ART failure: oocyte contributions to unsuccessful fertilization

BACKGROUND: The complexity of fertilization failure during assisted reproductive technologies (ART) is often under-appreciated, as this failure can occur at any number of essential mechanistic and cellular events. Importantly, successful fertilization is heavily dependent upon inherent qualities of the oocytes, and thus reliant upon fidelity of oocyte maturation. METHODS: Pubmed and medline were searched up to April 2008 for papers on oocyte fertilization and its mechanistic components. References to clinical/human studies were selected wherever possible. RESULTS: Successful oocyte maturation cannot simply be determined via visual assessment of polar body extrusion, but rather entails coordination of numerous cytoplasmic processes not readily observed. Proper regulation of intra-oocyte signaling cascades is crucial for sufficient production and storage of carbohydrates and proteins, successful relocation of organelles and regulation of metabolic pathways required for an apparently mature metaphase II oocyte to complete subsequent fertilization events; such as cumulus penetration, sperm/oocyte binding, fusion, oocyte activation, sperm processing and pronuclear (PN) formation. Regulation of oocyte maturation begins during oocyte growth and is intimately connected with events influencing folliculogenesis. Therefore, the oocyte is subject to a multitude of potential effector impacting fertilization potential and developmental competence long before encountering the artificial environment of the IVF laboratory. CONCLUSIONS: Although meticulous care and continued research is essential for future improvement, failure to fertilize and properly form PN following clinical ART is likely to be dependent on historical events in oocyte maturation, not easily explained or prevented through simple modification of contemporary laboratory protocols.