Oocyte maturation, fertilization and embryonic growth in vitro.

As the oocyte develops into an embryo, cytological and metabolic events follow one another in an accurate and successive sequence. Meiosis resumes in the ovarian follicle, parallel to cytoplasmic and membrane maturation, from the onset of the ovulatory LH discharge. Only a fully mature oocyte will be recognized and penetrated by a fertilizing sperm, to ensure rapid and synchronous male and female pronuclear growth and early embryonic development. In vitro, the resumption meiosis is easily obtained once the oocyte is withdrawn from the inhibitory influence of the follicle. Cytoplasmic and membrane maturation may however be impaired, leading to fertilization failures or anomalies such as triploidy and even impaired embryo viability. Human in vitro fertilization is nowadays routinely carried out with a high success rate, but in vitro embryonic growth to the blastocyst is still unsatisfactory even with oocytes matured in the ovary, and major improvements are needed to reach optimal viability. Many studies have now been published on human oocyte maturation, fertilization and the growth of embryos in vitro. We give only a brief account of them, due to limited space, and have therefore included topics of most relevance to assisted conception as opposed to those more involved with academic research.