Maturation of human oocytes in vitro and their developmental competence

Complete maturation of oocytes is essential for the developmental competenc e of embryos. Any interventions in the growth phase of the oocyte and the f ollicle in the ovary will affect oocyte maturation, fertilization and subse quent embryo development. Oocyte size is associated with maturation and emb ryo development in most species examined and this may indicate that a certa in size is necessary to initiate the molecular cascade of normal nuclear an d cytoplasmic maturation. The minimum size of follicle required for develop mental competence in humans is 5-7 mm in diameter. Maturation in vitro can be accomplished in humans, but is associated with a loss of developmental c ompetence unless the oocyte is near completion of its preovulatory growth p hase. This loss of developmental competence is associated with the absence of specific proteins in oocytes cultured to metaphase II in vitro. The comp osition of culture medium used successfully for maturation of human oocytes is surprisingly similar to that originally developed for maturation of ooc ytes in follicle culture in vitro. The presence of follicle support cells i n culture is necessary for the gonadotrophin-mediated response required to mature oocytes in vitro. Gonadotrophin concentration and the sequence of FS H and FSH-LH exposure may be important for human oocytes, particularly thos e not exposed to the gonadotrophin surge in vivo. More research is needed t o describe the molecular and cellular events, the presence of checkpoints a nd the role of gene expression, translation and protein uptake on completin g oocyte maturation in vitro and in viva. In the meantime, there are very c lear applications for maturing oocytes in human reproductive medicine and t he success rates achieved in some of these special applications are clinica lly valuable.