Oocyte maturation and embryonic failure

Embryonic development is readily compromised by imperfectionsintroduced during the process of oocyte maturation. We discussthe nature and causes of these imperfections, particularly inoocytes exposed to inappropriate hormonal regimes in vivo orto culture systems designed to induce the maturation of oocytesin vitro. The acquisition of developmental competence involvesthe synthesis and storage of a wide range of molecules duringoocyte growth followed by the reprogramming and ordered utilizationof these stored products during maturation, fertilization andearly embryogenesis. The regulatory signals for these molecularchanges are produced by the follicle cells in response to circulatinglevels of gonadotrophins; we report that some ovarian stimulationprotocols distort these signals thereby disrupting molecularreprogramming of the oocyte and reducing subsequent developmentalcompetence. The aspiration of immature oocytes from antral folliclesfollowed by their maturation in vitro is a potential alternativeto hormonal stimulation of patients in IVF treatment. Althoughrelatively successful in a variety of animals, the productionof fully viable human embryos by in-vitro maturation is stillunsatisfactory despite the use of a wide variety of cultureprotocols. Our data suggests that the key to maturation andembryo viability in vitro resides in the follicle cell compartmentrather than the oocyte. Because of rapid luteinization changes,follicle cells in culture probably fail to provide the maturingoocyte with the necessary ordered set of instructive signalsand nutrients needed for the acquisition of developmental competence.Although much remains to be discovered about the nature, concentrationand transmission of signals, nevertheless it is already clearthat different steroids, matrix metalloproteinases and growthfactors are involved in conferring viability on the maturingoocyte. Major improvements in the yield of viable embryos fromin-vitro matured oocytes can be anticipated from a systematicanalysis of somatic signals from the pre-ovulatory follicle.