set of instructive signals and nutrients neede d for the acquisition of developmental competence. Although much remai ns to be discovered about the nature, concentration and transmission o f signals, nevertheless it is already clear that different steroids, m atrix metalloproteinases and growth factors are involved in conferring viability on the maturing oocyte. Major improvements in the yield of viable embryos from in-vitro matured oocytes can be anticipated from a systematic analysis of somatic signals from the pre-ovulatory follicl e.