The daunting goal to rescue oocytes collected immature in conventional ovarian stimulation cycles

Abstract

The final maturation of the preovulatory oocyte is only a short chapter of the book of oogenesis. Yet, it is crucial for the oocyte to achieve full developmental competence. In preparation for fertilization and preimplantation development, meiotic maturation is accomplished, while the cytoplasm undergoes major rearrangements. Cumulus cells are key to maturation, supporting the oocyte with regulative and metabolic cues. For reasons not entirely understood, a minority of oocytes are still immature when collected from stimulated ovaries in assisted reproductive technology treatment. In intracytoplasmic sperm injection this material is discarded, as most oocytes are mature and in general sufficient for treatment. However, in selected cases in which the yield of mature oocytes is low, immature oocytes could be rescued by pursuing in vitro maturation. Indeed, animal and human studies have shown that oocyte in vitro maturation is not a “mission impossible”. Major hurdles, however, persist. Crucially, in the intracytoplasmic sperm injection procedure, cumulus cells are removed; with them, essential support to cytoplasmic maturation is also lost. So, while meiotic maturation may well occur in vitro, achievement of full developmental competence in a cumulus cell-free system remains a daunting task.